pH meter
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 but activities. To be more specific the pH meter does not measure the concentration of hydrogen ions.
pH meters are used to measure the activity of hydrogen ions in aqueous solutions:
pH = -Log10 {H+} or pH = -Log10 aH+.[H+]
Note: {H+} is the normal way of writing the activity of hydrogen ions but aH+.[H+] is also used often.
The activity of hydrogen ions is calculated into a pH value that is a measure of the acidity or alkalinity/basicity of the solution. Most pH meters have a digital display connected to the probe or 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 is
a 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-meter that a calibration is going to take place and then measuring pH in different colored aqueous buffer solution with pH values of 4, 7 and 10 or sometimes other pH values depending on pH meter type and manufacturer.
The software in the pH meter knows when the electrode is in the pH 4 solution and when the electrode is in the solution with a pH value of 10.
Note: For a detailed description of glass electrodes visit Wikipedia
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 one pH 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 pH meter 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 more than 1 foot.