pH Measurement - How pH Sensor Works?

pH is a measure of the acidity of a process liquid. Continuous measurements of pH of incoming sewage are frequently made, particularly in plants where drastic changes in pH (as a result of industrial discharges) cause treatment problems.

A glass electrode, which is sensitive to hydrogen ion activity, measures the pH of an aqueous solution.

The electrode produces a voltage related to hydrogen ion activity and to pH.

The pH is determined by measuring the voltage against a reference electrode.

While it is generally assumed that no other ions seriously affect the pH electrode in an aqueous system, sodium ions can have an effect.

Temperature corrections are also necessary, but are typically done automatically by the meter.

pH Measurement

A typical pH sensor is shown in Figure.

Source: WEF,2008

Precautions for O&M are as given below.

• Dirt on the electrode surface should be periodically removed and the surface cleaned

• Since the electrodes of the pH meter are made of glass, care is necessary to ensure that they do not break

• Due to long period use of glass electrodes, dirt sticks on them gradually, the zero point changes, and the electromotive force by pH reduces and it stops responding to changes in pH, making replacement necessary

• Standard liquid should be used in the pH meter and it should be calibrated. Calibration should include zero adjustment (standard liquid with pH7) and span adjustment (standard liquid of pH4 or pH9)