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Double Indicator Titrations

In this section you will learn about double indicator titrations involving CO3 and HCO3 mixture solutions.

Both CO]-and HC03 behave as base and react with acids. The CO3- reacts with acids in two stages.

1. CO3- (aq) + H+ (aq)à HCO3(aq)

2. HCO3 (aq) + H+ (aq) àH2O (l) + CO2(g)

In the first stage CO3 changes into HCO3 and in the second stage HC03 changes into CO2 and H2O. The first stage represents half neutralization of CO3. If the same acid with the same concentration is used, then the same volume of acid would be needed to provide an equal amount.

Phenolphthalein is used to locate the end point of stage 1 because there is a sharp pH change, at pH 8.3, within the PH range of phenolphthalein at that stage. Methyl orange is used to locate the end point of stage 2 since there is a sharp pH change, at pH 5 which is near its pH range.

In a solution of mixture of CO5- and HCO-3 the first titration using phenolphthalein as indicator represents the half neutralization of CO3 whereas the second stage of titration represents the reaction between the other half CO3 (aq). i.e .. HCO3 – (aq) from CO~- (aq) and the HCO) (aq) in the real mixture.

The end point of the second stage is indicated by methyl orange.

In a mixture of CO3- (aq) and HCO3 (aq) at the phenolphthalein end point there is a colour change of red to colourless. If the volume of the acid is V 1, this volume reacted with half of the CO3 to give HCO-3.

Thus, volume of acid required for complete neutralization of CO3- in the mixture = 2 X VI

On addition of methyl orange to the same reaction mixture the second end point can be located when it turns from yellow to pink. If V 2 is the volume of acid used from the start of the reaction then V 2 is the volume of acid required for complete neutralization of CO3- and HCO3. Volume acid required for complete neutralization of HCO3 = V 2 – 2V 1.

If V 3 is the volume of the acid used after the addition methyl orange indicator, then V 3 is the volume of the acid required for half neutralization of CO3- and complete neutralization of HCO3.

Volume of acid required for complete neutralization of HCO:J in that case is equal to V 3 – V 1

Knowing the volumes of acid required for complete neutralization of CO3- and HC03 in the mixture, their individual concentrations can be calculated.

Alternatively, this experiment can be carried out by titrating the CO3- and HCO3 mixture separately using phenolphthalein and methyl orange indicators.