Carbonates are the salts and esters of carbonic acid (H2CO3). Two series of salts are derived from the dibasic acid. The hydrogen carbonates, also called primary carbonates, with the general formula MIHCO3 and the (secondary) carbonates, with the general formula MI2CO3. The secondary carbonates are based on the double negatively charged carbonate ion CO32-. For primary salts, see hydrogen carbonates.
Carbonates are ionic salts and therefore generally crystalline solids at room temperature. The carbonate anion does not introduce its own colour into the compounds, so that their colour may be determined by the cation concerned. Carbonates are odourless. With the exception of alkali carbonates, they are only slightly soluble in water, so that most metals precipitate when reacting with alkali carbonates. Alkali carbonates are mainly used in surface technology. Alkali carbonates are formed in alkaline baths by the reaction of lye with air:
As the carbonate concentration increases, deposition is further prevented, which is why the value is monitored analytically. Baths based on sodium can be frozen to separate the carbonate from the remaining electrolyte. Potassium-based baths are usually diluted or discarded. Theoretically, precipitation with barium (e.g. barium cyanide in cyanide baths) is also possible, but in practice this is very rarely done due to the high workload.
Another possibility for precipitation is the use of calcium, for example as Ca(OH)2.
Sodium carbonate is used as a cleaning additive in degreasing solutions. Sodium carbonate is also used in alkaline pickling solutions for aluminium.