Adsorptive Stripping Voltammetry—A New Electroanalytical Avenue for Trace Analysis

Abstract

nickel can also be determined using a copper anode. Conditions are similar to those for nickel except that the applied potentials must be 200 mV more positive than for nickel. The anodic reaction rate constants with copper are typically somewhat greater than with nickel, however the applied potential results in increased background noise so that copper offers no signal-to-noise advantage over nickel. When the background electrolyte contains nickel as suspended nickel hydroxide as described above, copper, nickel, cobalt, silver, platinum, and gold electrodes yield similar analytical currents. These results indicate that suspended nickel hydroxide adsorbs to the metal electrode surfaces and essentially converts them to nickel oxide electrodes. Silver iodide in its room-temperature crystal form is sufficiently conducting so that it can carry the currents necessary for analytical amperometry. The electrode material is contacted using silver epoxy. The cathodic electrode mechanism involved generation of iodide at the silver:silver iodide interface yielding a current proportional to the rate of oxidation of iodide ions at the silver iodide:solution interface. Hypochlorous acid at sub-parts-per-million level thus yields cathodic currents for pH 6 solutions whereas pH 3 must be used for cathodic amperometry of monochloramine [4]. Accordingly a direct, linear response analytical technique for concentrations from more than 5 mg chlorine per liter down to about 10 gg chlorine per liter was developed for either monochloramine or the sum of hypochlorous acid plus monochloramine. The concentrations determined were sufficiently low to allow determination of the rate constant for monochloramine formation under realistic water treatment concentration, pH, and ionic strength conditions. The rate constant obtained was 3.2X 106 L/mol/s, in agreement with earlier reported values extrapolated from higher concentrations and less moderate pH solutions.