Gravimetric Determination of Carbon Dioxide in Calcium Carbonate

1 Gravimetric determination of carbon dioxide in calcium carbonate 

Weigh out accurately about 0.3-0.4 g of the carbonate into a weighed silica crucible. Heat in an oven at lOS C to constant weight and find die weight of any water lost. Then place in a muffle furnace and heat to constant weight at 1200 C. Repeat and calculate the % carbonate in the sample. 

When the residue is cooled to room temperature, use a weighed portion for the determination of calcium by dissolving in HCl using any suitable method (Sec.3.4.3). Compare the results of calcium and carbon. To a small portion of the residue add water drop wise and note any changes. 

When a solution of sodium carbonate is titrated against standard HCl, the titration curve shows two inflection points one at pH 8.5 when the carbonate is transformed to bicarbonate the other at pH 4 when the bicarbonate is neutralised to give NaCl and carbon dioxide. 

Prepare 0.1 M sodium carbonate using the purest anhydrous salt. Pipette 25.0 cm in a conical flask, add a few drops of thymol blue indicator. Cool to 0 C and stir magnetically as you titrate with standardised 0.2 M HCl until the colour changes from yellow to blue. Repeat and calculate the average titre. Repeat two titrations using bromophenol blue as indicator when the colour changes at the end-point from blue to yellow. Compare the two titres with the different indicators. 

---