Radiochemistry of Germanium

Home , Germanium disulfide, Germanium tetrachloride, Isotopes of germanium, Organogermanium compound

Jointly published by Journal of Radioanalytical and Nuclear Chemistry, Articles, Elsevier Science S. A., Lausanne and Vol. 202, Nos 1-2 (1996) 7-102 Akaddmiai Kiad6, Budapest

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RADIOCHEMISTRY OF GERMANIUM

S. MIRZADEH,* R. M. LAMBRECHT**

*Nuclear Medicine Group, Oak Ridge National Laboratory, Oak Ridge, TN, 37830 (USA) **Biomedicine and Health, Australian Nuclear Science and Technology Organisation, Menai,NSW (Australia)

(Received March 13, 1995)

Contents

1. Some general information 9 1.1 Natural occurrence 9 1.2 Environmental concentrations 13 1.3 Toxicity 14 1.4 Applications 15

2, General review of the inorganic and analytical chemistry of germanium 16 2".1 Germanium metal 16 2.2 Germanium compounds 19 a. Hydrides 21 b. Halides 22 c. Oxides 23 d. Sulfides 23 e. Organometallic compounds 24 2.3 Electrochemistry 25 2.4 Detection of germanium 25 a. Activation analysis 26

3. Production of germanium radioisotopes 27

4= A summary of the chemical behavior of carrier-free germanium-68 32

5, Hot-atom chemistry of germanium 34

6. Separation methods 38 6.1 Volatilization and gas chromatography 39 6.2 Precipitation and coprecipitation 41 a. Reaction with hydrogen sulfide 41 b. Coprecipitation with acid-insoluble sulfides 42 c. Coprecipitation with hydroxides of group III elements 43

6.3 Extraction 44 a. Extraction with organic solvents 44 b. Extraction of germanium complexes 48 c. Extraction with hydroxyl-containing organic ligands 51 6.4 Ion-exchange chromatography 52 a. Cation exchange 53 b. Anion exchange 56 c. Adsorption on inorganic exchangers 59 d. Thin-layer and paper chromatography 62 6.5 Electrodeposition 66

7. Applied radiochemistry o! germanium 67 7.1 68Ge--~6eGagenerator in biomedical research 67 7.2 71Ga[v,fl]71Ge: A radiochemical detector for the solar neutrino 72

8. Selected radiochemical procedures 75 8.1 Separation of germanium radioisotopes from various media 76 a. Fission products 76 b. Proton-irradiated RbBr target 76 c. Proton-irradiated Ga metal target 78 d. Proton-irradiated Ga4Ni target 79 8,2 Rapid separation of germanium isotopes from fission products 80 83 Determination of germanium by neutron activation 81 8.4 Thin-layer chromatographic separation of carrier-free 77As from 77Ge 82

References 85

Since the publication of Radiochemistry of Germanium_ (NAS-NS-3043) in 1961, there have been significant developments on the subject. During the period from 1970 to 1980, the diagnostic utilization of the 68Ge--.6SGagenerator system in nuclear medicine stimulated research in the field. In addition, over the past 30 years there have been many advances in the analytical chemistry of germanium (Go), owing to the rapid increase in application of Ge in the electronics industry and, most recently, as an important component in infrared spectrometers.

This latest review has been completely rewritten. A literature search has been completed through December of 1990. Literature for selected topics has been surveyed through September 1993. The first section contains general information about germanium and its radioisotopes, and relevant nuclear data in tabulated form. in the second section, a general review of the inorganic and analytical chemistry of Ge is presented. Following these two introductory sections, subsequent sections deal with the production and preparation of germanium radioisotopes, separation and determination of Ge, of particular interest to the radiochemist, and selected procedures for its determination in or separation from various media. The section on separation chemistry has been greatly expanded.

The review includes sections on hot-atom chemistry and the chemical behavior of carrier-free 68Ge. A section entitled "Applied Radiochemistry of Germanium" deals specifically with 68Ge .._.68 Ga generator systems, the role of 71Ge in the detection of solar neutrinos, and the preparation of 68Ge positron sources for studying dislocations in metallic lattices and calibration of Positron Emission Tomography (PET) cameras.

Two other noteworthy points follow. Throughout the text, the oxidation state of a metal ion having only one stable state, such as germanium, is not explicitly indicated. Therefore, "Go" typically represents Ge4+, Other ions such as arsenic and tin, however, are indicated with their appropriate oxidation states. The term "carrier-free" applies to radioactive preparations to which no isotopic carrier (stable isotopes) is intentionally added. S. MIRZADEH, R. M. LAMBRECHT: RADIOCHEMISTRYOF GERMANIUM

1. Some General Information

1~.1. Natural Occurrence 1.2. Environmental Concentrations 1.3. Toxicity 1.4. Applications

1.1 Natural Occurrence

Germanium (Ge) is a semiconducting metalloid of the Group IVA elements. It was predicted and called eka-silicon by Mendeleef, and was discovered in 1886 by Clemens Winkler. It is grayish-white, lustrous and brittle like glass. It is diamond-cubic in structure when crystalline. Germanium and many of its alloys expand on solidification. There are five stable isotopes of germanium: 7~ (20.55%), 72Ge (27.37%), 73Ge (7.67%), 74Ge (36.74%) and 76Ge (7.67%). A recent measurement of the isotopic abundance of Ge in a range of terrestrial materials using solid-source mass spectrometry revealed no variations in isotopic abundances in any of the reagents or minerals analyzed. 1

Among the nineteen known radioisotopes of germanium, seven are proton-rich and decay with I~+ and/or electron capture (64Ge - 71Ge with 7~ being stable), and nine radioisotopes are neutron-rich and decay by iT (75Ge - 84Ge with 76Ge being stable). There are only four very short-lived metastable isotopes of germanium: 71rnGe, 75raGe, 77mGe and 79mGe. Of the four, the last three are neutron-rich. Germanium-68, with a half-life of 270.8:t:0.3 days, is the longest-lived radioisotope of germanium. A summary of the nuclear and decay data pertaining to germanium isotopes and their production is given in Tables 1.1 and 1.2. The neutron capture cross sections for germanium isotopes and a summary of the fission neutron-averaged cross sections for [n,p], [n,a] and [n,2n] reactions are given in Tables 1.3 and 1.4, respectively. The data have been taken from references 2-59.

On a cosmic scale, germanium occurs as a trace element in iron meteorites, in stony meteorites, in the sun, and in the stars. The cosmic abundance of Ge is ~50 atoms per lx106 atoms of silicon. The crust of the earth is estimated to contain 1-2 g Ge per ton, or 1-2 ppm, so it is somewhat more abundant than gallium and lead, but less abundant than arsenic, beryllium, boron and bromine. Ge is not found in the free state on e~lrth, but always in combination with other elements. It may be found as argyrodite, a sulfide of germanium and silver, germanite, which contains 8% of the element, in zinc ores, and in other minerals. It also occurs in significant concentrations (up to 2-8 ppm) S. MIRZADEH, R. M. LAMBRECHT: RADIOCHEM]STRY OF GERMANIUM

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Table 1.2 Production reactions

Atomic Production Atomic Production number reactions Ref. number reactions Ref.

64 64Zn[3He,3n] [28] 73m 73As (1~---.) [50] 54Fe[12C,2n] [29] 75 74Ge[n,y] [47] 65 64Zn(3He,2n) [30] 75As(n, p) [48] 4~ [31] 75m 74Ge[n,7] [47] 66 64Zn[a,2n] [32-34] 75As[n,p] [51] 64Zn(3He,n) [35] 76Ge(n,2n) [2] 56Fe(t2C,2n) [2] 77 76Ge[n,y] [47] 67 64Zn[a,n] [36] [37] 77m 7SGe[n,y] [47]

68 66Zn[a,2n] [38] 78 82Se[n,en] [2] Zn[a, xn] [39] fission [52] 69Ga[p,2n] [40] Ga[p ,xn] [41 ] 79a fission [53,54] Ge[p,pxn] [42] Y,Rb,Br,As[p,spall] [43,44] 79b [n,a], [55] fission [53-58] 69 69Ge[d,2n] [45]

Co[ C,pn] [2] 81 fission [53-58] Ga[p,xn] [2] 7~ [2] 82 fission [53-58]

71 7~ [47] 83 fission [53-58] 71Ga[d,2n] [48] 72Ge[n,2n] [46,49] 84 fission [53-58]

in many coals of the world, Germanium is recovered worldwide as a byproduct of production of other metals, primarily zinc, copper and lead. Production of germanium in the U.S. was 2.0x109 g in 1986. 6~

1.2 Environmental Concentrations

Germanium occurs widely in food; seafoods such as canned tuna contain ~3 ppm of Ge, and canned tomato juice and baked beans contain ~ 5 ppm of Ge. 61 As a result of the relatively high fraction of Ge in coal, a significant amount of Ge can be found in urban atmospheres. It is estimated that 2000 tons of Ge are discharged annually in stack gases, flue dust and ashes from coal burning plants in the United Kingdom. Coal ash may contain 20-280 mg of Ge/kg. 62

13 S. MIRZADEH, R. M. LAMBRECHT: RADIOCHEMISTRY OF GERMANIUM

Table 1.3 Neutron capture cross-sections of germanium isotopes 4s

Natural Cross-section (barn) Target abundance Product nuclei (%) nuclei tl/2 Thermal Epithermal 7~ 20.5 71mGe 22 ms 0.28+0.07 71gGe 11.15 d 3.15+0.16 71m+gGe 3.43• 1.5

72Ge 27.4 73Ge stable 0.98:1:0.09 0.76

73Ge 7.8 74Ge stable 15:1:2 63.7

74Ge 36.5 75rnGe 48 s 0.17+0.03 0.41:t:0.07 75gGe 82.8 m 0.34+0.08 75m+gGe 0.51+0.08 1.0+0.2

76Ge 7.8 77mGe 53 m 0.10+0.01 1.2+0.2 77gGe 1.45 h 0.06• 0.8• 77rn+gGe 0.15• 1.8i0.4

Table 1.4 Fission neutron averaged cross-sections of Ge for [n,p], [n,a] and [n,2n] reactions 4e

[n;p] [n,a] [n,2n] Target Nuclei Product e (mb) Product o (mb) Product a (mb)

70Ge 69Ge 1.8+0.9

72Ge 72Ga (2.2+0.6)x10-2 69mzn (2.5+0.6)x10 2

74Ge 71mzn (2.5+1.3)x10-3

1.3 Toxicity

Among the subgroup IVA elements, Ge is considered the least toxic. Pharmacological inertness, diffusibility, and rapid excretion are responsible for low Ge toxicity in mammals.61 Germanium compounds also have a low order of toxicity. Only germanium hyddde has a hemolytic effect and is considered toxic with a safe exposure limit of only 0.2 ppm for a maximum time-weighted average of 80 h. Germanium does not accumulate in human or animal tissues, but plants readily absorb Ge. Excretion of Ge by normal human adults ranges from 0.56 to 3.0 mg/day. For rats, the biological half- life of Ge is 1.5 d for whole-body retention, 2 d for the liver and 4.5 d for the kidney. 62

14 S. MIRZADEH, R. M. LAMBRECHT: RADIOCHEMISTRYOF GERMANIUM

It was concluded from animal experiments that the acute and chronic toxicity of elemental germanium and GeO 2 islow by all routes of administration tested, including inhalation. 63 The LDs0 values for Ge and GeO 2 are 580 and 750 mg/kg, respectively. Toxicity of Ge compounds usually depends on other chemicals in those compounds.

1.4 Applications

Germanium was primarily used in the 1970s as a substrate for GaAs light-emitting diodes in the field of semiconductor electronics. The semiconductor properties of Ge are not employed in this application. Another significant application is in the field of photovoltaic solar energy conversion. Its earlier use in transistors and diodes in electronic devices has been in decline since 1960.

The fastest growing use of Ge is in the field of infrared (IR) optics, where its transparency to IR wavelengths longer than 2 pm and its high refractive index are of greater use than its electrical properties. Ge is highly transparent to infrared light and is used in the production of glass capable of transmitting IR light for IR spectrometers and other optical devices with extremely sensitive IR detectors. Although the principal applications in this area are military, nonmilitary IR applications include lenses for CO 2 laser.s and windows for intrusion alarms.

Germanium is used as a solid-state detector of gamma rays. A specially prepared pure crystal of intrinsic Ge, which does not have to be stored in liquid nitrogen, or a crystal doped with lithium (Li-drifted), is used. As such, Ge is a very important analytical tool in nuclear research and diagnostic nuclear medicine. In medical applications, Ge- based gamma cameras are routinely used for the assessment of uptake and biodistribution of radioactivity in humans. Small crystals of bismuth germanate (Bi4Ge3012) are under evaluation to replace the Nal detectors used in high resolution positron emission tomography (PET). 64'65

Germanium is also used as an alloying agent and as a catalyst. It is used in gold alloys such as dental alloys when precision casting is required. Germanium catalysts are used in some petroleum refining operations and processes and for the production of polyester fibers. Ge is also used as a red-fiuorescein phosphor in fluorescent lamps. There is considerable optimism concerning its application in the construction of optical wave guides, primarily in the 0.8-1.6 pm wavelength region, for use in telephone and data transmission cables.

15 S. MIRZADEH, R. M. LAMBRECHT: RADIOCHEMISTRY OF GERMANIUM

After 40 years of commercial use in the electronics industry, the market picture for germanium is changing significantly. In the U.S., its use in the field of IR optics surpassed its use in electronics during 1978, indicating a probable worldwide trend. U.S. consumption patterns of Ge in 1986 were as follows: infrared optics-60%, fiber optics systems-15%, detectors-10%, and semiconductors (including transistors, diodes and rectifiers), 5%. 6~

As a long-lived source of positrons, 68Ge (tl/2=271 d) is the only radioisotope of Ge with commercial applications. Diagnostic nuclear medicine and metallurgy constitute two main applications. As a sealed source, it is used for attenuation correction in PET6e- 7o and for the study of imperfections in metallic lattices 71-73 (see also Sect. 6.5). As a predecessor to the short-lived daughter 68Ga (tl/2 = 68 min), it is used for on-site generation of this isotope in a 68Ge/68Ga generator system.* This generator is discussed in some detail in Sect. 7.1. The potential of 68Ga radiopharmaceuticals as PET agents is therefore another commercial application of 68Ge. For examples of 68Ga radiopharmaceuticals, see references 74-76.

2. General Review of the Inorganic and Analytical Chemistry of Germanium

2.1. Germanium Metal 2.2. Germanium Compounds a. Hydrides b. Halides c. Oxides d. Sulfides e. Organometallic Compounds 2.3. Electrochemistry 2.4. Detection of Germanium a. Activation Analysis

2.1 Germanium Metal

Germanium is quite stable in air up to 400~ where slow oxidation begins. Oxidation becomes noticeably more rapid above 600~ Germanium is a base metal and thermodynamically unstable in the presence of water, tending to dissolve in aqueous solutions at all pHs. In neutral or acidic solutions, Ge is covered with rather insoluble oxides. The dissolution takes place mainly in the oxidation state of +4, with the

* The decay of 68Ge actually takes plat9 with 100% electron capture and the positron emission is from the daughter, 68Ga (see Fig. 7.1).

16 S. MIRZADEH, R. M. LAMBRECHT: RADIOCHEMISTRY OF GERMANIUM formation of germanate. In practice, Ge is practically unaffected by water, conc. HCI, dilute HNO 3, and dilute alkali hydroxide. Ge reacts readily with a mixture of nitric and hydrofluoric acid, molten alkali peroxides, nitrates, or carbonates, and is more slowly attacked by aqua regia, conc. HNO3 or H2SO4.

Table 2.1 Some physicochemical properties of germanium 2' 77, 78

Property Constant

Atomic number 34 Atomic weight 72.59 g.mol "1 Melting point 937.4 ~ Boiling point 2834 ~ Specific gravity 5.323 g.cm "3 at 25 ~ Atomic volume 13.2 cm3.mol 1 Ionic radii, M +4, in crystals 2.65x10 -9 cm Thermal expansion coef. 6.1x10"6 at ~25 ~ Thermal conductivity 0.14 cal.sl.cml.~ Specific heat 0.074 cal.gl.~ 1 (0-100 ~ Heat of fusion 8.1 kcal.gl.atom 1 Heat of vaporization 79.9 kcal.gl.atom -1, 334 kJ.mol 1 Hardness 6.0-6.5 (Mohs' scale) Electrical resistivity 0.089 ohms.cm "1 at 20 ~

Ionization potentials (eV): I st 7.900 2nd 15.9346 3rd 34.2241 4th 45.7131 5th 93.5

Atomic energy levels Level Trans. E (keV) lsl/2 K 11.104 2Sl/2 L 1 1.413 2Pl/2 L2 1.249 2P3/2 L 3 1.217 3Sl/2 M 1 0.181 3Pl/2 M 2 0.129 3P3/2 M 3 0.122 4d3/2&5/2 M4, 5 s "~9

K X-rays Trans. __Ex (keV) I x (Rel.) Ka2 9.8553 51.3 Kal 9.8864 100 K~I, 10.98 22.2 K~2, 11.10

Fluorescence yields, oJ 0.540

I7 S. MIRZADEH, R. M. LAMBRECHT: RADIOCHEMISTRY OF GERMANIUM

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18 S. MIRZADEH, R. M. LAMBRECHT: RADIOCHEMISTRY OF GERMANIUM

Table 2.3 Bond energies and bond lengths a

Bond energy, Do Bond Bond Compound distance, (KJ.mol"1) (KCal.rnol"1) r0 (A ~

H-Ge ' GeH 4 1.53 Ge-Ge 188 45 2.41 Ge=Ge - 272:1:21 65+5 Ge-F GeF 4 -452 -108 1.68 Ge-F GeF 2 481 115 Ge-CI GeCI 4 348.9 83.4 2.10 Ge-CI GeCI 2 -385 -92 Ge-Br GeBr4 276.1 66.0 2.30 Ge-Br GeBr2 325.5 77.8 Ge-I Gel 4 211.7 50.6 Ge-I Gel2 264.0 63.1

aAs compiled by James E. Huhey.78 The purpose of the table is for quick reference; for additional infor~mation, the original sources should be consulted 81, 82

The principal reaction with mixed acids is oxidation by one constituent followed by dissolution of the oxide by the other constituent. The reaction with fused alkali metals is a direct oxidation with the formation of alkali germanates. Ge is oxidized slowly by 3% H202 at room temperature, and rather rapidly at 90~ In fact, a solution of 0.1 M NaOH containing hydrogen peroxide (-0.5 M) is the best solvent for germanium. A mixture of NaOH and NaOCI also dissolves Ge powder. In the presence of air, water slowly dissolves a thin evaporated film of Ge, presumably because of the appreciable solubility of GeO 2. Germanium, less electronegative and more metallic than either carbon or silicon, stands in subgroup IV between silicon, which forms rare bivalent compounds, and tin, which forms a considerable range of bivalent stannous compounds. Some of the physicochemical properties of Ge are summarized in Table 2.1. Selected chemical thermodynamic properties of germanium are given in Table 2.2. In Table 2.3, the bond energies and bond lengths in some common compounds of germanium are tabulated.

