graphic techniques. To carry out these studies, a voltage sweep of 4 seconds or less will be substituted for the motordriven potentiometer in the present circuit. For extremely precise measurements of half-wave potentials, it would be possible to modify the X-axis to record a few tenths Of a instead Of volt as in the instrument described. The
cost of the polarograph, excluding the recorder and labor, was $103. LITERATURE CITED
(1) Arthur, P., Lewis, P. A,, Lloyd, N. A,, ANAL.CHEM.26, 1853 (1954). (2) Kolthoff, I. M., Lingane, J. J., “Polarography,” 2nd ed., p. 504,
Interscience, New York, 1952. (3) Lee, T. S., J. Am. Chem. SOC.74, 5001 (1952).
(4) Meites, L.,
“Polarographic Techniques,” Interscience, New York, 1955. (5) Pecsok, R. L., Farmer, R. W., ANAL. CHEM.28. 985 (1956). . ,
RECEIVEDfor review April 8, 1957. Accepted November 22, 1957. Division of Analytical Chemistry, 132nd meeting, ACS, New York, N. Y., September 1957. Work eu ported by Research Corp. Grant-in-lid.
Polarographic Determination of Tin in Zirconium AIIoys JOHN T. PORTER 11’ Knolls Atomic Power laboratory, General Elecfric Co., Schenectady,
b A method has been developed for determining tin in zirconium alloys polarographically. The procedure requires no separations from zirconium or the normally occurring alloy constituents.
T
application of zirconium to nuclear reactors has given rise to the Zircaloys, which contain tin, iron, nickel, and chromium in various combinations (6). Bryson and others ( 1 ) have described a titration procedure for tin, when greater than 0.5%. Because the general range of interest has been extended to about 0.25%, a more sensitive method was desired. In developing the polarographic method described, a further aim was to avoid separations. Because hydrofluoric acid is generally used in dissolving zirconium materials in amounts that are deleterious to the dropping mercury electrode, any excess must be removed prior to running the polarogram. This may be done by fuming with sulfuric acid, but the procedure is simplified if the use of hydrofluoric acid is avoided or if the fluoride is complexed prior t o running the polarogram. HE
EQUIPMENT AND REAGENTS
Equipment. A Sargent-Heyrovskf polarograph was modified t o fit a Brown recorder and a Leeds & Northrup Electrochemograph. An H-type cell was used with a saturated calomel electrode (S.C.E.) reference isolated by a sintered-disk agar bridge. Half-wave potential measurements were made with a large saturated calomel electrode and a Rubicon potentiometer. Reagents. Reagent grade chemicals were used throughout. The zirconium metal used in preparing synthetic samples was crystal bar zirco1 Present address, Research and Development Division, Corning Glass Works, Corning, N. Y .
