ABOUT URANIUM DETERMINATIONS in the Last Few

ABOUT URANIUM DETERMINATIONS

İhsan TOPALOĞLU and Nilüfer BAYÇIN Maden Tetkik ve Arama Enstitüstü, Ankara

ABSTRACT. — The most important methods in connection with uranium analyses published up to date will be briefly outlined in this article. Here each method is not taken separately. Instead, methods applied by various authors in the separation of uranium from its associated elements — Such as extraction, precipitation, electrolysis, column chromatography, separation by means of ion exchangers — and the determination proper — such as gravimetric, volumetric, coloriroetric and spectro — photometric, fluorometric, electrometric and radiometric — are discussed as a whole.

Of these methods those which were actually put to test and their degree of precision checked up, using standard specimens, are discussed in detail.

INTRODUCTION the determinations, and then this is fol- In the last few years, due to the rapid lowed by the determination proper. For development observed in the means for the uranium separation, either an extrac- putting to good use the atomic energy, tion method or one using precipitation, analysis of radioactive minerals— espe- electrolysis, column chromatography or cially the determination of elements Ura- ion exchangers is applied. As for the nium and Thorium—won a wide interest. determination, any one of the methods The physical and chemical analyses of that can be specified as gravimetric, these elements have, in pace with the mar- volumetric, colorimetric and spectro - ked development in this field in general, photometric, fluorometric, electronic or displayed considerable progress. Exten- radioactive may be chosen. sive work is being eagerly carried on and In all methods, in order to decompose the results regularly published. This the ore containing uranium, the sample article is written with the purpose of is treated with HNO3 or some mixtures briefly reviewing the current literature of acids such as (HNO3+HC1O4), (HNO8 on the methods of uranium analysis, + HF), (HN03 + HG104 + H2SO4+ considered particularly important, and HF) in given proportions. Those which to describe those among them which are not dissolved in acids are rendered are actually applied in our Institute. soluble by means of special mixtures like

