4th cd., p. 143, Semiconductor Products Department, General Electric Co., Liverpool, N.Y., 1959. (7) Grahame, D. G., Chem. Reus. 41, 441 (1947). (8) Heyrovsk$, J., Dzscussions Faraday SOC.KO. 1, 212 (1947). (9) Kelley, h l . T., Jones, H. C., Fisher, D. J., ANAL.CHEM.31, 1475 (1959). (10) IZolthoff, I. hI., Lingaiie, J. J.,
“Polarography,” 2nd ed., p. 52, Interscience, h’ew York, 1952. i l l ) Randles. J. E. B.. Trans. Faradav Soc. 44.327 f 1948). ’ (12) Ross, J. iV., DeMars, R. D., Shain, I., ANAL.CHEM.28, 1768 (1956). (13) Sevcik, A,, Collection Czechoslw. Chem. Cmms. 13, 349 (1948). (14) Streiili, C. A., Cooke, W. D., ANAL. CHEM.25, 1691 (1943); J . Phys. Chem. 57, 824 (1953). ~
(15) Vielstich, W., Delahay, P., J . A m . Ckem. Soc. 79, 1874 (1957). (16) Willard, H. H., Furman, N. H., Bricker, C. E., “Elements of Quantitative Analysis,” 4th ed., p. 69, Van Nostrand, New York, 1956. RECEIVED for review December 7, 1960. Accepted June 15, 1961. Division of Analytical Chemistry, 140th Meeting, ACS, Chicago, Ill., September 1961.
Determination of Uranium in the Presence of Molybdenum by Controlled-Potential Coulometric Titration H. E. ZITTEL, LOUISE 6. DUNLAP, and P. F. THOMASON Analytical Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tenn.
b A coulometric method for the determination of uranium(V1) in the presence of molybdenum has been developed. Uranium(V1) is titrated coulometrically in a solution of sodium tripolyphosphate and sodium sulfate (pH 7.5 to 9.5)a t a potential of - 1.40 volts vs. the S.C.E. Molybdenum(V1) does not interfere when the molybdenum-to-uranium weight ratio does not exceed 1:7. In the range from 1 to 5 mg. of uranium titrated, the error of the method is about * l % . Various possible interferences were studied; most of the interferences can b e eliminated easily. The method is relatively simple and rapid.
T
production of reactor fuels composed of uranium and various amounts of molybdenum has created the need for a method of determining uranium in such fuels. The method must be applicable to highly radioactive materials and must be more accurate than the conventional polarographic and fluorometric methods for uranium in molybdenum-bearing uranium fuels. Following the work of Harris and Kolthoff (5) and of Susic, Gal, and Cuker (I%’), Propst (8) developed a polayographic method for uranium in which the molybdenum intcrference encountered in acid media was prevented by use of a supporting electrolyte of p H 6 that was 0.5M in potassium phthalate and 0.0221 in potassium ascorbate. Propst ( 8 ) reported the precision of the method to be 2.5y0,. Pfibil and Blazek (7) w r e able to estimate uranium in an alkaline carbonate medium. Although the controlled-potential coulometric procedures for uranium in acid media that are discussed by Shults and Thomason (IO) and by Farrar, Thomason, and Kelley (1) are not applicable t o the HE
determination of uranium in the presence of molybdenum, the general method is useful for the analysis of highly radioactive materials and is much more precise than the polarographic method. The coulometric method described herein is precise and eliminates the molybdenum interference by the use of a complexing alkaline supporting electrolyte. The method is based on the facts that sodium tripolyphosphate in an alkaline medium complexes uranium (VI) sufficiently t o prevent its precipitation and that, in such a medium, molybdenum(V1) is not electroactive. Uranium(V1) is coulometrically reduced in this medium at a potential of -1.40 volts us. the S.C.E.The method is accurate to within =t0.5yoat the 5mg. level of uranium. Molybdenum (VI) does not interfere when i t is present in a weight ratio to uranium of not more than 1:7. The relative standard deviation is about 0.5% in the range from 1 t o 5 mg. of uranium. Trace amounts of iron(III), chromium(III), and nickel (11) do not interfere, whereas chromium(VI), copper (11))ruthenium(IV),
and nitrate do interfere. However, these interferences can be eliminated very simply prior to the coulometric titration. EXPERIMENTAL
Instrumentation and Apparatus. ORNL Model Q-2005 electronic controlled-potential coulometric titrator was used throughout this study (6). Potentiometer; Rubicon, 0- to 1.6volt range. pH Meter; Beckman Model G. Polarograph; ORNL Model Q-1673
($1
*
The titration cell assembly is shown in Figure 1. Reagents. Stock solution of suup o r t i n i electrolyte was prepared 6 y adding 40 ml. of 0.16M Na5P3010 solution t o 60 ml. of 0.1M NaZS04 solution and adjusting t h e resulting solution t o p H 10 by the addition of 1M KaOH. Commercial grade NasPsOla was obtained from hlonsanto Chemical CO. Standard stock solution of uranium (VI), approximately 25 mg./ml. was prepared by diqsolving iiational Bureau of Standards U308 in “Os. The nitrate mas destroyed by fuming with STIRRER
HELIUM---
BECKMAN CALOMEL ELECTQODE
D M E
TEFLON C A P 5 0 - m i BEAK W C O R TUBE
Hg 2 Figure 1.
