Q1(b). Glass-electrode errors

Answer in brief

Alkaline error usually makes the reading too low; acid error makes it too high.

Key points

• At high pH, low hydrogen-ion activity allows alkali-metal ions, especially Na⁺, to contribute to membrane response. The electrode indicates more hydrogen ion than is present.
• In strongly acidic solutions, the membrane response departs from its normal hydrogen-ion calibration; the indicated pH is higher than the true value.
• Use a suitable low-sodium-error glass, stay within its calibrated range and calibrate with appropriate buffers at the measurement temperature.