Hydrogen index (pH) characterizes hydrogen-ion concentration (more accurately – activity) in various solutions. Numerically equals negative common logarithm of hydrogen-ion concentration (gram-ion per 1 liter): pH= –lg[Н+] g-ion/l, where [Н+] – hydrogen-ion concentration.

Hydrogen index is usually defined by acid-based indicators with colors changing depending on the pH level, more accurately – with potentiometer method.

Hydrogen index impacts the direction and speed of many chemical reactions and bio-chemical processes. Definition of pH is required for flotation, hydro-metallurgy and waste water treatment.