because gold also dissolved to sonie extent and having itself a small catalytic action, interferred with that of mercury. Mercury was dissolved in about 10 minutes in concentrated nitric acid (3 drops). The solution was transferred to a graduated 25-ml. cylinder, using about 19 ml. of distilled water. Concentrated sodium hydroxide solution was added to obtain a weakly acid solution (pH about 5). Nitrosobenzene solution was added (2.5 ml. of 0.0042M solution) and 5 drops of an acetate buffer solution of p H 4.1. (Only a small quantity of the buffer solution was used to keep the negative salt effect as low as possible.) The volume of the solution was made u p to 22.5 ml. and the cylinder was immersed in a water thermostat at 20’ =k 0.05’ C.,
then 2.5 nil. of thermally equilibrated potassium ferrocyanide solution (0.005 M) were added, and the determination was carried out as described.
(2) BSperger, Smiljko, Murati, Ivo, z‘upahin, Oleg, J . Chem. SOC.1953, 1041. (3) ASperger, Smiljko, A i x . 4 ~ .CHEM. 28, 1761 (15
Six determinations were made after a complete destruction of the organic matter in urine. The accuracy of the procedure seems to be similar to the one obtained with platinum electrodes. ; i statistical treatment on a larger number of determinations will be made in due course.
(5) Davies, 0. L., “Statistical Methods in
(4) ASperger, Smiljko, J . Chem. Soc. 1955, 1
Research and Production,” Oliver ti Bol-d. London. 1949. (6) cynch, G. R., Analyst 79, 137 (1954). (7) Middleton, G., Stuckey, R. E., Ibzd., 79, 138 (1954). (8) Milton, R. F., Waters, W. A., “Methods of Quantitative Micro-Analysis,” p. 510, Edward Arnold, London, 1959. (9) Moldaa-skij, B. L., Zhur. Priklad. Khim. 3. 95.5 11930). (10) Stocg, A4ifred,’ NeuenschwanderLemmer, Kelly, Be?. deut. chem. Ges. 71B, 550 (1938). (11) Touden, W. J., “Statistical Methods for Chemist,,’ Wiley, Vew York, 1951.
ACKNOWLEDGMENT
The authors thank H. B. Ivekovi6 for helpful discussions. LITERATURE CITED
(1) ASperger, Smiljko, Murati, Ivo,
As.4~.
CHEW26,543 (1954).
RECEIYED for review August 14. 1958. Accepted December 10, 1958.
Accurate Determination of Uranium in the Presence of Small Amounts of Molybdenum Lead Reductor and Stannous Chloride Methods J. T. BYRNE, M. K. LARSEN, and J. L. PFLUG Rocky Flats Plant, The Dow Chemical Co., Denver, Colo.
b The conditions have been determined under which the lead reductor may be used for the accurate determination of uranium containing more than 50 p.p.m. of molybdenum. Results are low and erratic when the widely accepted method of preparing samples in perchloric acid is used, Accurate and precise results are obtained only in the absence of perchloric acid and, therefore, preparation of samples by dissolving in nitric acid and fuming with sulfuric acid is recommended. In addition, the application of the stannous chloride method to the determination of uranium in the presence of up to 6.6% molybdenum has been studied. Accurate results are obtained if the molybdenum content is separately determined and a correction is applied to the basis of a three-electron change. Previous work had suggested a two-electron change for the molybdenum. The effects of other common interferences are evaluated.
