pH
pH is a logarithmic measure of acidity defined as the negative base-ten logarithm of hydrogen ion activity in a solution. Activity accounts for the chemical behaviour of an ion in its surroundings; it is not always interchangeable with the ion's concentration. Lower pH corresponds to greater hydrogen-ion activity.[1]
Scale and interpretation
For dilute aqueous solutions, calculations commonly approximate activity using the concentration of hydronium ions. Because the scale is logarithmic, a decrease of one pH unit represents a tenfold increase in the activity used in the definition. This differs from a linear measure in which each numerical step would represent an equal added quantity.[1]
At 25 °C, neutral water has a pH of approximately 7. Acidic aqueous solutions have more hydronium than hydroxide ions, while basic solutions have the reverse relation. Neutrality is defined by the balance between the two, rather than by the number 7 under all conditions. The ionisation of water varies with temperature, so neutral pH also changes.[2]
The related quantity pOH expresses hydroxide-ion concentration on a logarithmic scale in elementary calculations. At 25 °C, pH and pOH sum to approximately 14 in the usual dilute-solution treatment. That numerical sum is temperature-dependent. Values outside the familiar zero-to-fourteen interval are possible; the interval is not an absolute mathematical boundary.[2]
Measurement
A pH definition expressed in terms of one ion's activity cannot be implemented as an entirely independent single-ion measurement. Practical measurement therefore relies on standards and agreed procedures. IUPAC's definition distinguishes this operational problem from the usefulness of pH as a reproducible quantity. Calibration and measurement conditions matter when results from different solutions or experiments are compared.[1]
References
Attribution
Adapted with reference to English Wikipedia, revision 1381177549 (contributors and history), under CC BY-SA 4.0. The prose has been rewritten and checked against the cited sources; changes have been made.