ACKNOWLEDGMENT
The author expresses indebtedness to Shoji Makishima for continual encouragement and helpful suggestions, to Toshio Nakai for permission to use laboratory facilities, to Fumio Aoki for his kind interest, and to Seishi Yajima, Yuichiro Kamemoto, Koreyuki Shiba, and Muneo Handa for help with gamma spectrometry.
a&, M., Anal. Chim. Ada 24,
LITERATURE CITED
(1) Hamaguchi, E., Tomura, K., Watanabe, K., “Determinationof Scandium in Silicate Rocks by Neutron Activation Analysis,” Symposium on Analytical Chemistry, Tokyo, May 1959. (2) Hughes, D. J., Harvey, J. A., U. 5. At. Energy Comm. BNG325 (1955). (3) Hughes, D. J., Magurno, B. A., Brussel, M. K., Ibid., BNL-325, Suppl. 1, 2nd ed. (1960).
R E C E I ~for D review July 5, 1960. hcc cepted September 1, 1961.
athodic Action of the Leadt the Dropping Mercury Electrode
X
TSAI-TEH LA1 and TEH-LIANG CHANG Analytical Chemistry Laboratory, Cheng Kung University, Tainan, Taiwan
b The polarography of lead in glutamic acid solution has been studied over the pH range 5.0 to 10.0. The half-wave potential of the complex is constant and equal to -0.476 volt vs. S.C.E. in the pH region 5.0 to 7.2; at higher pH the half-wave potential was influenced b y the pH. The diffusion current constant is 2.42 for p H 5.0 to 6.8, and 2.29 for pH 7.2 to .O. The formulas of the complexes and their dissociation constants were determined. Equations for the elecdrode reaction were proposed. The proportionality between lead ion concentration and diffusion current is valid for 1.0 X 10-4M to 1.8 X 10-3M with an average error of 1.5%. HE polarographic characteristics of lead ion in various supporting electrolytes including tartrate medium have been studied by Lingane (6). He has shown that the reduction of the lead ion in all these supporting electrolytes, except for the strongly alkaline tartrate medium, is reversible a t the dropping mercury electrode. As to polarographic studies of leadchelate compounds, Iwase (2) has reported the lead-tartrate system over the p H range of 2.0 to 7.0. Koryta and Kossler (8) have determined the stability constant of the lead-ammoniatriacetic complex polarographically. Since metal ions have a strong tendency to form stable chelate compounds with aminopolycarboxylic acids, glutamic acid was used as chelating agent to study the lead complex in our present work.
EXPERIMENTAL
Apparatus and Procedure. Conventional polarographic techniques were employed, involving manual
measurement with a Fisher Elecdropode with attached external potentiometer, A modified H-cell having a sinteredglass disk and agar salt bridge was connected to a saturated calomel reference electrode. Oxygen was removed from the electrolyte solutions by passing through hydrogen, which was scrubbed with an alkaline solution of potassium pyrogallate to free it of oxygen. All experiments were carried out in a water thermostat a t 30’ f 0.1O C. The p H values were adjusted with sodium hydroxide or nitric acid and measured with a Beckman Model H-2 pH meter. The dropping mercury electrode possessed the rnn’al1’* value of 1.39 a t -0.500 volt us. S.C.E. measured in 0.5M glutamate solution. A maximum suppressor was not used, as no maxima were observed. No other supporting electrolyte, except the chelating agent, was found necessary. Chemicals. The stock solution of 0.01M lead nitrate was prepared by dissolving 3.312 grams of Pb(T\J03)2 in distilled water, and diluting to 1 liter. The exact concentration of lead ion was determined complexometrically with standard Versene solution (I). A 2M aqueous solution of monosodium glutamate (99.65% purity) was prepared freshly every few days. All chemicals used were of reagent grade or equivalent and were not further purified. RESULTS AND DISCUSSION
