Studies in the polarographic behaviour

Abstract

A detailed investigation on the polarographic (d.c., a.c. and complex plane) and microcoulometric behaviour of m‑, o‑ and p‑nitrophenols in aqueous and 40% ethanolic solutions at various pH values is presented. In the case of m‑nitro‑ phenol one main d.c. wave is obtained at pH 1 and 2 and it is proved that reduction occurs to –NH₂ state via –NHOH stage on the basis of microcoulometric measurements. Beyond pH 5, the value of ‘n’ obtained is 4. On the basis of microcoulometric ‘n’ values in aqueous solutions, the value of ‘n’ in ethanolic solution is obtained by employing d.c.c. and Northrop‑Anson formula. o‑Nitrophenol is reduced to the amine stage over the entire pH range. In the case of p‑nitrophenol in aqueous solutions till pH 10.74 only one wave with n = 6 is obtained. At pH 11.45 and 12 two waves are obtained, the first corresponding to n = 4. In ethanol solutions split waves are produced. It is found that there is no stabilization of the nitroso stage of p‑nitrophenol. It is postulated that the formation of charged species –NO₂H and –NO₂⁻ from –NO₂H⁺ and NO₂ by electron transfer is the cause for obtaining a.c. waves and the variation in the height of the a.c. waves at various pH values is due to dependence of the rate constant on pH. Extensive analysis of complex plane polarographic data is made and rate constants are given.