( NaOH+Na2O2 ), (NaOH+NaNO3),

PART I (Na2O2+Na2CO3). URANIUM DETERMINATION METHODS Separation of Uranium Various methods have been devised for the determination of uranium. In 1. Extraction methods. general, first step in any method is to This is based on the fact that some free the uranium from its associated ele- uranium salts are more readily decom- ments, interfering with the precision of posed in certain organic solvents than 44 İhsan TOPALOĞLU, Nilüfer BAYÇIN in an aqueous solution of a given com- an ethyl acetate extraction have been position. The organic solution which , jointly used. In case the amount of U3 8 comes in contact with the one contain- exceeds 0.05 gr. in a solution of the ori- ing the uranium, either continuously or ginal sample, the tri-butyl phosphate gradually, after a while absorbs all of separation is dispensed with (12). the uranium. This is followed by ,evapor- According to a different method, the ation, of by destruction, of the organic extraction of uranium is done by first matter leaving uranium salt. adding sulfo-cyanide and then tin (II) One of the methods used to recover chloride in a medium of hydrochloric uranium from low-grade ores is the ether acid, followed by further addition, extraction method. In this method the within a given range of acidity (pH), of solution containing uranium is first dibutoxytetraethylene glycol plus tin acidified by adding nitric acid and, af- (II) chloride. As the latest publications ter being saturated with ammonium have shown a penta-ether to give a nitrate, is subjected to ether extraction purer product and do this more quickly in an extractor (1, 2, 3). An other method than tri-butyl phosphate, this latter may consists of ethyl acetate extraction from preferably be substituted (13, 14). nitric acid solutions having a high con- Apart from these, based on the fact centration of or being saturated with alu- that the complex substance obtained by minum nitrate (4, 5, 6, 7, 8). This method uranium (VI) with 8-hydroxykinoline is of separation for uranium is widely soluble in organise solutions, uranium is used and often applied prior to calori- extracted, within a given pH range, metric, spectro - photometric and flu- using 8 - kinolinol (oxine) and its deri- orometric determinations. Some authors vatives (such as di-chloro-oxine and di- prefer to apply ether acetate extraction bromo-oxine) and chloroform. The deter- method after adding hot aluminum mination of uranium follows using nitrate to the solution containing nitric spectro-photometry (21, 15, 16). acid or heating the solution to the boil- An other method, similar to the one ing point (9, 10, 11). In order to free above, is the cupferron or chloroform the uranium— obtained with either of extraction in a medium of sulfuric acid these two methods — from its organic or perchloric acid. In this method, in solvent, this latter is evaporated. contrast to the previous ones, uranium Again, prior to colorimetric and (VI) is left in the aqueous solution, fluorometric determinations, the tri-butyl while cupferrates of the associated ele- phoshate extraction method may be ap- ments pass into the organic solution. plied to solutions containing uranium This method of extraction is applied nitrate. In this method, in order to ob- either before (20, 87) or after (3, 17, 18, tain two better-defined phases than in 19) hydrogen sulfide group elements are one using only tri-butyl phosphate, separated; the advantage of the method tri-butyl, phosphate + methyl isobutyl being the possibility it offers for the ketone or tri-butyl phosphate + diethyl double cupferron extraction, as the ether or a 22 % solution of tri-butyl uranium may be of (IV) or (VI) valence phosphate in n-hexane must be used (12). (21,22). And this is based on the fact Lately, to extract uranium especially that in an acid medium cupferron and from ores or solutions where it exists in uranium (VI) do not give any preci- very small quantities, first a mixture of pitate, while uranium (IV) precipitates tri-butyl phosphase and n-hexane, then in the form of a cupferrate and decom- ABOUT URANIUM DETERMINATIONS 45 poses in an organic solvent. Usually, zinc amalgam and then treated with after such a separation, the determina- ammonium carbonate. During this pro- tion of uranium is made volumetrically. cess uranium (IV) carbonate is separated as a precipitate while iron (II) 2. Precipitation methods. remains in the solution. This method These methods make possible only is suggested mostly for the purpose of group separations, due to the fact that a rapid volumetric or gravimetric deter- uranium does precipitate with certain mination of uranium in the presence anions and cathions. Of course, they of- of excessive iron (26). Similarly, the ten are to be subjected to subsequent ore is rendered soluble with the aid of separation using any one of the extrac- potassium bifluoride and hydrofluoric tion methods. acid and then treated with an excess Frequently, uranium is precipitated of tin (II) chloride. In this manner, from uranyl solutions, under the action fluorides of uranium (IV), thorium, of ammonium diuranate. This method rare earth metals and earth alkali pre- makes possible the separation of uranium cipitate and are separated. This method from certain anions and some cathions, is applied mainly to complex minerals, such as Cu (II), Ni (II), which give such as, samarskite, columbite-tantalite complex substances with alkali, alkali and titano-niobite (27). earth (metals) and ammonia, and is 3. Separation by electrolysis. only applied to solutions containing no fluor, carbonate, citrate or tartarate. Electrolysis is used as an other ap- Thus the uranium precipitate, partly proach to freeing uranium from the separated from heavy metals, is decom- elements which might have misleading posed in a special acid, subsequently to effects during a determination. The undergo further separation either by sample, put in a solution form for this electrolysis or ether extraction. In this purpose, is subsequently electrolyzed, method pyridine or hexa-methylene tet- often in a medium of sulfuric acid, ramine are also used in place of ammonia using a mercury cathod. Under these (23,24,1). conditions, iron, cobalt, nickel, copper, zinc, gallium, germanium, rho- In still another widely used method, dium, palladium, silver, cadmium, in- hydrogen sulfide is passed through an dium, tin, iridium, platinum, gold, mer- acid solution containing uranium. Thus, cury, thallium, chromium, molybdenum, after the heavy metallic sulfides are rhenium, bismuth, arsenic, selenium, disposed of, cupferron - chloroform ex- tellurium, lead and osmium accumulate traction is applied to the filtrate. on the mercury cathod. Along with Instead of passing hydrogen sulfide these, manganese, ruthenium and anti- gas, the solution is treated, in an acid mony are also partially separated. Fol- medium, by sodium sulfide or, as prac- lowing this, uranium, columbium (VI), ticed lately, by thioacetamide (17, 18). tantalum, tungsten, titanium, vanadium A variation of this consists of preci- and zirconium, which are left ,in the pitating uranium (VI) with certain solution, are subjected to extraction for other metals in the form of phosphates, separation (28, 29, 30, 31). followed by the cupferron - chloroform extraction (25). 4. Column chromatography. Apart from these, as a special case, In this method separation is done the sample solutipn is reduced with in a column. The sample acidified with 46 İhsan TOPALOĞLU, Nilüfer BAYÇIN nitric acid is passed through a column dures are: (a) Amberlite 1R-4B, Amber- of specific dimensions which contains lite IRA-400, Amberlite IRA-410, a certain amount of active cellulose. Of Dowex-1, Dowex 2 (as ion exchanger the elements, totally or partiaily ab- resins) and (b) Amberlite IR-100, Amber- sorbed in the cellulose, in direct rela- ite IR- 120 and Dowex-50 (as cathion tionship with their speed in the column, exchanger resins). uranium is quantitatively extracted, using ether which contains nitric acid. ' Making use of the ion exchangers for the separation of uranium and its In order to reduce the speed of some associated elements, present in minute element atom ions in the column, quantities (trace), a solution containing certain substances such as Fe (NO ) 3 3 UO (NO ) is run through a column and Na HPO are added to the sample 2 3 2 2 4 which in turn contains Amberlite IR- solution. Thus foreign elements are 120. After this the extraction of U, Fe kept off being held in the column. and Cu is done with 0.5 N oxalic acid, As extraction solution, ether containing that of Ca, Ni, Go and Mn with normal 1-3 % nitric acid is used. After the HG1 and finally a 5 % ammonium cit- evaporation of the organic matter, rate is used for rare earths (42). the determination of uranium is done in any one of the followig methods; In an other method serving to sepa- namely, volumetric calorimetric, po- rate uranium from thorium, these two larographic or fluorometric (32, 33,34, elements are absorbed by Amberlite IRA- 35, 36, 37, 38, 39). Recently, it was 400 in ethylene diamine tetra-acetate. proposed to use a mixture consisting of Afterwards, uranium is extracted with 20 ml of petroleum ether + 190 ml of 0.02 N HC1 (43). ether + 10 ml of HNO (d. 142) as 3 In order to determine small quanti- an extraction solution (40). ties of uranium in the presence of an

