without an initial base line. By using a high temperature, contributions by Ck have been eliminated. Without an initial base line of solute the value (at one atmosphere) observed for D1,?is 0.565 cm2/sec; for a solute base line concentration of 0.031 mole fraction the value is 0.554 cmZ/sec, a deviation of 1.8%. This indicates that the even smaller concentrations (less than a mole fraction of 0.005) that have been used in all the other experiments contributed a negligible amount of error to the D1,z values. Values for D1,*at 25" C and one atmosphere for the pairs ethanol-helium, benzene-helium, and n-butane-nitrogen as measured by the chromatographic technique are summarized in Table I as well as values from the literature. The values measured chromatographically agree well with the literature values, the largest deviation being 4.4 %. Table I1 summarizes the values of 0 1 for , ~ six solutes in helium and in argon at four different temperatures. Each value of the interdiffusion coefficient is the average of four separate measurements. The data in Table I1 can be used to develop an expression for determining D1,2with reasonable accuracy at temperatures other than where measured directly. This is accomplished by fitting the data to an equation of the form:
\I
ref/
where Dr is the interdiffusion coefficient at To K and one atmosphere, Drefis the interdiffusion coefficient at TrefOK and one atmosphere, and a is a constant for a particular pair, Table 111 summarizes the values of a and Dreffor the pairs in Table I1 for a reference temperature of 273" K. The average deviations of the interdiffusion coefficients measured experimentally from those calculated using Equation 8 also are tabulated in Table 111. The chromatographic method, when used in conjunction with the initial base line technique, provides a rapid, convenient, and accurate means for measuring interdiffusion coefficients over a wide range of temperatures. In addition, the small deviations of the experimental points from the curve represented by Equation 8 indicate that over the temperature range studied, 25-200" C, the equation may be used with confidence to calculate D I , for ~ the twelve pairs summarized in Table 111. RECEIVED for review September 8,1966. Accepted November 18, 1966. Work supported by U. S. Atomic Energy Commission, Contract No. AT(ll-1)-34, Project No. 45.
Analysis and Identification of Uranium Fluorides T. A. O'Donnell and P. W. Wilson Department of Inorganic Chemistry, Uniaersity o f Melbourne, Parkuille, N2, Victoria, Australia
PREVIOUSLY there has been no generalized procedure for analysis of uranium fluorides; specific methods have been reported for analysis of particular fluorides. For example, there have been several analytical procedures proposed for uranium tetrafluoride. The present approach can be used for the analysis of any fluoride in the series UF3, UF4, U4F17, U2F9,UF6, and UF6 and can be applied to simple mixtures of uranium fluorides or to a mixture of a fluoride and an oxide fluoride. Furthermore, the methods reported hitherto for the analyses of uranium fluorides involve preliminary separation of the fluoride before analysis for that component. The present method does not require these tedious and difficult separation procedures. It uses an original approach to determination of the oxidation state of the uranium providing more precise characterization of the uranium fluoride to be analysed than is obtained by determining only the individual percentages of uranium and fluoride in the sample. A difficulty that normally attends the dissolution of lower uranium fluorides is used to advantage here. When fluorides such as U4Fli,U2F9,and UF5 are hydrolysed, disproportionation to uranium(1V) and uranium(V1) occurs and the insoluble UF, precipitates. In the present method, the precipitated uranium(1V) and the uranium(V1) are determined separately and the oxidation state of uranium in the original fluoride can be calculated much more precisely from these two values than in conventional procedures. In describing a cation-exchange separation of fluoride from uranium, Sporek ( I ) discusses the difficulties of separa(1) K. F. Sporek, ANAL.CHEM.,30, 1030 (1958). 246
ANALYTICAL CHEMISTRY
tion processes such as pyrohydrolysis and the Willard-Winter distillation. He as well as others (2) have commented on the inaccuracy of the thorium-alizarin titration, particularly when applied to a Willard-Winter distillate. Sporek concedes that in his method, as with pyrohydrolysis, use of an acidbase titration after separation is not specific for fluoride ; other anions-e.g., chloride-will appear as free acids in the eluate or in the pyrohydrolysis condensate. In the present work, uranium is determined by conventional methods and the null-point potentiometric titration (3) is used for fluoride analysis. In this method complexing of cerium(1V) by fluoride is used to change the potential of a cerium(1V)-cerium(II1) half-cell. In other titrimetric methods for fluoride, uranium interferes by being complexed by fluoride; but it has been established that, providing certain precautions are taken with the cerium(1V) concentration, the presence of uranium does not interfere with the null-point potentiometric method. Therefore preliminary separation of fluoride in the sample is not necessary. EXPERIMENTAL
Dissolution of Sample. The lower uranium fluorides are readily dissolved by reagents such as mixtures of ammonia and hydrogen peroxide or of sulfuric acid and ammonium persulfate (peroxydisulfate). They also dissolve readily in (2) T. A. O'Donnell and D. F. Stewart, in "Electrochemistry," A. Friend and F. Gutman, eds., pp. 333-336, Pergamon Press,
Oxford, 1964. (3) T. A. O'Donnell and D. F. Stewart, ANAL. CHEM.,33, 337 (1961).