484
ANALYTICAL CHEMISTRY
N. Y.
nium. Spectrographic analysis of this material shoiTed no significant impurities with the possible exception of iron (850 p.p.m.). PROCEDURE
Reduction by Solution Under Nitrogen. Weigh a sample of about 0.25 gram in a 25-m1. volumetric flask. Add 5 ml. of water, 3 ml. of hydrochloric acid, 1 ml. of sulfuric acid, and 1 ml. of fluoboric acid (48 to 50%). Place on a steam bath or hot plate, directing a stream of nitrogen into the flask. If evaporation requires the addition of further water, use deaerated water. In the course of solution a white crystalline solid may appear, which will usually go back into solution on dilution but will not interfere in any case. When solution is complete, cool (continuing the nitrogen stream), add 1 ml. of 0.125% peptone as a maximum suppressor, and dilute to volume with deaerated water. Transfer to the polarographic cell. Because the solution has been heated in preparation, this should be a thermostated cell. The solution may be conveniently stirred with nitrogen to speed thermal equilibration, but the purge is not required to remove oxygen, as the solution is thoroughly deaerated. Record the polarogram from -0.2 to -0.7 volt ZJS. S.C.E. In the work reported, the residual current correction was by baseline extrapolation. Reduction by Powdered Iron. Weigh a sample of about 0.25 gram into a 50-ml. flask. Add 5 ml. of water, 1 ml. of sulfuric acid, and 1 ml. of fluoboric acid (48 to 5Oye). Place on a steam bath or hot plate. Add 3 ml. of hydrochloric acid and continue heating to complete solution of the tin. If evaporation has reduced the volume substantially, add more water. Cool and add 0.5 gram of iron powder. Place a stream of nitrogen on the surface of the solution. When the reaction subsides, heat gently and finally boil to complete solution of the iron. Cool to room temperature, transfer to a 25-ml. volumetric flask containing
1 nil. of 0.125% peptone as a maximum suppressor, and dilute to volume with deaerated water. Continue as in nitrogen reduction. DISCUSSION
The use of hydrofluoric acid was avoided by dissolving the samples with a mixture of sulfuric, fluoboric, and hydrochloric acids. Despite prolonged use in this medium, the capillary behavior remains constant, as indicated by the diffusion current of the cadmium solution; dissolution was more moderate than with hydrofluoric acid. The total amount of fluoboric acid can be added initially. The presence of iron, which would interfere if present as ferric ion, combined with the more desirable behavior of stannous tin indicates that the analysis should be carried out with the metals present in the lower oxidation state. In the developmental work, the solution was reduced with iron powder and protected with a nitrogen atmosphere prior to polarographic analysis. The same result is obtained if solution of the sample is carried out under a stream of nitrogen. Under the conditions of measurement, stannous tin gives a well defined ware with a half-wave potential of -0.48 volt us. S.C.E. The average value of the reciprocal of the slope of the line obtained by plotting log ( i ) / ( i d i) z’s. potential for five measurements was 0.031, with 90% confidence limits of &0.002. The theoretical value for a reversible two-electron reduction is 0.030. In order to determine the lower limit of the procedure and whether the current was proportional to the tin concentration, synthetic solutions were made by adding tin to the zirconium solution prior to reduction with iron powder. The percentages in Table I are based on the amount of tin added
and refer to a 1-gram sample. The last two values were determined in final volumes of 50 and 25 ml., rather than in 100 ml. as were the remaining determinations. There is a possible discrepancy in the current values, but as the author no longer has access to the equipment or his records, it cannot be checked. I n application, the method is used with an external standardization procedure ( 3 ) . More recent data on different equipment give Ks,,/Kcd = 1.07, where K's refer to expressions of the form id = K x concentration (moles per liter). The standard deviation of the quantity idper mg. per ml., based on the first ten measurements, is 0.50. The results of the determination of three samples of Zircaloy 2 are presented in Table 11. The tin content for alloy A had been determined chemically as 1.&yo. I n the course of analyzing a large set of Zircaloy 3 samples, duplicate determinations were carried out on 12 samples, six having tin contents of about 0.250j, and six containing o.5y0 tin. The standard deviation computed from this data was 0.008%. Comparison of this value with those for Zircaloy 2 (Table 11) indicates that in this range the relative variation is more nearly constant than the absolute. To test the validity of eliminating the iron powder reduction by dissolving the sampleunder astream of nitrogen, tinwas determined in a Zircaloy 2 sample using both procedures. Because the concentration of zirconium in the final solution represents a significant contribution to the composition of the supporting medium, there was a possibility that sample size might affect the diffusion current constant through an effect on the
Table 1. Polarographic Results on Synthetic Solutions of Tin in Zirconium
Tin Tin in Added, Mg. id" idlMg.lM1.a 1.43 22.0 6.50 0.65 1.48 22.8 6.50 0.65 2.97 22.8 13.00 1.30 19.50 1.95 4.63 23.8 1.50 23.1 6.50 0.65 3.07 23.6 13.00 1.30 4.53 23.2 1.95 19.50 23.1 0.13 0.300 1.30 0.610 23.5 2.60 0.26 0.300 23.1 0.650 0.06 0.26 20 0.325 0.03 Current units arbitrary.
