26.1 What is pH?
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This article is from the Chemistry FAQ, by Bruce Hamilton B.Hamilton@irl.cri.nz with numerous contributions by others.
26.1 What is pH?
The pH scale determines the degree of acidity or alkalinity of a solution,
but as it involves a single ion activity it can not be measured directly.
pH = - log10 ( gamma H x m H )
where gamma H = hydrogen ion single ion activity coefficient
m H = molality of the hydrogen ion.
As pH can not be directly measured, it is defined operationally according to
the method used to determine it. IUPAC recommend several standardised methods
for the determination of pH in solution in aqueous solutions. There are
seven primary reference standards that can be used, including 0.05 mol/kg
potassium hydrogen phthalate as the Reference Value Standard. There is an
ongoing debate concerning the relative merits of having a multiple primary
standard scale ( that defines pH using several primary standards, and their
values are determined using a cell without a liquid junction ) or a single
primary standard ( that requires a cell with a liquid junction ). Interested
readers can obtain further information on the debate in [1]. Bates [2], is a
popular text covering both theory and practise of pH measurement.
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