Oxygen chemisorbed on zinc oxide: The determination of reactive oxygen

Abstract

The changes in excess zinc content which occur on heating zinc oxide in air are discussed, and are suggested as a means of estimating oxygen chemisorbed on zinc oxide. The published evidence for the existence of at least two varieties of chemisorbed oxygen is confirmed, and it is established that only one of these varieties is reactive chemically. The results deduced from the heating experiments are confirmed directly by a rapid photometric method which is described for the determination of reactive oxygen chemisorbed on zinc oxide. Oxygen is desorbed from the surface of the zinc oxide by etching with ammonium chloride solution in the presence of potassium iodide. By complexing the iodine liberated by reactive oxygen with starch prior to dissolution of the sample in acid, reduction of iodine by excess of zinc in the sample is avoided. Atmospheric oxidation of potassium iodide is overcome by close control of acid concentration (pH 5.2) throughout the determination. A reference solution is employed in which iodide is added only after dissolution of the sample in acid has been completed. The necessary compensation for oxidizable matter present in the zinc oxide and reagents is achieved by the addition of standardized hydrogen peroxide to both sample and reference solutions.    Reactive chemisorbed oxygen on a number of samples of zinc oxide ranged from 0 to 1.5 p.p.m. by weight. The method gives good reproducibility, and is sensitive to 0.03 p.p.m. of oxygen by weight.