Ionization in solution or electrolytic dissociation is a conception of the late nineteenth century that promises results of increasing importance, not only in the fields of pure physics and chemistry but also in industry and biology as well. Since Arrhenius first presented his electrolytic dissociation theory, there has developed about the concept "acidity" a new nomenclature, with new and interesting procedures for the determination of degrees of acidity.
Since this paper concerns itself with the application of these new procedures and names, new at least to the special field of dermatology, it seems necessary to explain the several principles underlying the observations hereinafter recorded.
Many solvents, especially water, effect, on solution of many substances (the solutes), a splitting of these substances into electrically charged units called ions. Thus, hydrogen chlorid [HC1] in dilute solution is H + — Cl -; for, when hydrogen chlorid dissociates, the hydrogen gives up a negative charge
SHARLIT H, SCHEER M. THE HYDROGEN-ION CONCENTRATION OF THE SURFACE OF THE HEALTHY INTACT SKIN. Arch Derm Syphilol. 1923;7(5):592–598. doi:10.1001/archderm.1923.02360110021003