solutions containing aluminum(III), cerium(IV), or boric acid. These reagents cause dissolution in either or both of two ways; they either remove the free fluoride ions from solution by complex formation or they oxidize the uranium to uranium(V1). Since., in the present work, fluoride is determined by complexing of the redox couple cerium(1V)cerium(III), the solution for fluoride analysis must contain no entity other than fluoride which can change the ratio of free cerium(1V) to free cerium(II1). Any dissolution reagent which acts by complexing the fluoride is unsuitable since all the fluoride must be available to complex the redox couple. Also, if the fluoride analysis were performed directly on a lower uranium fluoride, cerium(1V) would be reduced, and the ratio would be changed artificially. Therefore, in the analysis of a lower uriinium fluoride, the sample must be oxidized to uranium(V1) before fluoride is determined. However, if excess oxidant remains, this will oxidize the cerium (111) in the redox couple and again cause gross interference. So it is necessary to find an oxidant which will oxidize all uranium fluorides to uranium(V1) and be capable of being removed easily from the solution before the analysis for fluoride. The most suitable oxidants would appear to be a solution of hydrogen peroxide in ammonia or of ammonium peroxydisulfate in sulfuric acid. If either of these is used it should be possible to destroy excess oxidant by boiling and so prevent interference in the fluoride determination. It was found however, that even after prolonged boiling hydrogen peroxide could not be Icompletely destroyed. Since cerium (IV) oxidizes hydrogen peroxide gross interference resulted in the fluoride determination. Thus, the most suitable reagent for the dissolution of the uranium fluoride is the ammonium peroxydisulfate-sulfuric acid system. Dissolution is comparatively fast and any excess is easily destroyed by boiling. The oxidation is catalysed by silver(1) ions which do not interfere in the subsequent fluoride determination. However complete dissolution is obtained readily even in the absence of silver(1) ions. Determination of Uranium Content. There is no particular difficulty in the determmation of uranium in uranium fluorides since fluoride does not interfere with the methods normally used. Therefore in the experimental part of this project more work has been done on the determination of the fluoride content of the samples analysed than on uranium content. Total uranium content, that is uranium(II1) or uranium (IV) oxidized to uranium(VI), or uranium(V1) originally in solution, was determined spectrophotometrically as the peroxide complex in basic solution ( 4 ) . When uranium tetrafluoride was analysed or was formed during solution of an intermediate fluoride, ii: was determined by oxidizing with an excess of either cerium(1V) or potassium dichromate and back titrating with ferrous sulfate. Oxidation State of Uranium in Samples. The uranium contents of uranium tetrafluoride, of the intermediate fluorides U4F17 and U2F9, and of uranium pentafluoride vary only slightly from compound to compound, having values of 75.8x7,, 74.7%, 73.6%, imd 71.5%, for the four compounds. Therefore it is difficult to establish with certainty the exact identity of a sample if only total uranium and total fluoride contents are known. Eken a conventional determination of the oxidation state of uranium in the sample would provide surer identification than is given by analysis for components. The present work however gives still more precise information. When a sample of U4F17,U2F9 or uranium pentafluoride is hydrolysed it disproportionates to form a precipitate of uranium tetrafluoride and a solution containing uranyl and fluoride ions. The amount of uranium(V1) in such a solution (4) C . J. Rodden, “Analytical Chemistry of the Manhattan Project,” pp. 82-89, McGraw-Hill, New York, 1950.
