The experimental device consisting of two paste electrodes: ferrocene
and quinone at fixed current potentiometry was studied for the in-situ
control of the acidity level of hydrochloric acid solutions at moderate
concentrations between 0.6 and 8.0 M. The influence of different factors
affecting the variation of potential at imposed current were already
studied in previous papers published on H3PO4, H3PO4 and HClO4. Also,
the measured potential variations Ei between ferrocene and
orthochloranil (oQ) or parachloranil (pQ) electrodes were equal the
values as those E obtained by the voltamperometric curves at a fixed
current. The acidity function Ri(H) proposed previously was similar to
that proposed by Strehlow and named Ro(H); R (H) representing the H+
activity is the extended pH towards concentrated acid solutions. The
variation of potential determined at imposed current obeyed the Nernst
equation and the slope close 0.058 mV.