Table
111. Analyses of a Zircaloy under Varying Conditions
Sample Size, Gram 0.254 0.246 0.242 0.240 Soln. under nitrogen 0.247 0.284 0.240 0.240
Method of Reduction Iron powder
0.108
0.109 0.102
Tin Found,
%
1.46 1.43 1.43 1.49 1.45 1.47 1.48 1.44 1.44 1.41 1.48
0
Table 11.
Alloy A 1.44 1.46 1.52 1.50 1.50 Av.
1.48
Analyses of Zircaloys (% tin)
Alloy B 1.39 1.44 1.42 1.47 1.44 1.44 1.43
Alloy C 1.56 1.56 1.53 1.58 1.60 1.63 1.58
supporting electrolyte. Therefore, this sample was also analyzed using different sample sizes. Table I11 indicates that the method can be carried out omitting the iron reduction without affecting the diffusion current constant. All determinations were made with the same calibration constant. The change in sample size had no significant effect. Determinations of the alloy in Table I11 were not affected by additions of chromium and nickel.
In applying Zircaloy t o reactor fuel element development, determination of tin in the presence of uranium may be required. Two samples of the alloy in Table I11 were determined in the presence of the equivalent of 7.5Y0uranium. Vranium was added as uranyl sulfate and reduction was carried out using iron powder. The results were 1.43 and 1.46% tin. Because samples containing high uranium concentrations will require nitric acid for complete solution, the use of the iron reduction procedure will be mandatory. LITERATURE CITED
(1) Bryson, T. C., Goward, G. W., Mc-
Cay, J. R., Perrine, A. W., Rogers, J. F., Wilson, B. B., TT-estinghouse Electric Corp., Atomic Power Div., Rept. WAPD GTA(GLA) - 172 (1956). (2) Nucleonics 14, KO. 2, 45 (1956). (3) Porter, J. T. 11, ANAL. CHERI.29, 1638 (1957).
-
RECEIVEDfor review June 26, 1957. Accepted December 26, 1957. Work carried out under contract W-31-109 Eng-52, U. S. Atomic Energy Commission.
Pola rogra phic Determination of S ma I I Amounts of Tin SILVE KALLMANN, ROBERT LIU, and HANS OBERTHIN Research Division, ledoux & Co., 359 Alfred Ave., Teaneck, N. I .
b Steam distillation with hydrobromic acid effectively separates tin from all accompanying elements, except antimony and arsenic. The subsequent polarographic determination of tin in a bromide-chloride medium is not affected by arsenic. Antimony has a slight repressing effect.
D
of small amounts of tin in complex materials frequently is difficult. Of the methods using organic reagents, the dithiol method (8, 9, I 4 is enjoying increasing ETERMINATION
popularity. Unfortunately, it requires prior removal of arsenic and antimony, and destruction of the bromide was more time-consuming and involved than had been anticipated. The silicomolybdate blue spectrophotometric method (6) has not always yielded consistent results. Allsopp and Damerell ( I ) suggested a polarographic procedure for determination of tin in steels, which is considerably faster than the method suggested by Scherrer ( I S ) , but requires prior collection of the tin as the sulfide and is not directly applicable to other materials.
Lingane (11) reported the reduction of stannic ions a t the dropping mercury electrode in a supporting electrolyte of 1N hydrochloric acid, resulting in a wave a t -0.47 volt us. the standard calomel electrode. He ( I d ) also reported a well-defined doublet wave potential of -0.25 and -0.52 volt os. the saturated calomel electrode in a supporting electrolyte of 1N hydrochloric acid and 4N ammonium chloride. According to Lingane, stannic tin also produces a well defined double wave in 4N ammonium bromide as supporting electrolyte. VOL. 30, NO. 4, APRIL 1958
485
