Electrochemistry What is 3-point Calibration in pH Measurements?

A pH electrode needs to be calibrated regularly. It is recommended that you do this at least once a day before you start your pH measurements. This article gives you a step by step instruction.

What is calibration? It is the process of adjusting and validating the accuracy of measurement instruments or devices. It ensures that the outputs of these instruments correspond accurately to the known standards. This process is vital across fields, including science, engineering, and manufacturing, as it assures that measurements taken are reliable and consistent.

Technically speaking, calibration is the determination of the parameters of the calibration curve: slope and offset. Ideally, a perfect pH electrode has a slope of 100 per cent and an offset of 0 mV at pH 7.00.

But such ideal electrodes do not exist in practice. For this reason, it is essential to calibrate every pH electrode regularly. It is advisable to conduct this calibration at least once a day before commencing any measurements. During calibration, the real slope and real offset of the electrode are determined. These parameters are influenced by the unique characteristics of the sensor and may change over time as the electrode ages.

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

Calibration is essential for adjusting the slope and offset of an electrode to their accurate values specific to the measuring system being used. Once this calibration is complete, the resulting calibration curve can be utilized to relate the electrode's measured mV values to the corresponding pH value of the solution being analyzed.

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, 7.00, and 10.00, it is possible to obtain two curves, respectively, for the ranges of 4.01 — 7.00 and 7.00 — 10.00, as well as two slopes. This method better reflects the actual pH electrode response.

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: 2.00, 4.01, 7,00.

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 often not practiced. 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.

What is 3-Point Calibration?

A 3-point calibration is a specific method used primarily in pH measurement. It is known to be one of the most common calibration methods. This technique involves using three standard buffer solutions, each with a known pH value, to create a calibration curve for the pH meter. These calibration points are usually pH 4.00, pH 7.00, and pH 9.21 or 10.00. The pH scale typically ranges from 0 to 14, with 7 being neutral. 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?

Three-point calibration is commonly used in various segments where precise pH measurements are crucial. Some of these segments include:

Food and Beverage: The pH level is important for food safety, quality and stability. So manufacturers often use three-point calibration for accurate measurements:

• 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 (around 7). 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 following industries:

• Laboratory Research: In chemical and biological research, accurate pH measurement is vital for experiments and analyses.

• Pharmaceuticals: pH is a critical parameter in drug formulation and testing, requiring precise calibration to ensure product quality.

• Environmental Testing: In monitoring water quality and assessing soil pH, three-point calibration helps provide reliable data for environmental studies.

• Industrial Processes: Many manufacturing processes, such as chemical production and wastewater treatment, rely on accurate pH control, necessitating proper calibration.

• In the pharmaceutical industry, cosmetics, wastewater treatment, academia, etc.

You can learn more about how to calibrate a pH sensor accurately by registering for our webinar. Download our pH theory guide to learn all about pH measurements.

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