Neutralization of a polyprotic acid with a strong base (Key words: Phosphoric acid)

Determination of the concentration of phosphoric acid using NaOH

Theory

The shape of the titration curve will depend on the relative magnitudes of the various dissociation constants. If a dibasic acid H2A has two equivalent independent ionization processes with two dissociation constants K1 & K2, the ratio K1/K2 should be greater than 104 (K1/K2 > 104) in order to titrate the two hydrogen ions separately.
i.e. the acid behaves as a mixture of two acids.
The common polyfunctional acid used for titration is phosphoric (V) acid (orthophosphoric acid). The stepwise dissociation of the acid is represented by the following set of equilibria

Hence the acid will behave as a mixture of three monoprotic acids. The neutralization proceeds almost completely to the end of the primary stage before the secondary stage is affected, and the secondary stage proceeds almost to completion before the tertiary stage apparent.

The pH of the equivalence point is,
pH = 1/2pK1 + 1/2 pK2 = 4.6
At the second equivalence point,

pH = 1/2pK2 + 1/2pK3

and at the third equivalence point,

pH = 1/2pKw + 1/2 pK3 - 1/3 pC

for 0.1M phosphoric acid, pH at the third equivalence point = 12.35

In the third ionization stage, HPO42- behaves as an exceedingly weak acid and the titration curve is very flat (the magnitude of the free energy change is almost zero) and no indicator is available for direct titration. Therefore phosphoric acid behaves identical to a dibasic acid.

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Procedure

Pipette out 25.00 mL of the phosphoric acid into a titration flask and add few drops of bromocresol green indicator and titrate with the 0.20M NaOH solution.
Repeat the titration using thymolphthalein indicator.

I. Calculate the concentration of phosphoric acid.
II. Draw the titration curve and indicate the use of correct indicators in the titration.
III. Suggest an experimental procedure that you would carry out to determine the dissociation constant of the acid. Carry out the suggested procedure in consultation with your demonstrator. Thus determine the concentration of the acid.
IV. Discuss the possibility of using citric acid as a triprotic acid. The dissociation constants of the acid are:
K1 = 9.2 x 10^-4, K2 = 2.7 x 10^-5, K3 = 1.3 x 10^-6
V. You are provided with a mixture of two acids H2A and HB the corresponding dissociation constants are given below.

For H2A K1 = 2 x 10^-2, K2 = 1 x 10^-13
For HB K1 = 3.2 x 10^-7

a). Is it possible to determine the concentration of the two acids in the mixture by titrating with a standard solution of NaOH? Explain your answer giving reasons and calculations.
b). Draw the appropriate titration curve indicating all the necessary features.
c).Thus determine the concentration of individual acids in the mixture if the concentration of NaOH is 0.01M.