Q1(g). Alkaline glass-electrode error

Answer in brief

At very low hydrogen-ion activity the glass responds partly to alkali ions, especially sodium.

Key points

• The response is approximately related to a_(H⁺)+K_(H,Na)a_(Na⁺) rather than hydrogen activity alone.
• The extra sodium contribution mimics a higher hydrogen-ion activity, so the calculated pH is too low.
• This is alkaline or sodium error, particularly important in concentrated alkaline solutions.
• Use suitable low-sodium-error glass and calibration appropriate to the temperature and pH range.