V O L U M E 26, NO. 7, J U L Y 1 9 5 4 ACKNOWLEDGMENT
Grateful acknowledgment is extended to the Research Corp. for a grant made in support of this work.
1119 (6) Patton, A. R.,and Foreman, E. hl., Science, 109,339 (1949). (6) Thiele, J., and Gunther, O., Ann., 347, 106 (1906). (7) Wiesner, K.,Chem. Zisty, 36, 313 (1942). (8) Zimmerrnan, W., 2. physiol. Chem., 189,4 (1930).
LITERATURE CITED (1) Furman, N. H., and Sorton, D. R., ANAL. CHEM.,26, 1111
(1953). ( 2 ) Jones, T. 8. G., Biochem. J . , 42, ix (1947). (3) Klein, G.,and Linser, H . , 2. physiol. Chem., 205, 261 (1932). (4) Martin, A. J. P., and Nittelmann, R., Biochem. J., 43, 353 (1948).
RECEIVED for review December 14, 1953. Accepted .4pril 8, 1954. Presented before t h e Analytical Chemistry Section of the X I I t h International Congress of Pure a n d Applied Chemistry, New York, N. Y., 1951. Taken in part from t h e thesis presented by Daniel R. Korton t o the Faculty of Princeton University in partial fulfillment of the requirement for the Ph.D. degree, J u n e 1948.
Polarographic Study of lead in a Potassium Thiocyanate Supporting Electrolyte JAMES 0. HIBBITS and STANCIL S. COOPER St. Louis University, St. Louis,
Mo.
Because the diffusion coefficient, hence the diffusion current, of an ion being reduced at the dropping mercury electrode is a function of the medium in which the reduction is taking place, certain fundamental data must be determined with a wide variety of supporting electrolytes in order to extend the usefulness of the polarographic method. The purpose of this investigation was to examine the polarographic behavior of lead in potassium thiocj-anate. The half-wave potential, reduction step, diffusion coefficient, and diffusion current constant of lead in this medium were determined. As the diffusion current was found to be proportional to concentration ( i ~ 0 . 6 7 ~ between ) 0.50 and 2.00 m M lead in 0.1Jrl thiocj-anate, the use of a thiocj-anate supporting electrolyte for the determination of lead should be of analytical interest. The solubility product constant of lead thiocyanate was determined hy amperometric titration of lead with potassium dichromate.
I
(4)that when a metallic ion is reduced a t the dropping mercury electrode, the diffusion current observed is proportional to the molar concentration of the ion, the characteristics of the capillary used, the number of electrons involved in the electrode reaction, and the square root of the diffusion coefficient of the reducible substance. Since the diffusion copfficient of the ion being reduced is dependent upon the medium in which the reduction is taking place, it is necessary that certain fundamental data be determined with a wide variety of supporting electrolytes in order to extend the usefulness of polarography as an analytical method. One of the first major steps toward supplying these data was taken by Lingane (a), who reported the behavior of a number of different ions in various supporting electrolytes. However, an extensive literature search disclosed that relatively little attention has been devoted to polarography in thiocyanate media. The present investigation was undertaken in order to extend the work begun by Lingane and supplemented by others in order that the polarographic technique moy he more widely applied to analytical problems. T WAS first demonstrated by IlkoviE
