CouI ometric Determi nation of Uranium(VI) at Controlled Potential GLENN L. BOOMAN, WAYNE B. HOLBROOK, and JAMES E. REIN Atomic Energy Division, Phillips Pefroleum Co.,Idaho Falls, Idaho
b
Uranium(V1) can b e reduced to uranium(lV) a t a controlled-potential mercury cathode, by using a potassium citrate-aluminum sulfate or a sulfuric acid electrolyte. Essentially complete reduction is accomplished in 5 to 10 minutes a t room temperature b y using a 5-ml. capacity electrolysis cell. The method can tolerate mercury(ll), copper(ll), iron(lll), and large amounts of nitric acid. A procedure for the determination of uranium is capable of giving results reliable to less than 0.170 standard deviation in the range from 75 to 0.75 mg. and having sensitivity extending to a few micrograms.
T
HE FUNCTION of
the Idaho Chemical Processing Plant is to recover unburned uranium from spent, enriched uranium, fuel elements from various nuclear reactors. The aluminum-clad type of fuel element is dissolved in nitric acid with a catalyst of niercuiic nitrate (11). The main impurities in the aluminum cladding are iron and copper. A method for the determination of uranium in such a solution therefore, must tolerate nitrate, mercury(II), iron(III), copper(II), and the fission elements. Included in the main fission elements are zirconium, molybdenum, technetium, ruthenium, cerium, niobium, tellurium, and iodine (6). A rapid method, simple and reliable, was needed for such a solution. Because of the high radiation level, the aliquots taken for analysis are small, containing about 1 mg. or less of total uranium. Most of the procedures proposed for the determination of uranium(V1) in the milligram and submilligram range are rather involved and lack either speed or accuracy. Methods depending on absorbance of uranyl complexes in the visible region ( 8 ) , fluorescence measurement (9), and polarographic methods (10) are inherently not capable of reliabilities within better than about 1% and are subject to many interferences. The usual titrimetry of uranium, on both the macro and micro scale, is characterized by the necessity of metal reductors and nitrate removal, and in some cases by slow reactions requiring high titration temperatures and giving poor end point behavior ( 7 ) .
The isotope-dilution method using a thermal emission mash spectrometer is specific, but requires a long analysis time and expensive, complex equipment ( 3 ) . Indirect coulometric procedures have been developed using electrolytically generated cerium(1V) or bromine (a, 4). These methods require a preliminary reduction, absence of easily oxidizable species, and removal of nitrate ion. A direct, constant-current coulometric procedure for uranium(VI), using electrolytically generated titanous ion, has recently been presented (6). This method gives suitable accuracy in the high concentration ranges but requires a titration temperature of 85’ C. No interference study was made of the titanous titration but any easily reducible ion would be expected to cause bias. A study of the polarographic waves of uranium(V1) made the possibility of a coulometric titration a t controlled potential seem feasible, using a mercury cathode. This approach to the determination of uranium is able to remove the interference of substances which can be reduced a t a more positive potential than uranium. Also, the reduction potential for uranium can be shifted, by using an appropriate electrolyte, so that substances which normally reduce a t a more negative potential than uranium will not interfere. The direct coulometric titration also eliminates the need for standard solutions and the use of metal reductors. KO troublesome end point electrode system is needed, because with direct coulometric methods the titration is complete when background current is attained. EXPERIMENTAL DETAILS
Reagents and Solutions. The solution of 1 M sulfuric acid was prepared by dilution of the 96% reagent. The citrate electrolyte was made by dissolving 1.0 mole of citric acid and 0.10 mole of aluminum sulfate in distilled water, adjusting to p H 4.5 with potassium hydroxide, and diluting to 1 liter. The standard uranyl nitrate solutions were prepared from high-purity black oxide (UaOs) or from reagent grade uranyl nitrate hexahydrate. Each of the standard solutions was checked by gravimetric analysis.
