The negative log of the hydrogen ion concentration is called pH ( pH= - log₁₀ [H⁺]). The hydrogen-ion concentration is an important quality parameter of both natural water and wastewater. The pH of wastewater needs to remain between 6 and 9 to protect organisms. Acids and other substances that alter pH can inactivate treatment processes.

The pH of aqueous systems can be conveniently measured with a pH meter (see figure 4). Various pH papers and indicator solutions that change colour at define pH values are also used. The pH is determined by comparing the colour of the paper or solution to a series of colour of standard.

Figure 4: pH meter

Possible errors by measurement of pH can be followings:

• pH-sensor and meter are not connected properly,

• pH-glass electrode is polluted or damaged,

• Diaphragm of the reference electrode is black (Sulphide contamination),

• Reference electrolyte solution is used up,

• pH electrode is placed in distilled water and not in the same concentration and electrolyte solution,

• Wrong temperature for calibration and/or measurement,

• Wrong or old puffer for calibration,

• probe or cable is broken.