First pKₐ of Carbonic Acid

Below pH 6.35 a carbonate water is mostly dissolved CO₂; above it, mostly bicarbonate. That crossover, together with the second one at pH 10.33, produces the familiar three-region distribution diagram that governs alkalinity titration, scale prediction and CO₂ stripping. Because most drinking water sits between pH 7 and 8.5, well clear of both crossovers, bicarbonate is overwhelmingly the dominant species: typically over 95 % of total carbonate.

The value drifts with temperature and, more sharply, with ionic strength: in seawater at 35 salinity the apparent pK_a1 falls to about 5.86, which is why ocean-acidification models never use the freshwater constants. The Langelier saturation index takes the same equilibria and reduces them to a single number telling an operator whether the water will lay down calcium carbonate scale or strip protective film off the pipe wall.