A study of the polarographic reduction of methaqualone

Methaqualone 2-methyl-3-o-tolyl-4(3H)-quinazolinone] is reduced at pH 1.5–5 in a single two-electron step. At pH 1.5–3, wave i₁ appears and is gradually replaced by wave i₂ which predomiantes in the solution between pH 4.5 and 6.5. At pH > 5, the height of wave i₂ decreases in the shape of a dissociaton curve with increasing pH. For both waves, the diprotonated form of methaqualone (I_a) is reduced to 1,2,-dihydromethaqualone as the final product. In wave i₁, the monoprotonated form I_b is further protonated at the electrode surface before it accepts the first electron; in wave i₂, the free base form I accepts two protons at the electrode surface before the first electron uptake. Polarographic curves are complicated by the presence of three waves of catalytic hydrogen evolution. Wave i_{1, cata} appears at pH < 5, waves i_2,cata at ?1.5 V and i_3,cata at ?1.7 V at pH > 5. Citrate buffers pH 1.5–3 or Britton-Robinson buffers at pH 2.6–3.6 are most suitable for quantitative work with eitehr d.c. polarography or differential pulse polarography.