Many pH meters also have a separate reading range which gives results in mV. This can be used for a range of applications.
1.) Checking your pH electrode calibration. Most meters automate the calibration of pH which makes it simple to perform and ensures consistency in results. However, when issues arise the mV mode can be a useful tool. When measuring pH buffers, a standard pH electrode will give a known response in mV. So for pH the mV result should be 0 +/-25mV. The results at 4/10 should be 177 +/- 10% (these values are at 25 degrees, slightly lower values will be seen at 20 degrees). If these values are a long way from the expected normal then it is likely there is a problem with either the electrode or the buffers.
2.) Redox or ORP electrodes, for the measurement of Redox in solutions usually supplied with a BNC connector similar to a pH electrode.
3.) ISE electrodes or Ion Selective Electrodes can be used to analyse dissolved ions in water samples such as chloride, fluoride and sodium. The ion gives a similar response to a pH electrode according to the Nernst equation. So a monovalent ion such as Fluoride will give a response of 59.16mV per decade change (0.1 to 1.0mg/l, 1.0 to 10mg/l). To use this mode on the meter, you need an electrode with the correct cable or adapter, a series of calibration solutions (preferably covering at least 2 decade changes), and ISAB or ionic strength adjustment buffer.
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