March, 1951
POLAROGRAPHY OF LEAD(II)TARTRATES AND OXALATES
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(CONTRIBUTION FROM THE STERLING CHEMISTRY LABORATORY OF YALE UNIVERSITY]
Polarographic Studies of Metal Complexes. IV. The Lead(I1) Tartrates and Oxalates BY THELMA MEITESAND LOUISMEITES Lead(I1) forms a hydrogen tartrate complex, Pbz(HTart)s+, in acidic solutions, and, in strongly alkaline solutions, a complex containing two tartrate ions per lead atom. The latter complex is transformed at the dropping electrode surface into colloidal lead hydroxide which is then reduced. I n 1Fpotassium hydroxide solutions there is some evidence for the existence of a third complex in which the tartrate:lead ratio is 1. In strongly alkaline solutions containing oxalate, the biplumbite ion predominates. At PH values below about 12 a basic oxalate, 3Pb(OH)rPbC&O4, precipitates from 0.05 F oxalate solutions. At higher oxalate concentrations the Pb(C20,)t' ion predominates between PH 7.5 and 10.5.
The polarography of lead in tartrate media was first studied by Suchy,' who reported a welldefined wave for which E+ = -0.60 v. os. S.C.E. in an 0.5 F potassium sodium tartrate medium of unspecified pH. Lingane2 gave El/, = -0.50 v. in 0.5 F potassium tartrate, and -0.48 v. and -0.75 v. for the half-wave potentials in 0.5 F tartrate media of pH 4.5 and 13, respectively. PyatnitskiiS found that in strongly alkaline solutions the rate of change of El/, with pH indicated that two hydrogen ions were involved in the reduction of one lead atom. He gave the formula of the complex as Pb(OCHCOO-)2, and its association constant as 3.2 X 1014 from polarographic data, or 2.1 X 10lsfrom potentiometric studies. No previous studies of the polarography of lead in oxalate media have been published. Experimental -0.70 Measurements were made with the polafographic apparatus previously described,' using the, techniques discussed in earlier papers of this
As the pH is raised, Ell, becomes more negative, and finally becomes constant a t PH values between about 4.5 and 6, where it is equal to -0.440 v. in 0.05 F tartrate, -0.465 v. in 0.2 F tartrate and -0.482 v. in 0.5 F tartrate. From the fundamental equation for the reduction of a metal complex, then, assuming that the hydrogen tartrate ion is the complexing species AEi/*/A log [HTart-] = -0.042 = -O.O591(p/n)
where p is the number of hydrogen tartrate ions per lead atom in the complex and n is 2 for the reduction of lead(I1) to lead amalgam. This gives p = 1.42, so that the complex is presumably Pb2(HTart)3+. Comparing Ell, for this complex
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For this work the half-wave potentials were m; -0.60 measured by a simplified modification of a very precise method recently described.8 The residual current at the half-wave potential was secured by extrapolation from the preceding "zero- 2 current" portion of the wave, and the halfwave potential was found by interpolation be-0.50 tween measurements made at 5-mv. intervals along the rising portion of the wave. By this technique, results apparently precise to 2 mv. are easily secured.
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Data and Discussion -0.40 The Lead(11) Tartrates.-The halfwave potential of lead has been studied 2 4 6 8 10 12 14 as a function of PH in 0.050, 0.20 and PH. O . j o tartrate For fiH Fig. 1.-Half-wave potentials of lead(I1) in (a) 0.05 F (open about tartaric acid were circles), (b) 0.20 F (half-solid circles) and (c) 0.50 F (solid circles) titrated with sodium hydroxide; higher pH tartrate media. values were secured with potassium tartrate solutions titrated with potassium hydroxide. in 0.05 F tartrate with the value found by Linganeg The results, shown in Fig. 1, are generally similar in 0.1 F potassium nitrate, for which the ionic to those found with the copper(I1) tartratesb strength is about the same, we calculate a value for In strongly acidic solutions, where undissociated the formal dissociation constant of this complex tartaric acid predominates, El/, approaches that of about 3 X for the aquo-plumbous ion in media of comparable We have attempted to verify the formula for ionic ~ t r e n g t h . ~ this complex by an amperometric titration of a (1) K.Suchy. Collection Cucborlm. Chcm. Commrtns., 8, 364 (1931). solution of lead nitrate in 0.1 F potassium nitrate, (2) J. J, Lingane, I n d . Eng. Chcm.,Anal. E d . , 16,683 (1943). pH 3.4, with potassium hydrogen tartrate. The (3) I. V. Pyntnitskii, Zhur. Anal. K h i m . , 3, 331 (1948). difference between the diffusion coefficients of the (4) L. Meites and T.Meites, THIS JOURNAL. 73, 3086 (1950). aquo- and hydrogen tartrato- complexes is, how(5) 1,. Meites, ibid., 71, 3269 (1949). (6) L. Meites, i b i d , 79, 180 (1950). ever, so small that no break could be detected in the (7) L. Meiten, Ibid., 79, 184 (1960). resulting curve. The very low value found by ( 8 ) L. Meites, ibid , 74, 2293 (1950). Lingane2 for the diffusion current constant of lead (9) J. I. Lingane, ibid., 81, 2099 (1930). 1 '
THELMA MEITES AND LOUIS MEITES
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VOl. 73
in his "acid tartrate" medium is probably due to the gelatin present in his solutions.1° As the $H is raised above 6, the concentration of hydrogen tartrate ion in the solution becomes stnall compared to the lead concentration and, as with the copper coniple~es,~ the half-wave potential shifts rapidly to more negative values. There is no well-defined region of constancy corresponding to a normal tartrate complex, but rather a fairly sharp increase between pH 6 and 7.5, then a slower change up to pH 9..?, and finally, at still higher PH values, a linear change of /