excess of Fe, Al, Mg and SO4, this for- 5. Separation by means of ion mer is absorbed by a strongly basic exchangers. anion exchanger of a certain pH value In order to eliminate the ions having (such as Dowex-2) and thus separated a misleading effect on the determination from the rest (44). of uranium, this method is found to be Anion exchanger resins are also used of service. Often synthetic resins are to separate the other elements from used as ion exchangers. In general, or- uranium prior to its colorimetric deter- ganic cathion exchangers contain an acid mination. The uranium absorbed is ex- group (sulfonic acid, carboxyl group) tracted by HG1O4 (44,45). Here Am- and ion exchangers a basic group (such berlite IRA-400 which contains ammoni- as amino group). These resins, placed um group (NH ) is utilized. in a column will absorb cathions or ani- 4 4 ons that are present in the solution with Lately, by a new method, uranium which they come in contact, in accord- acetate is absorbed in a cathionic resin ance with their own character. To be formed by p - phenol - sulfonic acid and able to extract the desired cathions and poly-condensation of phenol and formal- anions from among those absorbed in the column, certain solution must be run dehyde. Its extraction is later brought through this latter (41). The name of about, using first ammonium sulfocya- some of the resins used in these proce- nide followed by hydrochloric acid (46). ABOUT URANIUM DETERMINATIONS 47

