654
ANALYTICAL CHEMISTRY
Table I. Calcium Saccharate us. a Synthetic Mixture of Magnesium Chloride and Calcium N i t r a t e hIg Taken Gram 0.0261 0.0232 0 0232 0.0237 0.0237
Calcium Saccharate Used M 1. 7.60 6.80 6.86 6.96 6.96
Mg Recovered Gram 0,0260 0.0232 0.0234 0,0238 0.0238
Difference Gram 0.0001 0.0000 0.0002 0.0001 0.0001
Ca Added Gram 0.0200 0,0200 0.0200 0.0200 0.0200
In order to see if this procedure would be effective in the presence of calcium, calcium nitrate was added to the magnesium chloride solution. As long as the amount of calcium WBS less than the amount of magnesium, it had no apparent effect on the reaction. Typical titration data are listed in Table I for a series of determinations which contained the maximum amount of calcium allowable. The results in this table were obtained by calculation rather than graphically. By selecting the linear values of resistance and titrant before and after the equivalence point, respectively, and substituting them in the equation for a straight line, a pair of equations may be obtained that when solved simultaneously will give the equivalence point. A complete description of the calculation has been given in a previous publication ( I ) . DISCUSSION
Less than 1% error is obtained by this method of determining magnesium and it is not necessary to separate calcium from the magnesium if the amount of calcium is less than the amount of magnesium. The titration can be made in 1 hour and the calculations can be completed in 10 minutes. CONCLUSION
A method for the volumetric determination of magnesium is described, the accuracy of which varies from 0.4 to 0.8 mg. of magnesium per 100 mg. of magnesium taken. The method is fairly rapid and can be carried out in the presence of limited amounts of calcium. Figure 1. Calcium Saccharate us. Magnes i u m Solution Containing Calcium Ions beaker and diluting with water to make approximately 200 ml. of solution. The beaker was placed in a constant temperature bath and the calcium saccharate was added in about 0.5-ml. portions from a microburet. Because the magnesium saccharate complex is somewhat soluble, 1 ml. of titrant was added and the solution was stirred constantly for 10 minutes before the fist resistance reading was taken. All subsequent readings were made when the resistance became constant or a t 10-minute intervals after each addition.
LITERATURE CITED
(1) Corwin and Moyer, IND.ENG. CHEM.,ANAL. ED., 18, 302 (1946). (2) Shead and Heinrich, Ibid., 2, 388 (1930). (3) Shead and Valla, Ibid., 4,246 (1932). (4) Tananaev and Lovi, J. A p p l i e d Chem. (U.S.S.R.), 10, 1112 (1937).
RECEIVED November 28, 1949. Work done under Grant-in-Aid from t h e Frederick Gardner Cottrell Fund, Research Corporation, New York, N. Y.
Application of the lead Reductor to Determination of Uranium WILLIAM D. COOKE', F R E D HAZEL, A N D WALLACE M. MCNABB University of Pennsylvania, Philudelphia, Pa.
A
NUMBER of metallic reducing agents have been proposed for the determination of uranium. Reduction of uranium (VI) in a Jones reductor gives a mixture of uranium(1V) and uranium(II1). The solution is aerated to oxidize uranium(II1) to uranium(1V). Birnbaum and Edmonds (1)used asilver reductor for reduction to the quadrivalent state. This procedure necessitated a high temperature, 60" to 90" C., and controlled acidity, 4 N hydrochloric acid. However, at least in small amounts, the extent of reduction depends upon the temperature and the rate of passage through the column ( 6 ) . Someya ( 2 ) found that uniform reduction of uranium could be carried out in strong acid solutions by means of liquid lead and bismuth amalgams. Koblic (4)reduced uranium quantitatively to the quadr,ivalent state with lead. However, his procedure is inconvenient because the solutions have to be boiled in an inert atmosphere of carbon dioxide in the presence of hydrochloric acid for a t least 0.5 hour. 1 Present address. Frick Chemical Laboratory, Princeton University, Princeton, N. J.