2.2 Germanium Compounds

In most Ge compounds, the germanium atom is quadrivalent and forms a large number of compounds in which the atom is tetrahedral (e.g., GeH 4 and GeCI4), as does carbon and silicon. Germanium also forms neutral or anionic octahedral complexes such

19 S. M1RZADEH, R. M. LAMBRECHT: RADIOCHEMISTRY OF GERMANIUM

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20 S. MIRZADEH, R. M. LAMBRECHT: RADIOCHEM'I,STRY OF GERMANIUM as GeF62 in aqueous solution or complexes with I~-diketones in organic solvents. For example, the cesium salt, Cs2[GeCle], has a crystalline structure similar to that of (NH4)2[PtCI6], and is formed by the addition of concentrated HCI and alcohol to a mixture of germanium tetrachloride and cesium chloride. A reaction of acetylacetone on germanium chloride in benzene or chloroform yields the following complex which is a neutral complex and not cationic like the silicon derivative of acetylacetone. H3C _o ljoCI //CH 3 H--C \ /\Ge /•C--H / C --0 0 C\ H3C CI CH3

structure of Ge-diacetylacetone

The +2 oxidation state of silicon is unstable but well-defined with germanium in various inorganic compounds. The germanous compounds are all solids and strong reducing agents in aqueous solutions, in contrast to tin and lead. Like silicon, germanium does not form any compound of sp2 or sp hybridization, analogous to acetylene or ethylene, respectively. Although the Ge-Ge bond has the same strength as the Si-Si bond, 42.5 kCal.moF 1, the chemistry of the germanates is not comparable to that of the silicates, since the link Ge-O is weaker than Si-O.

A number of the significant chemical properties of germanium and its compounds are illustrated by the reactions given in Table 2.4. a. Hydrides

GeH 4 is less stable thermally than either CH4 or Sill4. In the absence of oxygen, decomposition of GeH 4 to elemental Ge and hydrogen occurs at ~350 ~ The trend in the decomposition temperatures of the rnonohydrides of group IV is shown below:

MH4 CH 4 Sill4 GeH 4 SnH 4 PbH 4

Decomposition 800 450 300 150 0 temperature, ~

21 S. MIRZADEH, R. M. LAMBRECHT: RADIOCHEMISTRYOF GERMANIUM

Germanium undergoes self-linkage similar to that of carbon and silicon, and the first three members of the germane series, GeH4, Ge2H 6 and Ge3H 8 are known to exist, whereas tin only forms SnH 4 and Sn2H6, and lead only forms PbH4.

The germanes are less reactive than the silanes toward oxygen. Whereas the silanes are spontaneously inflammable in air, monogermane is not attacked by oxygen below 230 ~ and is unaffected by water and alkali solutions. The germanes are resistant to base hydrolysis, in contrast to analogous silanes and, to a lesser extent, stannanes... For example, ordinary glass vessels provide sufficient base to bring about the hydrolysis of a silane, but germanium is only hydrolyzed by a considerably stronger base. The group IV hydrides have been reviewed in two monographs. 83'84

b. Halides

Germanium tetrahalides, with the exception of germanium tetrafluoride, can be prepared by the reaction of the respective halogen with metal at elevated temperatures and by the reaction of GeO 2 with haloacid followed by extraction in CCI 4 or C6H 6. Germanium tetrahalides are nonpolar molecules and soluble in organic solvents such as carbon tetrachloride, chloroform, benzene, alcohols, etc. All germanium tetrahalides are

Table 2.5 Properties and preparation of germanium tetrahalides a

Properties GeF 4 GeCI 4 GeBr4 Gel 4

Appearance colorless colorless colorless red gas liquid liquid crystals Sublimation Temperature, ~ -37 m.p., ~ -49.5 26.1 144 b.p., ~ 86.5 186.5 Decomposition temperature, ~ >1000 950 440 Ge-X distance, A 1.73 2.10 2.32 2.57

Preparation BaGeF6--~ Ge + CI2 Ge + Br2 GeO2+HI BaF2+GeF4 100-180~ at 220~ T= ? at 700 ~

aAdopted from the original compilation by Durrant and Durrant 85

22 S. MIRZADEH, R. M. LAMBRECHT: RADIOCHEMISTRYOF GERMANIUM hydrolyzed by water to germanium dioxide and the haloacid. In the case of GeF 4 the final product is fluorogermanic acid (H2GeF6).

GeF 4 + 2H20 . ' GeO 2 + 2HF

GeF 4 + 2HF ~ ~ H2GeF 6

Germanium tetrachloride, which is of particular analytical value for the separation of germanium, is slightly soluble in HCI, with the solubility dropping rapidly with acid concentration and reaching a minimum at azeotropic HCI ( -6 M at 760 mm Hg). It is not soluble in H2SO 4 or decomposed by it. Germanium tetrachloride can be distilled and converted to germanium oxide by hydrolysis in water.

The properties and preparation of the germanium tetrahalides are summarized in Table 2.5.

c. Oxides

Germanium dioxide exists in three forms: the hexagonal soluble form (a), the tetragonal insoluble form (1~) and the amorphous vitreous form. The soluble form is made by hydrolysis of GeCI4 Or alkali metal germanates with water. The insoluble form is prepared by heating soluble oxides to a temperature greater than 200~ The vitreous form is made by rapid cooling of molten a or I~ forms.

The solubility of a-GeO 2 in water increases rapidly from 270 mg/100 g H20 at 0 ~ to 453 mg/100 g H20 at 25 ~ and to 1190 mg/100 g H20 at 100 ~ The solubility of the a-form also varies with aging, dehydration, exposure to heat and degree of contamination. The insolubility of the I~-form in conc. HNO 3 offers a unique method for separating germanium from other elements. The interfering elements include aluminum, silicon, zirconium, niobium, and tungsten, which all produce insoluble oxides. The a-form tends to form colloidal solutions when the concentration is above 0.05 M__. Germanium oxides in pure hydrogen are reduced to Ge metal at a temperature above 700 ~ and to v'~latile GeH 4 above 1000 ~

d. Sulfides

White germanium disulfide (GeS2), which is slightly soluble in 3 M HCI or 6 M H2SO4, is precipitated from strongly acidic solutions by hydrogen sulfide. It is readily soluble in alkali sulfides -- 6 M ammonium sulfide is the best solvent. In the presence

23 S. MIRZADEH, R. M. LAMBRECHT: RADIOCHEMISTRY OF GERMANIUM

of fluoride ions, germanium is not precipitated by hydrogen sulfide from acidic solution. Ions of As +3 can be separated from germanium in this way, since arsenic sulfide, As2S3, is precipitated in the presence of F-. The crystal structure of germanium disulfide is a three-dimensional tetrahedral structure of GeS4 units, similar to silica, in which the S-Ge- S angle is 103 ~ and the Ge-S distance is 2.19 A. 85

From an analytical point of view, germanium monosulflde is of less value. It is formed by heating a mixture of GeS2 and metallic Ge, or by reduction of GeS 2 with H2 at elevated temperatures, or it can be precipitated by H2S from a solution of GeCI2 in 1-2 M HCI. Crystalline GeS is slightly soluble in acid and alkali, but precipitated GeS is readily soluble in HCI with evolution of H2S. It is also soluble in alkali and alkali metal sulfides. It dissolves with difficulty in solutions of ammonium hydroxide and ammonium sulfide.

e. Organometallic Compounds

The organometallic chemistry of germanium has received considerable attention in the past two decades. Several reviews of the topic covering the period before 1970 are available. 8689

In general, organogermanium compounds are characterized as having low chemical reactivity and high thermal stability. From the point of view of their potential applications in radiochemistry and hot-atom chemistry, the most interesting complexes are those with phenols, 9~ 8-hydroxyquinoline, 91 I~-diketones (e.g., acetylacetone), 85 and phthalocyanines. 92 Phthalocyanine germanium dichloride, similar to many other metal phthalocyanines, shows high thermal stability (sublimes in vacuo at 450~ and is of special interest in studies of the Szilard-Chalmers process. 93 In this complex, the chlorine atoms may be substituted without destroying the complex. 94

Another organogermanium compound with potential applications in hot-atom chemistry is tetraphenyl germane, with substantial thermal stability (see Sect. 5). Aminocarboxylic acids form 1:1 complexes with Ge. The Ge complex with ethylenediaminetetraacetic acid (EDTA) is formed in weakly acidic solution with a formation constant of 2x105 (/1=0.5, 25 ~ The crystalline complex of GeEDTA,2H20 is made by boiling a solution of GeO 2 with a large excess of Na2EDTA. ' The crystalline form loses both water of hydration molecules at about 200 ~ The GeEDTA complex is stable up to 250 ~

24 S. MIRZADEH, R. M. LAMBRECHT: RADIOCHEMISTRY OF GERMANIUM

2.3 Electrochemistry

The electrical properties of germanium have received much attention; germanium's electrical properties have possibly been studied more than those of any other metal. 97 However, most" of these studies are of very little value to the radiochemist. The electrical potential values of selected electrochemical reactions of germanium are shown in Table 2.4, 80 and are comparedwith those of tin and lead in the following potential diagram. 94

ACIDIC SOLUTION BASIC SOLUTION

0.0 0.3 1.0 Ge ~ Ge+2-9~------~- GeO2 Ge ~ HGeO2

0.136 -0.15 0.91 0.93 SFI ~ SN+2~Sn +4 Sn HSnO- ~ Sn(OH)6-2..

0.126 -1.455 0.54 -0.28 Pb ~ Pb+2~PbO2 Pb ~ PbO -,=e------=- PbO2

The reduction of the Ge on dropping mercury has been studied in detail, and an overview of published work is given by Nazarenko. 95

2.4. Detection of Germanium

A detailed discussion of the determination of germanium by gravimetric, volumetric and photometric techniques is given by Nazarenko. 95 In Table 2.6, the detection limits for Ge by methods of instrumental analysis, including neutron activation, are summarized. The selectivity afforded by germanium hydride generation and purification in a gas chromatograph, in combination with the other instrumental techniques listed in Table 2.6, will further increase the sensitivity for Ge detection. The neutron activation method, in combination with radiochemical techniques, also offers very high sensitivity, depending on the neutron flux (see also Sect. 8.3).

25 S. MIRZADEH, R. M. LAMBRECHT: RADIOCHEMISTRY OF GERMANIUM

Table 2.6. Detection limits for germanium and its compounds

Method Detection limit Ref.

Spectrophotometry with 0.1-0.5 mg/I [95,98] phenylfluorone Atomic absorption 1.5 mg/I [99] Atomic absorption/ graphite tube atomizer 0.015 mg/I [100] Emission spectroscopy 1 pg [101] Microwave cavity plasma emission spectrometry 1 ppb [102] Spark source mass spectrometry 7 pg/Kg [103] Mass spectrometry 1 ppm [104] Neutron activation/radiochemical 0.02 pg [105]

a. Activation Analysis

In recent years, determination of elements including Ge in various matrixes with prompt X- or y-ray analysis has received much attention. The projectiles for these studies included not only the elementary particles but even some light elements. A comprehensive discussion of the subject is beyond the scope of this review, but a partial list of reported work using common projectiles is given in Table 2.7. l~

As discussed in Sect. 1.1, germanium in nature consists of five stable isotopes, but only three can be used in neutron activation analysis (NAA): 7~ 74Ge and 76Ge with natural abundances of 20.5, 36.5 and 7.7%. The thermal neutron capture of the

Table 2.7. A partial list of activation methods for Ge determination

Analysis Ref. Analysis Ref.

Neutron Activation Charqed Particle Activation

Reactor spectrum [105-108] Protons [115]

Epithermal [109] Tritons, 3.5 MeV [116] " (boron filtered) [110] Photon Activation Cyclic/Pulsed irradiation 40-44 MeV [117] 14-MeV [111,112] Reactor spectrum [113,114]

26 S. MIRZADEH, R. M. LAMBRECHT: RADIOCHEMISTRY OF GERMANIUM above isotopes yields three pairs of metastable/ground state nuclides. All three metastable nuclides are very short-lived (tl/2<60 s) and can only be used for on-line analysis. 111-114 The applicability of pulsed neutron irradiation has also been demonstrated by the use of very short-lived 73mGe (t1/2=0.50 s) for the detection of germanium. 114 Obviously, fast transfer lines and automated counting facilities are prerequisite for the application of such a technique.

The ground state nuclides, 11.2-d 71gGe, 83-m 75gGe and 11.3-d 77gGe, are formed with thermal neutron cross sections (aq+m) of 3.34 +0.2 b, 0.51+0.08 b and 0.15+0.02 b, respectively (see Table 1.3). The76Ge[n,~,] 77gGe reaction is the most suitable reaction. 1~ Considering the 11.3-d half-life of 77gGe and its ease of detection by y-ray spectroscopy (emission of 264 keV with an absolute intensity of 53%). The most sensitive reaction for the determination of Ge by neutron activation can, however, be expected from the 74Ge[n,y]75gGe reaction, if the facility exists to accommodate the relatively short half-life of 75gGe (82.8 m). The 71gGe decays with 100% EC and accordingly requires special attention with regard to its counting. When the utmost sensitivity is required, 71Ge can be analyzed in a miniaturized proportional counter loaded with GeH 4 gas as described earlier (see Sect. 7.2).

Without chemical separation sensitivity of NAA for Ge in copper ores is 3x10-3% at a thermal neutron flux of lx1012 n.s-l.cm -2. The attainable sensitivity increases substantially with the radiochemical NAA. At a ~Pn=lXl014 n.s-l.cm -2, quantities of Ge as low as 10-3 pg can be detected by radiochemical NAA.

3. Production of Germanium Radioisotopes

A list of references for nuclear reactions for the formation of radioisotopes of germanium was given in Table 1.2. Germanium-68 and -69 are the only two radioisotopes of germanium with sufficiently long half-lives (270.8 d and 11.2 d, respectively) and suitable physical decay characteristics (such as gamma-ray emission for ease of detection) to be useful in a typical radiochemical study. Germanium-71 (tl/2=11.2 d), which decays by pure electron capture with no gamma ray emission, is of interest in chemical studies of the hot atoms (Sect. 6), and in gallium solar neutrino detectors (Sect. 7.2).

Germanium-68, as the parent of the positron-emitter 68Ga (Sect. 7.1) is of particular importance, and its large scale production is discussed. Due to its rather long half-life, the large-scale production of 68Ge (100 - 1000 mCi) is currently limited to proton

2"7 S. MIRZADEH, R. M. LAMBRECHT: RADIOCHEMISTRY OF GERMANIUM

accelerators of high current and continuous operation modes such as the BNL 200-MeV LINAC (BLIP facility) and the LANL 800-MeV LAMPF.

The BLIP facility makes use of the 69Ga[p,2n] reaction with a metallic gallium target of natural abundance. The Ga target is last in the target array and is practically the beam stop for the typical 50-pA proton beam. The thickness and the types of the upstream targets are typically chosen in order to have an incident proton energy of ~20 MeV on the Ga target. At the BLIP facility, the yield of 68Ge is on the order of a few hundred mCi per run. 41

At LAMPF, eSGe is produced by proton spallation of a RbBr target with an effective cross section of -20 mb. 43' 118-120 In this case, the typical yield is on the order of 1 Ci per run. Cross sections from individual targets of Y, Rb, Br and As range from 6.8 to 11.1 with 593-MeV protons. 44 Germanium-68 is also produced by proton spallation of Mo, 121 Ag and Ta. 122 A summary of the production cross section of 68Ge via spallation reactions is given in Table 3.1.

Germanium-68, in the range of tens of mCi, could also be produced in a small cyclotron. 34'40'42'123-126 The excitation functions for the production of 68Ge and other proton-rich germanium nuclei with various charged particles are given in Figs. 3.1 to 3.3.

Table 3.1. Proton spallation yields of 68Ge from various targets

Ep(MeV) Target oeff(mb ) Ref.

450 Ta 0.88• 122

593 As 10.7 44 Br 11.1 44 Rb 7.8 44 Y 6.8 44

800 RbBr 19• 43,118 119,120 24 GeV Ag 17.3~3.5 122

A summary of the reaction types and the expected thick target yields is given in Table 3.2. Although the yield of 68Ge from proton bombardment of natural germanium is highest, the product is not carrier-free.

28 S. MIRZADEH, R. M, LAMBRECHT: RADIOCHEMISTRY OF GERMANIUM

103 ......

"~Ga (p,xn)

102 / ~'~.Z n (c~'xn) ' E o_Z F- (b w

o3 0 g 101 A

100 I BI, I C , , ,

0 10 20 30 40 50 60 70 Ep or Eo~ (MeV)

Figure 3.1. Comparison of various excitation functions for production of SSGe, As compiled by Horiguchi, T. et aL42 Reprinted from Int. J. Appl. Radiat. Isot. 34, 1531, Copyright (1983) with permission from Pergamon Press Ltd., Headington Hill Hall, Oxford OX30BW, UK. A) Porile, N. T. et al. 1963,125 B) Nagame, Y. et al. 1978,124 C) Horiguchi, T. at al. 1983. 42

Germanium-66 (tl/2=2~3 h, 100% EC) is the predecessor to carrier-free 9.4-h 66Ga, which is of interest in nuclear medicine for its therapeutic properties, 33 and it can be produced by 3He or an a-induced reaction on enriched 64Zn. The excitation functions for the 64Zn[a,2n] reaction were recently reevaluated by Mirzadeh et aL 33

Germanium-71 can be produced in a reasonable specific activity from neutron capture of 7~ with a thermal cross section of 3.2 b.* For example, at a thermal neutron flux of lx1014 n.s-l.cm "2 and thermal to epithermal ratio of 20, the expected specific activity of 71Ge would be on the order of 80 mCi per mg of 7~ at saturation and at the end of bombardment. It is also possible to produce this radioisotope in its carrier-free state from the decay of 71As (tl/2=64 h). The 71A8 precursor is best made

*See Tables 1.3 and 1.4 for cross sections for neutron-induced reactions on Ge.

