What is pH sensor calibration?

Calibration is the determination of the parameters of the calibration curve: slope and offset.

The perfect electrode has a slope of 100% and an offset of 0 mV at pH 7.00.

But do the perfect electrodes exist in real life? Not really! That’s why every pH electrode should be regularly calibrated. It is recommended to perform this procedure at least once a day before starting measurements. During calibration, the real slope and real offset of the electrode are determined. These two factors depend on the specific characteristics of the sensor and change in time as the electrode gets older.

The theoretical slope and offset are obtained from the Nernst equation:

E = E0 + 2.3RT / nF * log [H3O+] = E0 – 2.3RT / nF * pH

Slope = 2.3RT / nF

Calibration is necessary because it allows the slope adjustment and offset of the electrodes to their true values for a given measuring system. The calibration curve is then used to correlate the measured mV values of the electrode with the pH value of the solution being tested.

Correlation between mV value measured by pH electrode and pH value in the sample. The curves shown are for the theoretical behavior, for offset compensated behavior, and slope and offset compensated behavior. Source: Mettler Toledo

Since an electrode is characterized by both its zero point and its slope, it is advisable to do a minimum of a two-point calibration for reliable measurements and better precision. When measurements are performed over a large range of pH values, it is recommended that one takes at least three calibration points. Most pH meters can do three to five point calibrations. It is important to note that one should only measure samples within the chosen calibration region.

Linear or segmented?

There are two types of calibration: linear and segmented.

As a result of linear calibration, all calibration points are combined into one curve using linear regression, and one slope is determined. For wide measurement ranges, this method is less accurate because the electrode response is not linear in the range of 0 to 14 (due to acid and base error, among other factors).

Segmented calibration, on the other hand, allows for the determination of several calibration curves, respectively, for each segment, e.g., using solutions of 4.01, 7.0, and 9.21, it is possible to obtain two curves, respectively, for the ranges of 4.01-7.0 and 7.0-9.21, as well as two slopes.

This method better reflects the actual pH electrode response.

Source: Source: Mettler Toledo

How do you choose calibration points?

The calibration range should be wider than the pH range of the samples being measured. The number of calibration points depends on the range as well as several other factors:

• The distance between calibration points should not exceed 3 pH units.
• It also shouldn’t be less than 1 pH unit. Otherwise, the automatic buffer recognition function may not work.

For example, if the pH range of the samples is 3.5 to 5, calibration should be performed in three points: 4.01, 7.00, 9,21.

If the pH range of the samples is 3.5 to 10.5, calibration should be performed in five points: 2.00, 4.01, 7.00, 9.21, 11.00.

Although modern pH meters allow for calibration at five points, it is not recommended. Such a wide pH range means working with both strongly acidic and strongly alkaline compounds, and such extreme conditions significantly shorten the electrode's lifetime. In this situation, it is recommended to work with two separate electrodes: one for low pH and one for high pH.

Why is three-point calibration the most popular one?

In practice, the most common calibration method uses three points. These calibration points are usually pH 4, pH 7, and pH 9.21 or 10. This range covers many acidic, neutral and alkaline solutions. However, the specific calibration points may vary depending on the intended use of the pH meter and the measurement range of the electrode. It is important to follow the manufacturer's instructions for calibration and maintenance of the pH meter to ensure accurate and reliable measurements.

In which segments is three-point calibration used?

In juice production, raw fruits and vegetables (pH 3.2 to 7.5), final products (2.5 to 4.00), and water (6.5 to 7.5) measurements – in this case, the recommended calibration points are 4.01, 7.00, and 9.21.

The pH of beer is usually in the range of 3.5 to 4.5, but breweries are obliged to measure the pH of water as well. So again, calibration in 4.01, 7.00, and 9.21 should be performed.

In dairy factories, there are different kinds of products: milk (pH 6.5 to 7.00), sweet cream 6.00 to 6.7), and sour cream (4.2 to 4.5) – the recommended calibration points are 4.01, 7.00, and 9.21.

Three-point calibration is also common in the pharmaceutical industry, cosmetics, wastewater treatment, academia, etc.

To learn more about pH sensor calibration, register for this Mettler Toledo webinar or watch a quick guide here.

To contact the author of this article, email pHmatters@mt.com