What is the alkaline error in pH measurement

In strongly alkaline solutions measurement errors can appear due to a contribution to the potential from alkali ions – Lithium (Li+) and Sodium (Na+) ions in addition to that of the hydrogen ions. Na+ and Li+ ions interact with the glass membrane and can partially or completely replace the H+ ions present. The resulting low hydrogen ion activity results in the observed pH reading being more basic than the true value. This effect occurs in solutions with a pH value > 11 and increases with increasing temperature. The magnitude of the alkali error depends on the concentration of the interfering ion with respect to the H+ ion concentration. Membrane glasses with large alkali errors have only a short service life in the presence of high concentrations of alkali metal ions