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EXPERIMENT 3. ACID-BASE TITRATIONS: DETERMINATION OF CARBONATE BY TITRATION WITH HYDROCHLORIC ACID BACKGROUND Carbonate Equilibria In this experiment a solution of hydrochloric acid is prepared, standardized against pure sodium carbonate, and used to determine the percentage of carbonate in a sample. An aqueous solution of hydrochloric acid is almost completely dissociated into hydrated protons and chloride ions. Therefore, in a titration with hydrochloric acid the active titrant species is the hydrated proton. This species is often written H3O+, although the actual form in solution is more correctly (H2O)nH+. For convenience we designate it simply H+. Carbonate in aqueous solution acts as a base; that is, it is able to accept a proton to form bicarbonate ion. 2CO3 + H+ HCO3 (1)

Bicarbonate is able to combine with another proton to form carbonic acid: HCO3 + H+ H2CO3 (2)

Equilibrium expressions for the dissociation of bicarbonate and carbonic acid may be written 2[H+] [CO3 ] K2 = (3) [HCO3] [H+] [HCO3] [H2CO3]

and

K1 =

(4)

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where K1 and K2 are the first and second acid dissociation constants for H2CO3; the experimentally determined values are K1 = 3.5 x 10-7 and K2 = 5 x 10-11. When successive protonation reactions such as (1) and (2) occur, the extent to which the first reaction proceeds before the second begins depends on the difference between the two acid dissociation constants. By combination of Equations (3) and (4) with those for charge and mass balance, [H+] can be calculated for any ratio of hydrochloric acid to initial carbonate concentration, that is, at any point on a titration curve of carbonate with hydrochloric acid. Because complete and rigorous solution is time consuming, here only procedures for calculating the pH at several convenient points in a titration of 0.1 M sodium carbonate with 0.1 M hydrochloric acid