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Calibration and adjustment of a pH electrode

5/1/2023 | Dr. Klaus Reithmayer

Why do you have to calibrate? The pH value of an aqueous solution is determined with pH measuring instruments, also known as pH meters. The accuracy of the pH measurement depends on the calibration and adjustment. The adjustment is the setting of the pH meter to the data of the pH electrode (slope and zero point) determined by calibration. Therefore, regular calibration is very useful.

pH electrodes interact directly with the medium that is being measured. This has an influence on their properties during their lifetime. It is also worth noting that due to their nature, they are never "ideal" even when manufactured, and require calibration and adjustment. What do we mean by that?

We always speak of "calibration" but this only describes part of the procedure:

"Calibrating" means comparing a measurand, in this case pH, with a standard to determine the deviation.

The second term, which is often misunderstood here but is implicitly meant, is "adjustment". This means that the detected deviations are compensated so that the sensor shows the ideal values. This process must be performed regularly. The frequency can depend on the use, but also by internal company regulations.

Buffer solutions - standard solutions for pH measurement

Buffer solutions are stable aqueous mixtures of basic and acidic salts that are not completely dissociated and tolerate small additions of acids or bases by shifting the dissociation without a significant change in their nominal value. They have a defined value to which the potential of a pH electrode is compared.

The worldwide valid standard buffer solutions are referred to as “DIN/NIST Buffers” according to DIN or other standardization institutes listed in the corresponding standards with their formulation. Here are three examples of these buffer solutions:

bufferbufferbuffer

Fig 1: Standard buffer solutions for pH measurement.

The second type of buffers, which is much more common but, unlike DIN/NIST buffers, is subject to manufacturer-dependent formulations, are known as “Technical Buffers”.

bufferbufferbuffer

Fig. 2: Technical buffer solutions for pH measurement.

The difference is that with DIN/NIST buffers, the pH curve as a function of the temperature of the solutions is generally available because of the standardized composition and can be stored in all pH measuring instruments. 

In the case of technical buffers, each manufacturer may use their own formulations and determine the temperature dependence themselves. Therefore, these buffers are often not applicable to other instruments, but refer only to makes of their own manufacture. More on this later in the chapter Temperature dependence of pH measurement.

“Tip: It is important for all pH buffers that they are traceable to international standards. This means that all buffers have guaranteed values within the specified error tolerances and are comparable with other buffer solutions.”

This traceability is generated by comparison against a primary standard (a hydrogen standard electrode) and commercially available secondary standards derived from it. Due to its size and complexity, this type of pH electrode is only set up at metrological institutes such as the Physikalisch-Technische Bundesanstalt in Braunschweig and has nothing to do with a pH glass electrode. The buffer values and the deviations are documented for the customer by many manufacturers via batch certificates.

Suitable buffer solutions for your pH-measurement can be found here!

What happens during calibration and adjustment of a pH electrode?

As already noted above, there are two parameters that characterize an electrode: One is the zero point and the other is the slope. Usually, electrodes are calibrated so that the straight line passes through the neutral point (= pH 7, equivalent to 0 mV). The second parameter is the slope. It is fixed at 59.2 mV/pH, the value at 25 °C. This requires at least two buffer solutions. Depending on the range where the later measurements take place, one chooses either (for example) buffer 4 and 7 (neutral to slightly acidic) or buffer 7 and 10.01 (neutral to slightly basic). The often-used term “alkaline” should not be used, it fixes the origin of basicity to solutions of alkali or alkaline earth oxides. However, there are also compounds which generate OH- ions in aqueous solution.

(with buffer pH 7)

Fig 3: Determination of the zero point with buffer 7.

Fig 3: Determination of the zero point with buffer 7.

The instrument compares the measured values of the internally stored buffer set and determines the respective deviation. This is then electronically compensated, and the characteristic curve of the electrode is idealzed. This guarantees a reliable pH measurement. 

Important: Buffer solutions must always be discarded after use. Used buffer solutions must not be reused under any circumstances nor tipped back into the container!

The pH value and the temperature

The measurement of the pH value has two temperature dependencies.

One is caused by the Nernst equation. If you look at the term again, you can see that the slope depends on the temperature. If we consider only this value as a function of different temperatures, we get a deviation in the measured voltage which has nothing to do with the actual pH value:

Nernst-Gleichung Parameter pH

Fig. 5: Temperature dependence of the Nernst voltage.

Fortunately, this influence can be compensated by temperature compensation during pH measurement. This is done either automatically by a connected temperature sensor or manually. In any case, the temperature of the solution must be considered, otherwise incorrect measurements will occur.

The second dependence of the pH value concerns the measured solution. Its pH value also depends on the temperature, but: the dependence is usually completely unknown. Solutions whose pH value is determined as a result of temperature are buffer solutions.

“Tip: It is generally true that the pH value and the respective temperature must be indicated. pH values without documented temperature are unfortunately meaningless.”

Abhängigkeit der pH-Werte unterschiedlicher Lösungen von der Temperatur

Fig. 6: Examples of the dependence of the pH values of different solutions on the temperature.

The pH value also has an influence on the service life of the electrode. In principle, the rule of thumb is “the higher the operating temperature, the shorter the service life”:

  • Laboratory electrodes can achieve lifetimes of up to 3 years, depending on the model and type of sample
  • Process electrodes in the chemical industry sometimes only have lifetimes of a few days or weeks at temperatures of more than a hundred degrees and pressures of several bars

Buffer Solutions

Our buffer solutions are referenced against secondary standard reference material. Common certificates document the respective uncertainty of the pH value of the solution. 

Buffer solutions in plastic bottles 

Buffer solutions in glass ampoules

  • STAPL-4/7/9 precision DIN/NIST buffer in ampoules with ± 0.01 pH accuracy. 
  • QSC (Quality Sensor Control): With the QSC Kit consisting of three precision DIN buffers (pH 4.01, pH 6.87 and pH 9.18 with an accuracy of respectively ± 0.01 pH at 25 °C) in glass ampoules, an initial calibration can be carried out with IDS pH electrodes. Ideal for quality control: All following.calibrations are compared with this calibration

The pH guide with helpful practical tips

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Further questions are answered by our expert in the blog article FAQ pH measurement.

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