M
and other elements that are reduced to air unstable states in the Jones reductor, interfere in the determination of uranium by the usual Jones reductor method (6, 8) 942
OLYBDENUM,
ANALYTICAL CHEMISTRY
either by consumption of reagent or by inducing oxidation of uranium(1V) to uranium(V1) during aeration. Sill and Peterson (8) have shown that interference of this latter type can be reduced or eliminated by using a modification of the lead reductor method (1). The modified method consists of fuming the sample to dryness with either sulfuric or perchloric acid, taking u p the residue in 30% hydrochloric acid, and passing this solution through a properly prepared lead column. The effluent is either collected in ferric chloride and titrated with dichromate to a diphenylamine sulfonate end point, or collected in phosphoric acid and titrated with ceric sulfate to a ferroin end point. Fuming with perchloric acid is recommended over fuming with sulfuric acid to eliminate serious bumping caused by the separation of salts. I n determining uranium in the pure oxide (U308)in this laboratory the method suggested by Sill and Peterson gave reproducible results which checked n ell with other data on the same oxide. However, when the method n as tried on some metal samples, the results were low and erratic. Further investigation showed that reproducible and accurate results could be obtained on pure oxide fumed with either perchloric or
sulfuric acid and on metal fumed with sulfuric acid, but not on the same metal if i t was fumed with perchloric acid. Iron, molybdenum, and silicon were the impurities which were significantly higher in the metal than in the oxide. To determine whether any one, or a combination of these, affected the uranium determination, a series of experiments was run to study the interactions among these three impurity elements and the fuming agent (perchloric or sulfuric acid). I n a limited survey of the effects of interferences on the stannous chloride method, Main (4) found that the titration of 100 mg. of uranium oxide, (LTa08)containing 7 mg. of molybdenum consumed 21% more titrant than the uranium itself. This corresponds to a two-electron change for the molybdenum. I n Main’s (4) procedure the uranyl ion is reduced by stannous chloride in a hot, concentrated hydrochloric acid solution containing iron catalyst and phosphoric acid. After cooling, the excess stannous chloride is oxidized n-ith excess mercuric chloride. Ferric chloride, sulfuric acid, phosphoric acid, and diphenylaminesulfonic acid indicator are added and the solution is
titrated with 0.0214' potassiuni dichromate. This procedure is much faster than the solid reductor method, but in practice is applicable only to metals and other samples in which the uranium content is approximately known so that the proper volume of stannous chloride can be calculated. Because the stannous chloride method offers advantages in speed and does not present the problems encountered with the contamination of solid reductors, a study was made of the molybdenum interference to determine if it m-ere sufficiently reproducible to permit a correction to be made, without sacrificing accuracy in the uranium analysis, During this study some unanticipated results were found. EXPERIMENTAL
All uranium samples were Drepared from RE-ST USOS (99.93% pure). Distilled water and reagent grade chemicals were used. Solutions of potassium dichromate were prepared from reagent grade potassium dichromate and standardized against MS-ST UaOs. Apparatus. Potentiometric titrations were performed with a Beckman Model G pH meter and a platinumsaturated calomel electrode pair. T h e titrant was stored in a constant temperature bath (30' + 0.5' C.) and pumped t o a water-jacketed 100-ml. buret for titrations. Thus, errors due t o density changes were virtually eliminated. The lead reductor was prepared by placing a glass wool plug a t the bottom of a 45 X 2 em. glass column and filling the column about two thirds full with reagent grade, granular lead. Before use the lead was washed until bright with 3N hydrochloric acid. TT'hen not in use, the reductor mas filled with 3-Ir hydrochloric acid containing 0.1% ferric iron. It has been reported (6) that unless this iron is present. the first deterniination each da.v is several tenths of 1% low. Before use each day the reductor n-as washed with 0.8N hydrochloric acid, and between samples it was rinsed with distilled n-atcr to remove lead sulfate. Lead Reductor Method. Some of the ouide (U308) samples were dissolved in concentrated nitric acid and fumed to dryness with sulfuric acid. nhilp the rest were dissolved and fumed t o dryness with perchloric acid. The residues were taken up in 25 ml. of water. 10 ml. of concentrated sulfuric acid, and 15 ml. of concentratcd hydrochloric acid. The cooled uranium solutions were then washed through t h e lead reductor a t a rate of 15 to 20 ml. per minute with 0.81' hydrochloric acid. The reduced solutions were caught nndcr e ~ c e s s frrric sulfate. The ferrous iron fornicd by reaction of the ferric sulfate with uranium plus inipuritiPq n-as then titratrd n ith 0.05.Y potassium dichromate. The end points for the samples fumed with sulfuric Reagents.
