At a value above pH 10 the gel layer structure of the glass membrane of a measurement electrode is subject to certain changes which lead to a measuring inaccuracy. Known as alkaline error or sodium ion error, this issue is caused by the presence of a high concentration of alkaline ions, especially sodium ions (Na+). These ions replace, partly or completely, the hydrogen ions at the outer gel layer of the glass membrane, and by doing so, contribute to the voltage potential at the outer phase boundary. As a result, a lower pH value will be measured than the actual pH value of the measured solution.
In earlier days the alkaline error of glass pH sensors occurred starting between pH 9 and pH 10. Today, where the glass membranes contain lithium instead of sodium, the alkaline error is only noticeable from between pH 12 and pH 13. The alkaline error increases with increasing pH value, with higher alkaline concentration and with rising temperature. In order to counteract the alkaline error, electrode manufacturers use special membrane glasses with low alkaline errors for electrodes which are used in high pH applications.