Apparatus. The coulometer and electrolysis cell used have been described (1). Oxygen was removed from the nitrogen gas sweep by the use of n chromous chloride solution. Ordinary borosilicate volumetric ware was used. Uranium solution aliquots were obtained with micropipets. Procedure. The reduction potential for the citrate media was chosen as -0.60 volt with a prereduction potential of -0.20 volt, versus the silver-silver chloride-saturated potassium chloride reference electrode. I n 1M sulfuric acid, the electrolyte reduction potential chosen was -0.25 volt. The procedure used for the citrate media involved placing a measured amount of mercury into the electrolysis cell, pipetting in 5 ml. of citrate electrolyte, adding 0.5 ml. of the appropriate uranyl nitrate standard, and purging with nitrogen for 15 minutes. Prereduction a t -0.20 volt for 20 minutes was then followed by a 15minute reduction a t -0.60 volt t o obtain the uranium value. RESULTS
Reliability. Results obtained with 75- and 7.5-mg. samples are shown in Table I. The per cent standard deviation for six duplicate samples a t both levels was 0.03%. These results were obtained using sulfuric acid electrolyte with no prereduction step. Reduction in citrate is possible with this size of sample b u t requires about 30 to 40 minutes, compared to 8 t o 10 minutes required with the sulfuric acid electrolyte. No blank correction was necessary for either of these uranium levels, and the results showed no bias from the absolute value obtained from the gravimetric standardizations. Even though these samples gave answers a t only two values, 75.00 and 75.05 in one case and 7.500 and 7.505 in the other they are believed to be free of operator bias. Reduction was timed for exactly 15 minutes, thus eliminating any possible bias due to the operator’s watching the integrated current value. There is some reason to suspect that these exact agreements are due to the resolution of the potentiometer used. Some later studies a t the 0.750-mg. level using the citrate electrolyte gave a standard deviation of 0.01% and the data had a more normal distribution. These later studies were made with a potentiometer havVOL. 29, NO. 2, FEBRUARY 1957
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Table
I.
Sample Size, Mg. U(V1) Reduced 75 OOa
0
*
Coulometric Determination of Uranium(V1)
hIg. U(V1) Found 75.05 75.00 75.05 75.00 75.00 75 00 T 500a 7.505 7.500 7.500 7.500 7.505 7.500 0. 75O* 0.733 0.733 0.732 0.733 0.733 0.733 Sulfuric acid electrolyte. Citrate electrolyte.
%
Standard Deviation
0.0075ob 0.03
M g . U(V1)
Found 0.0730 0.0722 0.0722 0.0722 0.0724 0.0727 0.00767 0,00691
0.00720 0.00741
70
Standard Deviation 0.4
Table 11. Tolerance Level of Various Ions in Coulometric Uranium Method [Citrate electrolyte; sample of 0.750 mg.
U(VI) reduced in cell] Ion/Uranium Molar Ratio for No Bias at 95% Confidence Ion Level 6.0 0.8 0.5
2.2
0.00771
0,00708
~('111 j
175.0 700.0 >170.0
HS03 HC1
0.06
DISCUSSION
The use of a n electrolyte such as ANALYTICAL CHEMISTRY
Sample Size, Mg. U(V1) Reduced 0.0750b
0.03
ing a tenfold increase in resolution over the student potentiometer used in the studies presented in Table I. Results obtained n-ith 0.75, 0.075, and 0.0075-mg. samples of uranium are also shown in Table I. The standard deviation is 0.06% a t the 0.750mg. level, increasing to 2.2% a t the 0.0075-mg. level. Citrate electrolyte was used for these three lower concentration ranges. A background correction, necessary for these low levels, was obtained from the background current attained after a 15-minute reduction. Interferences. One of t h e outstanding advantages of this controlled-potential coulometric determination is t h e ability t o determine uranium in t h e presence of large amounts of nitric acid. Tolerance limits for nitric acid and several other common interferences for other uranium methods were obtained by finding t h e value of foreign material which would cause a single determination to be outside the 95% confidence limits. The tolerance limits for nitric acid, hydrochloric acid, copper(II), iron(III), aluminum(III), cerium(IV), and mercury(II), are shown in Table 11. The citrate electrolyte n-as used in this study. Molybdenum and ruthenium are known interferences. Being able to withstand u p to 4.2M nitric acid in a 0.5-ml. sample without bias makes the method suitable not only for nitric acid samples of uranium but also for directly determining uranium in solutions resulting from solvent extraction, where the uranium is in the nitrate form.