Treadwell (II) used a lead reductor instead of a Jones reductor for various metals but did not include uranium. The use of the lead reductor probably has been limited because of the difficulty of applying it to sulfuric acid solutions. When sulfuric acid solutions are reduced with lead, an adherent film of lead sulfate is formed which soon decreases the efficiency of the reducing agent. The formation of the lead sulfate film can b e prevented by the presence of hydrochloric acid. If the concentration of hydrochloric acid in the solution is greater than 2.5 N , no lead sulfate is formed even after continued use. The successful application of the lead reductor to the determination of uranium is described below. Solutions of uranium(V1) were reduced to uranium(1V) i n both hydrochloric and sulfuric acid solutions. The acidity was not critical and could be varied within wide limits. However, when sulfate ions were present, it was found necessary to add sufficient hydrochloric acid to prevent the formation of lead sulfate. The reduced uranium was caught in a solution of ferric sulfate.
V O L U M E 22, NO. 5, M A Y 1 9 5 0
655
Uranium can be reduced quantitatively to the quadrivalent state with lead. High concentrations of sulfuric acid can be tolerated, provided a sufficient concentration of hydrochloric acid is present. Reduction is rapid and amalgamation of the metal is unnecessary. The uranium(1V) is determined indirectly by the addition of an excess of hon(II1) and subsequent titration of iron(I1) with standard dichromate solution.
The iron(I1) formed during the oxidation of the uranium to uranium(V1) by iron(II1) was titrated with dichromate according to the method of Kolthoff and Sandell (6). Any iron present in the uranium solution will be reduced and interfere with the procedure described. Nessle (7‘) has proposed a method for determining uranium(1V) in the presence of iron(I1). This consists of titrating the solution potentiometrically with standard ferric sulfate solution at an elevated temperature in an inert atmosphere. PREPARATION AND STORAGE OF REDUCTOR
The reductor consisted of a column of reagent grade granulated lead 25 cm. long and 2 cm. in diameter. When the reductor was stored overnight it was covered with a solution of 10% hydrochloric acid containing about 0.1% ferric ion. Unless this small amount of ferric ion was added during storage, the firSt determination was a few tenths per cent too low. Before use the column was washed with six 25-ml. portions of 1 to 15 hydrochloric acid. PROCEDURE
About 50 ml. of solution containing 40 to 200 mg. of uranium and 0 t o 9 N in sulfuric acid concentration were made 3 N in respect to hydrochloric acid. The solution was poured through the reductor and caught in 10 ml. of 5y0 ferric sulfate solution. The reductor was washed with five or six 25-ml. portions of 1 t o 15 hydrochloric acid, 10 ml. of 85% hospharic acid were added, and the titration was carried out wit8 0.05 N potassium dichromate. Diphenylamine sulfonate indicator (0.5 ml. of 0.3% solution) was added a few milliliters before the end point. A correction of 0.10 ml. of 0.05 N potassium dichloromate was subtracted as the indicator blank. [Since the completion of this work, a new indicator, 5,6-dimethyl-l,lO-phenrtnthroline,has been proposed for the ferrous-dichromate titration (9). The small correction and the resistance to oxidation of this indicator would be of value in this titration. I
not evolve hydrogen from acids, amalgamation is unnecessary, and the reduction proceeds with theoretical efficiency. Complications caused by the evolution of hydrogen are not encountered (8). I n using a Jones reductor, air must be excluded from the column t o prevent low results. With the lead reductor such precautions are not necessary and solutions can be run through an air-filled column without introducing an error. Ammonium ion, which must be removed prior to treatment with amalgamated reductors, does not interfere. Solutions containing acetate ions can be reduced with lead, so that the indirect titration of sodium can be accomplished. Nickel does not interfere but, as in the case of the Jones reductor, nitrate ion must be removed. The breadth of the method is indicated by the results given in Table 11, where values obtained under different conditions are compared with the calculated value. Table I.
