Question

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

strongly basic solution containing Na+?

Answer 1

At high pH values (pH > 10), the gel layer formed at the membrane of a measuring electrode is subject to certain changes which leadto measurement inaccuracies, called “alkaline

error”. This alkaline error is caused by a high concentration of alkaline ions, such as sodium

ions (Na+). These ions replace, either partly or completely, the hydrogen ions in the outer gel

layer of the glass membrane, and by doing so,contribute to the potential at the outer phase

boundary.

As a result, a pH value that is lower than the actual pH value of the sample solution will be measured.