II. Determination of Uranium cadmium reducer, cadmium amalgam The uranium which is totally or (51), silver reducer (Walden-silver re- partially separated from the other ele- ducer) (52, 56), metallic copper, metallic ments— using one of the methods briefly aluminum spiral (53), titan (III) chlo- outlined in the previous paragraphs— is ride or titan (III) sulfide solution, tin now determined with the aid of various (II) chloride solution (54) and lead re- methods described below. ducer (18,20,55), are among the rather frequently used ones. In the course 1. Gravimetric determination of treatment with some of these reducing methods- agents, the valence of uranium does not Having first precipitated the uranium remain at 4, but reduces to 3, as in the with ammonium hydroxide and then cases with Jones Reducer and cadmium weighing it as oxide is an old method reducer. In such instances, the oxidation now rarely used. Because, to be able to of uranium (III) into uranium (IV) is precipitate the uranium using ammonia, done by sending a draught of air the solution must contain none of the through the solution. other elements. Consequently, uranium The titration of the reduced uranium is sometime precipitated from uranyl is done using such titrated oxidizing nitrate solution which is extracted using solutions as potassium permanganate, ether and then burnt under perfect oxi- potassium bichromate, cerium sufate, dizing conditions, prior to its weighing. potassium ferricyanide and sodium va- However, the fact that the precipitation nadate. For this purpose, one of the fol- of uranium (IV) with ammonia is quick lowing may be selected: Potassium and advantageous still holds its place in Bichromate Titration (29,54, current literature (1, 26). 56, 57, 63), using as indicator sodium or Besides these, according to methods barium salts of diphenylamine sulfonic devised recently, the precipitation of acid or diphenylamine or n-phenylan- uranium as phosphate, in the presence of thranilic acid (57), as calalyst FeCl3 complexon, prior to its determination, is solution; Potassium Permanga- possible (47); likewise, its determination nate Titration (17,50,56) using as as oxinate, using complexon as a shield- indicator ferroin (1,10) phenanthroline ing means (48), and finally application iron (II) sulfate; Cerium Sulfate of gravimetry for the same purpose, Titration (17,52, 55, 58,59,61,62) using salisilhydroxamic acid, are methods performed in room temperature, using actually in practice (49). as indicator ferroin or diphenylbenzidine and n - phenylanthranilic acid (59), as 2. Volumetric determination catalyst ortho - phosphoric acid H PO ; methods' 3 4 Sodium Vanadate (Na3VO4) Tit- Often a volumetric determination is ration (68,69), performed in room applied to uranium solutions which are temperature, using as indicator n-phenyl- freed by extraction from their foreign anthranilic acid or diphenylbenzidine elements. For this purpose, uranium is (in this case H3PO4 and H2C2O4 are first treated with, a reducing agent to added as catalysts). Only the reduction uranium (IV) and subsequently titrated of U (VI) into U (IV) is handled in a using an oxidizing solution. As a reducing different manner than the rest, involv- solution generally a solid zinc amalgam ing photo-chemistry. For this purpose, (Jones Reducer) is utilized (17,20, 50,63, Uranium (VI) solution is exposed, in a 70). Also liquid zinc amalgam (31),. medium of ethylalcohol and sulfuric 48 İhsan TOPALOĞLU, Nilüfer BAYÇIN acid, to sunshine or to Philip's Repro .methods is widely practiced. It is based lamps. The uranium thus reduced to on the absorption of light by the urani- valence (IV) is then titrated with sodium um (IV) and (VI) ions themselves, or vanadate (69). inorganic compounds or complexes of In addition to those mentioned uranium, in an aqueous or organic solu- above, potassium ferricyanide and cerium tion. This absorption is measured either sulfate titration is used indirectly. In in a colorimeter or, as it is often done, in this method, uranium (IV) is oxidized a spectro - photometer of the reflection type. into uranium (VI) by treating it with a certain amount of potassium ferri- A separation prior to a colorimetric cyanide and then, the ferrocyanide for- determination is usually required. Alkali- med during this reaction, in an equiva- peroxide method is applied to low-grade lent amount, is titrated back using concentrates following an ether extrac- cerium sulfate (53). tion (2, 3,24), or a cupferron ethyl acetate (or ether) extraction (71) or, for high- In the iodometric determination of grade concentrates, following the sepa- uranium, the solution of uranyl sulfate ration by ethyl acetate and taken into or uranyl chloride which is cleaned from aqueous solution (72). A method with foreign elements is acted upon with po- wide-spread application is that of thio- tassium iodate (KIO ) of a certain pH 3 cyanate. Uranium, after its separation by value, potassium iodide (KI) and chlo- ethyl acetate, is'treated with reagents roform. The iodine, which goes into a either directly in ethyl acetate or follow- free state and dissolves in chloroform, is ing its transfer from ethyl acetate into titrated using sodium thiosulfate or arse- water. Subsequently, spectro-photometric nite solution (64). measurement of the absorption for a There can also be mentioned a reduc- given range of wave-lengths takes place tion titration method, though seldom (73, 74, 75). used, consisting of uranyl solution being Using the yellow-colored complex titrated with chromium (II) chloride which uranium gives with dibenzoyl- and a precipitation titration method, methane, this element is spectro-photo- used for micro-determination, consisting metrically determined. This complex of uranium (VI) solution being titrated displays a maximum of absorption for a with sodium phosphate or potassium certain value of pH, in