29 S. MIRZADEH, R. M. LAMBRECHT: RADIOCHEMISTRY OF GERMANIUM

5" ~ ~" ~ ~

Ob ~ ,,4- ~

0 0 ~

o > :s 0 r CO 0 0 LO ~ 0,4 ~1 ~,1 t~ ~., [3. uJ U

E ~, o E 0 E, o .i t-" o3 E E E E U 0 E C}b.. Lo 03 .o I-- e"> C',-. C',.. ~ 0 ~ C,--

E 0

,9 13,.

=~ o ~ "6 ~ o ~ c c U U E 5 fi t~

c C C ~ ~7 ~C X

(.0 [..0 r (~ N I,- z

30 S. MIRZADEH, R. M. LAMBRECHT: RADIOCHEMISTRY OF GERMANIUM by 72Ge[p,2n] reaction. At the end of irradiation, 71As is quickly separated from the Ge target, purified and allowed to decay to 71Ge (t1/2=11.2 d). Authors are not aware of any published experimental work.

Germanium-75 (tl/2=83 m) is also produced in a nuclear reactor with a total neutron capture cross section of 570 mb and is suitable in neutron activation analysis of germanium. 1~ Germanium-77 (tl/2=11.3 h, 6") is the predecessor to carrier-free 77As (tl/2=40 h) which is of interest for cancer treatment because of its nuclear decay properties. The expected yield of 77Ge at a neutron flux of lx1014 n.s'l.cm "2 and thermal to epithermal ratio of 20 is ~5 mCi per mg of 76Ge. A discussion of the experimental results is given in procedure 8.4.127

103 I I I I

69Ge ~ ~o-O-O-o.o "67 G E :7 O I- O ' / \ 66Ge 111 09 r C~ .o~O--.-...O.o~. O~ 0 n~ 0 ./ "/ / 100 I I I I 10 20 30 40 Eoo (MeV)

Figure 3.2, Excitation functions for nat'zn(a,xn)SS'SeGe reaction.

Nagame et aL 124 Reprinted from Int. J. Appl. Radiat. Isot. 29, 615, Copyright (1989) with permission from Pergamon Press Ltd., Headington Hill Hall, Oxford OX30BW, UK.

31 S. MIRZADEH, R. M. LAMBRECHT: RADIOCHEMISTRY OF GERMANIUM

103 l I I I I

69Ge

..Q E 102

z 68Ge/o ' ~O.te~O.oo-e-o uO I--- O LU 03 03 03 O rr 101 0

100 I I I I l 1 0 10 20 30 40 50 60 7O Ep (aeV)

Figure 3.3. Excitation functions for nat'Ge(p,pxn)68'SgGe reactions.

Horiguchi, T. et al. 42 Reprinted from Int. J. Appl. Radiat./sot. 34, 1531, Copyright (1983) with permission from Pergamon Press Ltd., Headington Hill Hall, Oxford OX30BW, UK.

4. A Summary of the Chemical Behavior of Carrier-free Germanium-68

Little information can be found in the literature regarding the chemical behavior of carrier-free radiogermanium prior to 1970. During the period from 1970 to 1980, the diagnostic utilization of the 68Ge/68Ga generator system in nuclear medicine stimulated research in this field.

'Carrier-free 68Ge in dilute HCI (<0.1 _M), neutral, and dilute basic (<0.1 M) solutions is stable with respect to adsorption by Pyrex, quartz, Teflon and polyethylene, loss by evaporation and formation of radiocolloids, over a period of three months. 128 Germanium-68, free from chloride ions, is also stable in 0.1 __M HCIO 4 and 10 -4 IM NH4OH In strong HCl solutions, carrier-free 68Ge is volatilized at room temperature; the rate of volatilization from 8 M HCl solution is about 10% per month.

32 S. MIRZADEH, R. M. LAMBRECHT: RADIOCHEMISTRY OF GERMANIUM

If a chloride solution of 68Ge is added to concentrated perchloric acid, a loss of activity occurs. Therefore, to study the chemistry of germanium in strong acidic solutions (>6 M), it is advisable to use a chloride-free solution of germanium. This can simply be accomplished by the addition of concentrated nitric acid to the germanium-chloride solution in a Teflon beaker and evaporating the mixture to dryness under an IR heat lamp. Germanium activity can then be leached from the surface of Teflon with water or other appropriate chloride-free reagents. 129

In the absence of chloride ions, 68Ge solutions are stable when the concentration of HCIO4 or HNO 3 is below 6 M. When the concentration of either acid exceeds 6 M, 68Ge is adsorbed by Pyrex glass. In 13.5 M HNO 3, a significant fraction of activity (~35%) is lost to the glass within a 24-h period. 128'13~ Similar results are obtained with strong HCIO4, (>6 M) and the adsorption process is found to be largely irreversible. Depending on the glass type, some of the adsorbed germanium ~an be leached from the surface with water. After treating the surface with NH4OH followed by evaporation to dryness, however, the fraction of 68Ge leached with water increases significantly.128

A series of centrifugation experiments has indicated that 68Ge forms radiocolloids in strong nitric and perchloric acids. In a four-hour-old solution, the fraction of the germanium centrifuged, in 30 minutes and under a centrifugal acceleration of 1200g, increased from 7.5% to 46% when the concentration of perchloric acid was increased from 6.0 M to 9.8 M. About 86% of carrier-free 68Ge was removed from 9.8 M HCIO4 solution when centrifugal acceleration was about 10,000 times gravity. Similar behavior of carrier-free ~ has been observed in strong nitric acid solutions (>6 M). 128'130

Germanium-68 forms radiocolloids in moderately acidic solutions (0.1-5 M) in the presence of sulfide ions; these colloids can be centrifuged and filtered from solution. In general, reproducibility of the results of these experiments is rather poor, and the uncertainty in the duplicate runs in some cases is as high as 20%. In a number of experiments, the fraction of 68Ge centrifuged from 0.1-5 M H2SO4 or HCIO4 saturated with H2S increased with acid concentration, passed through a maximum (~50% in 2.6 M HCIO4 and ~60% in 1.8 M H2SO4), and decreased with further increases in acid" concentration. In 2.5 M HCIO4 in the presence of sulfide ion, the fraction of 68Ge centrifuged appeared to increase from 20% to 40% when the age of the solution increased from 1 to 24 hours. Similar results have also been obtained with ultrafittration experiments, utilizing 10-,urn Millipore filters. 128

The adsorption for carrier-free 68Ge by Pyrex, quartz, Teflon, polyethylene, powdered glass and platinum foil is negligible over the entire pH range. Carrier-free

33 S. MIRZADEH, R. M. LAMBRECHT: RADIOCHEMISTRY OF GERMANIUM

68Ge is almost quantitatively adsorbed by Fe20 3, ZnO 2, and AI20 3 at pH 5.9 and 8.8. A significant fraction of carrier-free 68Ge is adsorbed by Pyrex from strong nitric and perchloric acid solutions (> 6 M), and adsorption is nearly quantitative on 300 mg of Pyrex powdered glass within 4 hours. The adsorption process is found to be largely irreversible. Adsorption on quartz, Teflon and platinum foil is not significant from a strong acidic solution. Upon evaporation of.carrier-free 68Ge from a nitric or perchloric solution in an ordinary Pyrex vessel, activity is usually lost to the glass vessel and cannot be recovered unless the surface is treated with base. Typically, a few ml of 6 M ammonium hydroxide are added to the vessel and evaporated to dryness. The activity can then be recovered from the glass surfaces by leaching with a dilute acid.

In the presence of sulfide ions, adsorption occurs in moderately acidic solutions (0.1-5 M H2SO 4 and HCIO4). Carrier-free 68Ge is essentially quantitatively removed from 5.5M HCIO4 by adsorption on colloidal sulfur. The sulfur-adsorbed 68Ge is only slightly soluble in water (less than 2%). In distillation from acidic chloride solutions, carrieFfree 68Ge exhibits a behavior very similarto that of macroscopic quantities of Ge, and no novel phenomena are observed at the tracer level. In summary, carrier-free 68Ge quantitatively and conveniently distills from azeotropic HCI. 131

5. Hot-Atom Chemistry

Because Ge complexes are suitable for hot-atom studies, research in the field of hot-atom chemistry of Ge has focused primarily on studies with the objective of understanding the mechanism of such reactions. Early work was aimed at the chemistry of energetic Ge atoms produced in neutron-induced reactions. These studies were later extended to include investigations of the chemical fate of the decay product of Ge radioisotopes incorporated into either organic or inorganic molecules. 93'132"147 Reviews by Wiles and Baumgartner 148 and Halpern 149 describe most of these early studies.

One aspect of the hot-atom chemistry of Ge deals with the reaction of energetic tritium with the germanes, t5~ Another class of studies focused exclusively on the effect of the chemical environment on the decay constant of 71Ge.151"153 Several aspects of Ge hot-atom chemistry are listed in Table 5.1 and discussed briefly in this section.

Effects of the recoil process from [n,y] and [n,2n] reactions on a series of metal phenyl (9) compounds of Ge in solid and liquid states were studied by Merz and Riede193, 142-144 The distribution of the radioactive recoil products formed was compared with that of those produced after EC-decay of 68Ge~b4 and IT-decay of 77Ge~4. In the

34 S. M1RZADEH, R. M. LAMBRECHT: RADIOCHEMIS':I~Y OF GERMANIUM

Table 5.1. A summary of the hot-atom chemistry of Ge

Reaction References

Chemistry of energetic Ge atoms produced in nuclear reactions by nuclear decay

natGeO2[n,7177GeO2 (1~" --~) 132 77GEO2 (1~" ---*) 133-136

77GECI4 (1~" ---,) 137 77Gel4 (1~- --~) 133 natGe(Et)4[n,y]75'77Ge(Et)4 (1~- --,) 138-140 natGe~4[y, n]69Ge~4 (EC -* ) 141 natGer162 (1~" -*) 93,142,143 68Ger (EC --0 93,144 77Ge-Pc (1~" --~) 145

Reaction of hot Ge with germane and silane

75Ge + GeH 4 146,147 75Ge + Sill 4 146,147

Reaction of hot T with germane

T + GeH 4 150

Isotopic effect

71Ge/77Ge ' 69Ge/75Ge 143

Effect of chemical environment on the decay constant of 71Ge

71GeS (EC -,) 151-153 71GES2 (EC -.-,) 151-153

Et: C2H 5, ~: C6H s, Pc: Phthalocyanines neutron-induced reactions, the fraction of the inorganic form of Ge was found to be within'7-13% of the total activity produced. About 6% of the 77As formed from the I~'- decay of 77Ge~4, and 60-80% of 68Ga produced by the EC-decay of 68Ge~4, were found to be in inorganic form.

In a series of studies, Gaspar eta/. 146"147 studied the reactions of recoiling Ge atoms in germane, digermane and a germane-silane mixture. The recoiling Ge atoms

35 S. MIRZADEH,R. M. LAMBRECHT:RADIOCHEMISTRY OF GERMANIUM

were produced by 76Ge[n,2n]75Ge reaction.* A target filled with up to 3000 Torr of gaseous mixtures was irradiated with 108 n.s'l.cm "2 of fast neutrons produced by 9Be[d,n]l~ reaction using a 20-pA beam of 13-MeV deuterons. The irradiation periods were typically 45-60 min. The products formed in the target were analyzed by a radio- gas chromatograph. The absolute yields of 75GeH4, 75GeGeH6 and 75GeGe2H8 formed from the irradiation of pure germane are shown in Fig. 5.1. Below 3000 Torr, germane, digermane and trigermane were the only volatile radioactive species detected, and the remainder of the radioactivity produced was nonvolatile.

I t I I 1

75GeH4 /k/ LS Z~ 75Ge GeH6 / [] 75GeGe2H8 ~ 20

u~v 15 9uJ >- p-uJ =, -A [3 [] rO 10

o ~ -~

I I I I I 500 1500 2500 PRESSURE OF GeH4 (torr)

Figure 5.1. Variation of the absolute yields of 7SGeH4, 7SGeGeH6 and 7SGeGe2H8 with total pressure of GeH 4 147

Reprinted with permission from Gaspar, P. P. et al., J. Am. Chem. Soc. 91, 1574. Copyright (1969)American Chemical Society.

The threshold for this reaction is 9.35 MeV and cross section rises rapidly to 1.82 barns at 14.5 MeV.49

36 S. MIRZADEH, R. M. LAMBRECHT: RADIOCHEMISTRYOF GERMANIUM

As indicated in Fig. 5.1., the absolute yield of 75GeH4 changes insignificantly with a change in the total pressure. These results had led to the conclusion 147 that radioactive germane is formed by a mechanism other than direct activation of GeH 4. After formation, the recoiling 75Ge results in unimolecular decomposition of 75GeH4 leading to the formation of positively-charged species.* At different stages in the slowing down of the recoiled Ge, germyl (GeH3) and germylene (GeH2) free radicals are formed. These free radicals are believed to play an important role in the formation of the radioactive species (volatile and nonvolatile). It is important to realize that in the gas phase where intermolecular collisions are less frequent than in liquids and solids, the Columbic repulsion among the positively-charged species further invokes molecular fragmentation. Another aspect of the hot-atom chemistry of Ge deals with the reaction of energetic tritium with the germanes. Group IV hydrides have provided an ideal means for the study of hot T chemistry. 15~ In these studies, the recoiling T atoms are produced by 3He[n,p]T reaction in a nuclear reactor. Mixtures of GeH 4 and 3He were bombarded and then stored for a period of 30 d to allow 71Ge to decay. Analyses of the products were then carried out by a radio-gas chromatograph equipped with a flow-through gas- proportional counter. In a pressure range of 106-200 kPa, the yields of HT, GeH3T and Ge2H5T were rather constant at 71, 25 and 2%, respectively. Below 106 kPa pressure, the unimotecular decomposition of excited GeH3T became significant. The contribution of isotopic exchange to the product yields was ruled out based on the results of a separate experiment, where mixtures of HT and GeH4 or H 2 and GeH3T kept for 15 d at 330~ resulted in no exchange.

An isotopic effect on the recoil products of metallorganic compounds of Ge following [n,7] and [n,2n] reactions was observed by Merz. 143 In irradiation of Ge- tetraphenyl with thermal neutrons, the 71Ge/77Ge isotope ratios were up to 9% for the solid compound and up to 8% for the compound dissolved in cyclohexane or benzene. Similarly, in [n,2n] reaction, the 69Ge/75Ge isotope ratios were up to 10% for the solid Ge(p4, The effect of chemical environment on the decay rate of 71Ge was studied by Makariunas et aL 151-153 The relative change in the decay constants w obtained from the slope of the time-dependence ratio of 71Ge activity in two chemical environments (denoted as 1 and 2): Ro(t) = Ro(O) . [1-(AA/A)At]

where Ro(t) = Al(t)/A2(t), Ro(O) = AI(O)/A2(O) and Z~=]tl-~ 2

*The 4x104 eV recoil energy is sufficient to break all the bonds in GeH4

37 S. MIRZADEH, R. M. LAMBRECHT: RADIOCHEMISTRY OF GERMANIUM

The X-ray following electron capture of 71Ge was measured in a scintillation X-ray spectrometer. Decay constant changes on the order of 0.1% were observed between GeS and elemental Ge and between GeS and GeS2. Similar values also were reported for 53-d 7Be in BeF 2 and Be metal. 154

The data of Makariunas et aL, 151 reproduced in Table 5.2, support two general conclusions. First, in the +2 oxidation state, 71Ge decays faster than it does in either +4 or 0 states. Second, the values of Z~bl for sulfur compounds are larger than those for selenium .(;ompoundsl

Table 5.2. The effect of chemical environments on the decay rate of 71Ge151

Oxidation States Compound (Z~/A)x104

2 -0 GeS- Ge 9.2 • 2.3 GeSe - Ge 7.1 • 1.1

4 - 0 GeSe 2 - Ge 3.3 + 1.9 GeS2 - Ge -1:2 • 2.1

2 - 4 GeS - GeS2 10.4 • 1.0 GeSe - GeSe 2 3.8 • 1.5

Reprinted with permission from Makariunas, K., Dragunas, A., and Makariuniene, E., Hyperfine Interact. 36, 211. Copyright (1987), Elsevier Science Publishers.

6. Separation Methods

6.1. Volatilization and Gas Chromatography 6.2, Precipitation and Coprecipitation a. Reaction with Hydrogen Sulfides b. Coprecipitation with Acid-Insoluble Sulfides c. Coprecipitation with Hydroxides of Group III Elements 6.3. Extraction a. Extraction with Organic Solvents b. Extraction of Germanium Complexes c. Extraction with Hydroxyl-Containing Organic Ligands 6.4. Ion-Exchange Chromatography a. Cation Exchange b. Anion Exchange c. Adsorption on Inorganic Exchangers d. Thin-Layer and Paper Chromatography 6.5. Electrodeposition

38 S. MIRZADEH, R. M. LAMBRECHT: RADIOCHEMISTRY OF GERMANIUM

6.1. Volatilization and Gas Chromatography

The separation and purification of germanium based on the volatilization method take advantage of the volatility of GeH 4 and GeCI4. Both methods have been evaluated for their application with the carrier-free state.

Germanium hydride is best made by action of GeCI4 with LiAIH4 in ether, which requires only simple separation of GeH 4 from ether vapor, 155 or by reduction of GeO 4 by NaBH 4 in water solution. 156 In the latter method, GeO 2 is dissolved in 1 M HBr, and an excess of NaBH 4 solution (-5 g in 100 ml of H20) is added dropwise. The GeH 4 (with ~1% Ge2H6) is condensed from a stream of H2 in a series of five traps cooled to -196~ the yield is ~98%. 156 Thermal decomposition of GeH 4 occurs above 280~ and becomes rapid at 374~ with a deposition rate proportional to t,p GeH4)1/3 over 283-374~

The sequential separation and determination of Se, As, Sb, Ge and Pb has been achieved by reduction of these metals to their hydrides by the use of NaBH 4 followed by separation on a gas-solid chromatograph. Both on-line atomic absorption and emission spectrometry were used as detection systems.157-159 The use of gas- thermochromatography for rapid chemical separation of short-lived Ge isotopes from fission products has also been proposed. 16~

A standard procedure for the separation of macroscopic quantities of germanium from numerous other elements involves the distillation of Ge4+ from HCI solutions. The application of this method for purification of carrier-free 68Ge was studied in detail. 131

The effects of the initial concentration of an HCI solution on the extent of distillation of carrier-free 68Ge are presented in Fig. 6.1. In this study, at concentrations below 4 M HCI, essentially no activity was found in the first 15 ml of distillate (from an initial volume of 50 ml). Above 6 M HCI, the distillation of 68Ge was quantitative, while between 4-6 M, the recovery of the activity increased sharply as the concentration increased. No activity distills from either 6 M LiCI or 6 M HCIO4, indicating that the presence of both H+ and CI" ions is required to overcome Ge4+ hydrolysis.

