Entitled Submitted in Part Fulfilment of the Requirements for the Diploma Of

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DISSERTATION

entitled

THE ANALYTICAL CHEMISTRY OF ZIRCONIUM

P. R. S. SMITH

Submitted in part fulfilment of the requirements

for the Diploma of Imperial College.

Imperial College S.W,7. June, 1961. ACKNOWLEDGEMENTS

I wish to express my thanks to Assistant Professor L. S. Theobald for his assistance and advice in the writing of this dissertation.

riz. s. s, ..„------.

CONTENTS

Page No.

1. Introduction . o oe ow oo oo Os 04 so 1

2. Separation of Zirconium from Hafnium .. 3

3. Indirect Determination of Zirconium and Hafnium . • .. 11

4. Separation of Zirconium from most other Elements

(a) General Methods ...... 16

(b) "Selective" Reagents ...... 18 (c) Separation of Zirconium from Niobium and Tantalum .. 42

5. Volumetric Determination of Zirconium 46

6. Instrumental Methods ...... 60

7. Conclusion . .. 00 00 00 00 00 00 62 1. Zirconium is a transition metal, a member of sub— group IVA in the periodic table. The normal valency of Zirconium is 4, although valencies of 5,6,7 and 8 can be realised by co—ordination; valencies of 2 and 3 are known, but only in exceptional circumstances. Although normally quadrivalent, zirconium never exists as the Zr4+ ion but always in combination with some other element, usually oxygen or fluorine. for which it has great affinity. The most stable entity is the zirconyl ion, ZrO2+ , which can undergo normal ionic reactions. In sulphate solutions there is evidence that zirconium is present mainly as an ionic complex containing one or more zirconium atoms bound by oxygen linkings (1). In faintly acid solutions hydrolysis of the zirconium can give rise to some difficulty. However, the degree of complex formation or hydrolysis depends on both time and temperature, so that a freshly prepared solution of zirconium might give rise to different reactions from those of a solution that has been either set aside for some time or heated. This phenomenon must always be borne in mind, especially in the preparation of solutions for analysis (2). An exceptional feature of zirconium is the chemical similarity of hafnium, the next member of the sub—group. This is due to the lanthanide contraction; as a result, the difference in atomic volume between the two metals is only

2.2 per cent. M MO2 Molecular Volume Zr 13.97 21.50 Hf 13.66 21.70 Diff % —2.22 +0.93 Linear Diff % —0.74 +0.31

This means that these elements are more similar in character than two successive lanthanides (3). Zirconium has been shown by microscopical examina— tion to be one of the most constant of rock constituents, usually in the form of zircon (ZrSiO4) (4). It may be present up to a few per cent, but rarely reaches 0.2 per cent, and is normally less than 0.1 per cent. The ore minerals are zircon and baddeleyite (Zr02). Other minerals are: Oxides Silicates Brazilite Naegite Zirbelite Lavenite Uhligite Hainite Eudialyte Rosenbuschite G. Van Hevesy (5) found hafnium in all zirconium minerals, usually not greater than 2.0 per cent. except in alvite and cyrtolite, altered zircons. Only the rare mineral 3

thortveitite contains more hafnium than zirconium (6). Most analytical methods give results for zirconium plus hafnium. 2. Separation of Zirconium from Hafnium The separation of zirconium and hafnium is extremely difficult and, as yet, no analytical separation is known. Since pure zirconium is required for the preparation of standards, methods of separation are of interest to the analyst. Most early work in this field was concerned with the extraction of hafnium; recently more interest has been shown in the preparation of "hafnium-free" zirconium metal to accommodate the needs of nuclear technology, as it has particularly suitable properties (5), i.o. "transparent" to neutrons (Zr : 0.18 barns cf. Hf 120 barns). The methods available may be conveniently grouped as follows : (a) Fractional Crystallization (b) Fractional Distillation (c) Ion-Exchange (d) Liquid-Liquid Extraction (e) Miscellaneous. (a) Fractional Crystallization The earliest attempt at a separation was made by fractionally crystallizing the ammonium or potassium double fluorides, e.g. KZrF6, or KHfF6. As the hafnium salt is more soluble it concentrates in the mother-liquors, while zirconium is left in the precipitates. Von Hevesy and his co-workers (7) had to make 650 crystallizations to produce hafnium substantially free from zirconium. Van Atkel and De Boer (8) with a very similar method, had to make 180 crystallizations to concentrate hafnium from 10 per cent. to 50 per cent. of the mother liquor. Von Hevesy noted that hafnium secondary phosphate was appreciably less soluble than zirconium secondary phosphate but attempted no further work because of the difficult nature of the precipitate. De Boer (9) in 1926 managed to conduct a separation by dissolving the freshly precipitated phosphates in saturated oxalic acid solution and re-precipitating in the cold by adding hydrochloric acid. After only 26 precipitations, 97 per cent. hafnium was produced. In oxalic acid, the phosphates form soluble complexes that are easily decomposed by mineral acid. The hafnium complexes are less stable and are decomposed more easily. This factor helps in the separation. De Boer (10) studied many reagents that form similar complexes including sulphuric acid, orthophosphoric acid, hydrofluoric acid, dicarboxylic acid, o( -hydroxy carboxylic acids and polyhydroxy alcohols. By dissolving the precipitated phosphates concentrated sulphuric acid and re- precipitating by dilution with water, De Boer was able to obtain substantially zirconium-free hafnium in 12-15 precipitations. However, such a process is very expensive because of the large amounts of sulphuric acid required. By precipitating with sodium ferrocyanide from a mixture of zirconium and hafnium sulphates in the presence of dilute sulphuric acid, oxalic acid, and ammonium sulphate, Prandtl (11) obtained a precipitate enriched with hafnium. After three precipitations a product containing 95 per cent. of hafnium was obtained. Schumb and Pittman (12) in 1943 were unable to repeat Prandtl's work and with a revised procedure enriched a mixture containing 20 per cent. of hafnium to 80 per cent. of hafnium in four precipitations. To effect a separation using cheap materials, Larsen, Fernelius and Quill (13) used fractional precipitation of the phosphates from dilute sulphuric acid solution. To obtain a readily filterable precipitate they sprayed, similtaneously, zirconyl/hafnyl solutions and dilute orthophosphoric acid solution into 10 per cent. v/v sulphuric acid at 70°C. The action of an icc-cold solution of sodium hydroxide and sodium peroxide on the cold phosphate slurry yielded an acid-soluble hydrate which was reprecipi- tated as before. In seven operations the hafnium content - 6 -

of the precipitate changed from 13 per cent. to 93 per cent. By treating the mother-liquors, zirconium substantially free from hafnium was obtained. Willard and Freund (14) precipitated the zirconyl and hafnyl phosphates by hydrolysis of trimethyl phosphate. This yielded an easily filterable precipitate. Separation of the two elements was achieved after 5 to 6 precipitations. However, Hillebrand et al. (4) have been unable to obtain clear-cut separations. Today, the fractional precipitation of potassium fluorzirconate is used in Russia on an industrial scale to produce hafnium-free zirconium (6). In 16 to 18 recrystal- lizations zirconium containing less than 0.01 per cent. of hafnium is obtained. By re-cycling the mother-liquors an 80 per cent. yield of the zirconium added is obtained. (b) Fractional Distillation The tetrachlorides of zirconium and hafnium form addition compounds with phosphorus pentachloride and phosphorus oxychloride. Van Arkel and De Boer (15) fractionally distilled a mixture of crude zirconium/hafnium tetra- chloride and phosphorus pentachloride repeatedly at 340 to 410°0 in a stream of carbon dioxide and obtained a fraction, b.p. 416°, to which they assigned the formula 2ZrO14.FC15 and a fraction, b.p. 363, which was 2ZTC14.P0C13. The 7