a medium con- ferricyanide. The reading of the turning taining ethyl alcohol of a certain concen- points in these titrations is done poten- tration, and along a certain range of tiometrically (65, 66, 67). According to wave-lengths. The separation of uranium the method, in which the last point may is carried out either by ether or ethyl be read spectro-photometrically (80), the acetate extraction or by cellulose column- simultaneous determination of iron and chromatography (76,77). This method is uranium may be possible. In this pro- used to determine the minute quantities cedure a 90 % cadmium amalgam serves of uranium present in minerals and rocks. as a reducing agent and the subsequent An other way of determining the amount titration is done with cerium sulfate. of uranium spectro - photometrically is the one making use of the red-brown 3. Colorimetric and spectro-photo- complex (78) this former gives, in a me- metric determination methods. dium containing ammonia, by morine The determination of uranium using (3, 5, 7,2', 4' - pentahydroxy - flavanol) colorimetric and spectro - photometric which is an other organic reagent; or ABOUT URANIUM DETERMINATIONS 49 the orange-color complex it gives with 4. Fluorometric determination 8-quinolinal and its derivatives (79). The methods- latter is particularly advantageous as it The fluorometric methods of ana- is found to display a greater stability lysis used for the determination of ura- in colors and is better suited for appli- nium are of high speed and accurate. cation to low-grade ores. They are based on the fact that even In the course of direct spectro-pho- the least concentrations of uranium, so tometric determinations, the amount of small as to be called traces, will display absorption acquired by the uranium ions fluorescence in sodium fluoride. The in an aqueous acid solution, without presence of certain ions whose direct any prior separation whatever, plays a effect would decrease the intesity of significant part. The absorption values, fluorescence, is taken care of either by at 660 or 700 mm of reduced or unre- dilution or extraction. For this purpose duced uranium ions are measured in a ethyl acetate extraction is often used medium of sulfuric acid containing cer- (30, 96, 97, 98). In fluorometric deter- tain ions in a given concentration (81). mination, a very small quantity of the In a different method, after the decom- uranium containing solution is first pla- position of the sample, a direct deter- ced in a gold or platinum crucible and, mination is carried out on the uranium after evaporation is completed, the re- solution, using a Beckman spectro-photo- sidue is dissolved in a compound which meter (82). Similarly, the colorimetric contains sodium fluoride. At the end absorption value of a uranium solution of this procedure a tiny bead is formed is determined in a medium of perchloric and, under the ultra violet light, the acid, at 415-420 mm using a Beckman fluorescence it emits is measured by DU spectro-photometer (83). For small means of a fluorometer. As a solvent quantities of uranium', the absorption that will contain sodium fluoride, the values corresponding to a wave-length of following may be used: Sodium fluoride by itself, or a mixture of sodium and of 430 m[x are measured in a medium potassium carbonate and sodium fluo- of dilute sulfuric acid, with the aid of ride (89, 91, 92, 93) or a mixture of 98 % a highly sensitive spectro-photometer sodium fluoride and 2 % lithium fluo- (84). When using a differential spectro - ride (98). photometric method (85, 86), either the absorption at 430 mm wave-length of the 5. Electrometric and radiometric uranyl ion in a medium of sulfuric acid determinations. or its relative absorption at 418 mm. Before the list is brought to a close, wave-length is measured as compared a brief mention of these methods is also deemed proper. As compared with che- to a standard of high value. The dif- mical methods of determination, electro- ferent absorption values obtained offer metric determinations are much faster a dependable clue for a very accurate and more accurate. determination of the uranium concen- In a potentiometric determination, trations in the so1ution tested. These which is used to determine the true direct spectroscopic methods are prefer- point of equivalence, usually the ura- red to the others as being quick and jiium (IV) solution is titrated with such sensitive. oxidizing solutions as potassium perman- 50 İhsan TOPALOĞLU, Nilüfer BAYÇIN ganate, potassium bichromate, cerium ma rays. The intensity of radiation (or sulfate or iron (III) (99, 100, 101). emission) is quantitatively measured in an instrument such as a Geiger-counter The coulometric determination of or a scintillomcter, which are used for uranium is rarely applied (102, 103). In the purpose. This radioactive radiation the polarographic determination method, intensity is dependent upon the ele- — also rarely used — the uranium is ment bringing about this radiation; first freed from the other elements the mass of the active element present by means of one of the extraction me- in the mineral; the distance of the thods and then, with the aid of sensitive element to the measuring instrument measuring devices, its polarogram is ob- and finally the direction and com- tained in a suitable electrolytic medium position of the rays. As the analysis of (104, 105, 106, 107, 108). This method uranium through chemical methods and is successfully applied in micro - deter- procedures takes a long time, selecting minations, in particular. one of the radiometric or fluorometric The radiometric determinations methods, especially when a large num- make good use of the fact that all urani- ber of analyses must be taken up all at um minerals emit alpha, beta and gam- once, is duly suggested (110).

REFERENCES

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