Polarographic determination of acids by means of the reduction wave of quinones in methyl cellosolve solution

Methyl-p-benzoquinone (MQ) gives a single polarographic wave in methyl cellosolve media containing perchlorate as supporting electrolyte, but a prewave corresponding to the reaction Q + 2HA + 2e ái H₂Q + 2A^-, is found when an acid is added. The height of the prewave is proportional to the concentration of added acid. Traces (ca. 0.2%) of free perchloric acid can be determined in crude lithium perchlorate.The half-wave potential of the prewave depends on the pK_a value of the acid; if the dissociation constants of acids differ sufficiently, two prewaves are found and each acid species can be determined simultaneously. This method was applied to the MQ-HClO₄-CH₃COOH, MQ-CCl₃COOH-CH₃COOH and MQ-CHCl₂COOH- CH₃COOH systems.