PC1 residue 2ZrOl4 4 containing some hafnium was collected in several fractions that were analysed for hafnium by X-ray spectroscopy. They found that hafnium concentrated in the lower fractions. Gruen and Kratz (16) using phosphorus oxychloride obtained compounds of the type 3MC14.2P0C13. As the hafnium compound boiled at 355°C and the zirconium compounds at 360°C it was possible by careful fractionation through an efficient column (a glass perforated-plate column with 50 plates) to separate the two compounds fairly completely in relatively few fractions. For example, in a typical experiment, a zirconium compound containing 2.5 per cent. of hafnium yielded a first fraction (5 per cent.) containing 10 per cent of hafnium; the residue after distilling 40 per cent. away contained less than 0.2 per cent. of hafnium. Fractional distillation has not found any technical applications. (c) Ion-exchange Several papers on ion-exchange separations of the two elements appeared in the late forties and early fifties but very little has appeared recently. Solvent extraction appears to be more popular on an industrial scale. Street and Seaborg (17) were among the first to show that a cation—exchange separation was possible. A mixture of oxyehlorides (mg. quantities) was absorbed on to a column of Dowex - 50 (H+ form) and eluted with 6M hydrochloric acid. Hafnium was eluted first and a 66 per cent. yield of approximately 99.5 per cent. of hafnia was obtained. Newnham (18) applied the method to gram amounts and obtained a 42 per cent, yield of 99.9 per cent. pure hafnia. Kraus and Moore (19) obtained a partial separation of tracer amounts by adsorbing the fluoride and oxalate complexes of the elements on to the anion-exchange resin Dowex - 1. A mixture of 0.5Y hydrofluoric acid and 1.0M hydrochloric acid was used as eluant. Huffman and Lilly (20) adsorbed fluoro-ions on to Amberlite 1RA - 400 resin from dilute hydrofluoric acid solution and slowly eluted the tluoro-ions with 0.2M and 0.OlM hydrochloric acid. Combined fractions of eluant from 300 ml to 650 ml contained 69 per cent. of zirconia with no hafnium detectable by emission spectrography. In both these methods hafnium is eluted first. One of the factors adversely affecting cation- exchange separations is the tendency of zirconium and hafnium salts to hydrolyse; this prevents their adsorption on to the resin. After a critical survey of the subject, Lister (21) recommended collecting the metals from a nitrate solution on a column of Zeocarb - 225 and eluting with N-sulphuric acid solution, which removes zirconium well in advance of hafnium. Benedict, Schumb and Coryell (22) collected the metals from 12M sulphuric acid solution on a column of Dowex 50 and eluted with 0.09M citric acid in 0.45M nitric acid solution. They obtained a 99 per cent. yield of zirconium, which was eluted first, containing only a negligible amount of hafnium. Ion-exchange techniques in the technical production of reactor-grade zirconium were used in pilot plants set up at Harwell (6). There are no reports of these reaching the final production stage. (d) Solvent Extraction Huffman et al. (33) studied the extraction of zirconium and hafnium from perchloric acid solution with various fluorinated ; -diketones in organic solvents. For example, with an 0.025M solution of thenoyltrifluoro- acetone in benzene, hafnium containing 0.4 per cent. of zirconium was obtained after three extractions from 2M perchloric acid solution. Today, solvent extraction is the principal method used in the West for the production of reactor-grade zirconium (6). In the U.S.A., ammonium thiocyanate in hexane solution is used. Commercial zirconium tetra- — 10 — chloride containing 2 to 3 per cent. of hafnium is dissolved in hydrochloric acid and ammonium thiocyanate is added. This solution is passed through a four-stage counter-current extraction plant in which methyl isobutyl ketone (1.L.ex'crne)- is passed in the opposite direction. Hafnium is preferentially extracted into the organic solvent and zirconium is left in the aqueous phase. By various re-cycling and washing stages, a 95 per cent. yield of the charged zirconium is obtained containing less than 0.01 per cent. of hafnium. In France, a mixture of tributyl phosphate (60 per cent.) and de-aromatized white spirit (40 per cent.) and a small amount of nitric acid as solvent and unpurified zirconium in dilute nitric acid solution containing an equi-molar amount of sodium nitrate are fed counter-current- wise to each other through a six-stage mixer-settler appa- ratus. The zirconium is extracted into the organic phase and is subsequently stripped with water. An approximately 50 per cent. recovery of zirconium containing less than 0.01 per cent. of hafnium is obtained. (e) Miscellaneous Amongst other methods that have been suggested are adsorption on activated silica gel (24) paper chromatography (25) differential reduction (26), and vapour-phase — 11 —

dichlorination (6). Hansen et al. obtained zirconium containing less than 0.1 per cent. of hafnium by passing a mixture of the chlorides in anhydrous methanol through a column of 28 to 120 mesh, activated silica gel. Hafnium was preferentially adsorbed on to the column. The hafnium can be stripped from the column with a solution of anhydrous methanol in M hydrochloric acid. After stripping, the column is ready for use again. The main drawback is the difficulty of obtaining perfectly dry methanol. A chromatograph technique for separating micro— quantities of the two elements has been described by Wells and Kember; it depends on the greater rate at which zirconium diffuses from a solution of the nitrates down a paper strip when a 30:70 mixture of nitric acid and dichloro— triethenylglycol is used as solvent. The chronatograms are developed with an alcoholic alizarin solution.

3. The Indirect Determination of Zirconium and Hafnium Zirconium and hafnium in the presence of each other cannot be determined by straightforward classical analytical techniques. However, a number of indirect methods and instrumental methods are available. The first reasonably accurate estimations of -12 - hafnium/zirconium ratios were accomplished gravimetrically (5). One of the most widely known gravimetric methods is the Selenite method, investigated in this respect by Claasen (27). In brief, the method consists of precipitating the basic selenites of zirconium and hafnium from dilute acid solution with selenious acid. The mixture is then digested for up to 20 hours to convert the flocculent, hydrated, basic selenites into the normal selenites which are dense, crystalline and non-hydrated. The precipitate is filtered, washed free from reagent, dried and weighed. Then it is ignited to constant weight which converts it into the mixed oxides. Alternatively, the normal selenites can be dissolved in sulphuric acid and the selenium content determined iodometrically. In this gravimetric method, the percentage of hafnium can be determined from the formula:

Wt. of Wt. of ) 374.86()-oxides 0.35702 (selenites Hf per cent. Wt. of oxides obtained from simple simultaneous equations. The mean error of the estimation of hafnium is claimed to be 1 per cent. Similar methods have been proposed in which secondary ammonium phosphate (28) and mandelic acid are used (29). However, it should be borne in mind that such procedures — 13 — involve the assumption that hafnium behaves in the same way as zirconium, which, although very feasible, is not proven and also that there is always inherent error, i.e. small errors in weighing may produce large errors in the final calculation at low hafnium contents. With the development of refining processes for pure zirconium metal, less involved and more rapid procedures have been required. Methods investigated include X-ray fluorescence, emission spectrography, and colorimetry. A colorimetric method based on the use of Alizarin Red S has been suggested by Romans et al. (30). Zirconium and hafnium form a red lake in either hydrochloric or perchloric acid solution and the absorbancy of the lake is measured at 530 m on a spectrophotometer in the usual manner. Mixtures of pure oxides are used to construct a calibration graph. The zirconium/hafnium ratio can be determined with an accuracy of about 2 per cent. Freund and Holbrook (30) have attempted to im- prove this method by using a differential technique. In this more concentrated solutions are used, with increased absorbancy, and to provide additional light the slit width of the spectrophotometer is increased. Instead of using a water or reagent blank for comparison, a solution of similar - 14 - absorbancy is used. Some difficulty was encountered in trying to obtain uniform colour intensities in the blank but this trouble was overcome by using strips of coloured cello- phane instead. They obtained fair results but achieved no striking improvement on the normal colorimetric method. Emission spectrography had been used for a number of years as a qualitative but not quantitative method owing to a number of difficulties affecting reproducibility such as self absorbtion, non-uniform excitation and non-uniform vapourization. Feldman (31) overcame these difficulties by using the porous-cup technique on zirconium/hafnium solutions and estimated up to 10 per cent of hafnium in zirconium and vice versa by measuring the intensity ratio of the lines Hf 2641.406 / Zr 2761.911. The ratio of zirconium to hafnium was read from a standard curve calibrated from mixtures of pure Zr02 and Hf02. Fassel and Anderson (32) devised a method using conducting pellets of zirconium and hafnium oxides sintered with powdered graphite (1:4, oxides:C ) in conjunction with a spark produced from an overdamped condenser discharge. Using a series of internal standard line-pairs, the whole range of concentrations of hafnium 0 - 100 per cent.was covered. They claimed a precision of 1.5 to 2.0 per cent. for the hafnium/ zirconium ratios. Emission spectrographic methods are - 15 - especially useful in the determination of very small amounts of hafnium, i.e. .01 to 0.5 per cent. Far larger amounts of hafnium, i.e. 0.5 to 99.5 per cent., X-ray fluorescence techniques are especially suited as hafnium and zirconium are readily differentiated. Von Hevesy used this technique in his early work on hafnium but the lack of suitable instruments prevented its development for analytical purposes until about 1950. Mortimore and Romand (33) method for zirconium and hafnium was, in fact, one of the earliest analytical applications of X-ray fluorescence. The source of X-rays, in this method is a tungsten target X-ray tube working at 40 KV potential and 40 mA current. Almost any form of material can be analized but for reproducible results identical forms must be used. In this case, lg. of mixed oxides (ground to pass a 100 mesh) are mixed with lg. of corn starch in a mortar and briquetted at 800 p.s.i. The fluorescent X-rays produced are collimated on to a rock-salt crystal cut in a special way and the diffracted rays are detected by means of a geiger tube mounted on a goniometer. The intensity of the radiation is determined with a scalar. Hafnium/zirconium ratios are determined by measuring the intensities of the line pair 1st order Hf and 2nd order Zr Kfe1 after correction for background. The concentrations are then - 16 -

determined from standard curves calibrated from mixtures of pure oxides. Provided that the goniometer is aligned accurately and sufficient counts are taken to minimise the statistical errors of counting, precisions of 3 per cent. for 1 per cent, hafnium and 0.5 per cent, for 40 per cent hafnium can be obtained. Small amounts of hafnium cannot be determined as the background correction becomes too large. A number of other methods have been devised for determining zirconium/hafnium ratios. These include measurement of density (5), the effect on the optical activity of tartaric acid (34), and neutron activation (35).

