triprotinen

Triprotinen refers to substances capable of donating three protons in aqueous solution, in particular triprotic acids. A typical triprotic acid is denoted H3A and dissociates in three sequential steps: H3A ⇌ H2A− + H+, H2A− ⇌ HA2− + H+, and HA2− ⇌ A3− + H+. Each step is described by a dissociation constant Ka1, Ka2, Ka3, with corresponding pKa values.

In many triprotic acids the dissociation constants follow Ka1 > Ka2 > Ka3, so the first proton is released at relatively low pH, the second around neutral to mildly basic pH, and the third at high pH. This leads to three characteristic buffering regions around pKa1, pKa2, and pKa3, allowing the solution to resist pH changes over a wider range than monoprotic or diprotic acids.

Common examples of triprotic acids include phosphoric acid (H3PO4), citric acid (H3C6H5O7), and arsenic acid (H3AsO4). Approximate pKa values illustrate their behavior: phosphoric acid pKa1 ≈ 2.15, pKa2 ≈ 7.20, pKa3 ≈ 12.35; citric acid pKa1 ≈ 3.13, pKa2 ≈ 4.76, pKa3 ≈ 6.40. These values define the pH ranges where each successive proton is increasingly likely to dissociate.

Applications of triprotic acids include buffering in chemical analyses, biochemical assays, and environmental chemistry, where phosphate and citrate systems are commonly used. In titrations, the three deprotonation steps produce three inflection points on a pH versus added-base curve, reflecting the stepwise loss of protons.