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.
The carbonate error is an important consideration in double-indicator titration for quantitative analysis of soda ash. It occurs due to the presence of both carbonate (CO3 2-) and bicarbonate (HCO3-) ions in the solution being analyzed.
The main cause of the carbonate error is the reaction between the carbonate ions and the indicator used in the titration. In the presence of carbonate ions, the indicator undergoes an additional reaction, known as the secondary or carbonate reaction. This reaction leads to the consumption of acid, resulting in a lower concentration of acid available to react with the bicarbonate ions. As a result, the volume of acid required to neutralize the bicarbonate ions is underestimated, leading to an error in the calculations.
The effect of carbonate error on the calculated values obtained in standardization and sample analysis is that it causes an underestimation of the carbonate or bicarbonate concentration. This means that the concentration of these ions will appear lower than their actual values. Consequently, the calculated values of soda ash (which primarily consists of carbonates) will also be lower than the true values.
To minimize the carbonate error and obtain accurate results, several methods can be employed:
1. Pre-treatment of the sample: By converting the bicarbonate ions into carbonate ions through the addition of sodium hydroxide, the carbonate error can be minimized.
2. Use of two indicators: Selecting two indicators with different pH ranges can help compensate for the carbonate error. One indicator is chosen to detect the endpoint of CO3 2-, while the other indicator is used to determine the endpoint of HCO3-. By considering the readings from both indicators, a more accurate determination of the endpoint can be made.
3. Blank Determination: Performing a blank determination ensures that any inherent errors in the reagents or apparatus used in the titration are accounted for and subtracted from the final calculation.
4. Calibration: Regular calibration and standardization of the reagents, indicators, and glassware used in the analysis helps to reduce errors and obtain more precise results.
Applying these techniques can help reduce the carbonate error and improve the accuracy of the calculated values in the standardization and sample analysis of soda ash in double-indicator titration.