pH Buffer

A pH buffer is a aqueous reference solution of known pH value.

A pH buffer contains a weak acid and conjugate base, or the other way around, in a certain concentration. This constitutes the buffer system. Any addition of a small amount of even a strong  acid or base will have minimal influence on the pH value of the buffer.

The buffers used for calibration must be selected according to the pH of the sample or process.
For instance, if a sample is expected to have a pH of 7.45 or the process to have a pH range of 6.9 to 8.4 the calibration should include pH buffers 7.00 and 9.21 (or similar).
|Use a minimum of two fresh buffers for calibration in the lab and do a two-point calibration in a process environment.

pH Buffers are essential for the adjustment and calibration of a pH sensor to ensure accurate measurement.

It is not necessary to include pH 7.00 for calibration according to the US and European pharmacopoeias (USP and EP, respectively). But if the calibration is done in compliance with the Japanese Pharmacopoeia (JP), then a pH 6.86 buffer (phosphate) is mandatory. For calibration according to EP, only pH buffer 4.01 (phthalate) is compulsory. If not working according to any regulation, the principle is easy: For calibration, only those pH buffers are needed which bracket the pH range of your samples or your process. For example, in a quality control situation where every sample should have a pH between 3.5 and 3.8, a two-point calibration with pH 2.00 and 4.01 is sufficient. There is no need for calibration with pH 7.00 in such a case.

The quality of a pH buffer solution is only guaranteed for unopened bottles that have not yet expired. The following tips will help optimize the use of pH buffer solutions after opening and while in use in the laboratory. Fresh calibration solutions minimize the measurement uncertainty and thus optimize the reproducibility of results:

• Note the date on the bottle when the pH buffer is first opened and dispose of it after the recommended time.
• Keep the bottles sealed tightly and store them at room temperature.
• Never pour dispensed pH buffer solution back into the original bottle.
• Ensure that bottles containing pH buffer do not become contaminated.
• Replace pH buffer solutions that have reached the expiry date or may be contaminated.

Note: Fresh calibration solutions minimize the measurement uncertainty and thus optimize the reproducibility of results.

Certificates for all our calibration solutions and many electrolytes can be downloaded at www.mt.com/buffer. The respective material safety datasheets are available at the same place.

The expiry date indicates the date until which the certificate guarantees the value and the associated measurement uncertainty. After this date, the uncertainty cannot be guaranteed, so it is better to discard expired pH buffer.

Never calibrate directly in the solution bottle. Use a fresh aliquot of pH buffer for each calibration to ensure no contamination enters the bottle.

Regular calibrations will lead to more accurate measurement results. Some applications might require a calibration before every measurement, but in general, for laboratory measurements it is enough to calibrate every 24 hours. Always use fresh pH buffers for calibration. In-line measurement is different, the calibration interval depends on the application. Most commonly, the calibration interval is two weeks, but everything from days to even months is possible.