Coulometric Titrations with Electrolytically Generated Uranous Ion W. D. SHULTS
Ill,
P. F. THOMASON,
and
M. T. KELLEY
Analytical Chemistry Division, O a k Ridge National Laboratory, O a k Ridge, Tenn.
the amount of unknown substance titrated. The operation of the automatic coulometric titrator has been very satisfactory in the coulometric generation of several intermediates by this technique. Mechanical Apparatus. Titrations were made in a titration cell that was similar to the cell used by Furman, Cooke, and Reilley ( 4 ) . A 15-ml. weighing bottle was fitted tightly with a rubber stopper that supported two salt bridges, two electrodes, and a carbon dioxide inlet tube. The generating electrode system consisted of a 1 X 2 em. platinum flag (cathode) mounted vertically inside the cell with its “mast” protruding through the rubber stopper and a platinum gage cylinder (anode) 1 cm. high and 1 em. in diameter immersed in a 3% ammonium sulfate-3% sulfuric acid solution that was connected to the test solution by means of one of the 3% ammonium sulfate-+$% agar salt bridges. The indicating electrode system consisted of a &inch length of 16-gage platinum wire electrode (indicator), mounted vertically through the rubber stopper, and a saturated calomel electrode (reference) that was connected to the test solution through the second salt bridge. The solution was stirred with a magnetic stirrer. Reagents. -40.10005 potassium dichromate stock solution was prepared from National Bureau of Standards reagent No. 136a by weighing. Solutions of the desired chromium(V1) concentration were prepared from this stock solution by dilution. .4ceric sulfate stock solution was prepared from reagent grade ceric ammonium sulfate to be O.L\-. This solution was standardized against National Bureau of Standards arsenious oxide reagent Xo. 83a lvith osmium tetroxide as the catalyst The average normality, calculated from the results of five determinations, was 0.1015. Solutions of the desired cerium(1V) concentration were made from this stock solution by dilution. h 0 . 2 3 uranyl sulfate solution, which was 0.5N in sulfuric acid, was used t o supplv the uranium(V1) to be reduced. This solution was prepared bv heating approximately 13 grams of pure uranium trioxide in 50 ml. of water that contained 9 ml. of concentrated wlfuric acid until dissolution was complete, and then diluting the solution to a final volume of 500 ml. The final solution vas shown by gravimetric analysis to contain 21.9 mg. of uranium per milliliter. Calibrated volumetric glassware was used when standardizing the stock solutions.
Both dilute ceric sulfate and dilute potassium dichromate solutions were titrated with uranium(1V). The uranium(1V) was generated electrolytically with constant current at a platinum cathode, which was immersed directly in the test solution. The end point was detected by means of a platinum wire-saturated calomel electrode system, and the electrolysis time was measured automatically.
T
HE titration results show that cerium( IV) and chromium(VI) ions can be reduced with 100% current efficiencies by use of uranium( IV) as a coulometric intermediate. Belcher, Gibbons, and West ( 1 ) have reported the use of uranium(1V) as a volumetric reducing agent. They concluded that uranium(1V) in acid solution is a moderately strong reductant, which can be used for direct titration of the stronger oxidants, but which in practice is less convenient to use than are other existing reagents. This inconvenience is due to the manner in which uranium(1V) solutions must be prepared and stored and to their instability. The present work was undertaken in order to determine the feasibility of using electrolytically generated uranium(IV) as an intermediate in coulometric reductions, thus eliminating most of the disadvantages associated with volumetric solutions of this ion. Such a coulometric titration method should be especially useful in the determination of strong oxidants which may be present as minor constituents or impurities in uranium solutions; the uranium in the sample itself could then serve as the source of uranium( IV). The determination of uranium coulometrically has been studied by Carson ( 2 ) and by Furman, Bricker, and Dilts ( 3 ) . APPARATUS AND REAGENTS
Electrical Apparatus. The automatic coulometric titrator (ACT) used in this work was developed and built by Stelzner, Fisher, and Kelley of the Instrumentation Group of the Analytical Chemistry Division at this laboratory and is to be described fully in a forthcoming paper ( 6 ) . It electronically provides and maintains a constant-Le., to better than fO.l%-generating current, which can be adjusted continuously to any value between zero and 10 ma. .4n ammeter is included in the circuitry and indicates approximately the current flowing. The exact value of the generating current is computed from Ohm’s law by use of the measured potential drop across a standard 100-ohm (=k0.5%) resistor that is incorporated in the generating circuit; the potential drop is measured by means of a Rubicon potentiometer (Rubicon Co., Catalog No. 2731). The automatic coulometric titrator, in conjunction with a Leeds & Northrup pH meter (Catalog No. 7664), also provides for automatic stoppage of the generating current at any preselected end-point potential between -1.4 and 1-1.4 volts. The potential of the solution is constantly monitored during the titration. When the potential, as indicated on the pH meter, reaches the value that has been preselected as the end-point potential, the automatic coulometric titrator stops the flow of generating current. Should the potential fall below the cutoff value, the flow of current begins and continues until the cutoff potential is again attained. An electric timer (Standard Electric Time Co., Type S-10) calibrated in 0.1-second intervals is incorporated; it measures the time of current flow through the generating circuit. This instrument was designed for coulometric titrations of 0.5 to 20 peq. of substances. In practice, a portion of the substance being determined is first titrated automatically to the end point; the sample to be analyzed is then added and automatically titrated back to the end point. The same constant generating current is used in each of these titrations so that any errors caused of over- or undertitration are cancelled. At constant current, the time required to titrate the sample back to the end point indicates 1
Present addiess, U. S. Army.
