pH determination in anhydrous hydrogen fluoride and superacid media by means of the chloranil electro-chemical indicator system

The electro chemical systems of chloranil (quinone/semiquinone and semiquinone/hydroquinone) are shown to be usable as pH indicators in anhydrous hydrogen fluoride and in the superacid mixtures HF + MF_n (where MF_n = PF₅, BF₃, TaF₅, NbF₅, AsF₅ and SbF₅). The combined use of both hydrogen and chloranil electrodes has allowed the establishment of a complete potential-pH diagram of chloranil in the whole acidity range of HF. The quinone/semiquinone system can be used at very high acidity levels (solutions of AsF₅ or SbF₅) where the hydrogen electrode does not function correctly. A new value of the autoprotolysis constant of HF is reported (K_i = H⁺. F^-] = 10^-13.7 mol² l^-2) and compared with earlier values. The equilibrium constants of acid-base systems of the quinone (Q/QH⁺) and hydroquinone (QH₂/QH₃⁺ and QH²⁺₄) forms of chloranil, and the disproportionation constant of the semiquinone (QH₂⁺ form are also reported. The indicator system allows the acidity levels reached in HF to be placed on an R(H) scale (where R(H) = 0 corresponds to pH = 0 in aqueous solution). These levels were found to lie between R(H) = -14.2 for 1 M KF solution and R(H) = -27.9 for 1 M SbF₅ (i.e. 18% by weight).