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dilute acids or buffered acetate and a control potential where the 1-electron reduction to uranium(Ti) \vould occur was unsatisfactory, as very little disproportionation occurred. This disproportionation of uranium(V) into the (VI) and (IV) valence states was not sufficiently complete or fast to allow use to be made of the 1-electron reduction. Quantitative reduction of uranium(V1) to (IV) with 100% current efficiency was accomplished using 1M sulfuric acid or a 1M potassium citrate-0.1M aluminum sulfate electrolyte adjusted to p H 4.5. I n these solutions the disporportionation step is sufficiently fast to obtain the quantitative reduction to the (IV) valence state. With the citrate electrolyte, the presence of aluminum causes the polarographic half-wave potential for the reduction of uranyl ion to uranium(1V) to be shifted 150 mv. in the negative direction, with the p H being held constant. This change in the uranium-citrate complexes caused by the aluminum permits a 360-mv. separation of the polarographic half-wave potential for the uranium reduction wave from the half-n-ave potential for copper(I1) to (0) and iron(II1) to (II), the latter two overlapping at -0.08 volt us. the silver-silver chloride-saturated potassium chloride electrode. Iron(II1) forms a small additional wave which overlaps the uranium wave, limiting the tolerance of this colorimetric method to iron(II1). The polarographic half-wave potential for uranium(V1) to (IT') in 1.M sulfuric acid is -0.14 volt and in the citrate electrolyte is -0.44 volt. To obtain background currents of less than 5 pa., mercury free of oxides must be used. Commercial triple-
distilled mercury requires a dilute nitric acid rinse before use. Distilled mercury which has not been allowed to oxidize can be used directly. If the sample contains only uranium, the same electrolyte can be used repeatedly for 5 to 10 successive determinations. However, the presence of a n ion such as niercury(I1) was found to induce the air oxidation of the uranium(1V) from the previous sample, requiring new electrolyte to be used. A new batch of mercury for the mercury cathode \vas used with each new sample, as the presence of a small amount of copper, n-hich is reduced to copper amalgam, will sufficiently lower the hydrogen overvoltage to cause a n accumulative positive bias. A negative bias obtained a t low uranium levels was found to be dependent on the length of time the uranium sample was in the citiate media. By extrapolating back to the time the sample was introduced, this bias can be eliminated. The data given in Table I were not corrected for this source of bias. With samples containing no easily reducible impurities, the total analysis time could be shortened to 10 minutes in the citrate media, using a fritted disk nitrogen sweep for 2 minutes, followed by a n 8-minute electrolysis at -0.60 volt. The citrate electrolyte is to be pieferred for samples containing less than 2 nig. of uranium, since better separation is obtained from the copper(I1) and mercury(I1) reduction potentials. For samples containing more than 2 mg of uranium the 1X sulfuric acid electrolyte is preferred, because the reduction time in the citrate media becomes too long. CONCLUSION
Although a t the present state of development this direct coulometric determination of uranium does not possess the specificity of the isotope-dilution mass spectrometric method, it has the capabilities of higher precision and
analysis speed than practically all other uranium methods. Work is continuing on a simple means of further separating uranium from interfering materials. Using two complexing agents consecutively along with two prereduction steps would perhaps be useful, but some other approach, such as solvent extraction, will probably be needed. This further work will be published when completed. LITERATURE CITED
(2) (3)
(a) (5) (6)
(1) Booman, G. L., “Instrument for
Controlled Potential Electrolvsis and Precision Coulometric “In-
(7)
tegration,” U. S. Atomic Energy Commission, IDO-14370 (1956). Carson, W. X., ANAL.CHEV.2 5 , 468 (1953). Duffy, W.E., Tingey, F. H., “Uranium Determnation by the Isotope Dilution Technique,” U. S. Atomic Energy Commission, IDO-14301 (1955). Furman, N. H., Bricker, C. E., Dilts, R. V., A.u.4~.CHEC 2 5 , 482 (1953). Lingane, J. J., Iiyamoto, R. T., Anal. C h i m Acta 13, 465-72 (1955). Nehemias, J. V., Dennis, R. C., Ambo, E., “Calculated Distribution of Fission Product Nuclides,” Univ. of Michigan, - . IP-109,1955. Rodden, C. J., “Analytical Chem-
istry of the Manhattan Project,” pp. 51-77, McGraw-Hill, New
York, 1950.
