Where a pH meter is standardized by use of an aqueous
buffer and then used to measure the “pH” of a nonaqueous
solution or suspension, the ionization constant of the acid or
base, the dielectric constant of the medium, the liquid-junction
potential (which may give rise to errors of approximately
1 pH unit), and the hydrogen-ion response of the
glass electrode are all changed. For these reasons, the values
so obtained with solutions that are only partially aqueous in
character can be regarded only as apparent pH values.