Determination of Uranium
Uranium Taken Gam
Vol . Calculated
Vol. Found
M1.
MZ.
M1.
%
0.2128
33.52
33.45 33.46 33.49 33.48 33.53
-0.07 -0.06 -0.03 -0.04 $0.01
0.2 0.2 0.1 0.1 0.0
0.1425
23.48
16.76
4-0.06 -0.02 -0.05 0.00 0.00 -0.01 +n 03 -0.03 -0.03 -0.03
0.2 0.1 0.2 0.0 0.0
0.1017
23.54 23.46 23.43 23.48 23.48 16.75 16 79 16.73 16.73 16.73 6.74 6.71 6.72 6.74 6.74
$0.03 0.00 f0.01 +0.03 +0.03
0.5 0.0 0.1 0.5 0.5
0.04072
6.71
RATE OF REDUCTION
The rate of reduction of uranium(V1) to uranium(1V) was found to be high. Solutions containing 0.1 gram of u r a d u m were completely reduced when passed through the reductor at the rate of 175 ml. per minute. Higher rates of flow were not attempted because they could not be measured conveniently. T o find whether prolonged use would decrease the,rate of reduction, 50 samples (approximately 0.1 gram of uranium each) were determined and the above procedure was repeated. Reduction was complete at flow rates as high as 175 ml. per minute. ACCURACY
A solution of uranyl sulfate was standardized gravimetrically according t o the method of Hillebrand and Lundell (3)as modfied by Someya (IO). The uranium was precipitated from carbonate-free solutions as ammonium uranate, ignited t o the oxide, and weighed. Ten-milliliter Sam les of the solution yielded 0.2408, 0.2410, and 0.2408 gram of)U~O8. Aliquot portions of this solution were analyzed by the proposed method. Limits were set a t 0.04 to 0.2 gram. The higher limit was chosen arbitrarily at 0.2 gram, assuming that this would be as high a concentration as would be encountered ordinarily. The lower limit, 0.04 gram, was chosen because it corresponds, roughly to the beginning of the semimicro range. The results are shown in Table I. DISCUSSION
T h e use of metallic lead aa a reducing agent compares favorably with amalgamated zinc. Test lead can be obtained with a high degree of purity and a blank is not necessary. Because lead does
Table 11.
Error
2,:
“.L
0.2 0.2 0.2
Effect of Conditions
(Calculated value, 16.76 ml.) Solution M1. Required 3 N HC1 16.75 3 N HCL 3 N H1604 16.79 3 N HCI 9 N HzSOd .16.82 Air-fdled reductor 16.75 16.75 5 7 ammonium ion 5% acetate ion 16.75
++
LITERATURE CITED (1) Birnbaum, Nathan, and Edmonds, S. M., IND.ENG. CHEM., ANAL.ED., 12, 155 (1940). (2) Hillebrand, W. F., and LundeI1, G. E. F., “Applied,Inorganic Analysis,” p. 103, New York, John Wiley & Sons, 1929. (3) Ibid., p. 368. (4) Koblic, O.,Chem. Listy., 19, 1 (1925). (5) Kolthoff, I. M., and Sandell, E. B., “Textbook of Quantitative Inorganic Analysis,” p. 609, New York, John Wiley & Sons, 1943. (6) MacInnes, D. A., and Longsworth, L. G., U. S. Atomic Energy Commission, Document MDDC-910 (1942). (7) Nessle, G. J., et al., U. S. Atomic Energy Commission, Document MDDC-1123 (1947). (8) Nilakantan, P., and Jayaraman, N., IND.ENG. CHEM..ANAL. ED.,11, 339 (1939). (9) Smith, G. F., and Brandt, W. W., ANAL.CHEM.,21, 948 (1949). (10) Someya. Kinichi, 2. anorg. Chem., 152, 378 (1926). (11) Treadwell, W. D., HeZu. C h i n . Acla., 5, 806 (1922).
RECEIYED November
21, 1949.
