Carbonates and bicarbonates European Pharmacopoeia

The test identifies the presence of carbonate, COj , or bicarbonate, HCOj , in the substance to be examined. Reference to carbonates and bicarbonates is made in less than five monographs, all describing simple inorganic carbonate and bicarbonate salts. The carbonate ion forms insoluble salts with nearly all cations except the alkali metals and ammonia, and the bicarbonate ion, more commonly referred to as hydrogen carbonate, generally has an even lower solubility. Hydrogen carbonate is a weak acid and is unstable in an acidic environment. This characteristic and the insolubility of barium carbonate are used in the identification test. 

The solution or suspension becomes effervescent owing to the instability of the carbonate ion. 

Upon gentle heating, the gas is led to test tube B containing 5 ml of barium hydroxide solution R, and a white precipitate of barium carbonate is formed. 

Finally, this precipitate is dissolved upon the addition of an excess of hydrochloric acid Rl, thereby repeating the first reaction. The precipitate will. 

The excess carbon dioxide will lower the test solution pH, and this will release the carbonate by changing it to hydrogen carbonate, which does not form an insoluble salt with barium. 

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