proposed theories of anion induced adsorption. These theories differ primarily in that one considers that the adsorption is induced by adsorbed anions which remain adsorbed o n the surface in the presence of the adsorbed metal species whereas the other considers a competition between adsorbed anions and metal complex for available sites o n the mercury. The behavior of zinc in thiocyanate cannot be said to be sufficiently understood to add strength to either argument. The salient difference in the zinc system compared to those previously studied is the very sharp peaks observed in the amount adsorbed at anodic potentials. The sharp maximum may, perhaps, be due to the formation of ZnHg(SCN)4--i.e., there may be a light oxidation of the mercury at positive potentials in the presence of the adsorbed zinc and thiocyanate. At the potential where reduction of the ZnHg(SCN)r occurs, to leave the adsorbed zinc-thiocyanate, one could expect a high degree of disorder which would allow the cadmium to occupy some of the available sites, as observed. The potential of the peak is a function, however, of the concentration of SCN- in the solution as shown in Figure 8 and the shift, qualitatively, is in accord with what might be expected if a mercury-zinc-thiocyanate species formed at the surface. There is a distinct cathodic shift with increasing SCN- concentration; this shift does not correlate with the concentration of any of the thiocyanate complexes of zinc using published stability constant values (8).
The difference in charge o n the electrode, qm, we believe to be real. (See Table I.) However, the change in the charge (8) R. E. Frank and D. N. Hume, J. Amer. Chem. Soc., 75, 1736
(1953).
qm produced is much less than the quantity of adsorbed ma-
terial. This has been employed as a n argument by Anson and Barclay to indicate that the adsorbed species is neutral. However, Mohilner (9) pointed out that the relation
must be valid. Plots of qm 1;s. log C and nFr cs. E should have the same slope, as indicated by Equation 8 above. From Figure 3, a value of ZF[bI’/bEj‘ of about 5.3 x 10-5 C/V may be calculated. This would lead to a value of the term aQ/blogC, of about 1.6 X C per decade change in concentration of adsorbate in solution. Data in Table I1 were also used to construct such plots; subject to a considerable experimental scatter, values of about 4 X 10-5 CjV were obtained for both plots. Thus, the small change in qm obtained does not necessarily indicate the adsorption of a neutral species. Other arguments presented by Anson and Barclay, however, are in accord with the absorption of a neutral species. The data presented here do not conclusively distinguish between theories. ACKNOWLEDGMENT
We acknowledge the patience and skill of Richard Carpenter who obtained much of the data with enthusiastic resignation to his fate.
RECEIVED for review June 9, 1969. Accepted September 5, 1969. (9) D. Mohilner, Colorado State University, Fort Collins, Colo., private communication.
Determination of Chlorine in Selenium by a Distillation-Atomic Absorption Procedure Wladislaw Reichel and Laszlo Acs Canadian Copper Refiners Ltd., Montreal East, Quebec, Canada
CHLORINE, probably in the form of interstitial chloride ion, has a modifying role in the crystalline growth of selenium. In selenium destined for semiconductor application, the determination of chlorine, both in high purity and chlorinedoped products, is becoming increasingly important. A versatile and accurate procedure was sought to cover chlorine concentrations between several hundred ppm and the lowest attainable detection limit. I n the method of T. E. Green ( I ) , based o n dissolution of selenium in nitric acid in the presence of excess silver ions, the separation of the precipitated silver chloride and spectrophotometric determination of silver as sulfide results in consistently low recoveries, probably because of volatilization of chlorine during dissolution and/or the solubility of silver chloride in selenious acid- nitric medium. Because these losses were near 20 Hg C1 per 5-gram sample, another approach was needed, particularly for low chlorine concentrations. (1) T. E. Green, unpublished papers, as described by I. M. Kolthoff and P. J. Elving, “Treatise on Analytical Chemistry,” Part 11, Vol. 7, p 198 (1961). 1886
The separation of hydrogen chloride by distillation was chosen to avoid interference from impurities in the sample and to overcome the problem of silver chloride solubility in selenious acid. Chlorine, distilled as hydrogen chloride, was collected in a solution containing excess silver ions. The precipitated silver chloride was filtered off and redissolved in an ammoniacal solution. A number of published procedures describe the determination of silver in an ammoniacal solution (2). However, the sensitivity, specificity, and rapidity of atomic absorption spectrophotometry (3-6) in measuring silver ion concentration lead to the selection of this technique. (2) I. M. Kolthoff and P. J. Elving, “Treatise on Analytical Chemistry,” Part 11, Vol. 7, p 380 (1961). (3) V. L. Ginzburg, D. M. Livshits and G. I. Satarina, Anal. Abstr., 13, 15 (1966). (4) U. Westerlund-Helmerson, Atomic Absorption Newsletter, 5, 97 (1 966). (5) F. M. Tindall, ibid.,p 140. (6) J. B. Ezell, Jr., ibid., 6, 84 (1967).
ANALYTICAL CHEMISTRY, VOL. 41, NO. 13, NOVEMBER 1969
The procedure has been in use for over two years, and interlaboratory comparisons on hundreds of samples have been very satisfactory. EXPERIMENTAL Apparatus. Details of the distillation apparatus are shown in Figure 1. Selas No. 2001 crucibles were used for filtration. Atomic absorption measurements were made with a Techtron Model AA-100 unit, equipped with a 10-cm slot laminar flow burner head (Type AB-41). Fuel and supporting gases wer: acetylene and air at 15 and 10 psi, respectively. The 3281 A silver resonance line was used for measurement. Reagents. The chemicals used were all of reagent grade. Although reagent grade concentrated nitric acid is generally suitable, blank correction is suggested when very low concentrations of chlorine are to be determined. Chlorine-free distilled water (