4. Separation of Zirconium from most other elements. With large amounts of zirconium, separation from other elements is usually achieved by precipitating with a "selective" reagent. Many have been recommended and their merits and uses will be discussed in the following section. Ion-exchange, solvent extraction, chromatography, etc., have been used for separation but are in the main more useful when dealing with small amounts of material. First to be discussed will be the reagents and techniques which are common to many analytical investigations. (a) General Methods Zirconium is quantitatively precipitated from — 17 —

aqueous solution by ammonium hydroxide, The precipitate can be filtered and ignited directly to zirconium dioxide,

Zr02. Such a method is, naturally, suitable only for pure solutions on account of the many ions which are also preci— pitated by aqueous ammonia. It is, however, an excellent method for standardizing pure solutions. Hafnium inter— feres in all proportions. To a lesser extent, the same can be said of precipitation of hydrated zirconium oxide from aqueous solutions by phenylhydrazine (4). Zirconium is not precipitated as sulphide from acid solution. although the acid concentration should be fairly high to prevent hydrolysis. In this respect, zirconium is less prone to hydrolyse than titanium (5). Separation from the rare—earths can be achieved by dissolving the precipitate from ammonium hydroxide treat— ment in concentrated hydrochloric acid and evaporating to dryness (5). The residue is dissolved in water with just enough hydrochloric acid added to obtain a clear solution. An excess of saturated oxalic acid solution is then added to precipitate the rare—earths, leaving zirconium in solu— tion. Alternatively, the ammonium hydroxide precipitate can be treated with hydrofluoric acid, the residue filtered and zirconium washed free from the rare—earths with a weak solution of hydrofluoric acid. Niobium, tantalum, -18-

titanium and uranium would also be substantially removed. Large amounts of iron can be removed by the normal methods, i.e. the ammonium sulphide—tartrate method, thio— sulphate method, or by extraction with ether. Mercury cathode electrolysis can be applied (4). On electrolysing a solution containing zirconium, elements such as chromium, iron, cobalt, nickel, copper, zinc, gallium, germanium, molybdenum, palladium, arsenic, cadmium, indium, strontium, rhenium iridium, platinum, gold, magnesium, thallium and bismuth would be deposited on the mercury cathode and zirconium aluminium, titanium, vanadium, and uranium would be left in solution. (b) "Selective" Reagents (1) Precipitation as Secondary Phosphate One of the most effective methods for the separa— tion of zirconium from most other elements lies in its precipitation as the secondary phosphate from a solution containing about 10 per cent. by volume of sulphuric or hydrochloric acid and also hydrogen peroxide if titanium, niobium or tantalum is present. It is probable that very few elements save hafnium, protoactinium, niobium and tan— talum interfere if they can be obtained in sulphuric acid solution. Niobium and tantalum do not precipitate except on long standing when alone in sulphuric-hydrogen peroxide- — 19 — orthophosphoric acid solution at room temperatures, but are partially precipitated in the presence of zirconium. The method was first used by Hillebrand (36) who recommended it for the determination of the small amounts of zirconium usually found in rocks (0.02 - 0.2 per cent.). It affords a nearly ideal method of analysis, but unfort- unately, the precipitate tends to hydrolyse and lose P205 during washing. With small amounts such as 2 to 3 mg. the error is of no consequence but with larger amounts it may reach 1 to 2 per cent. of the total Zr02 content. Many of the earlier workers failed to realize this; they found the precipitate to be of variable composition and conversion factors ranging from 36 to over 50 per cent. were reported. A thorough investigation of the method by Lundell and Knowles (37) cleared up these misapprehensions. In their method, zirconium is precipitated from diluted (1 + 10) sulphuric acid solution, containing sufficient hydrogen peroxide to keep titanium in solution, by means of di- ammonium hydrogen phosphate ((NH ) HP0 ) added in up to a 4 2 4 ' 100-fold excess. Precipitation is completed by digesting the solutions at 4o to 5000 for two hours. If little zirconium is presents standing overnight or for minute amounts standing for 2 to 3 days may be necessary. The precipitate is filtered and washed with a 5 per cent. — 20 — solution of ammonium nitrate. This reduces hydrolysis. Very careful ignition is required owing to the tendency for the precipitate to decrepitate. The precipitate is weighed as ZrP207 after final ignition at a temperature of 1150°C. When much zirconium is present, or when accuracy of the highest order is required, the ignited residue has to be decomposed by fusion with sodium carbonate. The phosphate present is leached with water, the residue (con— taining all the zirconium) is fused with potassium pyro— sulphate the fused melt is dissolved in diluted (5 95) sulphuric acid and the zirconium precipitated by ammonium hydroxide. The precipitate is filtered, ignited to Zr02 and weighed. As it is difficult to remove all phosphorus in one carbonate fusion and to eliminate all alkali salt by one precipitation with ammonium hydroxide, it is better to repeat these operations twice. In tests of the method, Claasen and Visser (38) found that (1) tartaric acid is without effect; (2) that perfect separations are obtained from Al, Cu, Cd, Bi, Ni, Co, Mn, Mg, alkalis, alkaline earths, W, V, Mo and U; (3) that large amounts of fluoride are a disturbing factor and that tin always causes trouble since it tends to co- precipitate and (4) that when titanium is present in — 21 — solution containing hydrogen peroxide satisfactory values are obtained only when the amount of zirconium is small. Complete separation from thorium requires two precipitations and niobium and tantalum which are always partially precipitated are best removed beforehand. A better separation of zirconium from cerium results if the latter is kept in the tervalent state (4). The secondary phosphate precipitate has three undesirable features: (1) the tendency to hydrolyse on washing; (2) it is gelatinous, tends to carry down other ions and is difficult to wash; (3) it decrepitates during ignition. Ways have been sought for forming more granular precipitates to overcome these objections. Willard and Freund (14) in their investigation of the separation of zirconium and hafnium by fractional crystallization obtained a dense crystalline precipitate of zirconyl ethyl phosphate by the slow hydrolysis of tri- ethyl phosphate from homogenious solution. As complete precipitation takes at least 20 hours, Willard and Hahn (39) suggested the use of trimethyl phosphate which gave as dense a precipitate in 12 hrs. Although the precipitate obtained was more readily filterable there was serious interference from antimony, bismuth, cerous, stannic, uranyl and large amounts of ferric, thorium and titanyl -22— ions. Clean separations were reported from aluminium, arsenate, borate, cadmium, calcium, chromic, cobalt, cupric magnesium, manganese, mercuric, nickel, potassium, sodium, tartrate, vanadyl and zinc ions. Amounts greater than 60 mg. of zirconium were not recommended owing to the hydrolysis of the precipitate during washing. Metaphos— phonic acid was also suggested, although the precipitate was not as crystalline as that from trimethyl phosphate. The interferences are as for trimethyl phosphate except that determinations can be made in the presence of antimony if tartaric acid is added to prevent hydrolysis. It was claimed that up to 200 mg. of zirconium dioxide could be determined by igniting the precipitate and weighing as ZrP207. However, close scrutiny of their results show that analyses for amounts of about 100 mg. are up to 1 part in 500 low. Thus, for highly accurate work double fusion and precipitation with ammonium hydroxide would be required. In this respect, therefore, these methods show no advance on the original; the long digestion and slight loss of selectivity in fact make these methods less attractive. More recently Russian workers (40) have claimed that precipitation of the secondary phosphate by di—sodium hydrogen phosphate in the presence of E.D.T.A. from solu— tions three to four normal in acid produces a slow—forming, - 23 -