can be determined using any normal procedure such as spectrophotometric measurement of the peroxide complex in basic solution. The uranium(1V) content of the compound can be determined using an oxidant as described in the preceding section. In this manner the ratio of uranium(V1) to uranium (IV) can be determined. This ratio can be used very effectively to determine the nature of the uranium fluoride. Uranium hexafluoride provides no uranium(1V) fluoride on solution. For uranium pentafluoride the ratio uranium(V1) to uranium(1V) is 1 :1 ; for U2Fg the ratio is 1 :3, for UaF17the ratio is 1 :7, and of course uranium tetrafluoride contains no uranium(V1). It is seen that the ratio of uranium(V1) to uranium(1V) varies markedly with the nature of the particular uranium fluoride and is the best method for determining the identity of the uranium fluoride. Determination of this ratio is particularly valuable for identifying the intermediate fluorides, UIF17 and U2F9,since even though their empirical formulas are very similar, namely U4Fl7and U4FI8,their uranium(V1)uranium(1V) ratios are distinctly separated. It is also possible to use this ratio to determine the composition of simple mixtures of uranium fluorides. For example, the uranium(V1)-uranium(1V) ratio obtained for a mixture consisting of equal amounts of u4F17 and U2F9 would be 1 :5. This ratio is significantly different from that of either pure compound, whereas the percentages of uranium in either compound or in the mixture are very similar. Many samples of uranium fluorides are contaminated with uranium oxide fluorides. In an oxide fluoride the uranium content is not very different from that of related uranium fluorides. It is obviously advantageous to have the oxidation state (or the average oxidation state) of uranium in compounds for analysis, Uranium trifluoride is similar to uranium tetrafluoride and a determination of its oxidation number is necessary to establish its identity and purity. Effect of Uranyl Ions on Determination of Fluoride. Uranyl ions could interfere in the null-point potentiometric determination of fluoride by competing with the cerium for fluoride ions. However, it was found that if the conditions previously reported (3) in the null-point potentiometric determination of fluoride are adhered to, that is the cerium reagent solution is 0.01M in cerium(IV), 0.01M in cerium(III), and 0.5M in sulfuric acid, uranium present to the extent of 30 mg per 50 ml of cerium(1V)-cerium(II1) reagent does not interfere in the fluoride determination. This was demonstrated in two ways. After establishing that the potential difference between two half-cells each containing 50 ml of cerium(1V)-cerium(II1) reagent was zero, 30 mg of uranium as uranyl nitrate solution was added to one half-cell and an equivalent volume of distilled water was added to the other half-cell. The potential difference between the cells remained at zero. Also several titrations of known aliquots of standard fluoride solutions containing uranyl ions were performed. No departure from expected results was obtained. This shows there is no need to remove the uranium before performing the fluoride determination in the usual way provided the uranium content of the aliquot does not exceed 30 mg. If more uranium is present, then any interference can be overcome by increasing the cerium(1V) to cerium(II1) ratio proportionally. If the cerium(1V) molarity is increased to O.O2M, the uranium interference is insignificant provided the uranium content does not exceed 60 mg per 50 ml of reagent. By increasing the cerium(1V) content, toleration of quite large amounts of uranium can be achieved. However if the cerium(1V)cerium(II1) ratio exceeds 5 :1 , the sensitivity of the titration is decreased. In these cases it is advisable not to further increase the cerium(1V) content but to use the previously described technique of compensation (3). General Procedure for Analysis of Uranium Fluorides. The following procedure was used to obtain the results described in this report. After hydrolysis of the sample and VOL. 39, NO. 2, FEBRUARY 1967
247
Table I . Uranium Fluoride Analyses
z
Sample UF.5
Uranium(VI), Calcd. Exptl. 61.62
Uranium(IV), P: Calcd. Exptl.
61.69 61.19
UF4 Sample 1
75.81
75.18 15.88
UF4
Fluoride,
Exptl.
32.38
32.25, 32.42 32.42 24.13, 24.16 24.23, 24.11 24.14 24.19, 24.20 24.10
24.19 24.138
Sample 2 35.15
35.60 35.70
35.75
35.65 35.85
z
Calcd.