PROCEDURE AND RESULTS
Before automatic titrations can be made with the Stelzner titrator, the end point or cutoff potential must be established. This was done by coulometrically titrating a known quantity of the substance to be determined and plotting the indicated potential of the solution us. current generation time on rectilinear graph paper. The cutoff potential was then taken as that potential at which the rate of change in potential per unit time was greatest. In the present work, the cutoff potential was taken as 0.880 volt us. the S. C. E. for titrations of cerium(IV), whereas 0.180 volt us. the S. C. E. was used as the cutoff potential for titrations of chromium(V1). Although this cutoff potential for chromium(V1) differs from the indicated calculated equivalence point of 0.68 volt by approximately 0.5 volt, 0.18 volt was found to be the point of maximum potential inflection. The choice of the potential at which cutoff occurred was not very critical, because large (300 to 500 mv.) and rapid potential changes occurred at the end points of these titrations, and the electrolyte in which a sample was to be titrated was first brought to the cutoff potential before the sample was titrated. In order to make an automatic titration, the instruments were turned on and allowed to warm up for 10 minutes. During this time, the cutoff control was adjusted so that cutoff occurred at the desired potential, the generating current was adjusted to a value such that about 200 seconds would be required for titration of the sample, and the gas flow was adjusted to provide an adequate carbon dioxide blanket for the solution. Three milliliters of the uranium solution and 1 ml. of the sample were then placed in the weighing bottle together with a small glass-covered stirring
1750
1751
V O L U M E 27, NO. 1 1 , N O V E M B E R 1 9 5 5 bar. After the vessel was positioned so that contact was made between the solution and the various electrodes and after the stirring motor was adjusted to give rapid mixing, the generating current was turned on and the titration was allowed to proceed automatically to cutoff. During this preliminary titration, the generating current was adjusted to the exact value desired as indicated by the measured internal-resistance drop across a standard 100-ohm resistor. When the flow of generating current stopped, a 1-ml. portion of the sample was accurately pipetted in the solution in the cell. The clock was then set at zero and the electrolysis was begun again and allowed to continue automatically until cutoff was again attained. The time indicated on the clock and the known generating current were then used to calculate via Faraday's law the amount of suhstance titrated as follow: Equivalents electrolyzed
=
Z(amp.) X t(sec.) 96,493
Table 11. Results of Titrations of Ceric Sulfate
Micrograms of chromium(1-I) = 0.1796 X I(ma.) X t(sec.) Milligrams of cerium(1V) = 1.452 X Z(ma.) X t(sec.)
A second 1-ml. portion of the sample could then he added to the solution, and the titration of it performed as above. Consecutive titration of more than three 1-ml. portions of the sample in one pretitrated uranyl sulfate solution was not satisfactory because the total solution volume became too large for adequate mixing. The electrodes and vessel were rinsed thoroughly n-ith distilled water between each series of titrations, and the electrodes 11 ere left immersed in distilled water when they were not in use. The results of titrations of known quantities of chroniium(V1) and of cerium(1V) by the above procedure are given in Tables I and 11.
