The pH sensor composed of a Y-doped zirconia ceramics tubing
containing a dry Ni–NiO internal reference. The zirconia tube is
in contact with the studied solution, the pH of which influences
electrical reactivity of the zirconia tube, and formally acts as a
Ni/NiO/ZrO2(Y2O3)/H+ sensor. This method was originally
described by Niedrach [16,17], and was more recently reviewed
in Lvov et al.