Distillation of carrier-free 68Ge from azeotropic HCI was studied, and the cumulative yield of 68Ge as a function of distillate volume is shown in Table 6.1. These studies yielded a value Of 40+4 for the 68Ge distribution constant from the distillation of the azeotropic solutions of HCI. Furthermore, a simple correlation between the distribution constant of germanium, DGe, and the distribution constant of the hydrogen chloride, DHCP was observed for HCIO4-2 M LiCII HCI, and LiCI-2 M HCIO4 systems. It

39 S. MIRZADEH, R. M. LAMBRECHT: RADIOCHEMISTRY OF GERMANIUM

I I I I I I m 100 0.,_. 0 --

80 /

c3 4o

0 I I 2 3 4 5 6 7 8 HCI CONCENTRATION (M)

Figure 6.1 Dependence of distillation of carrier-free SSGe on initiel concentration of HCI. 131 Initial vol. = 50 ml Vol. of distillate = 15 ml

Table 6.1 Recovery of S8Ge distilled from azeotropic HCI as a function of distillate volume. 131

Distillate 68Ge yield a Distillate 68Ge yield a Vol. (ml) a (%) Vol. (ml) a (%)

1.0 58 5.0 99 2.0 83 6.0 99 3.0 92 7.0 99 4.0 97 8.0 100

aCumulative

was found that over a !imited range of the concentrations of HCIO 4, HCI, and LiCI, these simple correlations can be expressed as DGe=k[DHcI] n, in which k and n are constant for each system (see Table 6.2162). Since, by definition, the distribution constant of azeotropic HCI is equal to unity, from the data presented in Table 6.2, a value of 47+5 can be calculated for the

40 S. MIRZADEH, R. M. LAMBRECHT: RADIOCHEMISTRY OF GERMANIUM

Table 6.2 Relation between DGe and DHCI obtained from distillation of carrier-free 68Ge from acidic chloride solutions. 1s2

System Concentration Relation between range, M DGe and DHCI

HCIO4-2 M LiCI 2<[HCIO4]<4 DGe =(661+70)[DHci] 3"0

HCI 4<[HCI]<7 DGe = (47+5)[DHci] 19

LiCI- 2 M HCIO4 2<[HCI]<4 DGe = (41+5)[DHci] 18

Reprinted with permission from Mirzadeh, S., and Kahn, M, Radiochim. Acta 39, 73. Copyright (1986) Radiochimica Acta.

distribution constant of carrier-free 68Ge from the distillation of hydrogen chloride of various concentrations. This value is in fair agreement with 40+4, which is obtained directly from the distillation of azeotropic HCI.

6.2. Precipitation and Coprecipitation

Of the numerous published reactions for the precipitation of germanium, only a few are practical for use in radiochemistry. These reactions include precipitation in the form of sulfide, coprecipitation with acid-insoluble sulfides, and coprecipitation with hydroxides of Group III, especially with Fe(OH)3.

a. Reaction with Hydro.qen Sulfide

White germanium disulfide is formed in a strong acidic medium (3-4 M HCI or 5-6 M H2SO4) , and it thus differs from sulfides of most elements of the hydrogen sulfide group. The sulfides of other elements are precipitated first from a less acidic solution, and then the filtrate is acidified to precipitate GeS 2. The only other white sulfide is ZnS, which precipitates only in basic solutions. To further increase the selectivity, GeS 2 is precipitated after the separation of Ge by distillation from acidic chloride solutions in the presence of the oxidizing agents (see preceding section). In strongly acidic solutions, only germanium forms the yellow-orange precipitate of GeSe2. Therefore, the reaction of germanium with hydrogen selenide is a selective method for the detection of germanium.

41 S. MIRZADEH, R. M. LAMBRECHT: RADIOCHEMISTRY OF GERMANIUM

The solubility of GeS 2 is 4.45 mg/ml in water at 20~ and only 0.1 pg/ml in 2 M H2SO4 containing >0.02 M_M_H2S; 163'164 thus GeS 2 precipitate should be washed with an H2S-saturated acid solution. The GeS2 precipitate typically contains occluded sulfur which cannot be completely eliminated even by heating under CO 2 to sublime sulfur. To complete the determination, GeS2 should be oxidized to GeO 2 by dissolving the GeS 2 in 10 M NH4OH followed by addition of 3% H202.

b. Coprecipitation with Acid-Insoluble Sulfides

It is known that germanium, in concentrations greater than 10.6 M, can be coprecipitated with the sulfides which are insoluble in acidic or weakly acidic solutions (the sulfides of copper, arsenic, zinc, cadmium, etc.). Quantitative coprecipitation of macro amounts of Ge (0.25-20 mg/I) with copper and As s+ sulfides has been reported by Shevyakina 185 and Baraboshkin.166 Carrier-free 68Ge also can be coprecipitated with acid-insoluble sulfides. 128 The results of coprecipitation experiments performed with the sulfides of Sb 3+, As 3+, Bi, Cu, Hg and Pb formed in the presence of carrier-free 68Ge are summarized in Table 6.3. In general, the fraction of 68Ge coprecipitated with these sulfides increases with increasing cation concentration. Quantitative coprecipitation occurs with sulfides of Bi and Hg, when the initial concentration of these cations is 10-4 M, in 1.8 M__ H2SO 4 after allowing the solution to stand for 4 hours.

Table 6.3 Coprecipitation of carrier-free S8Ge with various acid-insoluble sulfides in 1.8 M H2SO4.12s

Germanium-68 coprecipitated, %

Precipitate Initial concentration of cation, M__

10.6 10.5 10-4

As2S 3 12 41 60 Bi2S 3 56 89 ~100 Sb2S 3 47 60 84 CuS 69 82 94 HgS 69 79 -100 PbS 7.8 13 55

After the addition of 68Ge, H2S was passed through the solutions for 5 minutes. After 4 hours the solutions were centrifuged for 30 minutes at 2500 g.

42 S. MIRZADEH, R. M. LAMI3RECHT: RADIOCHEMISTRY OF GERMANIUM

A comparison of the solubility products of the sulfides used (Bi2S3, 1097i Sb2S 3, 1095; PbS, 1028; CuS, 1048; HgS, 10"54) shows nearly the same molar solubility for the Group I sulfides Bi2S3, Sb2S3 and PbS, and the Group II sulfides CuS and HgS; and gives no indication that carrier-free 68Ge would be carried so completely with Bi2S3. These results are consistent with the results of the adsorption experiments, in which the highest adsorption of carrier-free 68Ge was observed on the sulfides of bismuth and mercury (see Sect. 4). 128

c. Coprecipitation with Hydroxides of_Group III Elements

The hydroxides of the Group III elements are known to coprecipitate microgram quantities Of germanium and, under certain conditions, the process is quantitative. 167173 Among the possible hydroxides, the most commonly used is Fe(OH)3, where quantitative coprecipitation occurs at pH>6 and the process is rather independent of the temperature and presence of foreign ions. The presence of ammonium salts improves the coprecipitation. A study of the effects of various factors is given by Agakova et aL 157 The required ratio of Fe:Ge for complete coprecipitation depends on the Ge concentration in the solution. In a solution containing 0.01 pg/ml of germanium, the required ratio is ~103:1.158 According to Novikov et aL, 169 the sequential separations of Zn, Ga, G e, As, and Se are also possible by coprecipitation with Fe(OH)3.

The separation of carrier-free 68Ge was briefly reported by Sewastjanow. 172 Accordingly, carrier-free 68Ge produced in a Ga target by [p,2n] reaction was separated from target solution by coprecipitation with Fe(OH)3 [~50 mg, added as Fe(NO3)3] at pH 11-12. Separation from Co and Ni impurities was achieved by the addition of hold-back carriers.

Quantitative coprecipitation of Ge with AI(OH)3 occurs only in neutral solutions (pH 6-8). 168'174 In a solution containing 0.01 pg/ml of germanium, the required ratio of Al:Ge is -10,000.168 The efficiency of the process decreases slightly at higher temperatures.174

The isolation of Ge from aqueous solutions by coprecipitation with copper gallate and tannate* is described by Andrianov et aL 175 The Ge gallates and tannates are more stable than the corresponding copper complexes and, hence, copper is readily replaced

* Gallic acid: 3,4,5-trihydroxybenzoic acid. Tannic acid: 076H52046

43 S. MIRZADEH, R. M. LAMBRECHT: RADIOCHEMISTRY OF GERMANIUM

by Ge in these complexes. The optimum pH ranges for quantitative coprecipitation of Ge with copper gallate is 6.5-6.0 and 3-11 for Cu tannate. The gallate and tannate precipitates can be dissolved in dilute mineral acids or in >6 M HCI + BuOH, respectively. It is also shown that freshly prepared copper tannate adsorbs Ge strongly (50 mg of Gel1 g adsorbent) and apparently Cu 2+, Zn 2+, Cd 2+, AI 3+, Mn 2'4+, As 3+, Sn 2+, and Sb 3+ are not adsorbed.

6.3 Extraction

a. Extraction with Orqanic Solvents

Extraction of GeCI 4 by nonpolar organic solvents from strong acidic chloride solutions is one of the most selective methods for separating germanium. 176178 In the CCI4/HCI system, only As 3+ as chloride, and Os 4+ and Ru 4+ as tetroxides are extracted together with germanium. Extraction of SnCI4 and SbCI 3 are not significant. Quantitative separation from arsenic can be achieved if arsenic is oxidized to its pentavalent state. Separations from Ru and Os are simply accomplished by evaporation from strong solutions of nitric acid or by preextraction of these elements into CCI4 from neutral or dilute acids. 179. Distribution constants for extracting germanium tetrachloride and several other metal chlorides from strong hydrochloric acid solutions are given in Table 6.4.

Table 6.4 Distribution constants for extraction of metal chlorides into CCI 4 from HCI solutions. 177

Metal [Mn+], N [HCI], _M Dext

Ge 4+ (0.14-3._5)xl 0 -4 8-9 50-500 a 5.8x10 "~ 8 45 (0.06-2.7)x10 "3 8 114+8 As 3+ 8 2 As 5+ 8 10 -4 Sn 4+ 8 10 -5 Sb 3+ 8 10 3 Se 2+ 8 5x 10 -5 Te 2+ 8 2xl 0 -5 B 3+ 8 2x 10 -5 Hg 2+ 8 2x10 -5

aData from ref. 176.

Distribution constant of OsO 4 in CCI4 from neutral or dilute acidic solutions is ~13.179

44 S. MIRZADEH, R. M. LAMBRECHT: RADIOCHEMISTRY OF GERMANIUM

I I I I I

100 --

uJ I- <0 60 - rr i- X I.,U

2 6 10 HCI CONCENTRATION (M)

Figure 6.2 Dependence of the degree of GeCl 4 extraction on the concentration of HCl. 177

Reprinted with permission from Sauvenier, Gh., and Duyckaer[s, G., Analytica Chimica Acta 16, 592. Copyright (1957) Elsevier Science Publishers.

Similar to distillation, the concentration of hydrogen chloride has a marked effect on the extent of germanium extraction. As shown in Figure 6.2, Ge is extracted into CCI4 only on the order of 2-3% from a 5 M hydrochloric acid solution but almost quantitatively when the concentration of HCI is equal to or greater than 8 M. Extraction increases very rapidly between 5-8 M. 177 In this experiment, 40 ml of hydrogen chloride solution containing 6.3x10-4 M Ge was equilibrated with 40 ml of CCI4 for 2 minutes. Germanium was detected by the polarographic technique. Furthermore, from a study of the extraction of Ge from HCI/H2SO4, HCI/CaCI2, and HCI/MgCI2 ,178 one can conclude that the extent of the extraction of germanium from acidic chloride solutions is a function of both H + and CI- ion concentrations.

The extraction of germanium from HCI, HBr and HI acid solutions into 18 organic solvents was studied by Siekierski et al. 18~ The distribution coefficients are summarized in Table 6.5. Germanium concentrations in these experiments were measured radiometrically with 77Ge. In general, for a given halide system, the extraction capacities of various solvents do not vary significantly. In all cases the distribution coefficients are

45 S. MPRZADEH, R. M. LAMBRECHT: RADIOCHEMISTRY OF GERMANIUM

Table 6.5 Distribution coefficients of germanium tetrahalides between aqueous and organic solvents, is~

GeCI 4 GeBr4 Gel 4 Solvent 7.3 M HCI 6.5 M HBr 5.2 M HI

n-Hexane 4.44 1.53 0.76 n-Heptane 4.22 1.55 n-Octane 3.72 1.40 0.79 Isooctane 3.16 0.94 n-Decane 2.98 1.38 n-Hexadecane Z48 1.06 0.71 Cyclohexane 5.14 2.52 2.28 Benzene 4.02 2.99 3.82 Toluene 4.35 2.83 o-Xylene 3.97 3.15 Chlorobenzene 3.34 2.87 2.83 Chloroform 6.17 3.07 - Bromoform 2.13 2.13 5.0 Carbon tetrachloride 6.76 3.27 - 1,2-Dichloroethane 2.71 2.15 - 1,2-Dichloroethylene 5.63 3.35 - 1,1,2,2-Tetra- chloroethane 2.67 2.15 - Trichloroethytene 5.50 3.02 -

[Ge4+]=10 -4 M, Vol. of each phase = 15 ml, T=20+2 ~

highest for the GeCI4/CCi 4 system for a given solvent, with the exception of the bromoform. Siekierski was able to show a relation between the distribution coefficient and solubility parameter 181'182 of the solvents. Due to the formation of H2GeF 6 in HF solutions, Ge in the presence of F is extracted by the organic solvents only to a very small extent. Back-extraction of Ge from the organic phase is typically achieved by water or dilute HCI. Other solvents which have been used include nitrobenzene, bis-2- chloroethyl ether, tributyl phosphate, acetophenone and saturated hydrocarbons.

In 48% HBr saturated with KBr, Ge is quantitatively extracted by diethyl ether. After washing the organic phase with 48% HBr, Ge is back-extracted into water. Adding TiBr3 apparently prevents the coextraction of Fe. t78 Extractions of Sn4+, As 3+ and Ge4+ from H2SO4-0.5 M_.M_Nal solutions into cyclohexane are shown in Fig. 6.3 (the curves are reconstructed from data of Tanaka and Takagi. 183

From a lO-ml solution of 10 M H2SO4-0.5 M Nal, 20-100 pg of Ge is quantitatively extracted into 10 ml of cyclohexane within one minute of mixing. From the data

45 S. MIRZADEH, R. M. LAMBRECHT: RADIOCHEMISTRY OF GERMANIUM presented in Figure 6.3, it is evident that the selective separation of Ge can be achieved by first extracting Sn and As from a 4 M H2SO4-0.5 M Nal solution and then increasing the acidity of sulfuric acid to about 6 M to extract germanium. The extent of the extractions in these experiments was measured photometrically, employing the metal- tetraiodide characteristic absorption in the ultraviolet region. The extinction coefficients for tin at 364 nm, arsenic at 282 nm, and germanium at 360 nm are 8700, 9700 and 8600, respectively. 183

I I I I I I

100 b--

LU 80 - o u)

W 60 - I-. O Sn I r~ l.- X W Z 40 - _o

20 -

0 I 0 1 2 3 4 5 6 H2SO 4 CONCENTRATION (M)

Figure 6.3 Extraction of Sn 4+, As 3§ and Ge 4+ from H2SO4-0.5 M Nal solutions into cyclohexane. 183

VoI. of phases = 10 ml, Element added = 100/Jg Mixing period = 1 min

Reprinted with permission from Tanaka, K., and Takagi, N., Analytica Chimica Acta 48, 357. Copyright (1969) Elsevier Science Publishers.

Based on the results of Grimanis et al. (see Table 6.6), 184 one could expect quantitative extraction of Sb 3+, and partial extraction of Se4+ and Hg 2+ under the above conditions.

47 S. MIRZADEH, R. M. LAMBRECHT: RADIOCHEM1STRY OF GERMANIUM

Table 6.6 Extraction of elements into benzene from 10.0 M H2SO4-0.03 M HBr and 5.0 M H2SO4-0.01 M KI systems 184

Metal extracted, % Element 10.0 M H2SO4 5.0 M H2SO4 0.03 M HBr 0.01 M KI

Ge 4+ 98.2 0.08 As 3+ 99.4 44.4 Sb 3+ 99.4 99.8 Sn 2+ 95.7 99.0 Se4+ 95.6 85.1 Hs 2+ 74.1 52.2 Bi ~+ 10.0 a Te4+' 5.5 a All others < 1 <1

Volume of each phase=10 ml, Extraction time = 2 min [M n+ ]total = 100 ,ug/ml. Percent extraction was determined using the appropriate radiotracers. aA thin precipitate is formed between phases.

Reprinted with permission from Grimanis, A. P., and Hadzistelios, I., Analytica Chimica Acta 41, 15. Copyright (1968) Elsevier Science Publishers.

b. Extraction of Germanium Complexes.

The extraction of anionic complexes of germanium into various organic amines using inorganic and organic ligands has been studied to some extent. 184"191 In the absence of a ligand, the weak metagermanic acid is not extracted by the amines. Some of the common ligands include F', 185 NO3", SO42-, pyrocatechol, 187'188 oxalic and gallic acids, 189 tannin, 19~ citric and tartaric acids. 191 The organogermanium complexes are typically formed in weakly acidic or basic solutions, which are then extracted into organic amines dissolved in typical organic solvents: CCl4, chloroform, xylene, benzene, "toluene. Tri-n-octylamine185,19~ is a typical extracting agent, but amines of different basicity also have been used. 185"188

The influence of the nature of the amine on extraction of GeF62-, as a function of HF concentration is shown in Fig. 6.4 for tri-n-octylamine (TOA), dibenzyloctylamine (DBOA) and tribenzylamine (TBA). 185 As seen, at the optimum concentration of HF (0.2- 0.5 M), the percentage of extracted Ge decreases sharply from TOA to TBA reflecting the decrease in the basic strength of the amines in the order TOA>DBOA>TBA.

