How does a pH-meter work?

A pH meter or pH meter is an instrument used to measure the acidity or alkalinity of a solution. The information provided by this equipment expresses the degree of acidity of an acid or a base in terms of the activity of the hydrogen ions.

The pH is the unit of measurement that describes the degree of acidity or alkalinity and is measured on a scale ranging from 0 to 14. An approximate indication of pH can be obtained using pH indicators or tapes, which change color depending on the variation of the pH level, but these indicators present limitations in terms of accuracy and can be difficult to interpret correctly in dark or colored samples, and this error can be corrected extensively through the use of a pH meter.

How is a pH meter composed?

A pH meter consists of three parts: a pH measuring electrode, a reference electrode, and a high input impedance meter. The reference electrode has a known, constant and stable potential.

The pH electrode can be considered as if it were a battery, with a voltage that varies according to the pH of the measured solution. The electrode that measures pH is a glass bulb sensitive to hydrogen ions, with an output in millivolts that varies according to changes in the relative concentration of hydrogen ions inside and outside the bulb. The output of the reference electrode does not change with the activity of the hydrogen ions.

The pH electrode has a very high internal resistance, which makes it difficult to measure the voltage variation with pH. Therefore, the impedance of the pH meter input and leakage resistances are important factors.

pH meter operation

A pH meter works through an electrochemical cell mechanism, where the H + in the chemical reaction allow determining the concentration of them in solution. When determining pH with a pH meter, a reference electrode and a measurement electrode are used, which are sensitive to analytes.

In other words, a pH meter is a high impedance amplifier that accurately measures the minimum electrode voltages and displays the results directly in pH units on an analog or digital display. In some cases, the voltages can also be interpreted for special applications or use with ion selective or oxidation / reduction potential electrodes.

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