pH Value

Author: Hans Lohninger

In layman's terms, the pH-value is an approximate number between 0 and 14 that indicates whether a solution is acidic (pH < 7), neutral (pH = 7), or basic (pH > 7).

The formula for calculating the pH is:

pH = -log₁₀ [H⁺]

[H⁺] indicates the activity of H⁺ ions (also written [H₃O⁺], the equivalent hydronium ions), measured in moles per litre (also known as molarity). In dilute solutions (like river or tap water) the activity is approx. equal to the concentration of the H⁺-ion.

In aqueous solution at standard temperature and pressure, a pH of 7 indicates neutrality (e.g. pure water) because water naturally dissociates into H⁺ and OH^- ions with equal concentrations of 1×10^-7 M. A lower pH number (for example pH 3) indicates increasing strength of acidity, and a higher pH number (for example pH 11) indicates increasing strength of alkalinity. Most substances have a pH in the range 0 to 14, although extremely acidic or basic substances may have pH < 0, or pH > 14.

In nonaqueous solutions or non-STP conditions, the pH of neutrality may not be 7. Instead it is related to the dissociation constant for the specific solvent used.

pH can be measured by addition of a pH indicator or using a pH meter. Universal indicator changes colour depending on the pH of the solution it is added to. Electronic pH meters consist of an electrolytic cell in which an electric current is created due to the hydrogen cations completing the circuit.