pH is measured to determine the activity of hydrogen ions[H+] in the solution (The p stands for “potential of” and the H is hydrogen). A pH sensor measures how acidic or basic the water is, which can directly affect the survival of aquatic organisms. pH ranges from 0 (very acidic) to 14 (very basic), with 7 being neutral. Most waters range from 5.5 to 8.5. Changes in pH can affect how chemicals dissolve in the water and whether organisms are affected by them. High acidity (such as pH of less than 4) can be deadly to fish and other aquatic organisms.
A pH sensor uses an electrode with hydrogen ion selective glass membrain that measures the difference in electrical potential between the sample and a reference electrode. The electrical potential is proportional to the hydrogen ions and must be corrected for temperature or the instrument may contain an automatic temperature compensation circuit.