THE DETERMINATION OF A STABILITY CONSTANT FOR THE AQUEOUS COMPLEX Zn(OH)₂⁰ USING ANODIC STRIPPING VOLTAMMETRY1

Abstract

The shift in the potential of the zinc stripping current maximmn observed in anodic stripping voltammetry is related to the free energy of the reaction between ionic zinc and a complexing anion. The model was tested using the zinc hydroxyl system; the agreement with experimental results was excellent.The findings suggest the existence of the aqueous complex, Zn( OH ):, which has been reported only once beEore. The stability constant calculated here is in good agreement with the single published value.Anodic stripping voltammetry from thin layers and hanging drops of mercury was developed and used for determining the concentrations and the extent of organic chelation of some trace metals in natural waters (e.g. Matson 1968; Fitzgerald 1970; Seitz 1970; Zirino and Healy 1972; Bradford 1972). A phenomenon associated with the technique that has received scant attention is the shift in the potential of the stripping current maximum (the so-called stripping peak potential, EP) with changes in the activity oE a complexing anion in the test solution. The phenomenon is implied by the equations of Roe and Toni ( 1965) and was discussed by Matson (1968) high intensity ultraviolet light (Armstrong and Tibbitts 1968) to break up the naturally occurring chelators, and thus suggested that shifts in E, could be caused only by inorganic ligands. Nicholson and Shain ( 1964) showed that in stationary polarography a shift in the potential of the peak oxidation current with varying complexing anion activity requires that the reaction between the metal cation and the complexing anion be fast relative to the rate at which the metal cation is oxidized from the mercury electrode. Bradford ( 1972)

Investigations on the use of anodic stripping voltammetry for the analyses of lead in saline environments

“…The downward potential trend could also be caused by lead complexed with the dominant labile carbonate anion. Bradford (1973) found that in the case of zinc the carbonate complex occurs in a relatively unstable form and causes the potential to shift. This same behavior should occur for lead, particularly in the case of a solution not purged of co 2…”

“…Recent work includes, as another alternative, studies in whic~ the chemical forms of certain trace elements are modeled using values derived from the laboratory analyses of sea water for stability constants, activity coefficients, and concentrations (Bradford, 1973;Zirino and Yamamoto, 1972;Ernst and others, 1975;Batley and Florence, 1976). The problem with this method is that the values come from samples which have been chemically altered for analyses and therefore do not represent ambient conditions.…”

“…difference in the total oxidation for lead in sea water as a free ion and for labile lead in sea water complexed with inorganics is the Researchers (Bradford, 1973;Ernst and others, 1975) d) Labile and non-labile lead adsorbed on organics and inorganics: Zirino and Yamamoto (1972, p.669) found that uncharged species of limited solubility behave in inert fashion at the working electrode and are the basis for colloid formation. Batley and Florence (1976, p.348) define this chemical form as lead occluded in or adsorbed on highly dispersed colloids.…”

“…Pb(OH 3 ) has a stripping potential of -0.815 ± 5%. Bradford (1973) and Long and Angina (1977, p.ll88) .…”

“…A number of different analyses systems follow these guidelines to varying degrees, but I selected anodic stripping voltammetry (ASV) because this method; (a) is theoretically simple; (b) uses a number of standard laboratory components; (c) theoretically makes species distinctions (Maney, 1972;Zirino and Yamamoto, 1972;Bradford, 1973 (Zirino and Lieberman, 1975, p.82); and (g) with certain changes analyzes samples without requiring the samples to be chemically altered beforehand.…”