active. In the presence of two molecules of cysteamine per one cobalt ion, Gonick (11) found a composition corresponding to Co(II)(RS)2, which has stability constants of pK1 = 7.68, pK2 = 7.03. The limiting current of the prewave has typical characteristics of an adsorption current, as has been shown in the experimental section. At potentials between about -0.7 and -0.82 V, the rate of electroreduction of the free (noncomplexed) cystamine in the presence of the adsorbed complex is no longer negligible, as is evident from i-t curves C and D in Figure 3. The current decreases abruptly at the time when the surface becomes covered with the complex and from then on the current increases parabolically with time, the rate becoming greater with increasing negative potential (compare C and D in Figure 3). At potentials more negative than -0.95 V, no adsorption of a cobalt complex occurs (Figure 7) and the current-time curves have the characteristics of a diffusion-controlled current ( E and F in Figure 3). In unreported experiments with mixtures of cysteamine and cobalt(II), the reduction of the latter started at about -0.85 V, the reduction waves having a similar appearance as the main cystamine waves in the presence of sufficient cobalt(I1) (curves 4,5,6, and 7 in Figure 1). In solutions 0.06M in borax and 1 x lO+Min cystamine, the surface at -0.75 V becomes completely covered with the cobalt(I1) complex after 3.2, 1.6, 0.9, 0.7, and 0.4 seconds in the presence of 0.8, 1.2, 1.6,2.0, and 3.0 X 10-4M cobalt(II), respectively (Figure 3, C). By extrapolation it is found that the surface is covered with the adsorbed species virtually immediately at the time of formation of a drop at a cobalt concentration of 8 X 10d4M. It is under this condition that the limiting current of the prewave is found proportional to the height of mercury in the reservoir (Figure 5). The validity of the expression 9c2 =
const.
(9 denoting the time of coverage of the surface) has been found to hold (Figure 6). This relation indicates that adsorption equilibrium is established rapidly and that it is diffusion-controlled (7, 8). Under this condition the maximum number, r,, of moles of adsorbed species per cm2 can be calculated (7, 8)
r,
=
7.36 X
c D,1’ztlr1‘2
D, denoting the diffusion coefficient of the adsorbed species. From the values of the cystamine diffusion currents in 0.06M borax at 25 “C, its diffusion coefficient, DRsBR, was calculated to be 3.5 X 10-8 cm2 sec-l. Assuming the diffusion coefficient of the cobalt(I1) cystaminate to be the same, the value of r, at a cobalt concentration greater than 1.2 X 10-4M was found to be 2.4 X 1Olo mole cm-2, which corresponds to an area of 69 APper adsorbed molecule of cobalt(I1) cysteaminate. The maximum number of moles adsorbed per unit electrode surface can also be calculated from the value of the mean (11) E. Gonick, Thesis, University of Pennsylvania, 1951.
adsorption current, provided the adsorption equilibrium is so favorable that the drop surface is fully covered as soon as the drop is formed, and that the reduction proceeds reversibly (6). From the height of the limiting prewave current, in = 0.13 pA (corrected for the residual current) in the presence of 1 X 10-3M cystamine and 1.6 X 10-4M cobalt(II), a value of r, of 2.6 X 10-lo mole cm-* is calculate!, corresponding to an area covered by one molecule of 61 A2. The size of the cobalt(I1) cysteamine molecule, CO(II)(RS)~,calculated from a stereo model, is of the order of 55 Az. If the molecule is spread flatly on the surface, this value is in satisfactory agreement with the above values. RECEIVED for review December 31,1968. Accepted March 18, 1969. This investigation was supported by Public Health Service Grant No. Ca-08723-02 from the National Cancer Institute.
Corrections Resolution of Overlapping Bands: Functions for Simulating Band Shapes In this article by R. D. B. Fraser and Eikichi Suzuki [ANAL.CHEM.,41, 37 (1969)l an error appears on page 38, column 2, Equation 16. The first equation should be: Yo = AX,
*
Y m exp (‘h)
Rapid Separation by Carbon Number and Determination of Naphthe,neand Paraffin Content of Saturate Petroleum Distillates up to 185 “C In this article by J. V. Brunnock and L. A. Luke [ANAL. CHEM.,40, 2158 (1968)l there is an error in the last sentence on page 2158. The sentence should read “It is essential to eliminate all traces of oxygen from the helium gas stream; otherwise, partial oxidation of the sample on the sieve will commence at temperatures in excess of 300 “C.
An Analyzer for the Dynamic Microdetermination of Carbon, Hydrogen, Nitrogen, Sulfur, and Uxygen In this article by George Dugan and V. A. Aluise [ANAL. CHEM.,41,495 (1969)l there is an error in the third sentence of the Abstract. It should read as follows: “Retention and separation of the combustion gases, Nz, COZ,SOP, and HPO are accomplished by using chromatographic columns in liquid nitrogen.”
VOL. 41, NO. 7,JUNE 1969
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