Micro-chemical test for carbonate

Author: Christine Bläuer

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Micro-chemical test for carbonate (CO₃^2-)

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

Procedure

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 CO₃^2- 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