When calibrating a pH meter for a staining solution, which standard should be used?

• A. A standard with a pH value closest to that of the staining solution
• B. A standard with pH exactly 7
• C. A standard with pH far from the staining solution
• D. A standard with pH 4

Answer: (A)

When calibrating a pH meter for a staining solution, you want the calibration point to be as close as possible to the pH of the solution you will measure. The electrode’s response is most accurate and most linear near the pH values around the calibration buffer, so anchoring the calibration near the staining solution’s pH yields the most reliable reading. If the buffer used for calibration is far from the staining solution’s pH, the slope and offset you determine won’t accurately describe the electrode’s behavior in the region where you’re measuring, leading to larger error in the reported pH. A neutral buffer (pH around 7) is fine when the staining solution is near neutral, but it becomes problematic if the solution is more acidic or basic. In practice, you’d also use a second buffer on the opposite side to bracket the expected pH, which helps stabilize the calibration and improve accuracy across the range of interest.