The determination of calcium carbonate by back-titration.

Since calcium carbonate is insoluble in water, a convenient way of finding its purity is to react it with a known amount of standard hydrochloric acid which is an excess, and then to titrate the unused acid with standard sodium hydroxide.

CaCO₃ + 2HCl à CaCl₂ + H₂O + CO₂

HCl + NaOH à NaCl + H₂O

1 Weigh accurately about 1.00g of calcium carbonate into a 400 cm3 beaker; add about 20 cm3 of pure water followed by 50.0 cm3 (pipette) 1.00 mol dm-3 hydrochloric acid.

2 When effervescence has finished, transfer the solution completely into a 250 cm3 graduated flask, rinsing the beaker into the flask with a little water. Make to the mark with pure water and mix well.

3 Pipette 25.0 cm³ of this solution into a 250 cm³ conical flask and add 4 – 5 drops of phenolphthalein indicator. Titrate the mixture with standard 0.1 mol dm-3 sodium hydroxide solution until the appearance of a pink colour. (Note that this endpoint will drift if the solution is allowed to stand owing to absorption of carbon dioxide from the atmosphere; the endpoint pH for phenolphthalein is around pH 9 – 10. The temptation to add more sodium hydroxide should be resisted!)

4 Repeat to obtain three consistent readings.

        Results:

Mass of calcium carbonate used/g:

Mean titre/cm³:

1 Find the original amount of acid used.

2 Find the amount of acid remaining, from your titration figures.

3 Hence find the percentage purity of the calcium carbonate.

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