Effect of Protolysis Reactions on the Shape of Chronopotentiograms of a Homogeneous Anion-Exchange Membrane in NaH₂PO₄ Solution

Single-pulse and double-pulse chronopotentiograms of a homogeneous anion-exchange membrane AX in 0.02 M solutions of NaCl (system 1) or NaH₂PO₄ (system 2) have been recorded in underlimiting and overlimiting current modes. It has been found that in the case of exceeding the limiting current (i > i_lim^Lev) calculated using the convection–diffusion model, the time required to establish a steady state in system 2 increases by more than an order of magnitude compared to system 1. The slow growth of the potential drop is due to a gradual transition of the membrane from the form in which the main counterion is H₂PO₄^– to the HPO₄^2– form. This transition is due to the deprotonation of a part of H₂PO₄^– ions forming HPO₄^2– and protons as they enter the membrane. The participation of H⁺ in charge transfer in the depleted diffusion layer at a given current density causes a lower value of the potential drop than in system 1 for the same i/i_lim^Lev ratio. In intense current regimes, chronopotentiograms of system 2 exhibit two inflection points. The first point corresponds to the classical Sand transition time and is due to reaching the limiting current of H₂PO₄^– ions (the main charge carrier for i < i_lim^Lev) in the depleted diffusion layer. The second point is associated with a critical current that can be called the second limiting current in the system with NaH₂PO₄ and has no analogue in the system with NaCl. This current, which is approximately 2i_lim^Lev, corresponds to the state when the membrane is completely transformed into the HPO₄^2– form. Meanwhile, the source of protons due to the transformation of H₂PO₄^– into HPO₄^2– ions as they enter the membrane is exhausted. After reaching this critical value of the potential drop, either the HPO₄^2– deprotonation reaction to give triply charged PO₄^3– ions in the membrane or the water splitting on fixed groups located at the membrane/solution interface may occur.