pHlike

pHlike is a term used in discussions of acidity in non-aqueous media and in computational studies as an analogue to the conventional pH scale. It refers to a dimensioned or dimensionless index designed to quantify proton activity in a given solvent or material, in a way that mirrors how pH relates to hydrogen ion concentration in water. Unlike pH, which is defined for aqueous solutions, pHlike is solvent-specific and may not be directly comparable across systems.

In practice, pHlike is defined as the negative logarithm of the proton activity a_H+ in the chosen medium: pHlike = -log10 a_H+. The activity is typically referenced to a standard state of the solvent (for example, unit activity of a solvated proton at unit molar concentration, or another convention chosen by researchers). Some approaches calibrate pHlike using indicators or spectroscopic probes adapted to the solvent, or by thermodynamic calculations based on acid-base equilibria.

Applications include evaluating acidity in ionic liquids, molten salts, non-aqueous electrolytes, or solid-state proton conductors, where conventional pH is undefined or misleading. pHlike provides a framework for comparing acid strength, driving force for proton transfer, and tuning catalytic or electrochemical processes in such media. It is often used descriptively rather than as a rigorous, universally standardized quantity; different groups may adopt different reference states or calibration methods.

Limitations include dependence on solvent, temperature, and measurement method; lack of direct comparability with aqueous pH; and potential ambiguity in interpretation. As a result, pHlike is best used alongside explicit proton activity values or equilibrium constants, with clear documentation of the reference state.

See also: pH, acidity, proton activity, Brønsted acidity, ionic liquids, non-aqueous solvent.