posted by mitch .
We did an experiment on Quantitative Analysis of Soda ash by Double-Indicator Titration.
What is the cause of carbonate error and its effect on the calculated values obtained in standardizatin and sample analysis?
Actually you have several questions rolled into one. The simplest answer is that CO2 is absorbed by the NaOH and that decreases the molarity of the standard base. IF the indicator is one that changes in the acid region (for example, bromocresol green or methyl red) then no error is created because two Hs have been used up in the reaction with CO2 and both hydrogens have been titrated with the indicator.
CO2 + 2H^+ ==> H2CO3.
However, if you are doing a double indicator titration, then one of the indicators is in the basic region (such as phenolphthalein) and only one of the hydrogens is titrated and that is the cause of the carbonate error.