Practical measurement of pH in amphiprotic and mixed solvents

Abstract

Procedures analogous to those on which a practical pH scale has been based can be used profitably to establish operational acidity scales in certain other amphiprotic and mixed solvent media. A universal pH scale relating proton activity uniformly to the aqueous standard reference state is not yet a practical possibility, but separate scales for each medium can be achieved and will fulfil most of the requirements. The best choice of unit appears to be pall* or — log (mH . yII), where sH is referred to the standard state in each particular medium s. Data for reference solutions in 50 wt. per cent methanol and in deuterium oxide are given. THE OPERATIONAL pH SCALE The well-recognized difficulty in reconciling a fundamental definition of the pH value with the practical experimental procedures for the routine measurement of acidity has led to the widespread adoption of an operational definition of the pH'-3 pH (X) — pH (S) = (Ri )/F (1) In this equation, X designates the solution of unknown pH and S the standard reference solution of known or assigned pIT, while E is the electromotive force of a suitable pH cell consisting of an electrode reversible to hydrogen ions (usually a glass electrode, hydrogen gas electrode, or quinhydrone electrode) coupled with a suitable reference electrode (commonly calomel-mercury or silver-silver chloride electrode). A bridge composed of a concentrated solution of potassium chloride usually connects the reference electrode with solution X or S when the cell is filled. The symbols R, T, and F represent the molar gas constant, the absolute temperature, and the faraday, respectively. INTERPRETATION OF THE MEASURED pH Under optimum conditions, the potential differences across the diffusion junctions Soln.X cone. KC1 and Soln.S cone. KC1 can be considered to be equal, and then the difference of e.m.f. E1—E8 is a useful formal measure