crystalline precipitate which does not carry down other ions. Just sufficient E.D.T.A. is added to form the Zr-E.D.T.A. complex (if larger amounts of E.D.T.A, were added difficulty could occur from the precipitation of E.D.T.A.). If precipitation is carried out in the presence of hydrogen peroxide, interference from titanium is greatly reduced, but not entirely eliminated. As a consequence of the difficulties encountered in the phosphate method, numerous alternatives have been suggested, the most important being precipitation by mandelic acid or its derivatives. These will be dealt with next. Separations based on the precipitation of zirconium as secondary phosphate have been used prior to the final determination of zirconium by some other method. Vinogradov and Shpinel (41) used it before precipitating with 8-hydroxyquinoline and determining zirconium by v-au- metric titration of the 8-hydroxyquinoline. Olsen and Elving (42) used it before titrating zirconium with cupferron to a superometric end-point. (2) Mandelic Acid Of the enormous number of organic reagents suggested for the separation of zirconium, precipitation by mandelic acid or its derivatives is the most useful. They -24- can be used in solutions at least 5N in hydrochloric or perchloric acid and effect the separation of zirconium from most other elements. They are less useful for sulphuric or hydrofluoric acid solutions owing to the formation of soluble zirconium-mandelate-sulphate or fluoride complexes. Mandelic acid was proposed as a selective reagent for zirconium by Kumins (43) who reported no interference from Ti, Fe. V, Al, Cr, Th, Ce, Sn, Ba, Ca, Cu, Bi, Sb, or Cd, although if large amounts of thorium, antimony, tin or bismuth were present there was some contamination of the zirconium precipitate. In Kumins' method 50 ml. of a 16 per cent, solution of mandelic acid are added to 20 ml. of the zirconium solution containing an unspecified amount of hydrochloric acid. The solution is diluted to 100 ml,, the temperature raised slowly to 85°C and held there for 20 mins. The resulting precipitate is washed with a hot solution containing 2 per cent, of hydrochloric acid and 5 per cent. of mandelic acid. This wash-solution is necessary as the precipitate is appreciably soluble in water. On ignition, the residue is weighed as Zr02. Kumins reported excellent results for amounts of zirconium dioxide (and Hf02) from 50 mg. to 300 mg. He indicated that amounts up to 5 per cent, of free sulphuric acid could be tolerated and that amounts in excess of this could be neutralized with sodium -25- hydroxide. However, results from such determinations were low by about 1 per cent. Neutralization by ammonium hydroxide produced even lower results probably because of the formation of a soluble ammonium-zirconium-mandelate complex. Hahn (44) investigated Kumins method and found that amounts much smaller than those originally claimed could be determined accurately. The lowest quantity he determined was 0.1 mg. He also showed that the concentration of hydrochloric acid was not critical and could range from 0.1 to 8M. The effect on the determination of the presence of Co, Mg Mn, Hg, Ni, U and Zn which had not been investigated by Kumins, was determined, and it was found that none of these interfered. The effect of sulphuric acid was further investigated by Mills and Hermon (45) on solutions containing 1 to 30 mg. of zirconium. From 65 ml. of solution containing 3 ml. of concentrated sulphuric acid, they found that recovery was incomplete for amounts less than 15 mg. of zirconium unless a prolonged period of digestion was use: (at least 18 hours). With prolonged digestion recovery at this concentration of acid was complete down to 3 mg. of zirconium. The precipitate corresponds to the formula (C H CHOHC00) Zr and Kumins suggested a simple salt-type 6 5 4 - 26 - of structure. Feigl (46) considered that the structure was of the chelate type:

H 0 - C = 0

H 0 \

4 and this was born out by experiments by Hahn and Weber (47). Many workers have attempted to devise means of weighing the tetramandelate precipitate directly. Astamina and Ostroumov (48) using 25 mg. concentrations, reported good results by washing out the excess of mandelic acid reagent with ethanol and drying the precipitate at 11000. Belcher, Sykes and Tatlow (49), using much larger quantities, obtained consistent but low results upon washing the precipitates with a saturated solution of zirconium mandelate and on drying at 110°. Their attempt to apply a correction factor was unsuccessful except over extremely narrow ranges of concentration. Hahn and Baginski (50) subsequently explained this lack of stoichiometry on the basis of contamination of the zirconium tetramandelate precipitate with basic salts such as ZrO(C0705)7 in dilute acid and -27-

ZrOH(C8H705)3 in stronger acid. They concluded that better results would be obtained by precipitating from strongly acid solutions in which hydrolysis is at a minimum. They recommended that precipitation should be effected by adding mandelic acid solution dropwise to a hot solution of zirconium at least 5M in hydrochloric acid. After digestion at 85 to 90°C, the cooled solution is filtered through a sintered glass crucible and washed successively with a saturated solution of zirconium-tetramandelate, 95 per cent. of ethanol and anhydrous di-ethyl ether. The precipitate is then dried to constant weight at 110°C. Oespar and Klingenburg (51 studied the behaviour of the following compounds related to mandelic acid to determine whether the glycolic acid group (-CHOH-COOH) was specific for zirconium: (1) p-methyl mandelic acid; (2) p-nitromandelic acid; (.3) p-bromomandelic acid; (4) p-ehloromandelic acid; (5) p-iodomandelic acid; (6) P( hydroxy decanoic acid; (7) benzilic acid (8) 2-naphthyl- glycolic acid; (9) 0-nitromandelic acid; (10) m-nitromandelic acid; (11) lactic acid; (12) glycolic acid. All these compounds contain the group: -CHOH-COOH. They all give precipitates with zirconium under suitable conditions but (9) to (12) are much less selective than mandelic acid. Large weighing effects are exhibited by -28-

(5), (6), (7) and (8) but these compounds are water insoluble. This somewhat complicates the analytical procedure and is undesirable. No particular advantage over mandelic acid was shown by (1) and (2). Only (3) and (4) which show greater sensitivity than mandelic acid were subjected to further investigation. The zirconium precipitates obtained with these compounds were found to be completely insoluble in water, as distinct from mandelic acid. This meant that precipitates could be washed with water and special wash solutions were unnecessary. Considerably less reagent was required to effect complete precipitation though this advantage is largely compensated by the higher cost of these reagents. Oespar and Klingenburg suggested the addition of 50 ml. of an 0.1M solution of either reagent as against 50 ml. of an 1M solution which is required when mandelic acid is used. In all other respects the procedures suggested are the same as for mandelic acid. Similarly, free sulphuric acid should not be present in amounts greater than 5 per cent.V/V. Oespar and Klingenburg claimed that direct weighing of the p-bromomandelate precipitate was possible, but this claim was not supported by Belcher, Sykes and Tatlow. However, thermogravimetric studies by Wendlandt (52) have indicated that a re-examination of the — 29 — precipitating conditions might be justified as the precipitate approximates to the formula Zr(C8H503Br)4 and has greater stability than any other organic precipitate of zirconium so far examined. So far no particular preference for either mandelic acid or p-bromo/chloromandelic acid has emerged. All three reagents have been used extensively in recent years for the separation and determination of zirconium in materials such as steels, alloys, ores, concentrates and fusion products. The range of determinations is from the parts per million level to high percentages. In the determination of zirconium as a minor constituent in certain alloys, special measures often have to be taken to prevent interference from the large quantities of the major constituent. For example, in the determination of zirconium in titanium-based alloys (53), titanium is co-precipitated with zirconium tetramandelate. It is removed by dissolving the precipitate in ammonium hydroxide (1 4), filtering off the Ti(OH)4 precipitate, acidifying the filtrate with hydrochloric acid and re-precipitating with mandelic acid. In such cases much less interference is encountered when p-bromomandelic acid is used. Other derivatives of mandelic acid which have been studied include, fluoro, trifluoromethylmandelic acid -30— and naphthylglycollic acid. None of these reagents shows any special advantages. (3) Other Reagents Numerous other methods have been proposed for the determination of zirconium gravimetrically. None of them is as selective as the phosphate or mandelie acid method. The selectivity of the latter is due in part to the high acid concentration used. At such concentrations ions one would normally expect to interfere remain in solution. With the majority of the other methods suggested a lower acid concentration has to be used and this, to some degree, accounts for the loss of selectivity. The methods most thoroughly investigated are: precipitation as basic selenite, cupferron methodl arenate method and arsenic acids methods. Each of them was proposed originally as an alternative to the phosphate method. They have all been largely replaced by the mandelic acid method, but still have their used where interferences are not likely to be serious. Within the last decade numerous precipitants have been proposed, mainly by Indian workers; they will be tabulated at the end of this section. (a) Cupferron (4) Precipitation by cupferron (ammonium salt of — 31 —

nitrosophenyl hydroxylamine; C6H5N.N0.ONH4 ,. followed by ignition to the oxide is an accurate process giving complete separation of zirconium from aluminium, chromium, sexi— valent uranium, boric acid and small amounts of phosphorus. However, many elements interfere, for example, titanium, thorium, cerium (and probably other rare earths), most of the hydrogen sulphide group, iron, vanadium, niobium, tantalum, silica tungsten and quadrivalent uranium. Preci— pitation by cupferron is usually carried out, after separa— tion of silica, tungsten, and the hydrogen sulphide group, in cooled (5 to 10°C) 10 per cent sulphuric acid solution. The precipitate cannot be dried and weighed as such but must be ignited to the oxide. The ignition must be done with very great care in the early stages owing to excessive liquefaction when the wet precipitates are heated and to the copious liberation of gaseous products when the dried precipitates are ignited. Final ignition is to 1200°C giving a precipitate which is not hygroscopic. (b) Precipitation as Basic Selenite A variant of this method has been described in the section on the indirect determination of hafnium and zirconium (p.11). Claasen1 s method for weighing directly as the normal selenite is time—consuming as a 12 to 15 hour digestion of the precipitate is required. Simpson and -32—