acid were detected potentiometrically. However, the end-point potentials were not well defined for the samples fumed with perchloric acid and they were titrated to a visual diphen) lamine sulfonate end point. For these titrations, 10 ml. of phosphoric acid were added to improve the end point. Results. The main purpose of this investigation uas t o determine why low and erratic results were obtained when uranium metal samples n ere analyzed by the lead reductor method after fuming with perchloiic acid. Previous work here and a t other laboratories (5, 8) had not indicated that a n y difficulty of this type should be encountered. To interpret the results in terms of accuracy as well as of precision, the work described here was done with uranium oxide sampleq of known impurity content. The only known differences between the metals that gave low and erratic results and those that gave apparently satisfactory results were the higher iron, niolybdenum, and silicon contents of the former. T o determine whether any one, or a combination, of these impurities interfered, various amounts and combinations of these impurities were added to the pure uranium oxide and the uranium was determined by the lead reductor method. The average recovery for 29 samples that were fumed with sulfuric acid, and that contained up to 2000 p.p.m. of molybdenum, 1000 p.p.m. of iron, and 200 p.p.m. of silicon was 100.00% n i t h a standard deviation of +0.03%. From these data it is apparent inimediately, that up to 2000 p.p.ni. of molybdenum. 1000 p.p.ni. of iron, and 200 p.p.m. of silicon do not interfere, if the samples are fumed in sulfuric acid. The titrations, of course, must be corrected for iron and niolybdenuni on the basis of one- and three-electron changes, respectively. Honever, low and erratic results are obtained when samples fumed in pcrchloric acid contain as much as 100 p.p.ni. of niolhbdenum, Table 1. It was of interest to determine next, the minimum concentration of molybdenum that causes Ion and erratlc results. Table I1 shons the results obtained on samples fumed n i t h perchloric acid, when the mol) bdenum content was varied from 20 to 500 p.p.m. The first indication of erratic results was a t the 100 p.p.ni. level so that the tolerance level appears to be a t 50 p . p m S o studies have h e n made to determine TI hether the sample size affects the tolerance level of rnolybdenum. Finally. to gain some knowledge of the mechanism of thiq interference, a n euperiment n as designed which nould indicate the step in the procedure a t n liich the interference occurred. To do this, pure uraniuni
Table 1. Effect of Impurities on Lead Reductor Method with Perchloric Acid Fuming TT-0. - . l - s
Impurity Added,
U308~
Taken 0.6004 0.6000 200 Si 0.5995 0.6008 200 Fe, 200 Si 0.5990 0.5995 100 1\10 0 6001 0.6007 100 110,100Fe 0 6000 0 6016 100 110,200 Fe 0 5993 0 6003 200 110 0 6016 0 6011 0 5993 0 6007 200 110,100 Fe 0 5997 0.6010 200 110)200 Fe 0 6001 0 6006 0 5988 0 6013 200 110,200Si 0 6004 0.6010 200 110, 200 Fe. 200 Si 0 5993 nm 9 n 6008 0 6006
P.P.M.
200 Fe
Found 0.6002 0.6002 0.5997 0.6005 0.5992 0.5994 0 5975 0.5984 0 5973 0 5994 0 5965 0 5984 0.6002 0 5951 0 5910 0.5850 0 5963 0.5959 0 5981 0 5959 0 5920 0 5974 0 5996 0 5954 0 5966
n 5977
o
5990
0 5976
Recovered, % 99.97 100.03 100.03 99.95 100.03 99.98 99.57 99 62 99 55 99 63 99 53 99 68 99 77 99 00 98 62 97.39 99.43
99 15 99.67 99.22 98 86 99 35 99.87 99 07 99 55 99 47 99 l o 99 50
Table II. Interference of Molybdenum in Uranium Determination by l e a d Reductor Method"
hlo UaOe Ua08 UaOa ReAdded, Taken, Found, covered, P.P.?\I. Gram Gram % 20 0.6010 0.6011 100.02 0.6005 0.6006 100.02 50 0.6009 0.6011 100.03 0.6007 0.6006 99.98 100 0.6001 0.5975 99.57 0.6007 0.5984 99.62 150 0.5997 0.5989 99.87 0.5993 0.5980 99.78 500 0 6001 0.5943 99.03 0.6021 0 5972 99.18 0 The Iiranium-molybdenum samples mere dissolved and fumed with HCIOI. Results corrected for molybdenum added.
oxide, samples (0.6 gram) were combined with 100 y of molybdenum a t various stages of the procedure. The results are tabulated in Table 111. It is clear from these data that low and erratic results are obtained when the uranium and molybdenum are passed through the reductor simultaneously in perchloric acid solution. This behavior could be explained on the basis that the molybdenum-catalyzed reduction of perchlorate by lead forms a product that oxidizes aranium(IV), thereby preventing the VOL. 31, NO. 5, M A Y 1959
943
Table 111. Effect of Adding Molybdenum to Uranium at Various Steps in Lead Reductor Procedure
Step Prior to Which 0 . 5 Gram of U and 100 y of M o Are Combined Fuming with HC104
Uranium Recovered,
%
99.60 99.67 99.27 99.10 99.38 99.39 99.42 99.38
Reduction
Titration
Table
100.02 99.98 99.98 100.02
V.
Stannous Chloride Method.
Using
Table IV. Effect of Molybdenum on Determination of Uranium by Stannous Chloride Method
U 0.05031 TheoPresMo N hlo, retical ent, Added, Titrant, Corr., Titer, y M1: M1. Gram 0.5084 200 85.01 0.12 100.00 0.5085 400 85.17 0.25 100.02 0.5083 600 85.25 0.37 100.01 0.5085 1000 85.54 0.62 100.02 0.5082 2000 86.09 1.24 99.99 $fter subtraction of 1.01 ml. blank. Q
Comparison of Uranium Analyses in the Presence of Molybdenum by Jones Reductor and Stannous Chloride Methods
Uranium rlv. yo Normalized SnClp J. R. 99.97 100.00 99.67 99.99 99.52 100.00
Molybdenum Concn., P.P.M.Q