48 S. MIRZADEH, R. M. LAMBRECHT: RADIOCHEMISTRY OF GERMANIUM

I I

uJ ,

20 . DBOA

0 0 1 2 HF CONCENTRATION (M)

Figure 6.4 Influence of the nature of the amine on the extraction of GeFs2" as function of HF concentrations. 1B5

TOA: tri-n-octylamine, DBOA: dibenzyloctylamine, TBA: tribenzylamine Solvent benzene Vol _ = VOl.aq = 10 ml, Extraction time = 5 min, " ~ "O/y [Amine] = 30 mM__, [Ge]totaI large excess. Reprinted with permission from Vasyutinskii, A. I. et al., Russ. J. Inorg. Chem. 18, 1312. Copyright (1973) British Library.

The extraction of germanium as hydrogen tris(pyrocatechotate)-germanate by amines of different basic strengths (octadecylamine [ODA], octyloctadecylamine [OODA] and dioctyloctadecylamine [DOODA]) as a function of pH for three organic solvents (octane, toluene and octanol) was studied by Kurnevich et aL 187 The influence of pH on the extraction for octanol as solvent is shown in Fig. 6.5. No significant differences between the extractior~capabilities of the three amines in octanol were observed.

Simiiarly, the extraction constants of hydrogen tris-(pyrocatecholate) germanium from the three amines in octane decreased very slightly, in the order primary to tertiary amine (see Table 6.7). This trend is more pronounced for octane and toluene.

49 S, M1RZADEH, R. M. LAMBRECHT: RADIOCHEMISTRY OF GERMANIUM

100

o ILl

25 C

I i I 2 4 6 8 10 pH

Figure 6.5 Influence of pH on the extraction of Ge tris-(pyrocatecholate) complex by amines in octanol. 187

A) 2.3 mM D_OODA, B) 1.8 mM OODA, C) 1.9 mM ODA, [Ge]total = 10 .3 M, Ge/pyrocatechol = 1/20, Vol. of phases= 10 ml.

Reprinted with permission from Kumevich, G I., Loiko, E. M. and Vishnevskii, V. B., Russ. J. Inorg. Chem. 24, 1067. Copyright (1979) British Library.

In these studies, the distribution constants were calculated from the equation

K = [(R3NH)2Ge(C6H402)3]org / [R3N]org 2 . 4[H2Ge(C6H402)3]aq3 (1)

based on the reaction:

2R3Norg + H2[Ge(C6H402)3]aq , ' (R3NH)2[Ge(C6H402)3]org (2)

50 S. MIRZADEH, R. M. LAMBRECHT: RADIOCHEMISTRY OF GERMANIUM

In equation 1, the ionic concentration of H2Ge(C6H402) 3 in aqueous phase is equal to 4c 3, where c represents the molar concentration of the acid, according to the theory of electrolytes.193

Table 6.7 Extraction constants for tris-(pyrocatecholate)germanate by amines in octane, toluene and octanol. 187

Extraction constant, Kd

Solvent Amines octane toluene octanol

octadecylamine (2.4+0.8)x1017 (6.0• (6.1+1.0)xl019 octyloctadecylamine (2.5+0.4)x1015 (1.2• (1.3• dioctyloctadecylamine (4.3• (1.5• (8.5•

Reprinted with permission from Kurnevich, G. I., Loiko, E. M. and Vishnevskii, V. B., Russ. J. Inorg. Chem. 24, 1067. Copyright (1979) British Library.

c. Extraction with Hydroxyl-Containing Organic Li.qands

In weakly acidic solutions, the cationic hydroxogermanium species, Ge(OH)n(4-n)+, reacts with hydroxyl-containing organic ligands. Among these ligands, 8-hydroxyquinolinoi (HOx) reacts with germanium to form a very stable complex, which can be used for extraction and preconcentration of germanium. 194-196 The electromigration studies of Ge-Ox compounds in organic solvents have shown that the principal species are neutral. 19T The overall heterogeneous reaction between germanium in aqueous phase and quinolinol in organic phase has been proposed as:

Ge(OH)22+ (aq) + 2HOx (org) . ' Ge(OH)2(Ox)2 (org) + 2H + (aq)

The fraction of germanium extracted in HOx/CHCI 3 from chloride solutions with an ionic strength of 1.0 as a function of pH is shown in Fig. 6.6.195 The concentration of germanium in these experiments was ~10-6 M and that of HOx ranged from 0.05 to 0.4 M, and the extent of extraction was determined radiometrically by the use of 71Ge. In the pH range of 3.5-10, quantitative extraction of germanium is achieved when the concentration ratio of quinolinol to germanium is about 10-5. From the above equation, it is evident that the sharp decrease in the extraction at both ends of the pH range is due

51 S. MIRZADEH, R. M. LAMBRECHT: RADIOCHEMISTRY OF GERMANIUM

I I I I 1 t i I I t I| J I t i i i i I i j i t A B 100

121 I'-m 0 < rr I--- X l.tJ I1)

0 2 4 6 8 10 12 0 2 4 6 8 10 12 pH pH

Figure 6.6 Extraction of germanium in chloride solution with 8-hydroxy- quinolinate in chloroform as a function of pH at ionic strength of A: 1.0, B: 0.1.195

[HOx], M = a: 0.05, b: 0.10, c: 0.20, d: 0.40, [Ge] = 10 -5 M

to an increase in the hydrogen ion concentration at low pH levels and removal of Ge(OH)22+ at high pH levels.

Considering the volatility of germanium halides formed from strongly acidic halide solutions, the need to limit contamination, and constraints on the use of corrosive acids in hot cells, extraction of the germanium complex from dilute acidic or basic solutions may prove to be more prudent, especially when dealing with high levels of radiogermanium, such as large-scale production of SSGe. The application of this extraction technique has not been shown for the separation and preconcentration of Ge radioisotopes at very low concentrations and in the absence of a carrier.

6.4. Ion-Exchange Chromatography

Prior to discussion of the behavior of Ge on ion-exchange resins, it is beneficial to briefly comment on the definitions used by various investigators. Adsorptivity is typically expressed by the implicit variable "distribution constant." Investigators have used the terms "volume distribution constant" and "weight distribution constant." Definitions for these terms follow:

D v = Volume distribution constant: (amount adsorbed per liter resin bed) / (amount per liter solution)

52 S. MIRZADEH, R. M. LAMBRECHT: RADIOCHEMISTRY OF GERMANIUM

Dw = Weight distribution coefficient: (amount adsorbed per kg dry resin) / (amount per liter solution)

These terms are related by Dw=Dv Ip, where p is the specific volume of the dry resin (ml/g).

Both column and batch techniques were applied to measure the adsorption distribution constants. When the adsorption is sufficiently low, column technique is employed. In this case, the Dw is given by the following expression:

D w = p [(VmaxlV)- I] = M (Vma x - Vo) where i = void fraction of the column Vrnax = peak elution volume (ml) V = total volume of column (ml) Vo = iV, interstitial volume of column M = mass of dry resin

Furthermore, the terms "distribution constant" and "distribution coefficient" have often been used interchangeably. Germanium-specific resin is discussed in Sect. 7.1.

a. Cation Exchange

The behavior of germanium on cation exchange resins (sulphonated polystyrene divinyl benzene) has not been extensively studied. From the fragmented information available, it is possible to conclude that germanium is not strongly retained by cation exchangers from common acids. A summary of systematic surveys of the behavior of elements including Ge by cation exchange resin is given in Table 6.8.198"209

In the region of 0.1-3 M HC1198 and 1-6 M HBr 199 the adsorptivity of AG 50W-X4 resin for Ge 4+ is low, with a distribution constant of less than 1. This behavior is consistent with the fact that in moderately acidic chloride and bromide solutions, Ge exists in negatively-charged halogermanium complexes which show the expected low adsorptivity toward cation exchangers. As shown in Fig. 6.7, however, in stronger HBr solutions (up to 9 _M_M,the azeotropic point of HBr), the distribution constant of Ge increases rapidly to a value of ~50.199 Above 9 M HBr, the determination of the germanium distribution constant is hampered by the serious loss of volatile germanium bromide. As with Au 3+, TI 3+ and Po4+, the adsorption of negatively charged halocomplexes of germanium on cation exchangers is a phenomenon that is not clearly

53 S. MIRZADEH, R. M. LAMBRECHT: RADIOCHEMISTRY OF GERMANIUM

102 I I I I I I Sn(IV)

I,- z LU

u_ 101 o

10 0

I I I 3 6 9 12 0 3 6 9 12 HBr CONCENTRATION (M) HBr CONCENTRATION (_M)

Figure 6.7 Adsorption of Ge 4+ and Sn4+ from HBr solutions on Dowex 50-X4 at 25oc. 199 Reprinted with permission from Nelson, F., and Michelson, D. C., J. Chromatogr. 25, 414. Copyright (1966) Elsevier Science Publishers.

....

Figure 6.8 Adsorption functions of the elements from HBr solution by Dowex 50- X4 cation exchange resin. 199 Reprinted with permission from Nelson, F., and Michelson, D. C., J, Chromatogr. 25, 414. Copyright (1966) Elsevier Science Publishers.

54 S. MIRZADEH, R. M. LAMBRECHT: RADIOCHEMISTRY OF GERMANIUM

Ga(lll) As(Ill) As(V) I IIII I Kd> 2.2 x 10s IIII Kd< 5 I III b I ii I, IIII I/ Sn(IV~ I I Sb(lll~ SbfV) I[111111III Kd> 5 i I\ IIIIIIIIIII in 3M l 1 il lllll tltll II II II I 11111 II I Illilllllil T(III) Pb(ll) Bi(lll) IIII LEGEND >4.8 x 104 >4.9 x 104 lli II 5 'L I,,r 4 I I-rl I ~3 IIIII IIIII

3 9 15 [HAC],M

Figure 6.9 The adsorption function of germanium and neighboring elements on AG50W-X8 from acetic acid solution. 2~176

Reprinted with permission from Jha, S. K., de Corte, F., and Hoste, J., Analytica Chimica Acta 62, 163. Copyright (1972) Elsevier Science Publishers.

understood. For comparison, the behavior of Sn 4+ in AG 50W-X4/HBr system is also shown in Fig. 6.7.

The data in Fig. 6.8 show that many elements exhibit strong retention on cation exchangers from 1-2 M HBr; germanium, which does not adsorb strongly in this case, can be separated from many elements which do adsorb, especially the Group III elements and Sn +4. Similar conclusions follow for HCI solutions.

The adsorption function of germanium and its neighboring elements on AG 50W- X8 from acetic acid solutions is shown in Fig. 6.9. 2~176The distribution constant of Ge remains rather constant at -50 and eventually reaches a value of -200 at 17 M acid. Neither As 3+ nor As 5+ show adsorption at lower acetic acid concentrations. The

55 S. MIRZADEH. R. M. LAMBRECHT: RADIOCHEMISTRY OF GERMANIUM

difference between the adsorptivity of Ge and As in cation exchange resin/acetic acid systems, therefore, could be used to separate these two elements from each other. Although the process seems attractive in the sense that separation is independent of the oxidation states of arsenic, the authors are only aware of one short report by Sewastjanow 172 for separation based on this system.

The cation behavior of several elements in hydrochloric, hydrobromic and nitric acids containing 0.1-0.2 M.M thiourea has been studied by Weinert eta/. 198'203'204 Germanium behavior was not examined in this system, but Sn 4+ shows strong adsorption in 0.1 M HCI (D = 3.3x103) and 0.2 M HNO 3 (D ~104) containing 1.0 M thiourea. Considering the chemical similarity of Ge4+ and Sn 4+, one could expect that separation of germanium from elements which do not form complexes with thiourea in acidic media could be achieved oncation exchangers.

Table 6.8 Summary of the systematic surveys of the behavior of elements including Ge by cation exchange resin

System (M) Resin DGe Ref.

HCI (1-12) Dowex 50W-X4 nm 202 HCI (1-12) MP-50 nm 201 HBr (1-12) Dowex 50W-X4 ~100 at 12 M__ 199 HCIO4 (1-12) Dowex 50W-X4 nm 202 HNO 3 (1-12) Dowex 50W-X4 nm 207 HNO 3 (1-12) MP 50 nm 207 HNO 3 (0.1-2) /thiourea AG 50W-X4 nm 198 HCI (0.1-3) AG 50W-X4 < 10 198 HCI (0.1-3) / thiourea AG 50W-X4 nm 198,203 acetic acid (1-15) AG 50W-X8 ~80 at 17 M 200 formic acid (3-23) Dowex 50W-X8 nm 205 HCI / acetone Dowex 50W-X8 nm 206 HCI / methanol MP 50 nm 209 HBr / acetone AG 50W-X8 nm 208 HBr / thiourea/acetone Dowex 50W-X8 nm 204

nm: not measured

_b. Anion Exchanpe

The systematic surveys of the behavior of elements on anion exchange resins are summarized in Table 6.9. 210-220 In a hydrochloric acid solution, the adsorption of Ge +4 on the strongly basic anion exchange resin Dowex 1-X10 becomes significant only above 4 M HCI. The distribution coefficient reaches a value of ~200 in 10 M HCI, where volatility of the germanium chloride is also appreciable. 21~

56 S. MIRZADEH, R. M. LAMBRECHT: RADIOCHEMISTRY OF GERMANIUM

Table 6.9 Summary of systematic surveys of the behavior of elements including germanium by anion exchange resins

System (M) Resin DGe Ref.

HCI (1-12) Dowex 1-X10 ~200 at 12 M 210 HF (1) / HCI (1-12) Dowex 1-X10 >100 at 0.1 M 210 HCI (1.2) / Acetone Dowex 1-X8 17 at 90% acetone 213 HCI (1-tl) Ambertite CG4B-I a 5 at 9 M 214 HF (1-24) Dowex 1-X10 nm 215 HNO 3 (0.1-14) Dowex 2 nm 216 HF/HNO 3 Dowex 1-X4 nm 217 H3PO4 (0.1-14) Dowex 1-X8 0 218 H2SO4 (0.01-4) AG 1-X8 nm 219 HSCN (0.01-1) Amberlite CG400-1 5.4 at 1 M 220 HAc Dowex 1-X8 ~50 at 16 M 221 HCI (pH 4.3) Dowex 21K/malonate 0 at pH 4.3 222 aweakly basic nm: not measured

In an HF-HCI mixture, however, the adsorption function of germanium on the same resin has the opposite trend. As shown in Fig. 6.10, in a solution of 1 M HF and 0.1 M HCI, DGe>100 and decreases rapidly with an increase in the HCI concentration to a value of <1 at [HCI]>4 M. 21~ Obviously, this behavior points to the formation of negatively-charged germanium fluorocomplexes presumably as GeF62- at low acidity. In this respect, the behavior of Ge4+ is similar to that of Ti4+, Zr4+, Hf4+ and Pa 5+. The lack of adsorption of Ga 3+ from a 1 M HF/0.1 M HCI mixture could, in principle, be used for its separation from germanium. The adsorption of germanium from pure hydrofluoric acid solution has not been studied in detail. 211 Some of the earlier reports, however, indicate significant adsorption of germanium on anion exchange resins from -1 M__ HF solution. 212 As discussed above, germanium shows no adsorption in 1.2 M HCI on anion exchange resins; however, in a 1.2 M HCI-acetone mixture, adsorption occurs to a measurable degree. The distribution coefficient increases by a factor of 2; from 8.9 at 20% acetone to 17 at 90% acetone. 213 Apparently, acetone promotes the formation of negatively charged germanium chlorocomplexes. On the weakly basic anion exchanger Amberlite CG4B (type 1), germanium tends to be adsorbed to a lesser extent than on Dowex 1 in HCI solutions. 214 In the CG4B-HCI system, the distribution coefficient of Ge4+ remains negligible over 1-6 M HCI, and reaches a value of -5 at 8.9 M HCI. In lower HCI concentrations (<6 M), Sn 4+ shows moderate adsorption on CG4B, where it could be separated from germanium.

5? S. MIRZADEH,R. M. LAMBRECHT:RADIOCHEMISTRY OF GERMANIUM

' I , ! , ' I , I t/ ,t Ge(IV) As(Ill)

In.In n I v I f .....In (III) t .....Sn(ll) t i

t" , I , ! . / ~1 . I . /

, I , I ' ' I I LEGEND 106 , r'~ 104 Ele'mentand1 ~olO2 Oxidation ] 101 ,Stat~ , t

0 4 8 12 [HCl] (_M) Figure 6.10 Adsorption of Ge and its neighboring elements from HCI and 1 M HF-HC! solutions on Dowex 1-X10 strongly basic anion exchange resin, 21~

Reprinted with permission from Nelson, F., Rush, R. M., and Kraus, K. A., J. Am. Chem. Soc. 82, 339. Copyright (1960) American Chemical Society.

Contrary to the report by Ichikawa, 215 germanium is not expected to exhibit adsorption in nitric acid solutions on anion exchangers. Sn 4+, Ga, In, As S* and Sb 5+ also show negligible adsorption in this system. 216 The behavior of Ge in a mixture of HNO3-HF has not been studied, where Sn 4+ shows very slight adsorption. 217 Germanium shows no adsorption on Dowex 1-X8 over the entire concentration of orthophosphoric acid (0.1 - 14 M). In this system, Ga, As3+ and Se4+ show low adsorption only at low H3PO4 concentrations. The distribution coefficient of Sn 2+ increases with an increase in acid concentration, reaching a value of ~140 at 3 _.M HCI, but decreases with a further increase in acid concentration. 218 The behavior of germanium in sulfuric acid on anion exchangers has not been studied, but no adsorption could be expected in view of the fact that germanium does not form or forms very weak complexes with sulfate ions (as with phosphate ions). In the H2SO4 - AG 1-X8 system, the neighboring elements gallium, indium, selenium, arsenic and germanium also show very little or no adsorption. 219 In dilute HCI solutions (pH=4), Ge is not absorbed on the anion exchanger Oowex 21K in malonate form, whereas Ga shows some adsorption. 222

58 S. MIRZADEH, R. M. LAMBRECHT: RADIOCHEMISTRYOF GERMANIUM

In thiocyanic acid and strongly basic anion exchange resins, the adsorption behaviors of Ge and As +3 differ significantly from those of Sn4+, Ga, In, and Sb 3+. (ref. 220) The distribution coefficients of these elements on Ambedite CG 400 in thiocyanic acid are shown in Table 6.10. As seen, germanium is only ,slightly adsorbed from this system, whereas Sn4+, Sb 3+, Ga and In show strong adsorption, and As 3+ is not adsorbed.

