Polyprotic Acids

Many acids contain two or more ionizable hydrogens. There are two in carbonic acid, H 2CO 3, and three in phosphoric acid, H 3PO 4. For any such multiple hydrogen acid, the first hydrogen is most easily removed, and the last hydrogen is removed with the greatest difficulty. These acids are called polyprotic (many protons) acids. The multiple acid ionization constants for each acid measure the degree of dissociation of the successive hydrogens.

Table 1 gives ionization data for four series of polyprotic acids. The integer in parentheses after the name denotes which hydrogen is being ionized, where (1) is the first and most easily ionized hydrogen.

Remember: The strongest acids dissociate most readily. Of the nine acids listed in Table , the strongest is sulfuric (1), with the highest acid ionization constant, and the weakest is phosphoric (3).

Here are the chemical equations for the three successive ionizations of phosphoric acid: 

Consequently, an aqueous solution of phosphoric acid contains all the following molecules and ions in various concentrations:


Consulting the table of the dissociation constants K a's for phosphoric acid shows that the first dissociation is much greater than the second, about 100,000 times greater. This means nearly all the H 3O + ( aq) in the solution comes from the first step of dissociation. The second and third steps add very little H 3O + ( aq) to the solution. So a solution of phosphoric acid will contain H 3PO 4 molecules in highest concentration with smaller, and nearly equal, concentrations of H 3O + and . The and ions are present in very small concentrations.

  • What is the principle species in a solution of sulfurous acid, H 2SO 3, a weak polyprotic acid? List H 2SO 3, , , and H + in order of decreasing concentration.