Question

Why would a glass pH electrode indicate a pH lower than the actual pH in a

strongly basic solution containing Na+?

Answer 1

Generally glass pH electrode determines H+ ion conentration and gives the pH accordingly.

When higher basic solution are tested where Na+ or Li+ ions are there in large quantities, the pH electrode starts measuring the Na+ or Li+ ions and the actual basic pH shown by the electrode will be lesser than the original value. This is called alkali ion error or sodium ion error. This error is more for Li+ ions compared to sodium ions. For potassium ions it is negligable.

It occurs only when alkali ions are too high in concentration in comparision to H+ ions.