This chapter elevates pH measurement from a qualitative tool to a quantitative instrument subject to rigorous calibration and quality-control procedures. The Nernst equation (E = E° + 0.0592 × pH at 25°C) is introduced to explain how the glass electrode generates a voltage proportional to the H+ activity, and why the theoretical slope of 59.2 mV/pH unit is the reference against which instrument performance is judged. The two-point calibration procedure uses certified pH 4.01 (potassium hydrogen phthalate) and pH 9.00 (borax) buffers, with the percent slope (95–105% acceptable) calculated as a metric of electrode condition. The critical electrode care sequence is specified as the single most important procedural step for reproducible results. A major section addresses electrode storage: the glass membrane must never be stored in deionised water, which draws KCl from the reference junction and permanently degrades the electrode; 3 M KCl solution is the correct storage medium. The chapter concludes with a systematic troubleshooting guide for common instrument faults.