(8) Ibtd., pp. 77-122. (9) Ibzd., pp. 122-35. (10) Ibzd., pp. 596-609.
(11) U. S. Atomic Energy Commission, “Chemical Processing and Equipment,” pp. 4-5, U. S.Government Printing Office, Washington, D. C. 1955. RECEIVED for review July 9, 1956. Accepted November 12, 1956. Conference on Analvtical Chemistrv and Aa-olied Spectrosc“opy, Pittsburgh, Pa., hiarch 1956. The Idaho Chemical Processing Plant is operated by Phillips Petroleum Co. for the U. S. Atomic Energy Commission under Contract No. AT( 116-1)-205.
Ferrous and Ceric Ions as Dual Intermediates in Coulometric Titrimetry Effect of Current Density on Titration Efficiency of Electrically Generated Ceric Ions A. JAMES FENTON, Jr., and N. HOWELL FURMAN Frick Chemical laborafory, Princefon University, Princeton,
.The coulometric generation of ceric or of ferrous ions from an acidified solution containing cerous and ferric sulfates offers numerous advantages in connection with oxidations or reductions that require an excess of one reagent followed by back titration. In studying the effect of current density upon the efficiency of titrations performed with ceric ion, approximate maximum and minimum limits were determined. More detailed measurements were made near these limits to find the range of error as a function of current density. Between 1 and 13 ma. per square centimeter, the apparent efficiency is 99.7% or better in the generation of ceric ion. There is a gradual decrease in current efficiency above and below this range.
D
in constant current coulometric titrations mere apparently first used by Swift and coworkers ( 3 ) . Their work dealt with the generation of cuprous ion or of bromine from aqueous cupric bromide mixtures. I n studying the equilibrium conditions in this system ( 7 ) , they applied i t t o back titrations after generating a n excess of bromine to complete the bromination of aniline. UAL INTERMEDIATES
N. J.
Cupious ion n a s generated to titrate the excess bromine ( 3 ) . Takahashi and coworkers (1I) have used ferric-cerous mixtures as dual coulometric intermediates in connection with the estimation of certain organic substances. They inyestigated the determination of micro amounts of oxalic acid and of 2-naphthylamine by generating excess ceric ion, then back titrating the excess with ferrous ion. Their results for oxalic acid n-ere from 2 to 4% high. They studied the effect of variations in the content of cerous ion and of current density as applied in the coulometric determination of hydroquinone with electrolytically generated ceric ions. They found that the current density should be below 0.1263 ma. per square centimeter when the cerous concentration was 0.00144N and below 0.50 ma. per square centimeter Kith a cerous concentration of 0.0048N in order to obtain 95 to 100% current efficiency. I n the present investigation, known amounts of ferrous ion were generated electrically in deaerated solutions under inert atmospheres a t current densities and current levels known from previous experimentation to operate at substantially 1 0 0 ~ otitration efficiency. Ceric ion was generated electrically under identical conditions, except t h a t the current density was varied above and
below that used in generating ferrous ion in the same solution. In this way numerous comparisons could be made of the difference Detn-een the equivalents of ferrous and ceric ions so generated, and the over-all efficiency of the titration with ceric ion was calculated from the difference thus found. The same current level was used in both ferrous and ceric generation, thus simplifying the calculations. Variations in the anodic current density n-eie made by using various anodes of measured areas. Preliminary experiments showed, however, that the presence of phosphoric acid was necessary for the attainment of stable galvanometer readings in the potentiometric indicating system. Kithout phosphoric acid apparent eirois of I to 2% were found, and with fairly large deviations. The phosphoric acid complexes both ferric and ceric ions and greatly improves the sharpness and steadiness of the readings near the equivalence point. Potentiometric titrations made in the generating medium showed clearly the improvement due to phosphoric acid in the form of the graph of e.m.f. us. ml. of reagent. There appears to be improved ease in generation of ceric ion near end points as well a s increase in the speed of reaction of the ceric ion with ferrous because of the VOL. 29, NO. 2, FEBRUARY 1957
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