Schumbes method is used more often and briefly stated the method consists in precipitation of the basic selenite with an excess of selenious acid in hot dilute hydrochloric acid, preferably 5 per cent. and not greater than 7 per cent, by volume, filtering, washing with hot dilute hydrochloric acid and igniting to the oxide at 100000. Iron, aluminium and certain rare-earths do not interfere. Moderate amounts of titanium can be tolerated if hydrogen peroxide is used. However, in the presence of vanadium, uranium and large amounts of titanium, there is some coprecipitation. The coprecipitated ions can be removed by dissolving the precipitate in hot 6N hydrochloric acid, provided the precipitate is not allowed to stand long enough to form the crystalline normal selenite which is difficult to dissolve, and then re-precipitating the zirconium with selenious acid on dilution of the hydrochloric acid solution. Thorium, phosphorus, niobium and tantalum interfere and must be absent. Fluoride and sulphate ions form soluble anion complexes with zirconium and must be absent. For the determination of zirconium in steel Simpson and Schumb (55) suggested a method consisting of a preliminary precipitation with selenious acid followed by solution of the precipitate in hydrochloric acid and subsequent reprecipitation as secondary phosphate. This is -33- claimed to give a purer final precipitate than is obtainable by either method alone. (c) Arsenate Method Zirconium is precipitated from a boiling solution of 2.5N hydrochloric or 3.7N nitric acid, but not in the presence of sulphuric acid, by the addition of arsenic acid or the secondary arsenate of ammonium or sodium. The (As0 ) is bulky precipitate, normal zirconium arsenate Zr3 4 4 and highly hydrated and usually contains some adsorbed reagent. Zirconium is weighed as oxide after the removal of arsenic and ignition. Moser and Lessing (56) removed arsenic by distilling off arsenic trichloride from a solution of the precipitate in concentrated sulphuric and nitric acids and hydrazine. Schumb and Nolan (57) and later, Claasen and Visser (58), obtained fair results by igniting the precipitate in the presence of a large amount of filter-paper pulp in a porcelain crucible. With a single precipitation zirconium is quantitatively separated from large amounts of bivalent metals, aluminium, chromium, ferric iron, uranium, vanadium and molybdenum; tungsten is partly precipitated, Double precipitation is required for a complete separation from cerium, beryllium, thorium, tin and titanium. The separation from titanium requires the addition of hydrogen peroxide and precipitation must be -34- carried out in hydrochloric acid solution. In nitric acid solution, there is probably perzirconate formation, which results in incomplete precipitation. Alternatively, the presence of titanium in the ignited precipitate can be determined colorimetrically. Niobium and tantalum interfere. (d) Arsonic Acids Work on the inorganic arsenates led to work being done on organic arsenic compounds. Those chosen were the arsonic acids which have the general formula: 7 OH R - As, OH Of the many investigated (59) only (1) phenyl, (2) n-propyl, and (3) p-hydroxyphenyl arsonic acid, (4) disodium methylarsonate (arrhenal) and (5) p-dimethyl amino- azophenyl arsonic acid have been proposed as reagents for zirconium and of these only the last one is mentioned in . recent literature (66). With the exception of p-dimethylaminoazophenyl- arsonic acid they are all used under the same conditions. Briefly, the general method consists of precipitating zirconium from up to 10 per cent. mineral acid by the addition of a 10 to 20-fold excess of reagent, filtering and -35- washing with very dilute acid. With (1), (3) and (4), the precipitate is firstly ignited in a stream of hydrogen to remove arsenic and then ignited to zirconium dioxide and weighed. With (2), n-propyl arsonic acid direct ignition in a porcelain crucible yields zirconium dioxide. Apparently arsenic volatilizes more readily than in the other cases. Quantitative separation is obtained from such elements as Al, Bi, Cd, Cr, Cu, FeII1 Ni, Co, Zn, MnII1 alkaline earths, etc. Double precipitation is usually required for complete separation from FeIII, Ti, Th, U, V. Antimony, tin, and sometimes bismuth, interfere. p-dimethyl aminoazophenylarsonic acid is a good reagent for separating microgram quantities of zirconium prior to a colorimetric determination. The zirconium is precipitated from hot (15 + 85) hydrochloric acid. Filtra- tion must also be carried out at about 70°C to reduce interference from titanium. Quantitative separation is obtained from Mo, Be, Ge, CrIII, UIV, and Pt. Under these conditions, V. Ta Nb and Th interfere. Vanadium, hydrogen peroxide and other strong oxidizing agents destroy the reagent. Small amounts of sulphate and phosphate can be present, but even microgram quantities of fluoride interfere. Under no circumstances must fluoride be used in the preliminary attack of the sample to be analysed as it is impossible -36-

to remove it completely.

The organic reagents that have been proposed in recent years are tabulated in the following pages.

Tartrazine has recently been proposed by Baiulescu and Turcu as a new reagent for zirconium (99). It is especially promising for the determination of zirconium as the precipitate can be dried at 110oC and weighed directly. Precipitation is from hot acid solution (pH 1 at 80°C), and the precipitate is washed with water. It appears to be reasonably selective, separating zirconium from Al, Ti, UIV, La, Ce, K, Cd, Zr, Co, Ni, Cu„ Mg Na, Li, Fe and Ca. Barium, thorium and mercury interfere. -37-

Ref. Reagent Separations Interferences Conditions Wash solution Weighed as: IV 61 Ammonium Al, Ce U Fe Ti, SnIV, Th, Boiling HC1 or HNO3 0.1% reagent Zr02 benzoate in presence of Bil Hf pH 1.0-1.5 H2SO4 Thioglycollic acid not recommended

62 Benzoic Bi, Mn, Zn, Ni, Co, Cu Ti, Hf Boiling 0.15N HC1 Zr02 acid UO22 , Al, rare-earths Double pptn: Th, V, Cr

63 m-nitro- Rare-earths, Al, Sb, Ti, Hf, Sn Boiling 0.2N HNO3 0.1% reagent Zr02 benzoic Bi, Cd, Pt, Li, Mg, Mn, acid conc. v. c acid Pd, K, Ti, Y critical

64 Ditto Double pptn. Fe, Cr Tn, Ti, Hf, Zr02 rare-earths and Th

65 Fumaric Al, Bi, U, Ni, Ba, Ca, Not given Boiling 0.35N HC1 0,2% reagent Zr02 acid Mn, cerite earths, Th and water and Fe. Double pptn. V, Cr, Ti, Sn

66 Cinnamic Th, Mn, U, Cerite Boiling 0.1N HC1 Zr02 acid earths, Re, Ni, Fe + 15% NH4C1 III 67 Diphenic Th, Zn, Fe Ti pH 2.0 HNO3 or HAl 0.02141 reagent Zr02 acid 0,1g. and hot water NOT HC1 or H2SO4

68 Benzilic Bivalent ions, cerite Sn, Ti 0.2N HC1 Hot water Zr0 2+ 2 acid earths, Al, UO2 or less Double pptn: Cr, V 9500 FeIII in presence of CN5 69 Ditto Higher conc. of acid with more reagent increases selectively -38-

Ref. Reagent Separations Interferences Conditions Wash solution Weighed as: 70 Phthalic Th, Fe, Al, Bi, U, Mn, Ti 0.3N HC1 0,1% reagent ZrO2 + NH acid Ni. Ce-earths 401 Double pptn: Ti, Vi, Boiling Cr, Sn 71 Tetrachloro- Double pptn: V only Ti 0.6N HC1 0.1% reagent + 10% NH C1 + 2% NH4C1 phthalic Th, Sn, Cr, Fe, U022+ 4 acid Al2 Ce-earths 72 Hydrazine Th, Be, Ni and pH 2.8 water ZrO2 sulphate rare earths in HC1/NH Cl soln. 73 Tannin Fe, Al, Cr, rare Ti, W, Sn 0.1N HC1 2% NH Cl ZrO2 earths. Be, Mn, Ni, Ta and Nb Cr, Zn, V 1N HNO Boiling water dry to 74 8-hydroxy- 3 quinoline and 2N Na0A1 Zr(C6H6ON)4 at 130° or Zr02 75 5:6 Benzo- Rare. earths, Cu, Cr, Zn, pH 1,8 1% NH4 ZrO2 NO soln. quinaldinic alkaline earths Ni, Mn, Cd, 3 acid Mg Hg 76 Quinaldinic As, Hg, Mg, Mn Th pH 2.8-3.0 Zr02 acid alkaline earths Double pptn: rare- earths 77 Phenolic Th Double pptn. p11 2 to 4 Zr00 acids

- 39 - a' (7,4

Interferences Ref. Reagent Separations Conditions Wash solution Weighed as 78 Anilic Th also Cu, Cd, Al, treat ppt. with cold Zr0 Cr, Be, Ca, Zn, Mg & 2 acids amyl acetate to Ti & cerite-earths (all remove Th. in small amounts). Some pH 2.7 4.2 double pptn. may be required. 79 Azo dyes of Cu, Cd, Co, Ni, Mg, Ca Th, Hg, Pb, Neutral to hot water Zr02 chromotropic Sr, Bu, Au, U, rare- Al, Cr, III congo red acid earths Fe 80 Maleanilic Ti, U, cerite-earths Pb, Hg, Fe, Hot soln. 80 - 90°C hot water Zr02 Double pptn: Ni, Co, Ce, Th, Au, acids pH 0.5 - 2 Cr, U Pt

81 2 hydroxy-3- Th Neutral congo red hot water Zr02 naphthoic 80°C acid Th, Ag, Hg, 82 Phenylglycine- acetic acid hot water Zr Sn, Fe, Ce, 2 0 & P carb- just below boiling and ethanol oxylic acids Ti, Pb 83 Aryloxy Hg Ce, Fe pH Zr2 Fatty acids 84 a) Salicylic Bivalent ions, Ti a)0.18N HC1 Zr02 acid and cerite-earths b) Phenoxy-- Double pptn: Th b) 0.24N HC1 acetic acid 85 2:4 Dichloro- Th, Fe, Ti Neutral to congo red 0.2% reagent Zr02 phenoxy- add 3.5 N HNO and 5g. 3 acetic acid ammonium acetate

- 40 -

Ref. Reagent Separations Interferences Conditions Wash solution Weighed as:

86 3-ethylphen- Double pptn: Cr & V 0.15N HCl 0.1% reagent Zr02 oxyacetic Zr, Bi, Ca, Ba, Hg, hot soln. acid UIV, Th, Mn, Cr, Ni, Al, Ce, Ti, Fe, Cr, V and Sn 87 m-acroxy-- Bi, Al, Ca Ci, Ba, SO " 0.2N HC1 0.1% reagent Zr0 4 2 acetic acid UO2 Ce-earths. Double Boiling pptn: Ti, Sn, Cr, V

88 o-phenylene Ca, Ba, Zn, Hg, Al, Ce 0.3N HC1 0.1% reagent Zr02 diacetic Ti, Th, UO22+ , Mn, Fe, Boiling acid Co & Ni Double pptn: Vi, Cr 0.08N HNO Boiling 0.1 % reagent Zr0 89 m-phenylene Cu, Be, Hg, Ca, Zn, Sn, V, Th 3 2 IV, Mn", SO di-oxy- Cd, Pb, U HC1 and H2 4 diacetic Double pptn: Al, Ce, not suitable acid Cr, Fe 90 Thiodi- Co, Ni, Zn, Mg, Cd, Al Zr0 Not H2SO4 2 glycollic Mn, Pb, Ti, Be 0.4N HCl acid alkaline earths

91 Thio- Alkaline earths, rare pH 4.5 5% NH4N05 Zr02 glycollic earths, Mn, Mg, Be, Al, 85 - 90° acid Ce, Cr, Fe, V. Copptd: Sn, Hg, Zn, Cd, Ti, Th Cu Pb, Ni, Co, Bi 92 Thiomalic Hg on standing 0.2N HNO 0.2N HNO Zr0 3 3 2 acid Bi 0.1N HC1 and hot water Not H 50 2 4 -41- 4ir

Ref. Reagent Separations Interferences Conditions Wash solution Weighed as: 93 o-cresotic Be, Ca, 3a, Mg, Zn, Cd pH 3.7 Zr0 acid Hg, Pb, Co, Cu, Mn, Bi, 2 Ni, Li, Co, Al, Double pptn: UO22 -1-, 3+ Cr , Fe3+ and Ti (with H202)

94 p-cresotic Double pptn: Fe, Cu, pH 4.0 Zr0 acid UIV 2

95 Ethanol- 25% HNO3 85 - 90°C I% reagent Zr0 amini 2 96 Benzene- Rare-earths, Be, Ni, Cr 1.5N HC1 Zr02 sulphinic Nb, Ra and Fe (small or HNO3 at 20°0 acid amounts). Double pptn. sometimes desirable for rare-earths 97 Ammonium Alkali, alkali earths in HNO3 scan. water Zr02 mercapto- pptn. at pH 6 - 7.6 benzothiazole 98 N-benzoyl-N- Fe, Al, Ti, Cr, Nb and 2,4N HC1 or 1% HC1 Zr02 phenyl- Ta do not interfere 3.6N HS 04 hydroxylamine -42-

(c) Separation of Zirconium from Niobium and Tantalum Fortunately, biobium and tantalum rarely occur with zirconium but when they do they always interfere in the analysis of zirconium. The reagents already mentioned do not completely remove the interference due to them and so a preliminary separation must be conducted. (1) Schoeller and Powells Tannin method Schoeller and his co-workers (5) have made a thorough investigation of the behaviour of tannin in various media, particularly with reference to the determination of niobium and tantalum. The principal applications proposed by them may be grouped under four headings according to the medium in which the reaction takes place: (1) In oxalate solution, tantalum, titanium, niobium (Group A) are quantitatively separable from group B - zirconium. thorium, aluminium, uranium, etc. (2) From neutralized tartrate solution, tannin precipitates quantitatively all the above elements; a number of other elements, notably beryllium, rare-earths and manganese (Group C) are precipitated only from ammoniacal tartrate solution. (3) A solution of tannin in dilute sulphuric acid in contact with a bisulphate melt dissolves titanium, zirconium and metals with soluble sulphates, leaving an insoluble residue containing niobium and tantalum. — 43 —

(4) Hydrochloric acid added to an alkaline tungstate solu— tion containing tannin precipitates the tungsten complex. Schoeller and Powell recommended that zirconium be separated from niobium, tantalum and titanium by adding a 5 per cent. solution of tannin (in 15—fold excess) to an oxalate solution (5 g. ammonium oxalate per 600 ml. of water) of these elements. Niobium, tantalum, and titanium (Group A) are precipitated while zirconium (Group B) remains in solution. If the amount of zirconium is less than 0.1 g., the filtrates are evaporated to 50 ml. and treated whilst boiling with 1 g. tannin and a slight excess of ammonia. After digestion at 60°C, the precipitate is filtered and washed with a dilute ammonium nitrate solution. Schoeller claims that the precipitate flocculates readily, is easy to handle and wash, and is easily ignited to Zr02. If the quantity of zirconium is substantial, or large amounts of Th, Al, U, etc., are present, the ammonium salts and tannin in the solution are destroyed by evapora— tion with 50 ml. of concentrated nitric acid and 10 ml. of concentrated sulphuric acid until white fumes are evolved. After cooling the acid is diluted with 100 mls. of water, the solution filtered and the zirconium precipitated either with ammonia if no interfering elements are present or with a reagent such as di—ammonium hydrogen phosphate - 44 - or mantelic acid if the presence of interfering elements is suspected. If a small quantity of zirconia is to be separated from large quantities of earth acid, the tannin precipitate becomes inconveniently large and a combination process involving fusion with potassium carbonate as the first step is then desirable. (2) Barium Fluorozirconate Method Milner and Barnett (100) have reported that excellent separations of zirconium from niobium and tantalum in uranium-base tertiary alloys ( U - Zr - Ta and U - Zr Nb) can be obtained by the precipitation of barium fluorozirconate, Ba Zr F6. The method is applicable to fluoride solutions and is important because of the need to use hydrofluoric acid to produce complete solutions of zirconium metal and zirconium-containing alloys. The separation is effected in solutions 2M in HF and less than

3M in HNO3 (or HC1) by the addition of a 5 per cent. solution of barium nitrate (or chloride). Zirconium is precipitated as Ba Zr F6. The precipitate is washed with an aqueous solution; 1 per cent. in barium nitrate, 2 per cent. in hydrofluoric acid and 10 per cent. by volume in nitric acid (or H01). Normally nitrate solutions are used except in the presence of tin when chloride solutions are - 45 -

necessary to avoid the formation of metastannic acid. The barium fluorozirconate precipitate can then be dis— solved in a mixture of boric and hydrochloric acids and the zirconium determined by precipitation with cupperron. Alternatively, the barium fluorozirconate precipitate can be dissolved in a solution of saturated boric acid contain— ing 20 per cent. by volume of nitric acid and a second precipitation of barium fluozirconate effected by the addition of 5 per cent. barium nitrate followed by 40 per cent. hydrofluoric acid solution. This precipitate is then dissolved in (1 + 1) hydrochloric acid and evaporated with 60 per cent. perchloric acid to fumes of perchloric acid, diluted with water and evaporated once more to fumes of perchloric acid to ensure complete removal of fluoride ions. Zirconium present can then be detemined by an E.D.T.A. titration method. Good separations for 10 to 150 mg, of zirconium from large amounts (up to lg.) of U, Ti, Mo, Pb. Fe Cu, Nb, Ta W and Sn are claimed. Alumi— nium forms an insoluble precipitate in fluoride solution in the presence of barium ions and bismuth is also preci— pitated from fluoride solution. Since both these elements interfere with the E.D.T.A. titration, they must be absent. Sulphuric acid cannot be used on account of the insolu— bility of barium sulphate. - 46 -

(3) Other methods Vinogradov and Shpinel's (41) procedure for the determination of zirconium by precipitation with phosphate and then with 8-hydroquinoline is claimed to produce nearly complete separation of small amounts of zirconium (less than 3 mg.) from niobium and tantalum. Russian workers have also reported that benzene- sulphuric acid (96) and N-benzoyl-N-phenylhydroxylamine (98) precipitate zirconium free from niobium and tantalum.

