Micro-chemical test for carbonate

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Author: Christine Bläuer

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Micro-chemical test for carbonate (CO32-)

For more information on the necessary equipment and materials see the Micro-chemistry article.


Solubility and pH of carbonate salts

alkaline earth metal carbonates

Ca and Mg carbonates, unless under in a hydrated phase, are hardly soluble in water. Solutions of hydrated magnesium carbonate such as Lansfordit (MgCO3.5H2O), and Nesquehonit (MgCO3.3H2O), are alkaline, with a pH of about 10.

alkali carbonates

The most common ones are natron (Na2CO3.10H2O) or thermonatrite (Na2CO3.H2O). Solutions of alkali carbonates have an alkaline pH of about 12. For mixed salts of alkali carbonates and hydrogen carbonates (e.g., trona (Na3H(CO3)2.2H2O) the pH is around 10.

Carbonate test using mineral acids

When adding a drop of diluted hydrochloric or nitric acid (2 M) to the test drop a bubbling will be visible from the CO2 (gas) released by the acid decomposition of the carbonate.

Adding a drop of diluted (2M) hydrochloric or (2M) nitric acid to a grain of carbonate salt instantly leads to the dissolution of the salt showing an intense effervescence. The only exception to this rule in the present context are dolomite (CaMg(CO3)2) and magnesite (MgCO3). These carbonates will only dissolve in more concentrated or heated acids and the reaction is far slower.

Carbonate test using calcium chloride solution

Adding a drop of CaCl2 solution (1% aqueous 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