Appreciation of pH as an ecological index has varied considerably over its past history, influenced by perceptions of chemical rigour, ease or difficulty of measurement, and multiple chemical and biological correlations. These factors, and especially the last, are considered in relation to the extensive range of pH in inland waters. Emphasis is placed upon the role of the CO₂ system, the components of which are subject to biological metabolism (photosynthesis, respiration) and are extensively determined by products from rocks (e.g. limestone) and soils.

Titration alkalinity, or acid neutralising capacity, is a most valuable summarising and reference measure. For this, and CO₂ variables, Potentiometric Gran titration opens new possibilities - including the definition of negative alkalinity (acidity). The relationship of pH and titration alkalinity is close and semi-logarithmic for waters in equilibrium with atmospheric CO₂. Very high pH, above 10, can develop from the photosynthetic depletion of CO₂ and by the evaporative concentration of bicarbonatecarbonate waters in closed basins. Very low pH, below 4.5, results from the introduction of strong acids by volcanic emissions, pyrite oxidation, ‘acid rain’ and cation exchange; here the CO₂ system lacks influence, biological diversity is reduced, and ionic aluminium often exerts toxic biological effects.

Situations of pH excursion are discussed and illustrated; they operate over day—night, seasonal and longterm time-scales. A summer rise of pH is widespread in productive near-surface waters. There is also a seasonal pH rise in the anoxic deep water of many lakes, as a consequence of the interaction of acid-base and oxidation-reduction systems. These can be regarded as two ‘master systems’ of environmental chemistry and — dating from pioneer studies of wetland soils and waters — of much freshwater ecology.