K z S 0 4 -AGAR PLUG
\P+ SEALED IN CONTACT TUBE
Titration cell assembly VOL 33, NO. 1 1 , OCTOBER 1961
1491
H2S04. The solution was diluted to volume with 0.5M HzSO4. The final
ground current of 50 pa. Stop the titration and measure the readout voltage (Eo). Calculate the amount of uranium(V1) titrated by multiplying Eo by the coulometric factor for uranium. The derivation of this factor is given by Farrar, Thomason, and Kelley ( I ) .
solution was standardized volumetrically by potentiometric titration with ferric sulfate (4). Standard stock solution of molybdenum(VI), approximately 5 mg./ml. was prepared by dissolving reagent grade Na2Mo04.2H20 in distilled water. The solution was standardized by the colorimetric thiocyanate method (9). All other reagents were A.C.S. approved grade and, when necessary, were standardized by conventional methods. Procedure. Transfer approximately 10 ml. of the stock solution of Na5P3010 and Na2S04 to the titration cell and add the mercury that is to be used as the cathode. Deaerate the supporting medium for several minutes; then reduce (pretitrate) it a t - 1.40 volts us. the S.C.E. until the current decreases to 50 pa. Transfer a test aliquot of the sample solution to the titration cell. Check the pH of the resulting solution; if the p H is not in the range from 7 . 5 to 9.5, readjust it to bring i t within this range by adding 1JP NaOH. Stir and deaerate the solution for 15 minutes, prereduce the sample a t -070 volt us. the S.C.E.; then reduce the sample it - 1.40 volts DS. the S.C.E. to a back-
Table 1.
RESULTS AND DISCUSSION
An acid electrolyte is normally used for the coulometric titration of uranium(V1) (1, 10). The reduction potential of molybdenum(V1) in such an electrolyte is approximately the same as that of uraniuin(V1) ; therefore molybdenum must be remored before uranium(V1) can be determined electrometrically. Alkaline electrolytes have not been used extensively in the electrometric determination of uranium(V1) although they would seem to offer the advantage of eliminating interferences such as molybdenum. Frankenthal and Neuberg (3) observed the masking effect of sodium tripolyphosphate on various ions. Van n7azer and Campanella ( I S ) showed that the condensed phosphates as a class are excellent
Precision and Accuracy of Titration of Uranium(V1) in Na6P3OIo-NazSO4 Medium
Conditions, see Procedure section. pH of supporting medium, 8.5 - - I'ranium, Mg __ Molvbdenum Relative Taken, Found Number of Standard Zlg. rakm (av.) Titrations Deviation, 5 3 I; 2 37 4 i3 1 73
Yone 0 25 Yonr
0 i0
2 2 1 4
37 37 73 73
9
0 0 7 0
20
s
16
7 6 3 4
Table 11.
