The alkaline error:
It is remember that the ordinary glass electrode becomes sensitive to alkali ions and gives low reading in high pH range - above 9 or 10 pH units. The purpose for the error is which whilst the glass membrane is selective to hydrogen ion, it also responds to other ions. That becomes more important while the activity of the other ions is higher to activity of the hydrogen ion.
Figure shows the error produced by different cations at different concentrations. You may have noticed that alkaline error is relatively more in case of sodium ion. This is since of higher selective coefficient of sodium ion. This because of sodium ion can be reduced by the use of Li2O glass in place of Na2O glass.
Figure: Deviation in pH measurement using a glass electrode under alkaline conditions