IC Controls Ltd.

pH Theory & Measurement


Courtesy of IC Controls Ltd.

pH is a short form for the Power (p) of Hydrogen (H). pH is defined as the negative log of the hydrogen ion activity, aH~ or the effective hydrogen ion concentration.

Descriptive Definition
pH is a unit of measure which describes the degree of acidity or alkalinity of a solution. Acidity is defined as the concentration of hydrogen ions [H+] in solution and alkalinity' as the concentration of hydroxyl ions [OH-] in solution. As seen above the actual theoretical definition of pH is -LoglO aH-h But. since the activity coefficient (a) for hydrogen (H+) is 1 the practical definition for pH can then be defined as -LoglO [H-]. Below is a chart showing the relationship between pH and the Hydrogen ion concentration.

There are two ways of measuring pH. The first is a Coloiimetric Method using color indicators to indicate the pH of the sample. There are limitations to this measurement technique. For instance, visual measurement by an operator is subject to variation. As welL this technique is done by grab sample which is not suitable for continuous on line measurement. A more effective way to measure pH in an industrial setting is the Potentiometric

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