Effects of Trace impurities in Uranium-Molybdenum iueis on the Coulometric Titration of Uranium(V1) in a Na~PsOl,J-Na&O~Medium
('onciicioris
baint: 5s
for data of Tzble I
bLch value
IS
thi. result of
81
'w-t 1 trlnis
complexers. Subbaraman, Josh, and Gupta (11) studied the polarographic reduction of uranium(V1) in an aqueous solution of sodium tripolyphosphate and state that, in such a medium of p H 9, the uranium(V1) undergoes a oneelectron reduction at an El,* of about - 1 volt us. the S.C.E. In the prehminary stages of the investigation reported herein, various alkaline media were tested as supporting media for the coulometric titration of uranium(V1) in the presence of moll bdenum. They were all of the same t>pe, that is, an alkaline buffer medium that contained a complexing agent to keep the uranium in solution. The following were tried: sodium sulfate-h> drouylamine sulfate solution of p H adjusted to about 8 by a solution of sodium hydrouide, a potassium phthalate-ascorbie acid solution whose p H was adjusted to 6 to 9 with sodium hydroxide solution, and a series of sodium tripolyphosphate-sodium sulfate solutions of various pH values. All but the latter gave poor results. Initial work with pure solutions of XasP3010 as the supporting medium did not give satisfactory results due to the fact that the background currents were high. Sodium sulfate was added to increase the ionic strength of the solution. T h e n the sohuni sulfate concentration of the medium was greater than 0 I M , it mas very difficult to reduce the current to the background value ti e., 50 pa.) After some experimentation a solution composed of 4 nil. of 0 l f i l i Na,P3010And 6 ml of 0 1 M Y A , S O n~ w found to be the b e d solution F w corilometrtu purpose>, thls solution Adjusted t o the proprr D H I ~ i - i + i .I, iil the studic>s r f p r l t d 1'1 pip1 r Precision and Accuracy. ' n$> I + -tuclirs the prerwor1 tnd ,i t h e methoif ~ n n a ' ~nn , -11 i t i' ) r i t a i n t ~ ~+itht,, l Ira1)f
11
11'
I '
0.05 10 'I. 25 I)
1492
ANALYTICAL CHEMISTRY
1
s
d
~1,
irtii
3loc . Hot Irltrrr.
, i i i " + ~
't i t
(,>
2
u+5+ U+6 + u+4
This two-electron change, as expected is not in agreement n-ith the oneelectron change that was reported b y Subbaraman, Joshi, and Gupta (11) to occur in the polarographic reduction of uranium(V1) in a solution of sodium tripolyphosphate. The results of studies of the effect of p H on the coulometric titration of uranium(V1) show t h a t the p H of the supporting medium should be kept between 7 . 5 and 9.5. Effect of Trace Impurities. Since other trace elements would be introduced in t h e burnup of t h e uraniunimolybdenum fuel, for whose analysis this method was developed, jtudies \wre carried o u t t o determine t h e effect of some of these interferences. All interferences n-ere studied polarographically prior t o t h e coulometric study. T h e results are gi~7en in Tahle 11. The d a t a of Table I1 indicate that nickel(I1) does not interfere, and t h a t the interference of trace amounts of iron(II1) can be eliminated by selection of the proper pretitration voltage. If only a small amount of iron(II1) is present, interference from i t can be eliminated by a pretitration at -0.75 volt us. the S.C.E. The interference of chromium(VI), which is not obvious from the data of Table 11, is eliminated b y following the steps outlined in the Procedure section. Since chromium (VI) exhibits an Ellz of about -0.2 volt us. the S.C.E. in this electrolyte, the pretitration at -0.70 volt reduces the chromium(TI’1) t o chromium(III), which does not interfere. The mechanism of the interference of copper(I1) is complex; however, interference from copper(I1) can also be eliminated b y a proper pretitration