1 Present address, Carbide and Carbon Chemicals Co., Y-12, Oak Ridge, Tenn.
REAGENTS
The materials used in the experimental work were of analytical reagent grade. Stock solutions of lead were prepared from weighed amounts of dried lead nitrate. A few drops of nitric acid were added to prevent hydrolysis. The potassium dichromate solutions used for the amperometric titrations were prepared by dissolving a weighed amount of dried reagent. Potassium thiocyanate solutions were prepared by appropriate dilution of a stock solution, approximately 2 M , which had been standardized by the Volhard method (11). APPAR 4TU S
Data for the polarograms were obtained with a Fisher Elecdropode. The galvanometer and potential scale were calibrated according to the procedure given by Kolthoff and Lingane (6); the maximum sensitivity of the galvanometer was found to be 2.00 X 10-2 Fa. per division. The arrangement of the H-type cell used was similar to that shown by Kolthoff and Lingane ( 7 ) , in which a saturated calomel electrode served as the reference anode. The cell was placed in a water bath maintained a t a temperature of 25' f 0.1 O C. I n all cases, oxygen was eliminated by passing a stream of nitrogen through the cell for 10 to 15 minutes. Calibrated microburets were used in the amperometric titrations. All potentials presented are with respect to the saturated calomel electrode. EXPERIMENTAL RESULTS
Polarographic waves obtained with lead in potassium thiocyanate solution exhibited maxima when methyl red was used as a suppressor, but were well defined when 0.01% gelatin was used; consequently this concentration of gelatin was used in all cases. When solutions of lead nitrate, varying in concentration from 1 to 1 0 m X were prepared in 0.1M potassium thiocyanate, precip itation of lead thiocyanate occurred in those solutions in which the lead concentration was above 2mM. -2s there is disagreement in the literature ( 1 , 3, 10) regarding the solubility of lead thiocyanate, it seemed advisable to determine its solubility in order to find the maximum concentration of lead which may be determined polarographically, with potassium thiocyanate as the supporting electrolyte. K,, of Lead Thiocyanate. The solutions of lead nitrate and potassium thiocyanate listed in Table I were prepared, shaken for 38 hours (25' f 1' C.) to establish equilibrium, and filtered, and the diffusion currents of the filtrates were determined a t -1.0
ANALYTICAL CHEMISTRY
1120
A diffusion current constant mag be calculated using either the equation given by Lingane (8) or that of Lingane and Loveridge (9) which corrects for the Initial Lead Initial KCKS i d at -1.0 Volt Lead Concn. curvature of the electrode surface. The Concn., Concn., u s . SCE, K1Ci-207 Detd., id/Pb Concn., n.Mb AMb pa. (0.025M), M1. m M pa./niM two equations yield values for this con1.285 2.570 8.88 5 0.10 22.80 stant of 4.08 and 3.63, respectively. 10 0.10 28.88 1,647 3.294 8.77 0.538 I . 076 8.89 10 0 25 9.56 A diffusion cocfficient for lead can be 10 0.50 5.41 0.304 0.608 8.90 calculated from the IlkoviE equation ( 4 ) 0.237 0,474 8.85 10 0.75 4.20 to be 10.1 x 10-6 cm.2 sec-1, which 0 25O f 0 . l o C. b Initial lead and thiocyanate on concentrations are concentrations which would have existed had agrees well n i t h the value 9.8 X no precipitation occurred. 10-6 em.2 set.-', obtained from equivalent conductance data ( 5 ) , the 1attc.r value calculated for the condition of infinite dilution. The number of electrons involved in the rlrcvolt. Bmperometric titrations ( 5 ) were then performed with trode reaction (assumed t o be two in the calculation of the 0.025M potassium dichromate a t -1.0 volt on 25-ml. aliquots, diffusion coefficient) was determined by plotting Ed vs. log with the results given in Table I. z/(& - i). Values of Ed , the applied e.m.f, were corrected for Solubility products obtained from the data in Table I are the cell resistance, determined as 1300 ohms (the rcsistancxe tabulated in column 3 of Table 11. The solubility product conof the shunted galvanometer was l o x and considered neglistants listed in the last column were calculated using activity gible). The slope of the resulting straight line, equivalent coefficients obtained from the Debye-Huckel “extended equato -0.059/n volts, gave a value of 2.00 for n, and the point a t tion” ( 3 ) . which the log term became zero. led to a value of -0.385 volt for the half-wave potential. This value of the half-wave potential is similar t o that for lead Table 11. Solubility Product Constant Data for Lead in a noncomplexing electrolyte such as nitrate. Since the halfThiocyanate“ wave potential was unaffected by a n increase or decrease in thioKSP,(E) cyanate concentration, the reduction of lead in this medium is Solution Ionic Strength K,, ( X 105) ( X 108) b considered to be that of a “simple metal ion.” 5 m M Pb in Table I. Amperometric Titrations of Lead in Potassium Thiocyanate Solution (Containing 0.01% Gelatin) with 0.025M Potassium Dichromate at -1.0 Volt vs. SCE“
~
0.1M K C S S lOnM Pb in 0.1M KCNS 1OmM Pb in 0.2511.1KCNS 1 0 m M Pb in O.50M KCNS l O m M Pb in 0 . 7 5 M KCNS
0
b
0.1077
2.33
3.80
0,1099
2.47
3.92
0.2532
5,81
4.76
0.5018
14.06
6 46
0.7514
25.35
7.20
c. Calculated from Debye-HUckel extended equation.