solution, while above pM 10.0 lead ion precipitated. Well defined reversible waves were obtained through this pH range. The reversibility of the electrode reaction was examined by plotting log {/(id - i) us. Ed.#.as shown in Figure 1. The points of the log plot formed good straight lines, whose reciprocal slopes between 0.029 and 0.032 were in excellent agreement with the theoretical value of 0.030 volt for %electron reduction. For most polarograms, EW El,r were also employed to interpret the reversibility immediately. The obtained values between 0.026 and 0.032 were also close to the expected value of 0.029 for a !&electronreduction. A satisfactory proportionality was obtained between the limiting current and the square root of the height of the mercury column, indicating that the reaction was entirely diffusion-controlled (Table I). Effect of pN. The effects of p H on half-wave potential and diffusion current constant were studied with a series of polarographic solutions, which contained 1.00 x lO-3M lead
-
Table I. Variation of Limiting Current with Height of Mercury Column (Lead nitrate 1.00 X 10-$M, glutsmio acid 0.498M) Hei ht, h,
pH 6.0
Nature of Reduction. The electrolyte solutions, which contained 1.00 X 10-3Mlead nitrate and 0.498M glutamic acid, were polarographed a t various pH values from 5.0 to 10.0. The lower limit of p H was restricted by the solubility of glutamic acid in the base
8.3
8m. (Corr.) 82.4 70.8 59.2 39.5 82.1 71.5 89.4 53.6
i, pa. 4.16 3.76 3.37 2.85 3.83 8.56 3.26 2.96
VOL. 33, NO. 13, DECEMBER 1961
i / c 0.46 0.45 0,44 0.48
0.42 0.42 0.42 0.40
1953
nitrate and 0.996M glutamic acid a t various pH. Figure 2 represents the results obtained by plotting the halfwave potential and the diffusion current constant vs. the pH. At pH lower than 7.2 the half-wave potential .was independent of pH. It kept constant a t -0.476 volt, and shifting from the half-wave potential of the simple lead ion, -0.405 volt (I?) was found. At pW higher than 7.2, the half-wave potential became pH-dependent, and shifted to the negative side as the pW value increased. The increment rate, -0.030 volt per p H unit, indicated that one hydroxyl ion was involved in the electrode reaction. Figure 2 also shows that the diffusion current conetant was kept constant a t 2.42 a t pN lower than 7.0 and at 2.29 at pH.between 7.2 and 9.0. The sudden drop of the diffusion current constant from 2.42 to 2.29 a t pH around 7 , where the electrode reaction began to involve one hydroxyl ion, shows that the lead chelate species has been changed with the addition of one hydroxyl ligand. Effect of Glutamic Acid Concentration. For ascertaining the formula of the lead-glutamate complex, the effect of glutamic acid concentration on the half-wave potential was studied a t p H 6.0 and 8.3. Keeping the lead nitrate concentration a t 1.00 x 10-aM and varying the glutamic acid concentration, the electrolyte solutions were polarographed. The half-wave potentials were plotted against the logarithm of the concentration of glutamic acid (Figure 3). As seen from Figure 3, there are two breaks in each curve, indicating the possibility of the existence of three different complex species, depending on the chelating agent concentration. The three slopes obtained from curve .I in Figure 3 were 0.029, 0.064, and 0.091 for glutamic acid concentration lower than 0.4M, 0.4M to 1.2M, and higher than 1.2M, respectively. Introducing each of the slopes into the Lingane equation (4) A(E1n)c
-
Z-GijE -
-0.060
(at 30'
1.53 2.23 3.06 8.76 4.46 6.16 5.96 AV. I d r= 2.70 Av. dev. 1.5%
Id
2.60 2.77 2.69 2.77 2.71 2.69 2.66 2.69
- i ) vs. E 1
b-: ab5-
0 v)
ui
' 0.50-
-k
u I
0.45
1 1 ' 5.0
I
1
6.0
TO
1
4 1.6
#
8.0
3.0
10.0
PH Figure 2.
Variation of half-wave potential and diffusion current constant with
0.4
I
0.2
X
pH at 6.0
0
2 i, pa. 0.36
Plots of log i / ( i d
I
e.)
Table It. Diffusion Current as a Function of Lead Ion Concentration at pH 6.0
Concn. of Pb, XnM 0. 10 0.40 0.60 0.80 1.00 1.20 1-40 1.80
Figure 1.
0
-
0.2
-
0.4
- 0.6 1
Figure 3.
-
1
1
- 0.45 E$
-0.50
vs.
1
- 0.60
S.C.E
El/, of lead-glutamate as a function of glut~micacid Concentration
p becomes equal to 1, 2, and 3 for the given chelating agent concentration. The pKI and pKz of glutamic acid are 4.3 and 9.7, respectively (6). Thereby, and from Figure 2, the leadglutamate complex species and electrode reactions a t pH between 5.0 and 7.2 may be expressed as
+ + Ilg = + HCPb(HG)t + 2e + Hy Pb(Hg) + 2HCPb(HG)f 2e Pb(Hg)
[G-]
