Dreiprotonigen

Dreiprotonigen, or triprotic species, are chemical compounds that can undergo up to three successive proton transfer steps in aqueous solution. In acid–base chemistry a triprotic acid can lose three protons in a stepwise process: H3A ⇌ H2A− + H+, H2A− ⇌ HA2− + H+, and HA2− ⇌ A3− + H+. Each step has its own equilibrium constant, reflected in three dissociation constants (pKa1, pKa2, pKa3). The distribution of protonation states depends on pH and can be described by fractional concentrations (often denoted α0 to α3).

Common examples include inorganic acids such as phosphoric acid H3PO4, which has pKa values around 2.15, 7.20, and 12.35, and organic acids such as citric acid, with pKa values near 3.13, 4.76, and 6.40. Triprotic behavior allows these substances to buffer solutions over wide pH ranges, particularly between adjacent pKa values, contributing to stable buffering around pH 2–3, 4–7, and beyond.

Applications and significance: triprotic acids are important in chemistry, biology, and environmental science because they provide multiple buffering steps and influence metal complexation, solubility, and nutrient availability. Phosphate buffers, which rely on the H2PO4−/HPO4^2− system, are widely used in laboratory and physiological contexts. Citric acid–based buffers serve in food, cosmetic, and pharmaceutical formulations. In some cases, triprotic species can form multiple coordinated or polymeric structures with metals, affecting reactivity and transport.

Notes: not all molecules with three hydrogens act as triprotic acids in solution. For example, boric acid behaves as a weak monobasic species rather than a typical triprotic acid in water.