The two most important measurements in water quality.
If there were only two measurements I could take in water to assess that water's health they would be pH and ORP. The reason is relatively simple. Almost all major and important chemical and biological processes that happen in water are either directly influenced by, or conversely alter the pH and ORP. If all of these processes are relatively well balanced or 'behaved' then the pH and ORP values for your water should fall within acceptable limits for that water type. For example, if you want to drink the water the pH - ORP values should lie within the long dashed area in the next diagram.
Some practical applications of this are in water treatment and water quality management. For example if you were trying to reduce iron in water and the ORP was very low, pH would have to be very high to make the iron relatively insoluble. I've drawn a line on the diagram that shows approximately at what ph / ORP combination the iron will become less soluble. Therefore both pH and ORP have to be considered. Another good example is in chlorine disinfection. Sometimes ORP is used as a measure of oxidizing power in the water rather than measuring the chlorine level. But disinfection capability of chlorine is also pH dependent so pH has to be measured.
pH - ORP phase diagram for a selection of water types. Adapted from Langmuir, D. (1997). Aqueous Environmental Chemistry. New Jersey, Prentice-Hall, Inc.
There are many ways to measure pH in water from test strips to test kits, instruments and testers. You will find many examples of these on this website. ORP is a little more tricky. It is usually done with an electronic tester or meter. ORP is measured in millivolts. Here is an example of a tester that conveniently combines these two important factors. See it at Hanna Instruments 98121 pH / ORP tester.
For a more detailed explanation of ORP (REDOX) in water see What do your water test results mean?
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