Calibration of the pH electrode requires which of the following?

• A. Two phosphate buffers stored anaerobically at room temperature
• B. Two gases of known pCO2
• C. Two gases of known pO2
• D. Two solutions of known pH

Answer: (A)

Calibrating a pH electrode hinges on anchoring its electrical response to solutions whose pH is precisely known. Using two buffer solutions with known pH values lets you determine both the offset and the slope of the electrode’s response, which is what the Nernst equation describes for pH measurements. Phosphate buffers are a common, reliable choice because their pH values are well characterized and they buffer effectively around the neutral range. Storing the two phosphate buffers anaerobically at room temperature helps keep their pH stable by minimizing uptake of atmospheric CO2, which can drift the pH and spoil calibration. This approach gives you accurate, reproducible calibration data. The other options describe calibration for gas sensors (pCO2 or pO2) and are not relevant to calibrating a pH electrode.