Calcium Carbonate percent purity determination Back Titration

CaCO3 % Purity Determination Since CaCO3 is not soluble in water, it cannot be titrated directly by dissolving it in water. For this reason, back titration process is applied. In this method, excess amount of reagent is added onto analyte to react with it. Then remaining unreacted reagent is titrated with another reagent. In this experiment excess amount of standardized HCl is added to CaCO3 to dissolve and react with it. Then remaining HCl is titrated with NaOH. A sample of 0.1-0.2 g is carefully weighed and transferred to erlenmeyer flask. Then, exactly 50 mL of standardized HCl is added. Erlenmeyer flask is then heated in order to remove CO2 which is produced during reaction. Otherwise CO2 could results in inaccuracies as it is an acidic oxide. Finally, 1-2 drops of phenolphthalein is added to erlenmeyer flask and remaining HCl is titrated with NaOH until pink color. CALCULATIONS 1) Milligram equivalent of CaCO3 is calculated from: VHCl x NHCl = Milligram equivalent of CaCO3 + VNaOH x NNaOH 2) Then mg of pure CaCO3 is calculated from: Milligram equivalent of CaCO3= mg of pure CaCO3 / Equivalent amount of CaCO3 Milligram equivalent of CaCO3= mg of pure CaCO3 / (100/2) 3) Finally percent purity is calculated as; % purity = (mg of pure CaCO3 / mg of CaCO3 weighed) x 100 CaCO3 HCl NaOH REFERENCES: Analitik Kimya Pratikleri – Kantitatif Analiz (Ed. Feyyaz ONUR), 2014