procedure. Copper gives two polarographic waves in the Na5P3010-Na2S04 medium. The first wave appears as a rounded maximum at about -0.2 volt vs. the S.C.E., whereas the second occurs at about -1.0 volt vs. the S.C.E. JVhen the sample n-as pretitrated at a voltage which corresponded to that of the maximum of the peak of the first copper wave, the second wave eventually disappeared, and the copper itself was titrated t o a background current; the copper interference vias thus eliminated. The potential of the maximum of the first copper n-ave depends on the p H and on the copper concentration. Therefore, in each case, the potential should be scanned from about -0.1 t o -0.5 volt until the potential at which the highest current occurs is determined; this potential is t h a t at which the sample should be pretitrated. The peculiar coulometric bphavior of copper in this medium is being studied further. Ruthenium(IV), \\hen polarographed alone, did not exhibit behavior t h a t n-ould be expected to cause interference n-ith the uranium(T’1) determination; however, when present with uranium (VI), i t caused the uranium wave to become very erratic. The coulometric study confirmed that ruthenium(1V) interferes Rith the coulometric titration of uranium(V1). By fuming the test portion with perchloric acid, the ruthenium n-as eliminated and thus was kept from interfering with the uranium(V1) determination. Nitrate in small concentration interferes; i t n-as eliminated b y fuming the test portion n i t h sulfuric acid prior t o the coulometric titration. As indicated b y the data given, the method is relatively precise and accurate and should be of use n-hen i t is desired t o determine uranium in the
Estimation of Ascorbic PotentiaI Coulometry
presence of limited amounts of molybdenum. Most of the impurities that might be introduced do not interfere or can be eliminated easily. The method has been used routinely with no difficulties on simulated uraniummolybdenum reactor fuels. LITERATURE C I T E D
(1) Farrar, L. G., Thomason, P. F., Kelley, hf. T., ANAL.CHEM. 30, 1511
(1956). (2) Fisher, D. J., “Polarograph, ORNL Model &-1673, High-Sensitivity, Diode Filter, Derivative, Recording,” Method Nos. 1 003042 and 9 003042 (2-13-57), ORiL’L Master Analytzcal Manual TID-7015,Sec. 1. (3) Frankenthal, L., Neuberg, C. N., Ezpt2. M e d . Surg. 1, 386 (1943). (4) Ginocchio, B. J., “Uranium, Automatic Potentiometric Ferric Sulfate Method,” Method Nos. 1 219224 and 9 00719224 (2-24-58), ORSL ilf‘aStW Analytzcal h’ununl TID-7015,See. 1. (5) Hairis, W. E., Kolthoff, I. &I., J . Am. Chem. SOC.69,446 (1947). (6) Kelley, RZ. T., Jones, H. C., Fisher, D. J., ANAL.CHEDI.31, 488, 956 (1959). ( 7 ) Pfibil, R., Blazek, A,, Collectzon Czechoslov. Chem. Communs. 16, 567 (1951). (8) Propst, R. C., “Polarographic Determination of Uranium in the Presence of Molybdenum,” DP-236 (Sept. 1957). (9) Sandell, E. B., “Colorimetric Determination of Traces of Metals,” 3rd ed., pp. 644-650, Interscience, New York, 1959. (10) Shults, W. D., Thomason, P. F., ASAL.CHEM.31, 492 (1959). (11) Subbaraman, P. R., Joshi, N. R., Gupta, J., Anal. Chim. Acta 20, 89
RECEIVEDfor review May 10, 1961. rlccepted July 17, 1961. Division of Analytical Chemistry, 140th Meeting, ACS, Chicago, Ill., September 1961. The Oak Ridge National Laboratory 1s operated by Union Carbide Carp. fox the U. S. Atomic Energy Commission.
Acid by Controlled
K. S. V. SANTHANAM and V. R. KRISHNAN Sri Venkateswara University Chemical laboratories, Tirupufi, South India Ascorbic acid has been estimated by coulometric oxidation at an optimum controlled anode potential of 1.090 volts vs. S.C.E. a t p H 6.03. Quantities of ascorbic acid ranging from 15 to 100 mg. were determined with an average error of A0.7 mg. The optimum potential decreases linearly with increase of pH. The presence of oxalic acid, added as an anti-
autoxidant, or of citric, succinic, or tartaric acids does not interfere with the estimation.
A
several methods have been proposed for the estimation of ascorbic acid, none of them yield very accurate results. The potentiometric method, among the electro.metric methods reported, does not yield accurate LTHOUGH
or even reproducible results (1, 4). The polarographic method yields results with a comparatively large error ( 2 ) . Reasonable accuracy has been claimed for a coulometric titration method (3). No attempt, however. has been made so far by earlier workers to estimate this acid by controlled potential coulometry involx-ing anodic oxidation. The results reported in VOL. 33, NO. 1 1 , O C T O B E R 1961
1493