250 i=
10
The values obtained for the solubility product constant,
K w ( E ) ,are in good agreement with each other (particularly in solutions having a low ionic strength), considering the method employed in their determination. The calculation of activity coefficients in this ionic strength region is only an approximation. Masaki (10) has published experimental values for the activity coefficients of lead and thiocyanate, obtained from POtentiometric measurements, which lead t o a K,, of 10.3 X 10-8. Bottger ( I ) , in his discussion, gives a solubility value for this salt on the basis of conductance measurements which is of the same order of magnitude as Masaki’s value and the values of Kw(E)in Table 11. However, in his table, he gives the same value given in the “Handbook of Chemistry and Physics” ( 3 ) ; this latter value appears t o be erroneous. Polarographic Properties of Lead in Potassium Thiocyanate. On the basis of the data obtained from the amperometric titrations, the solubility of lead ion in 0.1M potassium thiocyanate is about 2.5 m X . A diffusion current constant was determined for lead below this concentration by determining the diffusion currents of aliquots of two separately prepared stock solutions of lead, in 0.1JI potassium thiocyanate containing 0.01% gelatin. At - 1.0 volt, the diffusion current-concentration ratio was 8.24 i 0.05 pa. per millimole per liter between 0.50 and 2.00 m.ll lead. This value of id per millimolar lead is somewhat lower than the corresponding value in Table I because of the higher mercury head used in the amperometric titrations. The value of the capillary constant mz’atl’swas determined as 2.02 rng.%ec.-1’2 a t - 1 0 volt.
SURIM4RY
The reduction of lead in potassium thiocyanate involves a %electron change. I n 0 . l M thiocyanate, the half-wave potential and diffusion coefficient were found to be -0.385 volt os. SCE and 10.1 X cm.*set.-*, respectivelv. As the half-wave potential did not change 75-ith thiocyanate concentration, the reduction is that of a “simple metal ion.” Diffusion current constantLC for lead in 0.1M thiocyanate were calculated to be 4.08 (8) and 3.63 (9). The solubility constant of lead thiocyanate was determined by amperometric titration with potassium dichromate: values are presented for the simple K,, and for a Kw(E)obtained from the Debye-Huckel extended equation. ACKNOWLEDGMENT
The authors are greatly indebted to Robert C. McIlhenny for his help in preparing the manuscript. LITERATURE CITED
Bottger, W., 2. physil;. Chem., 46, 603 (1904). Glasstone, S., “Thermodynamics for Chemists,” pp. 410-20, New York, D. Van Nostrand Co., 1947. Hodgman, C. D., “Handbook of Chemistry and Physics,” 33rd ed., p. 514, Cleveland, Ohio, Chemical Rubber Publishing Co., 1951-52.
IlkoviE, D., Collection Czechoslor. Chem. Communs., 6, 498 (1934).
Kolthoff, I. Af., and Laitinen, H. d.,“pH and Electro Titrations,” New York, John Wiley & Sons, 1948. Kolthoff, I. M., and Lingane, J . .J., ”Polarography,” 2nd ed., p. 320, New York, Interscience Publishers, 1952. Ibid., p. 354. Lingane, J. J., IXD.ENG.CHEM., AX.AL. ED.,1 5 , 5 8 3 (1943). Lingane, J. J., and Loveridge, B. A , J . Am. Chem. Soc., 72, 438 (1950).
Masaki, K.. Bull. Chem. Soc. Japan. 6 , 163 (1931). Pierce, W. C., and Haenisch, E. L., “Quantitative Chemical Analysis,” p. 300, New York, John Wiley & Sons, 1948. RECEIVED for review July 1, 1953. Accepted February 27, 1954. Abstracted from a thesis submitted by J. 0. Hibbits to the Graduate School of St. Louis University in partial fulfillment of the requirements for the degree of master of science.
