pH

pH is the negative base-10 logarithm of hydrogen-ion activity in a solution. Lower pH indicates greater hydrogen-ion activity; higher pH indicates lower activity.

Hydrogen-ion activity: The effective concentration of hydrogen ions in a solution, accounting for interactions among dissolved particles. pH is defined from this quantity rather than hydrogen-ion concentration alone.
Glass electrode: An electrochemical sensor whose voltage across a glass membrane responds to hydrogen-ion activity. It is the standard practical sensor for measuring pH in many solutions.
Hydronium: The hydrated proton, commonly represented as H₃O⁺, formed when a proton associates with water. In aqueous solutions, hydrogen-ion activity is commonly expressed through hydronium.
pOH: The negative base-10 logarithm of hydroxide-ion activity in a solution. It tracks hydroxide activity rather than hydrogen-ion activity.
Logarithm: A mathematical function that gives the exponent to which a base must be raised to produce a number. The base-10 logarithm makes each pH unit correspond to a tenfold activity change.
pH indicator: A dye that changes color over a characteristic pH range because its molecular forms absorb light differently. Indicators provide a visible estimate of pH without an electronic meter.
Acid: A substance that donates protons or accepts an electron pair, depending on the acid–base framework. Acids increase hydrogen-ion activity in water and generally lower pH.
Titratable acidity: The amount of base required to bring a sample to a specified endpoint pH. Unlike pH, it measures an acid reserve rather than current hydrogen-ion activity.
Acid dissociation constant: An equilibrium constant that quantifies an acid's tendency to donate protons in solution. Acid dissociation equilibria determine hydrogen-ion activity and therefore solution pH.
Ocean acidification: The long-term decline in ocean pH caused mainly by uptake of atmospheric carbon dioxide. Ocean pH tracks a major consequence of carbon dioxide entering seawater.

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