Table 1. Results of Titrations of Potassium Dichromate (Three I-ml. aliquots of four solutions containing varying amounts of potassium dichromate stock solution were titrated consecutively in 0 . 2 N uranyl sulfate. Total volume was 5 t o 7 ml. Cutoff potential was 0.180 volt u s . the E.C.E.) Relative M a r . Dev. ti^^ ChromiumiVI), Y yo of Std. Dev. from AT. .1hCurrent, Found quote Ma. Taben (av.1 Y % Y %b 12 7.000 260.1 2131.1 0 48 0.18 -0.9 -0.3 +0.8 +0.3 I? 5.000 173.4 173.6 0.64 0.37 -1.4 -0.8 +0.7 ~ 0 . 4 -0.4 -0.6 ~~
The data were analyzed statistically to determine whether a bias existed in the method. The bias B wa~! calculated bj- the formula B
=
uranium concentration resulted in somewhat smaller potential changes at the end point, but the cutoff potential remained essentially constant. The titration was therefore not adversely affected. The free acid concentration in the uranyl sulfate solution may vary from 0.2 to l.OdV, the optimum being 0.5.V. Hydrolysis sometimes occurred at acidities less than 0 . 2 N , whereas acidities greater than 1 . O S resulted in a slight loss in precision. Therefore, the acidity of the sample should be such that no large change in acidity occurs upon the addition of the sample to the pretitrated uranyl sulfate solution.
E
=IC
1S
(Three I-ml. aliquots of four solutions containing varying amounts of ceric sulfate stock solution were titrated consecutively in 0 . 2 N uranyl sulfate. Total volume was 5 to 7 ml. Cutoff potential was 0.880 volt us. the S.C.E.) M a x . Dev. N ~ of. ti^^ Cerium(VI), Mg. Relative from -41,. 91iCurrent, Found S t d . X Mg. 70 hfg. Go quots Ma. Taken (A>.) 12 7.500 2.277 2.276 0,OOE. 0 . 2 7 -0.011 - 0 . 5
10
5.000
1.423
1.421
0.005. 0 . 3 7
11
2.500
0.712
0.716
O.OO?, 0 . 4 6
15
0.500
0.142
0.146
0.002
1.42
+0.008 -0.008 +0.009 -0,006
-4-0.005 -0.004 +0.002
+0.4
-0.6 +0.6 -0.8 10.7 -2.5 +1.6
Some work on the determination of iron(II1) was done. Because of the sluggishness of the reaction between iron(II1) and uranium(IT), it was necessary to carry out this titration at an elevated temperature. .4t 60" C. the speed of the reaction was increased to the extent that titrations could be made, but only with poor precision. T1'ith the setup described herein, elevation of the temperature above 60' C. caused bubble formation and volume losses by evaporation. This titration should prove feasible if a setup of larger scale than that of the present one could be used; it would then be possible to use 1argl:r volumes of solution, and the temperature could be elevated to 80" to 90' C. without difficulty. iilthough the work reported here was done entirely with apparatus designed for the titration of small quantities of eubstances in small volumes of solution, there is no apparent reason why this method could not be used on a larger scale. There is evidence that some uranium(TI1) is formed during the electrolysis-e,g., upon titration past the end point, the potential of the solution falls to a much lower value than would be expected if only uranium(1T) were generated. A uranous sulfate solution was prepared by passing the uranyl sulfate solution through a Jones reductor and then aerating it to give a uraniuni(IT) solution. When this solution was electrolyzed the potential dropped to -0.150 volt vs. the S. C. E., indicating that some uranium(II1) was produced. Further study of the reduction of uranium(1V) is warranted. ACKNOWLEDGMEXT 3
T
= =
I'
The authors are indebted to H. H. Willard for his helpful suggestions and to R. IT'. Stelzner and others Tho supplied the coulometer.
av. value known value
tS -
\/AT= relative qtandard error Because E ( = -0.3%) was less than
LITERATURE CITED tS
d% ( = +0.5%) no bias
a-as considered to exist hetween the amount of chromium and
cerium found and the known amount that was titrated. DISCUSSION
The concentration of uraniuni in the uranyl sulfate supporting electrolyte solution was varied from 0.1 to 1.O.V. An increase in
T.5.. ANAL. C m x , 26, 1025 (1954). ( 2 ) Carson, R7. S . ,Jr., Ibid., 25, 406 (1953). (3) Furman, i i.H., B r i c k e r , C. E., and Dills, R. \*., Itid.,25, 4S2 (1953). (4) Furman, K. H., C o o k e , IT. D., and Reilley. C. N., I b i d . , 23, 945 (1951). (5) S t e l r n e r , R. IT7., Fisher, D. J., and Kelley, 11. T.. u n p u b l i s h e d (1) Belcher, R.. Gibbons, D., and West,
report.
RECEIVED for review April 21, 1955. -4ccepted July 29, 1965.
