Analysis Mixture of Sodium Hydroxide and Sodium Carbonate by Warder Titration

Theoretical Background / Context

• The pKa1 and pKa2 values of H2CO3 are quite distinct and so when a carbonate solution is titrated against hydrochloric acid in sodium carbonate titration, there occur two distinct regions of sharp pH change. 
• The first corresponds to the formation of HCO₃^- (pH 8 to 10) and the second due to complete neutralization at pH 4-6. 
• The first is in the pH range in which the color of phenolphthalein changes from red to colorless and the second is that at which methyl orange changes from yellow to orange-red. 
• The sharp change of pH occurs over a range of pH that includes the regions of the color change of both the indicators, so both of them give the end point correctly in the Warder method.
• So When we have both sodium carbonate and sodium hydroxide present together in a solution for analysis of a mixture of carbonate and bicarbonate, a titration using phenolphthalein gives the titre (volume at the equivalence point) corresponding to sodium hydroxide reaction plus half the carbonate and the titre obtained with methyl orange corresponds to the total alkali. 
• The individual sodium carbonate and hydroxide concentrations may be calculated from the data.

Principle Work of Warder Titration

In Warder titration experiment, NaOH is a strong base which can be determined directly by HCl using phenolphthalein indicator where the color change from pink to colorless at the endpoint:

NaOH + HCl→NaCl + H2O

• Carbonate ion reacts with hydrogen ions in steps:

CO32- + H+→HCO3 –

HCO3- + H+→H2CO3 →CO2+ H2O