5. Volumetric Determination of Zirconium Titration methods for the determination of zirconium have been revolutionized since the introduction of ethylene-diaminic-tetra-acetic acid in analysis. Both direct and indirect procedures are available, and reason- able selectivity is obtained in these methods because they are carried out in solutions having low pH values. The stable zirconium - E.D.T.A. complex persists in acid solutions conditions under which bivalent and many tervalent ions are not complexed. Solutions of the reagent are usually prepared from the di-sodium salt and standardized in the normal manner. (a) Indirect methods involving E.D.T.A. Methods; in which the excess of E.D.T.A. over the -47-

amount required to form the complex with zirconium is titrated, are preferred as the risk of low results caused by the hydrolysis of zirconium is avoided. The elements, used for the back-titration include Bi, FeIII Th, Co, Cu which form stable complexes with E.D.T.A. in solutions having pH values from 2 to 3. For visual end-point detection the use of a bismuth solution as back-titrant and xylanol orange as indicator is probably the best, a sharp change from yellow to red being obtained at the end-point. Titrations are usually carried out at 0.02M to 0.05M concentrations. Kinnumen and Wennerstrand (101) recom- mend the addition of the excess of E.D.T.A. to a solution of zirconium (as nitrate or perchlorate) adjusted to a pH between 1 and 2 and titrated the excess with bismuth nitrate solution. Milner and Edwards (2) report that for all except the smallest amounts of zirconium it is preferable to add the excess of E.D.T.A. to an acidic solution (approx 1N in HNO3) of zirconium before adjusting the pH to between 1 and 3 with dilute aqueous ammonia and carrying out the back-titration. This procedure should not be applied to chloride solutions because of the risk of precipitating bismuth as oxychloride. However, at pH values between 2 and 3 chloride solutions can be back-titrated with a thorium solution. Both fluoride and sulphate ions -48-

interfere. Milner and Edwards (102) applied the method to the determination of zirconium in zirconium-thorium alloys and Lassner and Scharf (103) applied it to zirconium- tungsten alloys. In the procedures using a bismuth solution as back-titrant, thiourea and potassium iodide have been used as indicators (2). With thiourea as indicator, the back-titration is usually carried out at pH2. How- ever, the optimum pH and the thiourea concentration are closely related. In Fritz and Johnson's method (104) an aliquot containing 13 to 36 mg. of zirconium per 100 ml. is complexed with 10 ml. of an 0.05M solution of E.D.T.A., the pH adjusted to 2 and the excess titrated with bismuth

, nitrate solution (0.05M) with 1.3 g. of thiourea as indicator. Most complexing anions, including fluoride, sulphate, phosphate, oxalate, thiocyanate and tartrate inteffere. Fluoride can be removed by fuming the solution with perchloric acid. Some slight modifications of the procedure permit the detection of zirconium in the presence of moderate amounts of Th, Ti, NB and Ta without preliminary separation. Some operators find difficulty in detecting the end-point clearly as the colour change is from colourless to yellow. The bismuth-potassium iodide system behaves similarly and can be used over the pH range 1 to 5 depending on the concentration of iodide (2). - 49 -

For the determination of zirconium in sulphate solutions a solution of ferric ion can be satisfactorily used to titrate the excess of E.D.T.A. (2). Salicyclic acid (105), tiron (106) and benzohydroxamic acid (107) are available as indicators. With salicyclic acid as indicator, the end-point is difficult to distinguish visually as the colour change is from brown to yellow, but this difficulty is overcome if a photometric technique is used. Sweetzer and Bricker (105) first proposed such a method. A special quartz titration cell was used in conjunction with a Beckman model DU spectrophotometer. Ten millilitres of a standard solution of E.D.T.A. (0.001 to 0.IM concentration) and 15 mis. of a sodium acetate solution (0.05 g.of Na0Ac per ml.) were placed in the cell followed by an aliquot of zirconium solution (3.6N in HCl). After adjusting the pH to 4 with dilute aqueous ammonia and addition of 1 ml. of a 0.1 per cent, solution of salicyclic acid in acetone, the excess of E.D.T.A. was titrated with a ferric sulphate solution (0.018M) added from a micro- burette. The wavelength of the spectrophotometer was set at 520 - 525 m and the end-point interpolated from a graph of titre against absorbancy. As can be seen a very sharp decrease in absorbancy is observed at the end-point. Amounts up to 80 mg. of zirconium can be determined. -50-

Milner and Phennah (107) used a similar method for the determination of zirconium in zirconium-uranium alloys. They recommended a pH range of 5 to 6 for the titration obtained by using an ammonium acetate buffer and detected the end-point visually. With tiron the titration is conducted at pH 4.8 and the colour change is from yellow to purple. The end-points obtained with salicylic acid and tiron suffer slightly from fading caused by the ferric iron slowly displacing zirconium from its complex with E.D.T.A. The end-point with benzohydroxamic acid is said to be satisfactory when determined photometrically and to be superior to salicylic acid (2). For the detection of the end-point an EEL absubliometer can be used with an ordinary uat 400-m1. beaker as titration vessel. The titration is carried out at pH 2 to 3 and amounts of zirconium up to 100 mg. can be determined with an accuracy of ±0,6 per cent, or better. Milner and Barrett (100) applied the method to the analysis of zirconium-uranium alloys. However, even at the low pH used there is interference from some elements including Al, Bi, Th, snII, SnIV, Ti UVI and Mo. Other indirect methods proposed include titration with cobaltous chloride solution (00012) with potassium thiocyanate as indicator (108), thorium nitrate with - 51 -

arsenazo (109) and copper sulphate with 1-(2-pyridylazo)- 2-naphthol (110). In all these methods the recommended concentration range is 0.01 to 0.05M. With cobaltous chloride, the titration is conducted in the presence of ammonium acetate and concentration of acid up to 2N can be tolerated. ghe indicator (KSCN) changes from pale rose to purple at the end-point. In the procedure using thorium nitrate with arsenazo as indicator, the titration is conducted at pH 2.3 to 2.4 and it is claimed that 2 to 40 per cent. of zirconium in ores can be determined with an accuracy of ±0.6 per cent. The indicator changes from bright pink to blue-violet at the end-point. The colour change in the titration with copper sulphate as titrant and 1-(2-pyridyl azo)-2 naphthol, conducted at pH 3, is from colourless to red. When choosing a method for a particular analysis, consideration has to be given to the selectivity of the various procedures. As far as possible, methods applic- cble to solutions having low pH values should be used, as there is then no interference from nearly all bivalent and many tervalent metals. However, even under the best conditions interference may usually be expected from Al, FeIII1 CrIII CeIV, Th Ti Bi Vv, Nb, Ta, Cu, Ni and also from sulphate fluoride, phosphate tartrate, citrate, oxalate — 52 —

and molybdate. The presence of highly coloured ions in solutions may also cause some difficulty and oxidants and reduodants in solution may affect the indicator. It is possible, however, to overcome several of these interferences by making use of masking reactions, thereby avoiding preliminary separations. This technique is best illustrated by reference to Fritz and Johnson's work (104) in which the bismuth-thiourea system is used. In the titration interferences from small amounts of niobium, tantalum sulphate and phosphate can be prevented by using ammonium tartrate, and interference from thorium and titanium (in small quantities) is overcome by adding sulphate after the initial formation of the complex with E,D.T.A. More generally; difficulty caused by the presence of ferric ion in complexometric titrations can be removed by reducing the iron to the ferrous state, ascorbic acid being a satisfactory reducing agent for this purpose. In the bismuth - K1 system (2), interference from fluoride ions was prevented by preferential complex formation with beryllium. Difficulties caused by the presence in solution of coloured substances that do not react with E.D.T.A. can often be avoided by detecting the end-point photometrically.

Electrometric methods are available for end- -53-

point detection in back-titration procedures; Vladimirova (111) used an amperometric titration of the excess of E.D.T.A. with bismuth under conditions similar to those in the bismuth-thiorurea method. This titration is glower to perform than those in which the end-point is detected visually. A potentiometric method (112) is also available; the excess of E.D.T.A. is titrated with a solution of copper in acetate buffer solutions having pH values between 4 and 5.5 and a mercury electrode is used as the indicator electrode. In general, however, methods in which indicators are used are preferable. They are applicable to the determination of zirconium in amounts varying from several milligrams to about 100 micro-grams. Prelim- inary separation of zirconium, if necessary, can be achieved by methods described on pp. 16 - 46. (b) Direct methods involving E.D.T.A. Some workers have reported direct-titration procedures for zirconium, the titrations being carried out in mineral acid solutions to prevent possible losses of zirconium by hydrolysis. Korbi and Pribil (113) described a titration procedure applicable to M nitric acid solutions; xylenol orange was used as indicator. The titration was carried out•at 90°C to speed up the formation of the Zr - E.D.T.A. complex; boiling was undesirable, because of the -54- risk of hydrolysis. However, Milner and Edwards (102) found that this procedure gave a slow end-point and low results for zirconium. They encountered a slight fading at the end-point which resulted in the final titres being slightly lower than the theoretical value. Good results were obtained by titrating to a preliminary end-point, neutralizing 80 per cent. of the acid with aqueous ammonia and continuing the titration to a permanent end-point (2). This modification reduces the selectivity of the method and thorium, for example, which does not interfere in M nitric acid, causes serious interference at the lower acidity. Russian workers have subsequently reported direct titration in solutions approximately 2N in hydrochloric acid. Eriochrome black T (114), carmininic acid (114) or 4-(4'- nitrophenylazo)-pyrocabchol (115) being used as indicator. The E.D.T.A. is standardized against a solution of zirconium containing the same amount of hafnium as is present in the sample. Solochrome violet R has also been proposed as an indicator for the direct titration of zirconium in hot N hydrochloric acid solutions. It is reported that 1- to 50-mg. amounts of zirconium in 50 ml. of solution can be determined with a precision of ±1 to 0.1 per cent. and 10- to 100-mg. amounts in 10 ml. of solution with a precision of ±8 to 0.6 per cent. Even under these acid -55- conditions, FeIII, Sn, VV, Mo, W, Li, oxidants and reductants interfere, as also do phosphate, fluoride and large amounts of sulphate and nitrate. The natural colours of copper, chromium, cobalt and nickel may obscure the colour change of the indicator if these ions are present in large amounts. Interference from ferric iron can be decreased if the iron is reduced to the ferrous state by the minimum amount of stannous chloride. Other indicators for direct titrations are of the lake-forming type and require less acid solutions. Under these conditions, there is greater risk of zirconium being hydrolysed and the colour change at the end-point is not generally good. The titration is best carried out in cold solutions at a pH between 1.3 and 1.5 with Eriochrome cyanine R as indicator (116), at pH between 1.5 and 2.5 with 2-p-sulphophenylazo-l:3 dihydroxy-naphthalene-3:6-di- sulphonic acid (SPANDS) (117) as indicator or in approximately 0.5N hydrochloric acid with Chromazasol S as indicator (118). With the last-named indicator, it is recommended that the zirconium solution be run into a known amount of E.D.T.A. solution, thus reversing the usual order. The titration with Eriochromic cyanine R as indicator can readily be changed into a back-titration procedure. -56-

