Electroanalytical monitoring of free acidity in concentrated organic acid solution for skin exfoliation

It is well known that skin reaction to AHA and TCA peeling solutions can be widely tuned by modulating the acid type, concentration, and pH, as well as the solvent formulation. Unfortunately, in the case of concentrated peeling solutions, several drawbacks impair the significance of traditional pH measurements on concentrated and/or nonaqueous peeling solutions. As a convenient alternative, we propose electrical conductivity, accounting for the (concentration × mobility) products of the free ions in the investigated solution, and particularly the H+ ones, being the fastest on account of their unique “jump” motion ability across the water network. As a test, we have performed a systematic investigation on concentrated solutions of glycolic, pyruvic, and trichloroacetic acid at 20-50% concentrations in a mixed aqueous/organic solvent series, the organic cosolvent (an innovative formulation by General Topics S.r.l.) ranging 0 to 33% in weight. Conductivity clearly and neatly accounts for the differences in free acidity resulting from the different strength and concentration of the organic acids in the tested solutions, consistently with clinical tests (whereas parallel pH determinations proved to be hardly significant). Moreover the neatly linear dependency of the conductivity values on the organic cosolvent weight fraction could be usefully exploited for formulation and quality control purposes, and it is also intriguing from a more fundamental point of view, suggesting the proton to be regularly slowed down with decreasing number of the water network sites available for jump motion. Actually, in speculating on the role of free protons in the mechanism of acid peeling, it could be useful to take into account both their concentration and their mobility (particularly, as a function of the skin polar structure).