upon the minimum value of BH to attain a constant ii. Actually this is found under specified conditions, when the presodium current is small enough as not to cause too much uncertainty in evaluating the BrdiEka current (Figure 7). The maximum in the BrdiEka current, which with cysteinelike compounds is usually observed at a potential of the order of -1.6 V (~s.SCE) is probably related to adsorption properties of Co(0)RS and/or Co(0)RSH. In spite of the negative charge of the former and the negative charge of the electrode, both species apparently are partly adsorbed on the surface of the mercury. With increasing negative potential, there is increasing desorption of these active compounds. It is fair to assume that the rate of disproportionation (Equation 4) of these active compounds in the non-adsorbed state is much greater than when they are adsorbed on the surface of the electrode. The adsorption may even greatly contribute to their chemical stability. The increasing desorption and subsequent increasing rate of disproportionation tends to decrease the BrdiEka current; on the other hand, increasing negative potential increases the rate of electroreduction (1). The overall effect apparently is a decrease of the BrdiEka current when the potential becomes more negative than it is at the maximum. The first product of the disproportionation reaction (Equation 4 or 4a) is metallic cobalt, Co(O), which is very unstable in the presence of mercury and rapidly yields cobalt amalgam and Co,Hg, (18, 19). Direct formation of Co (0)RS by reaction of Co(0) with RS, if any, would be expected to be very slow and not to occur under our experimental conditions, when the life period of metallic Co(0) is short. This conclusion is substantiated by the fact that with Co(II1) hexammine instead of Co(I1) hardly any BrdiEka current is observed. Co(II1)hexammine is much more stable than Co(1II)RS complexes. The first reduction product, Co(I1) (NH3)e2+ apparently is electroreduced so rapidly to Co(0 that it does not form or forms only a trace of Co(I1)RS complex at the surface of the electrode, whereas the Co(0) does not react with RS- at the electrode surface. This accounts for the fact that extremely small BrdiEka currents are observed with CO(III)(NH&~+,whereas with Co(1II) (RS),, the BrdiEka currents are identical with those observed with Co(II)(RS),. The large increase of the Brdizka current in the presence of small concentrations of surfactants (Figure 9) apparently is related to a stabilization of adsorbed Co(0)RS and/or Co(0) RSH at the surface of the mercury. The presence of adsorbed compounds on the mercury surface can greatly increase the lifetime of freshly deposited cobalt, nickel, or iron atoms, because of the changed physical conditions at the (18) W. Kemula and Z. Galus, Bull. Acad. Pol. Sci.,Ser. Sci., Cliim., Geol., Geograph., 7,729 (1959). (19) B. K. Hovsepian and I. Shain, J. Electronrial. Chem., 12, 397 (1966).
metal-surfactant-mercury interface (20-22). Similar stabilization may occur when the Co(0)RS complex is deposited in the presence of adsorbed surfactant. An increased lifetime of the complex results in an increase in BrdiEka current, which actually is observed (Figure 9). At large concentrations of the surfactant, when the surface coverage by the adsorbed surfactant is close to complete, the BrdiEka current becomes suppressed, the surfactants then replacing the adsorbed complex from the surface of the electrode. At potentials where the surfactant is desorbed, the BrdiEka current decreases, to increase again to its normal maximum value of a potential near - 1.6 V in the absence of surfactant. Thus in the presence of small concentrations of surfactant two maxima in the BrdiEka current are observed, resembling the characteristics of BrdiEka currents in protein solution. These two maxima are not observed in the presence of gelatin which is not desorbed in the potential region where the BrdiEka currents are observed (Figure 10). RECEIVED for review June 26, 1970. Accepted September 4, 1970. This investigation was supported by Public Health Service Grant No. Ca-89723-03 from the National Cancer Institute. (20) V. F. Ivanov and 2. A. Ioffa, J. Plzys. Ckem. USSR, 38, 563 (1964). (21) Zbid., 36, 571 (1962). (22) G. N. Babkin, Izo. Vyssh. Ucheb. Zaaed., Kliim. Khim. Tekhnol., 7 , 9 0 (1964).
Corrections Improved Exponential Dilution Flask for Gas Chromat sgra phy In this article by L. J. Lorenz, R. A. Culp, and R. T. Dixon [ANAL.CHEM., 42,1119 (1970)l the height of the Teflon stirrer was in error and should be 40.8 mm. Stirrers much higher than 40.8 mrn are unstable and difficult to balance.
A Complexometric Titration for the Determinationof Sodium Ion In this article by James D. Carr and D. G. Swartzfager [ANAL. CHEM.,42, 1238 (1970)l there is an error in the caption for Figure 3, page 1240. It should read as follows: “Effect of varying potassium ion concentration on the titration curve at a constant pH of 13.00.” The value of pH 12.00 is incorrect and misleading when compared to the correct figure and caption of Figure 1.
ANALYTICAL CHEMISTRY, VOL. 42, NO. 14, DECEMBER 1970
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