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Measuring the pH of a Solution

Another frequently used measure of water quality along with electrical conductivity and turbidity is the pH. pH is a measure of the hydrogen ion activity in a solution. Solutions with high hydrogen (H+) or hydronium ion (H3O+) activity are called acids and solutions with low hydrogen ion activity are called bases. Another way of looking at it is by looking at the reciprocal quantity of hydroxyl ions (OH-). The more hydroxyl ions, the more basic a solution is and so on. pH is a mathematical way of quantifying these concentrations and placing them on a relative scale. It is defined as:

In a parallel fashion the quantity pOH is defined as:

Conversely, to determine the activity or concentrations given the pH or pOH, we have:

The relationship between pH and pOH at standard temperature conditions (25 degrees Celsius) is:

pH + pOH = 14

From here, we can see that the relative scale used is from a pH of 0 to a pH of 14 with 0 being very acidic, 14 being very basic, and 7 being neutral.

There are 2 basic types of techniques for measuring pH. The first type is called colorimetric measurement. The use of litmus paper is a colorimetric technique. The second type is called electrometric and is done using an electrical instrument. The instrument is calibrated with 3 standard solutions at known pHs of 4, 7, and 10. Then the instrument can be used to measure unknown solutions. The typical range for drinking water is ideally around 6 – 8.5.