LEYBOLD® Analysis of Triprotic Phosphoric Acid by Titration

LEYBOLD® Chemistry Experiments.

Phosphoric acid is a triprotic acid. When dissolved in water, it first gives up one proton and dissociates to dihydrogen phosphate, i.e. it reacts like a monoprotic acid. The addition of sodium hydroxide, e.g. during titration, first leads to complete dissociation of the phosphoric acid into dihydrogen phosphate.

The second protolysis, i.e. the reaction to hydrogen phosphate, occurs only after a high pH value has been reached, approx. pH 9. The third protolysis requires a considerably higher pH value. The first two protolysis steps of phosphoric acid are determined in an automatic titration.