Table 6.10 Distribution coefficients of Ge, Sn 4+, Ga, In, As 3+ and Sb 3+ on amberlite CG 400 in thiocyanic acid ~20

Distribution coefficient, Dw

[HSCN], M Element 0.01 0.03 0.1 0.3 1.0

Ge <1 <1 <1 <1 5.4 Sn 4+ ppt ppt 2.3x103 1.0xl04 9.3x103 Ga 2.3 14 77 2.2x102 1.5x102 In 2.5x103 2.2x103 1.4x103 5.9x102 1.5x102 As 3+ <1 <1 <1 <1 <1 Sb 3+ ppt ppt ppt ppt 2.1x104

Reprinted with permission from Kiriyama, T., and Kuroda, R., Analytica Chimica Acta 101,207. Copyright (1978) Elsevier Science Publishers.

c. Adsorption on Inorqanic Exchan.qers

The studies of the adsorptive properties of germanium on inorganic ion exchangers have primarily focused on the development of the 68Ge-68Ga biomedical generators [see Sect. 7.1]. In this context, many researchers have limited their~ investigations to a search for suitable inorganic supports which irreversibly adsorb 68Ge but not the 68Ga daughter. In general, most hydrated metal oxides in neutral or acidic solutions show a tendency to adsorb germanium. 13~ Typically, the magnitude and the irreversibility of the adsorption increases when the metal oxides are prepared from acidic solutions.

The results of some experiments studying the adsorption of 68Ge on various hydrated metal oxides (summarized in Table 6.11) show that the carrier-free 68Ge is

59 S, MIRZADEH, R. M. LAMBRECHT: RADIOCHEMISTRY OF GERMANIUM

almost quantitatively adsorbed by 100 mg of Fe203, ZrO 2 at pH 5.9 and 8.8 within an hour. Under similar experimental conditions, the adsorption on AI203 was about 94% and 96%, respectively; only 23% and 27% of 88Ge was adsorbed by SiO2, respectively.13~ The pH-dependence of the adsorption of carrier-free 68Ge and 68Ga on AI203, AI(OH)3 and Fe(OH)~ is shown in Fig. 6.11. In this experiment, 500 mg of dry AI203 was

Table 6.11 Adsorption of carrier-free 68Ge on 100 mg of various hydrated metal oxides 13~

Set I: Initial pH = 5.9 Set I1: Initial pH = 8.8 Oxide Final 68Ge Final 68Ge pH adsorbed, % pH adsorbed, %

AI203 8.1 94 8.8 96 Fe203 6.1 99 6.5 100 SnO2 5.8 12 6.4 15 TiO 2 6.1 71 6.5 73 SiO2 4.2 23 4.6 27 ZrO 2 4.6 99 4.8 99

Reprinted with permission from Mirzadeh, S., and Kahn, M., Radiochim. Acta 39, 189. Copyright (1986) Radiochimica Acta.

I J I ' I ' I = I ~ I ~Ge 68Ga 1.0 a w li1 0 u3 111 < z 0.5 <(3 n- IJ.

0 0 2 4 6 8 10 12 14 0 2 6 8 10 12 14 pH(~) PH(eq) Figure 6.11 The pH-dependence of the adsorption of carrier-free 68 Ga and 68 Ge on AI203, AI(OH)3 and Fe(OH)3" 224

o - A1203, [] - AI(OH)3 , n - Fe(OH)3

Reprinted from Kopecky, P., and Mudrova, B., Int. J. Appl. Radiat. Isot. 25, 263. Copyright (1974) with permission from Pergamon Press Ltd., Headington Hill Hall,Oxford OX30BW, UK.

60 S. MIRZADEH, R. M. LAMBRECHT: RADIOCHEMISTRY OF GERMANIUM shaken for 20 minutes with 20 ml of 68Ge or 68Ga solution of known pH. The aluminum and iron hydroxides, however, were prepared in situ from solutions containing 5.4 mg of AI and 11 mg of Fe. 223 The presence of radioactivity before or after the formation of the hydroxides apparently did not have any effect on the adsorptive behavior of either element. From the data shown in Fig. 6.11, it is evident that the separation of carrier- free 68Ga from Ge could only be achieved on preformed AI20 3 in acidic solutions. Adsorption of germanium on silica gel from acidic solutions is a selective and quantitative method of separating germanium from other elements. 13~ tn strongly acidic solutions, adsorption of carrier-free 68Ge on the walt of a glass vesset also becomes significant13~ (see Sect. 4). As shown in Table 6.12, pretreated silica gel with concentrated nitric acid selectively removes germanium from 9.4 M HNO 3, whereas all other elements (with the exception of W and Ag) are eluted in one column volume. The adsorbed germanium is retained strongly and cannot be eluted with either 1.0 M HNO 3 or 6 M NH4OH.225 In addition, it is also shown that carrier-free 68Ge is nearly quantitatively adsorbed on 300 mg of powdered Pyrex from 9.6 M HCIO4 solution within four hours, and the adsorption process is largely irreversible. 13~ Caletka et al. 227 has indicated that the germanium adsorption on silica gel occurs from all common mineral acids or their mixtures at concentrations of > 8 M. When [Ge]<10 "6 M, the adsorption was found to be independent of the germanium concentration. In mixtures of 11 M HCI and various alcohols (e.g., ethanol or 1-

4 I I I I (~ | (',,.| I I 0.2

o~3 \ E,OJ/ - H --I I 0.1

, C I I I I 0 ;./, 0 20 40 60 80 100 0 20 40 60 80 1(30 VOLUME PERCENTOF ALCOHOLS VOLUME PERCENT OF ALCOHOLS

Figure 6.12 Adsorption function of Ge on silica gel (a) and solubility of GeO z (b) at 25~ in mixture of 11 M HCl and various fractions of alcohols. 227 o - EtOH, - BuOH

61 S. MIRZADEH, R. M. LAMBRECHT: RADIOCHEMISTRY OF GERMANIUM

butanol), the germanium adsorption on silica gel shows a distinct minimum with an increase in the mole fraction of alcohol. Based on Caletka's data, 227 the minimum of the adsorption function coincides with the maximum of the solubility of GeO 2 in the same mixture (Fig. 6.12). The above results.are qualitative since there is a vast difference in the concentrations of germanium in the two studies. Furthermore, the volatility of germanium and adsorption on the wall of the reaction vessel both vary significantly in these experiments.

Table 6.12 Retention of various elements on silica gel from 9.4 M HNO322s

Radio- [M n+] Fraction Radio- [Mn+] Fraction element (N_) eluted (%) element (N_) eluted (%)

110mAg 4.2x10-6 58 2~ 7.4x10 -5 97 77As c.f. 95 115mln c.f. 98 198Au 2.7x10-6 98 14~ 1.8x10-6 97 1150d 6.0x10 -5 97 54Mn c.f. 95 6~ 1. lx10 -5 100 99Mo 1.3x10 -4 96 51Cr 4.0x10 "3 96 75Se 6.9x10-4 99 134Cs 7.2x10 6 99 46Sc 9.0x10 -5 97 640u 4.8xl 0-4 99 99mTc c.f. 93 59Fe 8.6x10 -3 96 65Zn 5.1x10-4 99 72Ga 2.5x10 -5 97 187W 2.8x10 -6 4 77Ge 8.2xl 0-3 0

Column: 0.9x3 cm, ~ 1 g of dry silica gel, 0.05-0.20 mm, pre-treated with conc. HNO 3. Eluent Volume: 1.5 ml. Flow rate: 1.6 ml.min -l.cm -2. c.f.: Carrier-free.

Reprinted with permission from Lievens, P., and Hoste, J., Analytica Chimica Acta 70, 462. Copyright (1974) Elsevier Science Publishers.

d. Thin-Layer and Paper Chromato.qraphy

The paper and thin-layer chromatographic (PC and TLC) behaviors of a number of elements have been systematically investigated in binary mixtures consisting of various mineral acids and organic solvents. 127'228-236 The results are generally comparable to th3se from column experiments, but TLC offers better and faster separation for analytical PUrposes. The Rf values of a number of cations in HCI-acetone (1:1) mixtures on silica gel are shown in Fig. 6.13. 228 As seen, over the HCI concentration range of 0.1-6 M, the germanium af is very small and differs markedly from those of its neighboring elements. Indeed, Maki et aL 127 were able to separate

52 S. MIRZADEH, R. M. LAMBRECHT: RADIOCHEMISTRYOF GERMANIUM carrier-free 77As from neutron-irradiated germanium by the TLC technique. After evaluating various mixtures of HCI with acetone and methanol, separation was carried Out from a mixture of 5 M HCI-methanol (1:2) where the Rf values for Ge, As 3+, and As 5§ were 0.00, 0.50 and 0.94, respectively [see Sect 8.4].

Li, 8e I,~!0 8 C N 0 f ,+ @@NN@ ;rljl] L,,

r//z//'//~l V/z,Tzz./~l NN++ + N|174 |174174 ttg Sr Y lr Nb No Tc Aa Rh PcL A! COL [n Sn Sb Te I I/'r-'-Fl~'l'~ll.'l'l'l ' ' " -,r.....~. .~/.X/.q/ff/zgl~.~.,~,,. i. I JLLLJ~Lw~,F.,I--~ I ]. I, II. I. I. I~ F'EP~Et:~ E]~-t ~51d t:2;::ldM-:~ M:.MItM-:-t ~ ~ CS 8(1 R.E. Hr T~ W Re 05 IP PL ALL HQ T1. P~ 8L Po A~,

Ljl~[_jl I.Ifl II I I~11 I~L 11 Ijrl II I~y II tn'l li~f~_.~.:~l IJrl I~I__ULL]I I/I IL..~LLJLJJI.L_jL_j,L]_jr~f::i~./_~r~..;]~ I I I I ~ ~ V fP Ko. Ac Th p~ LI

Figure 6.13 Rf functions of cations in HCl-acetone (1:1) mixtures on silica gel (KSK). 228

As indicated in Fig. 6.13, the significant difference between the Rf of Ge and that of Ga in an HCI-acetone system, offers a possible technique for the determination of the 68Ge breakthrough from the 68Ge-68Ga generator (see Sect. 7.1).

The separation of microgram quantities of Ge, Sn 2+, and Pb was reported by Johri et at. using silica gel G- and silica gel GF-coated plates and solvent systems iso-BuOH-HCI-Et-COMe (10:8:1) and iso-BuOH-HOAc (3:1), respectively. 229

The paper chromatographic separation of Ge and As was also studied in mixtures of Me2CO, iso-BuCOMe, or dioxane with HCI, HNO 3, or HBr. The Rf values for Ge and As § were found to be close, while those for Ge and As +5 were far enough apart to allow their separation. 23~

The adsorbents for TLC are not limited to alumina and silica gel, as a rather large number of synthetic ion-exchangers have also been evaluated. 232"235 Among the ion-

63 S. MIRZADEH, R. M. LAMBRECHT: RADIOCHEMISTRY OF GERMANIUM

Jil!ls

Figure 6,14 The Rf functions of Ge and other elements on DEAE-cellulose in methanoI-HCI (20:1). 234 Reprinted with permission from Kuroda, R.; Yoshikuni, N., and Kawabuchi, K., I. Chromatogr. 47, 453. Copyright (1970) Elsevier Science Publishers.

00 3 6 g 12 HQ WI

Figure 6.15 The Rf functions of Ge and other elements on DEAE-cellulose in cyclohexanone-HCI (20:1). 234 Reprinted with permission from Kuroda, R., Yoshikuni, N., and Kawabuchi, K., .1. Chromatogr. 47, 453. Copyright (1970) Elsevier Science Publishers.

64 S. MIRZADEH, R. M. LAMBRECHT: RADIOCI-IEMISTRY OF GERMANIUM

B3+

'"tI I , ....

AI3+

I !

. \v \ ,i Ge4+ As Se4+ G'a'A~ ~ f I I I

in3+ Sn

I I f I 1.0 , , _~ , , , , -1 0 1

~- 0.6 t Hg~+~ 0.2 I I/ I I I~ I I -3 -2 -1 0 1 -2 - t 0 -2 -1 0 0 1 Log [HCI] Figure 6.16 Rf functions of Ge and its neighboring elements on paper treated with 0.1 M HDEHP in cyclohexane, 236

Reprinted with permission from Cerrai, E., and Ghersini, G., J. Chromatogr. 24, 383. Copyright (1966) Elsevier Science Publishel~.

exchangers, the weakly basic anion exchanger diethylaminoethylcellulose (DEAE- cellulose) has been used extensively. A systematic survey of the behavior of elements on DEAE-cellulose and microcrystalline cellulose, Avicel SF, in various binary systems was given by Ogauma and Kuroda. 232-234 The Rf values as 'a function of HCI concentration for Ge and other elements in methanoI-HCI and cyclohexanone-HCI are given in Figs. 6.14 and 6.15, respectively.

Reverse-phase chromatography using TLC plates and papers treated with di-(2- ethylhexyl) orthophosphoric acid (HDEHP) has been a very effective tool in the separation of many cations including Ge. A discussion of all the investigations on this subject is beyond the scope of this review. The potential user is referred to a good systemaUc study by Cerrai et aL 236 The Rf values of Ge and its neighboring elements

65 S. MIRZADEH, R. M. LAMBRECHT: RADIOCHEMISTRY OF GERMANIUM

on paper treated with 0.1 M HDEHP in cyclohexane as a function of hydrochloric acid are shown in Fig. 6.16. The behavior of most cations in this case compares well with that obtained from liquid-liquid extraction [see Sect. 6.3].

6.5. Electrodeposition

Electrolysis of Ge4+ from a basic or an acidic solution, or from a solution containing complexing agents (e.g. phosphates, oxalates, tartrates, etc.) is typically incomplete and results in only a thin black layer of germanium which adheres poorly to the electrode. After the cathode is covered with a thin layer of Ge, deposition ceases and simultaneous evolution of hydrogen and germanium hydrides takes place. In alkaline solutions, Ge can be deposited simultaneously with other metals such as Cu, Ni and Co. Deposits of Ge from solutions containing F" and CN" ions cannot be obtained.8~

For current densities of 1-10 mA.cm "2 the hydrogen overpotential of Ge is sufficiently high (~ 0.4 - 0.6 V in 2 M H2SO4) to explain the inertness of Ge in solutions which are free from oxidizing agents. The overpotential values of Ge are comparable to those for Fe, Ni and Co, but substantially smaller than Zn. The fact that the electrodeposition of Zn in an acid bath is hindered by the presence of trace amounts of Ge is attributed to lower overpotential of Ge. 8~

Satisfactory deposits of Ge can be obtained in nonaqueous electrolytes. From solutions of GeCI4 or Gel4 in ethylene glycol, diethylene glycol and glycerin, it is possible to obtain thick deposits of Ge which adhere strongly to the electrode. 237'238 Under optimum experimental conditions, the electrolyte consisted of a solution of 7% GeCI4 in ethylene glycol (by volume) at 59~ and deposition was carried out on a rotating electrode (~600 rpm) under 0.4 A.cm "2 current density using a graphite anode. The deposition rate was 0.025 mm in 3 h, and a shiny deposit of Ge with a thickness of 0.12 mm was obtained. 238

Quantitative electrodeposition of 68Ge (in amounts up to 10 mg) from saturated solutions of ammonium chloride was reported. 71 Foils of Ni, Cu, Pd and Pt, were used as a cathode under an EMF of 4 volts and a current density of ~150 A. Carrier-free 68Ge, in amounts up to 2 mCi, has also been deposited successfully by this procedure, and a radioassay of the spent electrolyte indicated that >99% was deposited. 71 Unfortunately, no analytical details were given.

The suitability of 68Ge as a positron source has been proposed for positron trapping in metallic lattices, 71"73 which is a well-recognized technique to study vacancies,

66 S. MIRZADEH, R, M. LAMBRECHT: RADIOCHEMISTRY OF GERMANIUM

dislocations and other imperfections in a metallic lattice. 239-242 An intense source of 6SGe was prepared on a 5/Jm thick Ni foil. After electrolysis, the Ni foil was washed with water to remove residual NH4CI, and then washed with acetone. The foil was heated for 1/2 hour at 200~ in vacuum (10-5 mm Hg), and was then annealed at 1050~ for 3 h. After annealing, no significant quantity of 68Ge could be removed by wiping. About 30-40% of the 68Ga daughter was lost during the initial annealing process, and no further loss of activity occurred in the subsequent heat treatments. After a series of experiments with a Ge-Ni source, 73 it was shown that under high temperature and high vacuum, however, germanium was volatilized from the source. Covering the source with a thin film of evaporated quartz inhibited the loss of germanium. 72

7. Applied Radiochemistry of Germanium

7.1. Ge(68)-,Ga(68) Generator in Biomedical Research 7.2. Ga(71)[v,l~-]Ge(71): A Radiochemical Detector for the Solar Neutrino

7.1. Ge(68)--*Ga(68) Generator in Biomedical Research

Positron emission tomography (PET), which noninvasively provides information regarding blood flow and metabolism in patients, is a common procedure in clinical nuclear medicine. The detection of the two positron-annihilation photons (511 keV) in coincidence results in a significant reduction in background radiation, thus providing sharp tomographic images. The use of very short half-life positron emitters for PET studies provides sufficient levels of radioactivity to permit adequate statistical sampling in the reconstruction of the cross-sectional images while minimizing the radiation dose to the patient.

The availability of short-lived radionuclides from radionuclide generators provides an inexpensive and convenient alternative to in-house radioisotope production facilities such as cyclotrons. Gallium-68, which has all of the physical characteristics desirable for PET, is produced from the decay of 270-d 68Ge (100% EC). In the form of a radioisotope generator, 68Ga is separated from its long-lived parent and used as needed. Gallium-68 decays with a 68 min half-life to'stable Zn with 100% I~+ emission. A simplified decay scheme of the 68Ge---,68Ga system is shown in Fig. 7.1. Production'of 68Ge was discussed in some detail in Sect. 3.

Over the past three decades, a number of 68Ge--*68Ga generators have been developed in an attempt to provide high yield of 68Ga and low breakthrough of 68Ge. The efforts in this field have been reviewed by Lambrecht. 243'244 Due to the simplicity of the operation, chromatographic-based generators have been the method of choice,

67 S. MIRZADEH, R. M. LAMBRECFIT: RADIOCHEMISTRY OF GERMANIUM

0+ 0.O 270.8 d

Eo.,~.O~c.11,/ 1+ 9,~ ~( 68.1 rain .~ . /- ~,Ge=

(2+) ~_ . ~ 0.0094%

(2+) /I-<+" 0.8,1, I ~ i (2+) ,+,~ o.@~/ o.~,,% II .~%~Y"" / (o*) ll" -' o o o,

(2+) ~1 lli~ 1077.36 g

(2+) 0.0 / ~+ 88.0%, EC 8.94%,Eft+ mex= 1899

Figure 7.1 Decay scheme of S8Ge and 68Ga

although generators based on solvent extraction and volatilization have also been proposed. Both inorganic absorbers and synthetic resins have been evaluated as the chromatographic supports, with inorganic supports having the advantage of being rather resistant to radiation damage.