28.50
28.60, 28.56 28.46
Certified value.
separation of the solution of uranium(V1) and fluoride ions from any precipitated uranium tetrafluoride, uranium in the filtrate was estimated spectrophotometrically as the peroxide complex in basic solution. Uranium(1V) was determined by dissolving the uranium tetrafluoride in excess oxidant, usually potassium dichromate, and back titrating with ferrous sulfate. Usually it was more convenient to determine the uranium(V1) and uranium(1V) contents on separate portions of sample. However, if necessary, it is possible to use only one aliquot. Since fluoride does not interfere in most normal methods for determining uranium any of these would be suitable, for example, precipitation with 8-hydroxyquinoline. The sample for fluoride determination was hydrolysed in slightly ammoniacal solution to prevent loss of fluoride ions as hydrogen fluoride. After neutralizing the solution, a two-fold excess, with respect to uranium(IV), of (NH4)&Os in 1M sulfuric acid was added, and the solution stirred until the sample, including precipitated uranium tetrafluoride, was completely dissolved. After neutralizing again, excess (NH4)2SzOswas destroyed by gentle boiling. The solution was then transferred to a standard flask and a portion removed for fluoride analysis. This analysis was performed using the null-point potentiometric method as previously described (3) excepting that modification of the cerium(1V)cerium(II1) ratio was necessary to prevent uranium interference if uranium was present in a moderately large amount. If the uranium content of the sample is less than 30 mg, no modification is necessary, while for uranium contents up to 60 mg, a cerium(1V) concentration of 0.02M is suitable. For amounts of uranium exceeding 120 mg it is best not to further increase the cerium(1V) concentration but rather to use a compensating technique as described previously. In a typical analysis of uranium pentafluoride 50 mg of sample was hydrolysed in about 40 ml of ice water. After removal of precipitated uranium tetrafluoride by filtration, the filtrate was transferred quantitatively to a standard flask and diluted to 100 ml. The uranium(V1) content of this solution was determined spectrophotometrically or gravimetrically as indicated above. Uranium(1V) was determined by hydrolysing a further 50 mg of sample in approximately 40 ml of water and separating the precipitated uranium tetrafluoride by filtration. The residue was then dissolved in 20 ml of 3 X 10-*M potassium dichromate solution in 1M sulfuric acid and excess dichromate was back-titrated with 0.1M iron(I1) solution. The fluoride content was
248
ANALYTICAL CHEMISTRY
determined by hydrolysing 200 mg of sample in 50 ml of chilled, slightly ammoniacal solution. This solution was then neutralized with sulfuric acid and approximately 20 ml of freshly prepared 5 X 10-2M(NH4)&08 in 1M HzS04 was added. The solution was stirred until all the sample was dissolved, neutralized, and gently boiled to destroy excess (NH4)&Os. The solution was then transferred to a 100 ml standard flask. Fluoride was determined on a 20-1111 portion of this solution. Using these quantities no modification of the null-point potentiometric method was necessary. RESULTS
Samples of uranium hexafluoride, uranium pentafluoride, and uranium tetrafluoride, mixtures of uranium trifluoride and uranium tetrafluoride, and mixtures of U4FIi and UzF9 have been analysed using the procedures reported here. The uranium hexafluoride sample was carefully purified by collection over sodium fluoride and repeated distillations from Kel-F traps. Uranium pentafluoride was prepared by the interaction of this pure uranium hexafluoride and pure uranium tetrafluoride. The first sample of uranium tetrafluoride analysed was prepared by passing a mixture of hydrogen and hydrogen fluoride over oxide-free uranium metal heated to 400°C. Extremely rigorous precautions were taken to ensure no oxygen or water could enter the system and contaminate the sample. In addition uranium tetraff uoride (sample 2) was obtained as an analysed sample (No. 17B) from the New Brunswick Laboratory of the U.S.A.E.C. to compare the fluoride analysis results obtained using the method described in this paper with results obtained independently using previously reported, but more tedious procedures. Results of analysis of several of these fluorides are shown in Table I. Analysis of the intermediate fluorides is not reported since they behave similarly to uranium pentafluoride on solution. Mixtures of uranium trifluoride and uranium tetrafluoride have also been analysed.
RECEIVED for review July 5 , 1966. Accepted September 26, 1966. Financial support for this project was received from the Australian Atomic Energy Commission.
