Displacement titrations (Key words: Bronsted bases, acetates, carbonates, borates)

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Neutralization of anions of weak acids (Bronsted Bases) with strong acids

Theory

Titrations are also possible with weak bases (Bronsted bases) such as acetates, carbonates, borate ions. Here the OH- produced due to hydrolysis of the salt will react with the strong acid.

The weak acetic acid was apparently displaced by the strong HCl, and the process was referred to as displacement titration.

The so called titration of solutions of hydrolysis is merely the titration of a weak base with a strong acid (highly ionized).

Titration of carbonate ions with a strong acid

Determination of the concentration of carbonate using 0.1M HCl

Theory

The dissociation constant for H2CO3 are,

K1 = 4.3 x 10^-7, pK1 = 6.37
K2 = 4.67 x 10^-11, pK2 = 10.33
A solution of carbonate ion can be titrated to the hydrogen carbonate stage with HCl acid.

Equation 1 -The equivalence point for the primary stage of ionization
Equation 2 -The solution may also be titrated until all the carbonic acid is displaced.
Equation 3 -The same end point is reached by titrating hydrogen carbonate solution with HCl.
The pH of this equivalence point can be calculated if the concentrations are known. It is approximately 3.7 (for equal volumes of 0.1M HCl and 0.1M Sodium hydrogen carbonate)

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Procedure I

Pipette out 25.00mL of the given carbonate solution into a titration flask, add few drops of phenolphthalein indicator and titrate the solution with 0.1M HCl. Repeat your titration using methyl orange indicator instead of phenolphthalein.

I. Calculate the concentration of carbonate solution using the data obtained for the two indicators separately.
II. Comment on the results obtained.


Procedure II

Pipette out 25.00 mL of the carbonate solution into a flask, add few drops of phenolphthalein indicator. Titrate the solution to end point with 0.1M HCl and then add few drops of methyl orange indicator. Continue the titration to its end point.

1. Calculate the concentration of carbonate solution.
2. Comment on the two titration readings.
3. Suggest an experiment to determine the dissociation constants of the carbonic acid. Draw the corresponding titration curve for your suggested procedure.
4. You are provided with a solution containing a mixture of sodium hydroxide and sodium carbonate, a solution of standardized HCl and some common indicators. Describe a method to determine the concentration of sodium hydroxide and sodium carbonate in the given mixture.