The inorganic absorbers primarily include the hydrated metal oxides (e.g., AI203, SnO 2, Fe203). As discussed in Sect. 6.4, most metal oxides show a tendency to absorb Ge as well as Ga. In dilute acidic solutions or in the presence of a chelating ligand, however, the adsorption of Ga decreases much faster than that of Ge. Hence, under a narrow range of acidity and ligand concentration, Ga can be separated from Ge. Under optimum conditions, it is hoped that Ge is retained on the support irreversibly to the extent that less than 10"2% of Ge is co-eluted with Ga. Another important parameter that needs to be considered here is the solubility of metal oxide supports in aqueous

68 S, MIRZADEH, R. M. LAMBRECHT: RADIOCHEMISTRY OF GERMANIUM solutions. In general, the solubility of metal oxides increases with the alkalinity of the solutions. Methods of oxide preparation also have significant effects on their solubilities.

A few extraction-based 68Ge--~68Ga generators have been proposed. In one of the first generators, Ga (not carrier-free) was extracted into acetylacetone (dissolved in cyclohexane) from a buffered solution.containing Ge. The Ga was then back-extracted into 0.1 M HCI.245 In another procedure, 68Ge was extracted from HCI solution into CCi4, whereas Ga remained almost entirely in HCI solution 246 (see Sect. 6.3). Erhardth et al. reported an extraction generator where 68Ga was complexed with 8- hydroxyquinoline (oxine). 247

The only reported volatilization-based generator takes advantage of the substantial vapor pressure of the chlorogermanium compounds from strong aqueous HCI.248 An azeotropic solution of HCI (6 M) containing 68Ga' and 68Ge in secular equilibrium was evaporated to dryness under a heat lamp, in a stream of N2. During this step 68Ga was retained quantitatively by an evaporation dish; the fraction of 68Ge retained with Ga was <5x10 -4. Subsequently, the 68Ga was removed from the surface of the evaporation dish with 0.1 M HCI or 0.1 M NaCI. The 68Ge was recovered essentially quantitatively in a cold trap at -40~ An apparatus for cyclic operation is shown in Fig. 7.2.

The most widely used generator was proposed by Green and Tucker. 249 In this system, Ge is adsorbed on aluminum or zirconium oxides, and Ga is eluted with a neutral solution of 5 mM Na-EDTA. The initial Ga yield from this generator was -70% but decreased considerably with time. In a comparative study, Karpeles 126 evaluated

TEFLON STOPCOCK7

"•)--•'•" COLD -12 cm f ~ -- TRAP I OIL BATH, -13ooc LIQUID N2

Figure 7.2. Apparatus for cyclic operation of volatilization-based 6SGe--,S8Ga generator

69 S. MIRZADEH, R. M. LAMBRECHT: RADIOCHEMISTRY OF GERMANIUM

several generators which included an Ai203/EDTA system. Alternatively, carrier-free 68Ge was adsorbed on Sb20 5, and 68Ga was eluted with an oxalate solution at pH 5-11. 25o The generator in which gallium is obtained in the form of Ga complex is commercially available in the U.S. and Europe.

For the preparation of 68Ga radiopharmaceuticals other than 68Ga-EDTA, it is necessary to prepare 68Ga in ionic form. For this purpose, the 68Ga-EDTA complex (or any other complex) is usually destroyed with concentrated HCI after the addition of a carrier. The 68Ga is then separated in ionic form by ion exchange or extraction. Because of the rather fast decay of 58Ga, the final yield is significantly reduced.

In a two-part study, Kopecky et al. 224'251 reported the adsorption behavior of carrier-free 68Ga and 68Ge on alumina, AI(OH)3 and Fe(OH)3. A part of this study is presented in Sect. 6.4. Based on this research, a generator was proposed from which the 68Ga could be eluted directly in an ionic form. Malyshev et al. 252 studied the feasibility of efuting ionic 68Ga with mineral acids from generators where Ge was adsorbed on hydrated ZrO 2, SnO2, and TiO 2. Subsequent studies 253 confirmed that the SIlO 2 was a superior support for germanium. In this system, Ga was eluted with 1 M__ HCI. Among the three systems (ZrO2/HNO 3, SiO2/HNO 3, and TiO2/dilute NaOH or Na3PO 4 buffer) which were studied by Neirinckx and Davis 254 for generating ionic 68Ga, only the silica gel/HNO 3 system was found to be suitable. Lewis and Camin 255 suggested eluting the Ga from an alumina generator with 0.1 M__ NaOH. Ambe 256 examined the adsorption of carrier-free 68Ge on a-Fe20 3 and found that 50-70% of Ga can be eluted with an HCI solution of pH 2.0.

The significant difference between the behavior of Ga and Ge from dilute hydrofluoric acid solutions on anion exchangers (Kd=27 and >4000, respectively) was the basis for a generator proposed by Neirinckx and Davis. 257 The reported breakthrough of 68Ge was >10 -4 for up to 600 elutions of the generator with a 65Ga yield of ~90%.

A germanium-specific synthetic resin was prepared and evaluated for 68Ge/68Ga generators. 258'259 This chelating resin was prepared by condensing 1,2,3-trihydroxy- benzene (pyrogallol) with formaldehyde.

D OH OH OH

HO~DH J

CH2 rl

70 S. MIRZADEH, R. M. LAMBRECHT: RADIOCHEMISTRY OF GERMANIUM

The batch distribution coefficient (Kcl)* of carrier-free 68Ge for the resin from solutions of 0.9 M NaCI and dilute HCI (0.1-0.5 M__)is >-5x103, whereas, the Kd for 68Ga is 50 for 0.9 M NaCI and 1.8-0.3 for dilute HCI. The yield of 68Ga from the generator was reproducible at 60% after 600 elutions, while the breakthrough of 68Ge remained at <10 ppm per elution. The pyrogallol-formaldehyde resin was found to be rather resistant to dissociation from radiation. At a radiation dose of 3.8x108 rad/g, no evidence of physical or chemical change was observed. Schuhmacher and Maier-BOrst 259 independently synthesized the pyrogallol-formaldehyde resin for the same purpose. They reported an average yield of 68Ga of 75% during a period of 250 days of study. The Ge breakthrough was <0.5 ppm with no detectable radiolytic byproducts for a 10- mCi generator.

12 D TIN DIOXIDE / 1M HCI

~ E 10 ALUMINA / 0.1M NaOH _J UJ 8 ALUMINA I 0.005M EDTA oo

6 .J ..J < 0 4

o 0 2 4 6 8 10 12 14 ELUTION VOLUME (ml)

Figure 7.3 Elution profiles of S8Ga from 68Ge/SSGa generator systems. 261

Reprinted from McEIvany, K. D., Hopkins, K. T., and Welch M. J., Int. J. Appl. Radiat. /sot. 35, 521. Copyright (1974) with permission from Pergamon Press Ltd., Headington Hill Hall,' Oxford OX30BW, UK.

* Kd= (activity of 68Ge per gram of resin)/(activity per gram of liquid phase)

71 S. MIRZADEH, R. M, LAMBRECHT: RAD1OCHEMISTRY OF GERMANIUM

The strong adsorption capacity of the pyrogallol-formaldehyde resin for Ge is due to the formation of very stable complexes of Ge with phenolic groups. 26~ Indeed, tannic acid consisting of polyhydroxybenzene is a very selective reagent for precipitation of germanium.

In a comparative study, McEIvany et a/. 261 evaluated three types of commercially available 68Ge----,68Ga generator systems (namely SnO2/HCI , AI203/NaOH and AI203/EDTA) with respect to Ga elution profiles and yields, parent Ge breakthrough levels and amounts of dissolved support over a period of 1 year. Their results, together with the data compiled by Neirinckx, 258 are reproduced in Table 7.1. The elution profiles of 68Ga as measured by McEIvany are shown in Fig. 7.3. In these studies, the levels of activity were 25, 30 and 50 mCi, and the maximum of the elution curves (Fig. 7.3) corresponded to 14, 8 and 28% for AI203/EDTA, AI203/NaOH and SnO2/HCI , respectively.

7.2 Ga(71)[v,6"JGe(71): A Radiochemical Detector for the Solar Neutrino

Based on the standard solar model it is believed that the heat of the sun is generated by thermonuclear reactions which fuse the light elements into heavier ones. During these processes, which take place well within the interior of the sun, many elementary particles are released. Radiation from solar fusion processes, however, is emitted in the form of neutrinos (v), which have the ability to penetrate from the center of the sun up to its surface and into space. All other particles lose their information load during their journey to the surface. 263 The principal reactions involved in the sun's fusion process are shown in Table 7.2.

The flux of high-energy neutrinos* from the sun was measured by Davis and co- workers 264 in a very difficult radiochemical experiment using a process called "inverse 6" decay". Specifically, the high-energy neutrino is captured by a 3701 atom to produce radioactive 3TAr (an inert gas) and a electron, 37Cl[v,e']37Ar. The neutrino flux is then calculated from the experimentally measured 3TAr radioactivity. Because of the extremely small cross section of neutrino capture by matter (<10 "38 cm 2) 264 several tons of chlorine were required to produce only a few atoms of 3TAr (tl/2=35 d) at saturation. In addition, to redu~.e the contribution of cosmic rays to 3TAr production, it was necessary to conduct this experiment deep underground. The results have indicated a lower

*Emax=14 MeV, generated primarily from aB decay, reaction 7 in Table 7.2

72 S. MIRZADEH. R. M. LAMBRECHT: RADIOCHEMISTRY OF GERMANIUM

Oe~ ~"'- .:'- O tO d , V V | V , 04 V ~ V ~ :

0 0 O0 0 000 0 000 r.O 0 O~ (D ; cO 04 , "~" 09 09 ,

bb obbb ob ~bb b bob XX XXXX ~ ~ ~ Xx ,vv v~v ~ Oxx

(.9

(.9 o~ ~ ~o O~ 0 ~ "O O

.9. ,? ,z- O "- CO - 000 , ~ d , , cO c~I , , , ~-

E e O , ~ , ~ t' , ; , o c5 , , ,. ~ ~-

._q u') e O CD &9 &. i- ~D -o

f- d~ oooo ~ 6dd d d~ O ~o ../., O _e ,-,--,- ~ ooo o 0_ l--

73 S, M/RZADEH, R. M. LAMBRECHT: RADIOCHEMISTRY OF GERMANIlSM

neutrino flux at the earth than the flux predicted from the standard solar model. This discrepancy, over the past two decades, has stimulated the development of other radiochemical detectors for solar neutrinos. 265267 Currently there is considerable interest in the gallium neutrino detector. This detector is based on the capture of neutrinos by 71Ga to produce radioactive 71Ge, which decays to 71Ga by 100% electron capture with a half-life of 11 d.

71Ga + v ~--- 71Ge (t1/2=11 d) + e"

The 233 keV threshold of this reaction is ideal for observing the low-energy neutrinos produced in the proton-proton (p-p) reaction (reactions 1-4, Table 7.2). As indicated in Table 7.2, the rate of the p-p reaction is directly related to the solar luminosity and apparently is insensitive to alteration in solar models. 263'268

Table 7.2 Principal reactions involved in the sun's fusion process a

Solar Neutrino No. Nuclear reaction terminations Ev rnax radiochemical (%) (MeV) detector

Main p-p chain (95%)

1 2p --, d + e + + v e 99.75 0.420 71Ga[ve, e'] 71Ge 2 or (2p + e" --* d + v) 0.25 1.44 b 3 d+ p ---* 3He + X 4a 2~He --* 4He + 2p 86

Net 4p --~ a + 2e + + 2v

Side chain (-5%)

4b 3He + 4He ---. 7Be + 2p 5a 7Be + e --* 7Li + v 0.861(90%) b 0.383(10%) b 6 7Li + p --* 8Be* ~ 24He 14

Net 4p--*a+2e ++2v

Side chain (rare)

5b 7Be + p .~. 8B + y 7 8B--,SBe +e ++v e 14.06 37Cl[ve,e']37Ar 8 8Be* --* 24He 0.02

Net 4p--. a + 2e* + 2v

aAdopted from the original compilation by Bahcall and Davis263 bmonoenergetic

74 S. M/RZADEH, R. M. LAMBRECHT: RAD1OCHEMISTRY OF GERMANIUM

The recent results from the Ga detector, 268 as well as an independent measurement by neutrino-electron elastic scattering employing a water Cherenkov detector, 269 have confirmed the low neutrino capture rate. The significant suppression of the low-energy neutrino flux from the sun points to a new fundamental property of the elusive neutrino. An international collaboration, the Soviet-American Gallium Experiment (SAGE), 269 has operated a 30-ton Ga detector in the Baksan Neutrino Observatory in the Soviet Caucasus since early 1990, In this experiment, metallic Ga (m.p.= 28~ is contained in four Teflon-lined tanks, each holding ~7 tons of Ga. Germanium-71 produced in the Ga is separated by heterogeneous extraction into dilute HCI in the presence of H202 after the addition of 160 ,ug of Ge carrier to each tank. The extractants are combined and condensed by vacuum evaporation. After the addition of strong HCI, GeCI4 is purged from solution by a stream of Ar carrier gas and collected in a water trap (1.2 L). Germanium is then further purified by extraction into CCI 4, back- extracted into 100 ml of low-tritium H20. The overall extraction yield is 80~-6% which is astonishingly good, considering the magnitude of the experiment. Finally, Ge is reduced to germane and purified by GC. The purified germane was then mixed with Xe and filled into a miniature proportional counter with an internal volume of ~0.75 cc (similar to counters used in the 3TAr experiment). The proportional detector is placed into a weil- type Nal detector. The activity of 71Ge is measured from the 10.4-keV K-shell auger' electrons signal (produced, from decay of the electronically excited 71Ga daughter) in anticoincidence with the signals from the Nat detector. The typical net count rates of 71Ge (after correction for background) is on the order of ~0.1 count/d for 3-4 weeks of exposure of 30 tons of Ga to solar neutrinos under 4700 m of water-equivalent shielding.

A German-Italian-French collaboration (GALLEX) 27~ operates a 30-ton detector using GaCI3 in dilute HCI, and is located {n the Gran Sasso Underground Physics L~boratory in Italy.

8. Selected Radiochemical Procedures

8.1. Separation of Germanium Radioisotopes from Various Media a. Fission Products b. Proton-Irradiated RbBr Target c. Proton-Irradiated Ga Metal Target d. Proton-Irradiated Ga4Ni Target 8.2. Rapid Separation of Germanium Isotopes from Fission Products 8.3. Determination of Germanium by Neutron Activation 8.4. Thin-Layer Chromatographic Separation of Carrier-Free 77As from 77Ge

In this section, no "recipes" are given for radiochemical procedures. Rather, the essential chemical features of reported procedures are summarized.

75 S. MIRZADEH, R. M. LAMBRECHT: RADI()CHEI~flSTRY OF GERMANIUM

8.1. Separation of Germanium Radioisotopes from Various Media

a. Fission Products 271

Targets of depleted 238U or 232Th as nitrates (2-10 g) were irradiated with 14.8- MeV neutrons with durations ranging from 5 min to 1.5 h. The fast neutrons with fluxes of I09-101~ n.s'l.cm "2 were produced by the T[D,n]4He reaction.

The irradiated targets were dissolved in conc. HCf containing 15 mg each of As +3 and Ge4+ carrier. Germanium was extracted into 30 ml of CCI4 for 5 min, the organic phase was washed with conc. HCI for 2 min, and then Ge was back-extracted into 10 ml of H20 for 5 min. One drop of conc. H2SO4, 0.8 g of NH4SO4 and 10 ml of freshly prepared 5% tannic acid solution were added to the aqueous phase to precipitate Ge tannate. The precipitate was allowed to coagulate for 5 min in a water bath at 95~ While hot, the precipitate was collected on a Whatman #-41 filter paper and then transferred to a porcelain crucible (precipitate only). Germanium was converted to GeO 2 by igniting the precipitate at 900~ in a muffle furnace. With the aid of ethanol, the oxide was transferred to a glass fiber filter, weighed, mounted and I~ and y counted. The time required for separation was about 1.5 h. The chemical yield averaged ~30% with an overall decontamination factor of ~3x107 from other fission products. With the exception of the iodine and bromine, all fission products were separated in the extraction and back-extraction steps. The precipitation step further afforded separation from As, I and Br. Although the distribution coefficient of As +3 is considerably less than that of Ge (4.1 vs 594 at 12 _M HCI), for complete separation it would be advisable to oxidize As +3 to As +5 by adding KBrO 3 or purging the target solution with CI2 gas prior to the extraction step.

b. Proton-irradiated RbBr Tar.qet11,9_

A RbBr pellet was prepared by pressing high-purity powder under 9000 psi for 10 minutes. The pellet (2-3 g, 13 mm in diameter and 6 mm in height) was irradiated with 500- and 800-MeV protons with integrated intensities of 1-2 pA.h.

The irradiated target was dissolved in 50 ml of 6 M HCI in a semiclosed apparatus. After dissolution, 20-25 ml of solution was distilled into an ice-cooled receiver containing 5 ml of 3% H202. The rate of distillation was about ~0.5 ml/min.

* Apparently the presence of filter paper during ignition will reduce GeO2 to volatile GeO.

?6 S, MIRZADEH, R. M. LAMBRECHT: RADIOCHEMISTRY OF GERMANIUM

DISSOLUTION DISTILLATION RbBrTARGET H,..., Y ~-- Ge, As, Se SM_HCl 8.5MHcI1 H202 Icc,.soLvE..I EXTRACTION J As, Se ~ AQUEOUS I I ORGANIC ~ Ge ! ",OBAC I EXTRACTION]

I AQUEOUS Ge

Figure 8.1. Schematic showing the chemical steps for separation of carrier-free SSGe from proton-irradiated RbBr target. 119

Concentrated HCI was added to the distillate to adjust the HCI concentration to 8.5 M_. Germanium-68 was then extracted into 50 ml of CCI4 and back-extracted into 50 ml of H20. These chemical steps are schematically shown in Figure 8.1.