Other Titration Methods Several indirect methods available do not involve the use of E.D.T.A. In general such methods are not as useful as those involving the use of E.D.T.A. and are not of much importance. Acidimetric titrations based on the formation of a complex between zirconium and fluoride ions have been studied. In Sawaya and Yamashita's method (119) a solu— tion of a zirconium salt is neutralized with potassium hydroxide with bromocresol purple as indicator and to this neutralized solution is added an excess of potassium fluoride and a measured excess of 0.1N nitric acid. The following reaction takes place: Zr (OH)4 + 6KF + 4HNO3 = K2ZrF6 + KNO3 + 4H20

The excess of nitric acid is titrated with potassium hydroxide solution. Bezier (120) suggests a method in which two identical 50—m1. aliquots of a zirconium solution (6 to 9 g. of Zr02 per litre) are taken. To one aliquot is added a measured quantity of 0.2N sodium hydr— oxide solution in excess, the suspension boiled and the excess of hydroxide titrated with 0.2N hydrochloric acid. The second aliquot is treated with 30 ml, of a 2 per cent. solution of sodium fluoride (at pH 8) and then titrated directly with 0.2N sodium hydroxide with phenol red as -57- indicator. The difference in volume of the two quantities of 0.2N sodium hydroxide used is a measure of the zirconium concentration. A precision of ±0.5 per cent. is claimed for pure zirconium solutions. Ions that form complex fluorides or are precipitated as hydroxides at pH 1 to 2 or anions that precipitate zirconium interfere. Molotkova and Zolotuklin (121) suggest that zirconium in solutions as its basic salt can be determined by complexing zirconium with fluoride and titrating the liberated hyd- roxyl ions with standard acid. The reaction suggested is: 2+ 2- Zr0 + 6F- + H2O + ZrF6 + 20H. Cations that react with caustic alkali interfere. Other procedures involve the precipitation of zirconium with mandelic acid (122), p-bromomandelic acid (123), iodate, 8-hydroxyquinoline (124) or selenious acid, (124) solution of the precipitate after separation and subsequent titration of the anion. The p-bromomandelate ion is determined by oxidation with an excess of ceric sulphate and then titration of the excess with ferrous iron, ferroin being used as indicator. Similarly, mandelate ion is determined by oxidation by an excess of potassium permanganate. An accuracy of ± 1 per cent. is claimed for 2 to 20 mg. amounts of zirconium. Iodate is determined in the usual way by treatment with potassium iodide - 58 -

and titration of the liberated iodine with sodium thiosulphate. Similarly, selenite is determined by adding potassium iodide and titrating the liberated iodine with thiosulphate. For the determination of the 8-hydroxyquin-:. oline an excess of potassium bromide and potassium bromate is added, 2 bromo-8-hydroxyquinoline being formed. The excess of bromate is determined by adding potassium iodide and titrating the liberated iodide by thiosulphate. All these methods are applicable only to solutions containing less than 20 mg. of zirconium. The best accuracy obtainable is ± 1 per cent. For amounts greater than 20 mg. the uncertain nature of the precipitates results in precision being of the order ± 4 to 5 per cent. Amperometric procedures involving the use of cupferron(42), fluoride (125), 1-nitro-2-naphthol (126) and m-nitrophenylarsonic acid (127) as titrant have been described. The cupferron method is not valuable as it can be directly applied to fluoride solutions of zirconium and even to suspensions of zirconium phosphate. The zirconium is titrated in 10 per cent. V/V sulphuric acid solution against 5 per cent. V cupferron solution, the diffusion current being measured at -0.84 volt against a saturated calomel electrode. Little preliminary treatment of the - 59 - sample is needed, but a disadvantage of the method is the instability of the cupferron solution. In the fluoride method the zirconium is titrated at pH 2.2 in chloride solutions containing 50 per cent, of ethanol, ferric ions being present to help in detection of the end-point. The titration with 1-nitroso-2-naphthol is carried out in acetate buffered solutions; the titrant is stable for about two weeks, as compared with 1 day for a cupferron solution. Several elements, including Cu, Co, Fe, Pd, are precipitated by 1-nitroso-2-naphthol and interfere. Substances known to cause interference include Al, Ca, Co,

An, Mg Ti Zr, SO4", NO3" and F'. With very careful control of acid concentration (1.5N in HC1), good results can be obtained by titrating zirconium with m-nitrophenyl arsonic acid. The end-point is readily obtainable as the reduction of the nitro-group in the excess of reagent produces a good titration line. The graph for the determination of the end-point is in the shape of a reversed - L. -60-

6. Instrumental Methods In general, instrumental methods of analysis are not sufficiently accurate to be used for the determination of major constituents. However, the techniques of differential spectrophotometry and X-ray fluorescence have been applied to the analysis of high percentages of zirconium with some success. A differential colorimetric method has been devised by Manning and White (128) using Alizarin Red S. A special application of the method by Freund and Holbrook has been mentioned on page 13. The normal alizarin red S method suitable for small amounts of zirconium was developed by Green. The zirconium-alizarin red S lake is formed in either. 1M hydrochloric or 1112 perchloric acid solution. The absirbancy is measured at 530 m after 30 min. for colour development and the lake is stable for 3 hours. Anions such as fluoride, sulphate and phosphate interfere and also aluminium. If iron is present it must be in the ferrous state and one normally reduces any ferric iron present with ascorbic acid. At the acid concentrations used, titanium and thorium do not interfere. In the differential method, the spectrophotometer is set at zero using a highly coloured solution, in this case a solution containing 1 mg. zirconium per 25 ml. - 61 - of lM perchloric acid. To provide additional light the slit—width is increased. A standard curve is constructed by measuring the absorbancy of solutions containing 1.00, 1.05, 1.10, 1.20, 1.30, 1.40 and 1.50 mg. of zirconium per 25 ml. of lM perchloric acid. Analyses of solutions con— taining 30 to 43 per cent. of zirconium were compared with analyses by the mandelic acid method. The average deviation was 0.5 per cent. indicating that the method can be nearly as accurate as a gravimetric method. One disadvantage is that calibrations must be made daily; otherwise variations up to 2 per cent. can arise. The X—ray fluorescence method of Mortimore and Romans (33) can be applied to the analysis of large amounts of zirconium. The method has the advantage that analyses can be conducted without interference from hafnium. However, other elements may interfere seriously and would have to be characterized beforehand. Where samples are all of the same type the method may have a use for rapid routine analysis. -62-

7. Conclusion This study has attempted to cover only the methods established for the accurate determination of zirconium as a major constituent. The difficulty of obtaining pure zirconium for use as a standard has now been overcome, though by necessarily elaborate methods, but the problem of separating zirconium and hafnium quantitatively still remains. Fortunately, as there is rarely more than 2 per cent. of hafnium accompanying zirconium this is of little importance as a number of rapid instrumental methods are now available to determine these small amounts with reason- able accuracy. However, the problem still remains where large amounts of hafnium are present. Nowadays, such cases occur more frequently especially in the course of research on problems concerned with atomic energy. Of the gravimetric methods available, mandelic acid and p-bromomandelic acid are now firmly established as reagents and have largely replaced the phosphate method with its uncertainties. However, the phosphate precipitation still has its uses especially for separations in the presence of sulphuric acid and when phosphate can be tolerated in the final determination. Except in special cases, most other methods are of little use. E.D,T.A has been shown to be a good titrant with -63- a large range of available indicators covering most circumstances. Methods in which the fluoride ion can be tolerated in the final determination are also useful since hydrofluoric acid must be used in the dissolution of zirconium metal and many zirconium alloys. In this respect the amperometric titration method of Olsen and Elving is especially noteworthy. Too little work has been done on instrumental methods with large amounts of zirconium for any clear conclusion to be reached. Many instrumental methods are available for the determination of small amounts of zirconium. Alizarin red S is probably the best—known colorimetric reagent though many more reagents are known. Further details may be obtained from Milner and Edwards (2) and Sandell (129). Polarography is not well suited to the deter— mination of zirconium although an indirect method has been suggested by Graham et al. (130). Other methods avail— able for the determination of small amounts of zirconium are neutron activation (35), emission spectrography (131), and X—ray fluorescence (33). -64-

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