Q1(j). Alkaline (sodium) error of the glass electrode

Answer in brief

At high pH the glass membrane also responds to Na⁺ (and other alkali ions) in its hydrated gel layer, so it senses more H⁺ than is present: the measured pH is too low.

Key points

• Membrane: thin Na₂O–CaO–SiO₂ glass with hydrated gel layers; H⁺ exchanges with Na⁺ at the silicate sites.
• Normal response: E = const + 0.05916 log aH⁺ (59.16 mV per pH at 25 °C), H⁺ only.
• Above pH ≈ 9 to 10, aH⁺ is tiny while aNa⁺ is large; Na⁺ competes for the exchange sites: E = const + 0.05916 log(aH⁺ + kH,Na aNa⁺) (Nikolsky equation).
• Apparent H⁺ too high ⇒ measured pH lower than true (negative error).
• Remedy: lithium glass usable to pH ≈ 14; avoid Na⁺-rich media or apply a correction chart.