Mechanism of nitrite-catalysed oxidation of chlorpromazine with hydrogen peroxide and/or dissolved oxygen used for the determination of nitrite

Abstract

The mechanism of nitrite-catalysed oxidation of chlorpromazine with hydrogen peroxide in an acetic acid medium is inferred from absorption spectra, absorbance change and the properties of gas generated by reacting solutions under different conditions. (i) In a nitrite chlorpromazine hydrochloride (CPH)–dissolved O₂ system, nitrite is reduced by CPH to NO, which in turn is oxidized to NO₂ by the dissolved O₂. Much of the NO₂ oxidizes the CPH directly, and is converted into NO. The residual NO₂ disproportionates to HNO₃ and HNO₂. Nitrite and NO₂ oxidize residual CPH cyclically, until they are consumed. (ii) When a fresh CPH–H₂O₂ mixture is added to the nitrite-containing solution, peroxonitrous acid (HO₂NO) instantly forms, oxidizes CPH and is reduced to HNO₂, which reacts again with H₂O₂. The HO₂NO is also partially converted into its inactive isomer, HNO₃. The CPH is oxidized cyclically until the nitrite ions are consumed. (iii) When CPH and then H₂O₂ are added to the nitrite-containing solution, nitrite is reduced by CPH to NO, and then NO is oxidized by the H₂O₂ to produce mainly HNO₂ and some NO₂. The newly produced HNO₂ reacts with excess H₂O₂ to form HO₂NO, followed by the same cyclic reaction as in (ii). In reactions (ii) and (iii) a maximum absorbance proportional to the nitrite concentration is rapidly attained.