What buffers are used for pH meter calibration?

• A. Use buffers pH 6, 8, and 9.
• B. Use buffers pH 9, 9.5, and 11.
• C. Use buffers pH 7, 4, and 10.
• D. Use buffers pH 3, 7, and 12.

Answer: (C)

Calibrating a pH meter relies on known, stable buffer pH values so the instrument can set its slope and zero offset accurately. The best approach is to use three buffers that bracket the range of samples you’ll measure: one acidic, one near neutral, and one basic. This combination lets the meter map the full response of the electrode, accurately determining how many millivolts correspond to each pH unit across the range you’ll encounter.

Using buffers with pH 7, 4, and 10 provides that balance: pH 7 gives a solid neutral reference for the offset, while pH 4 and pH 10 anchor the acidic and basic ends to define the slope. With these three points, readings between them are more reliable, and you’ll get good accuracy for typical samples that fall on the acidic–neutral–basic spectrum.

Other sets miss essential coverage. For example, using only high-pH buffers lacks a proper acidic anchor and can skew the slope near neutral, while omitting the neutral point or using unusual extremes can reduce accuracy across the main measurement range. Remember to use buffers at the measurement temperature or apply temperature compensation, since pH readings shift with temperature.