First Dissociation Constant of Phosphoric Acid

Phosphoric acid gives up its first proton readily — K_a1 = 7.11 × 10⁻³ makes it about four hundred times stronger than acetic acid, strong enough that a 0.1 M solution is roughly 25 % dissociated and the usual weak-acid approximation starts to break down. This step is what gives cola its tartness and its pH of about 2.5, and what lets dilute phosphoric acid strip light rust while leaving a passivating iron phosphate film behind.

Each successive proton is roughly five orders of magnitude harder to remove than the last: 7.11 × 10⁻³, then 6.32 × 10⁻⁸, then 4.5 × 10⁻¹³. That regular spacing is a general feature of polyprotic acids and it makes phosphate unusually versatile, since one substance provides three separate buffering regions spread across almost the whole pH scale.