Micro-chemical test for carbonate

Author: Christine Bläuer

back to Micro-chemical testing

More information about the necessary equipment and materials see Article Micro-chemistry.

Procedure

Solubility and pH of carbonate salts

alkaline earth metal carbonates Ca and Mg carbonates that are not containing crystal water are hardly soluble in water. Solutions of magnesium carbonates containing crystalline water (Lansfordit=MgCO3.5H2O, Nesquehonit=MgCO3.3H2O) are alkaline, with a pH of about 10. alkali carbonates Solutions of alkali carbonates have a alkaline pH of about 12.

Carbonate test using mineral acids

When adding a drop of diluted hydrochloric or nitric acid to the test drop the carbonate will decay to CO2 and H2O which can be recognized from the visible bubbling. Adding a drop of diluted hydrochloric or nitric acid to a grain of carbonate salt instantly leads to the dissolution of the salt showing an intense effervescence. The only exception from this rule in our context are dolomite (CaMg(CO3)2) and magnesite (MgCO3), which only will dissolve in more concentrated or heated acids.

Carbonate test using calcium chloride solution

Adding a drop of CaCl2 solution to a sample solution containing CO32- leads to an immediate precipitation of white, finely crystalline calcite (CaCO3). This forms a sort of a white veil in the solution and will dissolve quickly showing effervescence when a drop of hydrochloric acid is added.

Carbonate test using silver nitrate solution

When a drop of silver nitrate solution is added to a test drop that was not acidified a white precipitation of silver carbonate is formed, that will turn brown on exposure to light.

Literature

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