The chemical yields of Ge in each step (the average of eight independent experiments), together with those of As and Se, the main contaminants, are given in Table 8.1. Small fractions of As and Se also co-distill with Ge, but quantitative separation was achieved in the extraction step. The final Ge preparation, however, contained a small fraction of radioselenium. Although not explicitly given, the separation factors from the other spallogenic byproducts were sufficiently high to use the procedure in large-scale production. Apparently, the above procedure yielded better quality 68Ge from a 50-g RbBr target irradiated at a proton current of 340 pA for 2 weeks.

The following observations have been made under hot cell conditions: At the higher dose, the target was found to be rather insoluble in H20, but readily dissolved in 3 M HCI, yielding a brownish-yeilow solution. In the distillation step, a significant fraction of radioselenium adsorbed on the condenser wall, with no apparent effects on the yield of Ge. Some loss of Ge by volatilization may occur during adjustment of HCI concentration to 8.5 M HCI and prior to extraction.

77 S. MIRZADEia &. M. LAMBRECHT: RADIOCHEMIS'fRY OF GERMANIUM

Table 8.1. Behavior of carrier-free Ge, As and Se radioisotopes in the chemical processing of the proton-irradiated RbBr target 119

Overall chemical yield (%)a Chemical Step process Phase Ge As Se

1 dissolution aqueous 100 100 100 (6 M_ HCl)

2 distillation residue 0 4+2 14• (6 M HCI) distillate 98:1:2 91:1:2 59•

3 extraction aqueous 0.5+0.2 90+3 51+20 (CCI4/8.5 M HCI) organic 87+3 0 7:1:3

4 back extraction organic 2+1 0 9• (H20) aqueous 87• 0 1.3+0.3

aRelative to the activity in the original target solution

c. Proton-Irradiated Ga Metal Tarqet 41,172'272_

There are three reported approaches for processing a Ga metal target for the recovery of radioisotopes of Ge.produced by proton-induced reactions. 41'172'272 The Ga targets can be dissolved in an appropriate solvent followed by commonly used techniques for Ge and Ga separation (e.g., extraction with organic solvents or distillation), or Ge can be extracted directly from Ga metal liquid by heterogeneous extraction. These procedures are also equally applicable for the separation of 71Ge produced in a Ga target by neutrino interactions, which was discussed in some detail in Sect. 7.2.

In the first approach, the proton-irradiated Ga metal (6-8 g) was dissolved in 50 ml of concentrated HNO 3 with an addition of 5% H202 .172 The second approach took advantage of the low melting point of metallic Ga (28~ and Ge was isolated from liquid Ga metal by multiple extractions with 5 ml of 4 M HCI containing 0.5 ml of 30% H202 .41 Alternatively, Ge was extracted with a mixture of 0.25 _M NaOH and 0.05 M H202 from Ga metal at 40~ 272 The presence of hydrogen peroxide and the contact area (interface) between the two phases were important parameters in these processes.

Pure metallic Ga in the liquid state has a tendency to dissolve metals. Capsules made from niobium, however, have been successfully used as containers for Ga metal during long irradiations, typically 60-90 days irradiation with ~15 juA of ~20-MeV protons. 41

78 S. MIRZADEH, R. M. LAMBRECHT: RADIOCHEMISTRY OF GERMANIUM d. Proton-Irradiated Ga4Ni Target4~

The Ga4Ni (m.p. ~900~ was prepared by fusing 3 g of Ni with 12 g of Ga at 1600~ in a high-frequency induction furnace. The molten alloy was allowed to solidify into a slug in a copper mold. The slug was then pressed into a copper backing (under a pressure of 40 tons). The copper alloy unit was then annealed at 4500C for 2 h, and after cooling, the face of the target was machined to an alloy thickness of 3 mm.

The Ga4Ni target was irradiated for 60 h with 1-2 pA of 19.5-MeV protons. During irradiation, the target unit was cooled by water from the back and with a current of He from the front. The entire assembly is shown in Fig. 8.2.

O RING

COPPER TARGET

Ga4NiALLO'

TiFOI STAINLESS STEEL SUPPORT COOLING SYSTEM

SHUFFLE

Figure 8.2. Ga4Ni target assembly. 4~

Reprinted from Loc'H C. et al., Int. J. Appl. Radiat. /sot. 33, 267. Copyright (1982) with permission from Pergamon Press Ltd., Headington Hill Hall, Oxford OX30BW, UK.

79 S. MIRZADEH, R. M. LAMBRECHT: RADIOCHEMISTRYOF GERMANIUM

After a 1-month cooling period, the target unit (Ga4Ni and copper backing) was dissolved in 250 ml of cold concentrated HNO 3. The target solution was first mixed with 50 ml of CCI 4 for a few minutes, then 500 ml of 9 M of HCI was added. After 15 minutes of mixing, the phases were separated. Extraction was continued with two additional 50- ml portions of fresh CCI 4. The organic phases were combined and Ge was back- extracted into 2x20 ml of 1 M HCI. Recovery of 68Ge was 93%, and the remaining activity was found to be 4.3% in the 9 M HCI fraction and 0.5% in the CCI 4 fraction. It was estimated that 2.2% of the activity was lost by volatilization. Decontamination factors from Co, Ni, Ga and Zn were measured by the addition of traces of 57Co, 57Ni, 68Ga and 65Zn to the target solution in a cold run. The decontamination factors achieved were ~1x104 for 57Co and 65Zn, and 2x103 for 57Ni and 68Ga. From the latter two factors, it was estimated that the final eSGe preparation contained 40/zg of Ni and 250/Jg of Ga. No value was given for separation from copper.

8.2. Rapid Separation of Germanium Isotopes from Fission Products 273

Thirty mg of 235U, as uranyl nitrate, were irradiated for 30 rain in a neutron flux of 4x1011 n.s-l.cm-2 in a nuclear reactor. The apparatusfor the fast separation is shown in Figure 8.3. After cooling for one hour, the target was dissolved in 2 ml of conc. HCI, and together with Ge carrier (0.1-10 rag) and a trace amount of 68Ge (for chemical yield determinations), was placed in flask B, Flask A contained 12 ml of 9.5 M_M_HCI, and prior to adding the contents of flask B to A, flask A and the exit tube were partially evacuated.

B VACUUM I

CI2 i VACUUM ' ~ll! !!::', I r.~ D

RIE I C A

Figure 8.3. Apparatus for fast chemical separation of Ge from fission products. 273

80 S. MIRZADEH, R. M. LAMBRECHT: RADIOCHEMISTRYOF GERMANIUM

After the addition of B to A, a burst of CI2 gas was passed through solution A. The volatile species formed were then sucked through the drierite in C ( 20 g in a U tube of 1.4 cm O.D.) and Zn powder in D (40-200 mesh, contained in a 1.4x3-cm tube).

In the presence of a small amount of carrier, Ge radioisotopes were completely retained by the Zn powder, although the -50% of Ge was also stopped at the drierite trap. In the absence of the drierite trap, the Ge yield (as measured on the Zn trap) was a function of the CI2 burst duration and increased rapidly to ~ 70% in the first 20 sec. These data were obtained by the use of 68Ge as a tracer added to the Ge carrier solution. In the actual run, the presence of 68Ge also provided an excellent means for yield determination, whereas it did not interfere with y-ray activities of the short-lived isotopes since its I~,+-emitterdaughter, 68Ga (which was quantitatively retained in flask A) had not sufficiently grown in the first few minutes.

In the presence of the drierite trap the decontamination factors were better than 105 for Ga, As, Mo, Sn, Sb, I and Cs, and were lx104 for Tc and 3x103 for Br. All the fission-produced noble gases and some of the bromine escaped from both traps.

8.3 Determination of Germanium by Neutron Activation

In a procedure for determining germanium in geological material1~ a 400-rag sample of powdered geological material was irradiated in a reactor with a thermal neutron flux of 2.5x1012 n.s'l.cm'2 for a period of 15 h. The irradiation sample was transferred to a polypropylene beaker containing a mixture of 4 ml conc. HF, 2 ml conc. HNO 3, 2 ml conc. H2SO4 and 1 ml Ge carrier [-1 mg/ml, added as (NH4)2Ge(C204) 3, 4H20 ]. The mixture was heated on a steam bath to dissolve the sample, and cooled. Three ml of H20 were added, then the mixture was evaporated to near-dryness. The residue was dissolved in 5 ml of H20, brought to near boiling for a few minutes, then transferred to a 40-ml centrifuge cone. It was then cooled to 20~ and 1 ml of 0.1 M KCIO3 and a sufficient amount of conc. HCI (ca. 15 ml) were added to make the solution about 9 M in HCI. (Potassium perchlorate oxidizes As to +5, which is not significantly extracted into CCI4.) The solution was centrifuged and filtered into a separatory funnel containing 10 ml of CCI4. Germanium was extracted into the organic layer for 2 minutes. The organic phase was then washed twice with a mixture of 10 ml of conc. HCI and 1 ml of 0.1 M KCIO3, and once with 10 ml of 9 M HCI. Germanium was then back- extracted into 10 ml of H20 for 2 minutes, and the aqueous phase was transferred to a 40 ml centrifuge cone, and H2SO4 was added to make the solution 2.5 M in acid. Germanium was then precipitated as GeS2 by H2S passage, centrifuged, and the

81 S, MIRZADEH, R. M. LAMBRECHT: RADIOCHEM1STRY OF GERMANIUM

precipitate was thoroughly washed with water and acetone, dried at 110~ and weighed as GeS2 for chemical yield determination. A chemical yield of-95% can be expected from the procedure.

8.4. Thin-Layer Chromatographic Separation of Carrier-free 77As from 77Ge127

Germanium-77 was prepared by neutron irradiation of high-purity GeO 2 of natural abundance. A 100-mg target was irradiated with thermal neutrons of 5x1011 n.s'l.cm "2, for 5-6 h. After irradiation, the target was allowed to decay for 25.5 h prior to dissolution. Under the above conditions, 77Ge (tl/2=11.3 h) and its daughter, 77As (t1/2=38.8 h) are the main radioactive species with a small percentage contribution from 71Ge (tl/2=11.4 d, 100 % EC).

The target was dissolved in 2 ml of 1.5 M KOH. The yield of 77As at this time was 4.5 pCi. A 10opl aliquot of the Ge target solution was spotted on a silica-gel TLC plate (5x20 cm, ~250 pm in thickness) and developed against various mixtures of methanol/water, methanol/HCI, acetone/water and acetone/HCI. Autoradiography of the plates indicated three distinct spots on the plates which were assigned to Ge, As 3+ and As 5+ in order of their Rf values.

In the case of the hydrochloric acid mixtures, activities were concentrated in small spots with no overlapping. The best conditions were found to be 2:1 mixtures of methanol/5 M HCI or 1:1 mixtures of acetone/1 M HCI. For both cases, the Rf values were 0.0 for Ge, 0.5 for As 3+ and 0.94 for As 5+.

In the absence of an As 3+ carrier, only 6% of the As was found in the +3 state. Whereas the fraction of 77As3+ increased sharply to -70% when the target dissolution mixture contained 2 mmol/I of As3§ carrier. The presence of a carrier from 2 to 30 mmol/I resulted in only a slight increase from 75 to 80% in the fraction of 77As3+. These results are consistent with previous data of Genet et al. 135'136 who employed electrophoresis to separate As 3+ from As 5+. The portion of the TLC plate containing As 5+ was scraped off and carrier-free 77As was nearly quantitatively eluted from silica gel with H20 or 0.1 M HCI.

82 S. MIRZADEH,R. M. LAMBRECHT:RADIOCHEMISTRY OF GERMANIUM

The work at Oak Ridge National Laboratory was performed under the auspices of the United States Department of Energy under Contract DE-AC05-84OR21400 with Martin Marietta Energy Systems, Inc. The work at Australian Nuclear Science and Technology Organisation (ANSTO) was supported by the Commonwealth of Australia. The authors wish to acknowledge Dr. F. F. (Russ) Knapp, Jr~, ORNL Nuclear Medicine Group, for his continuous support and review of the final draft, K. Ambrose for proofreading, and K. S. Brown for technical editing and formatting of the article. The authors gratefully acknowledge permission from various journals to reproduce the numerous figures and tables contained in this monograph. The authors would also like to acknowledge the editorial assistance of Jacqueline Mirzadeh. Without her aid, completion of this monograph would have been very difficult if not impossible.

References

LIST OF BOOKS, CHAPTERS and JOURNAL REVIEW ARTICLES: Germanium or Germanium Compounds

General:

Glocking, F. The Chemistry of Germanium, Academic Press, Inc., London, (1969).

Germanium (Supplement), Gmelin Handbook of Inor.qanic and Or.qanomettallic Chemistry, Gmelin, Frankfurt (reprint 1958). (in German, Literature closing date 1954).

Germanium, Gmelin Handbook of Inorganic and Organomettallic Chemistry, Gmelin, Frankfurt (reprint 1961). (in German, Literature closing date 1931).

Analytical:

Henze, G., "Germanium -- 100 Years. Development of Analysis," Fresenius' Z. Anal. Chem. 324, 105 (1986).

Nazarenko, V. A., Analytical Chemistry of Germanium [translated by N. Mandel], John Wiley & Sons, Inc., New York (1974).

Musgrave, J. R., "Germanium" in Treatise on Analytical Chemistry, Part II, Vol. 2, [1. M. Kolthoff, P. J. Elving, and E. B. Sandell, Eds.], John Wiley & Sons, Inc., New York (1962), pp. 208-245.

Inorganic:

Rochow, E. G., Germanium, in Comprehensive Inorganic Chemistry Vol. 2, [J. C. Bailer Jr., H. J. Emeleus, R. Nyholm, and A. F. Trotman-Dickenson, Eds.], Pergamon Press Ltd, Oxford (1973), pp. 1-41.

83 S. MIRZADEH, R. M. LAMBRECHT: RADIOCHEMISTRYOF GERMANIUM

Samsonov, G. V., and Bondarev, V. N., Germanides, [translated by A. Ward], Primary Sources, New York, (1970). Stone, F. G.A. HydroQen Compounds of the Group IV Elements, Prentice-Hall, Inc., Englewood Cliffs, N.J., (1962), pp. 63-76.

Johnson, O. H., "Germanium and Its Inorganic Compounds," Chem. Rev. 51,431 (1952).

Organometallic:

Tetraorganogermanium Compounds from Ge(C2HT)3R to GeR4, Gmelin ~landbook of InorclanLc.and Or,qanomettallic Chemistry, Gmelin, Frankfurt (1990). (Literature closing date 1987).

Ge(CH~)3R and Ge(CzHs)3R Compounds, Gmelin Handbook of Inor,qanic and .Or,qanomettallic Chemistry, Gmelin, Frankfurt (1989). (Literature closing date 1985).

Tetraorganogermanium, Gmelin H.andbook of !nor,qanic and OrganomettaUic Chemistry, Gmelin, Frankfurt (1988). (Literature closing date 1985).

Lesbre, M., Mazerlles, P., and Satg6, J., The Orqanic Compounds of Germanium, John Wiley & Sons, Ltd., London (1971).

Dub, M., Or,qanometallic Compounds 2nd ed., Vof. 2, Springer Verlag, Berlin, 1967.

Hooton, K. A., Organogermanium Compounds, Preparative Inor,qanic Reactions," [W. L. Jolly, ed.], Vol. 4, John Wiley & Sons, Inc., New York (1968), pp. 85-176.

Hagihara, N., ed., Handbook of Or,qanometallic Compounds, W. A. Benjamin Inc., New York (1968), pp. 449-467.

Quane, D., and Bottei, R. S., "Organogermanium Chemistry," Chem. Rev. 63, 403 (1963).

Radiochemistry:

Madnsky, J. A., The Radiochemistry of Ge.rmanium, National Academy of Sciences, NAS-NS-3043 (1961).

Mirzadeh, S., Some Observations on the Chemical Behavior of Carrier-Free 68Ge PhD Thesis, The University of New Mexico (1978).

84 S. MIRZADEH, R. M. LAMBRECHT: RADIOCHEMISTRY OF GERMANIUM

Electrochemistry:

Efimov, E. A., and Erusalimchik, I. G., Electrochemistry of Germanium and Silicon, [translated by A. Peiperl], The Sigma Press Publishers, Washington, D.C. (1963).

De Zoubov, N., Deltombe, E., Vanleugenhaghe, C., and Polurbaix, M, "Germanium," in Atlas of Electrochemical Equilibria in Aqueous Solutions [M. J. N. Pourbaix, Ed.], National Association of Corrosion Engineers, Huston (1977).

Geochemistry:

Weber, J. N., Geochemistry of Germanium, Dowden, Hutchinson and Ross, Stroudsburg, PA (1973).

List of References

1. Green, M. D., Rosman, K. J. R., and De Laeter, J. R., "The Isotopic Composition of Germanium in Terrestrial Samples," Int. J. Mass Spectrom. Ion Processes 68, 15 (1986).

2. Lederer, C. M, and Shirley, V. S., editors, T__ableof Isotopes, 7th ed., John Wiley and Sons Inc., New York, 1978.

3. Reus, U., and Westmeier, W., "Catalog of Gamma-Rays from Radioactive Decay", At. Data Nucl. Data Tables. Parts I and II 29 (1983).

4. Wapstra, A. H., and Bos, A., "The 1977 Mass Evaluation, Part I., Atomic Mass Table," At. Data Nucl. Data Tables 19, 175 (1977).

5. Halbert, M L., "Mass Chain: 64," Nucl. Data Sheets 28, 259 (1979).

6. Singh, B., "Mass Chain: 64 Update," NucL Data Sheets 82, 688 (1991).

7. Auble, R. L., "Mass Chain: 65," Nucl. Data Sheets 16, 376 (1975).

8. Auble, R. L., "Mass Chain: 66," Nucl. Data Sheets 16, 409 (1975).

9. Bhat, M. R., "Mass Chain: 66, Update," NucL Data Sheets 61,510 (1990).

10. Auble, R. L., "Mass Chain: 67," Nucl. Data Sheets 16, 440 (1975).

11. Kearns, F., "Mass Chain: 68," Nucl. Data Sheets 33, 540 (1981).

12. Kearns, F., and Ward, N. J., "Mass Chain: 69," Nucl. Data Sheets 35, 151 (1982).

13. Bhat, M. R., "Mass Chain 69, Update," Nucl. Data Sheets 58, 59 (1989).

85 S. MIRZADEH, R. M. LAMBRECHT: RADIOCHEMISTRY OF GERMANIUM

14. Bhat, M. R., "Mass Chain: 70," Nucl. Data Sheets 51, 131 (1987).

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