Background

pH meters have been used since 1936 when they were invented by the Danish company Radiometer.In contrast to many other measuring devices pH meters do not measure concentrations, rather they measure activities. Tobe more specific the pH meter does not measure the concentration of hydrogen ions.

pH meters are used to measure theactivity of hydrogen ionsin aqueous solutions:

pH = -Log₁₀{H⁺} or pH = -Log₁₀a_H+^.[H+]

Note:{H⁺} is the normal way of writing the activity of hydrogen ions but a_H+^.[H+] is also used often.

The activity of hydrogen ions is calculated into a pH value that is a measure of the acidity oralkalinity/basicity of the solution. Most pH meters have a digital display connected to the probeor electrode where both the pH and the temperature in the solution can be read.

Advanced pH meters even have conductivity meters build in measuring conductivity in mS/cm. Below isa couple of pictures showing different types of pH meters:

Calibrating a pH meter

Calibrating a pH meter is done semi automatically with the help of the technician (or you) using the pH meter.Usually 2 or 3 solutions are used for the calibration procedure that involves telling the pH-meterthat a calibration is going to take place and then measuring pH in different colored aqueous buffersolution with pH values of 4, 7 and 10 or sometimes other pH values depending on pH meter type andmanufacturer.

The software in the pH meter knows when the electrode is in the pH 4 solution and when the electrodeis in the solution with a pH value of 10.

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Other pH meter relevant information

Cleaning a pH meter:

pH meter should be cleaned regularly by applying a cleaning solution on the glass electrode

In between experiments:

The glass electrode should be submerged in a aqueous solution provided by the manufacturer; usually a solution with a pH of 3.

Considerations before buying a pH meter

If you are going to buy pH meters for your department you might as well consider buying at least onepH meter with a build in conductivity meter.

Also, you might choose a pH meter with a build in thermometer so you don’t have to type it into the pHmeter yourself when you are going to calibrate. Secondly, and depending on what you are working with,you might also consider having a wire from the electronic pH meter to the probe of a length morethan 1 foot.

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pH strips

Background Substances with the property to change color when they come into contact with an either acidic or basicsolution are called pH indicators. This is used in pH strips that changes color when they come into contactwith hydrogen ions (H+) and hydroxide ions (OH–). Usually, pH strips are bought in packages with either 50 or 100 strips. pH strips are most useful for demonstration experiments or experiments where accuracy of the measured pH is of minor importance. Using pH strips is simple. The strip is simply submerged in the aqueous solution and the reading of pH is done by comparing the color on the pH strip with the color on the package that the pH strips came in. The accuracy of any pH reading is within 0.5 pH units.

Web resources

Technical explanation