Method for correcting the temperature dependence of field-type glass electrode pH sensors

Seawater pH measurement is essential for monitoring ocean acidification and its biogeochemical impacts. Glass electrode pH sensors are widely used for in situ applications due to their ease of operation, low power consumption, and no reagent requirements. However, their accuracy can be affected by temperature-dependent effects, particularly those associated with the internal reference solution. Conventional calibration methods based on the Nernst response do not explicitly account for temperature-induced variations in the internal solution, which may lead to systematic bias under conditions with large temperature gradients. In this study, a correction method is proposed to address temperature-dependent errors by modeling the internal pH (pH_in). The model incorporates two primary temperature-dependent contributions: the intrinsic temperature dependence of the phosphate buffer and potential shifts caused by activity changes in saturated KCl. These effects are combined through the electrode response factor (f_R), defined as the ratio of the observed electrode slope to the theoretical Nernst slope, to estimate pH_in as a function of temperature and correct measured pH values to a reference temperature of 25 °C. Application of the method to vertical seawater profiles demonstrated a reduction in deviation from colorimetric reference measurements. The corrected results showed reduced temperature-dependent bias across depth. This approach provides a practical framework for improving the reliability of in situ glass electrode pH measurements under varying thermal conditions. The proposed method addresses equilibrium temperature-dependent effects of the internal solution but does not explicitly account for transient hysteresis, pressure effects, or long-term sensor drift.