Micro-chemical test for carbonate: Difference between revisions
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'''Micro-chemical test for carbonate (CO<sub>3</sub><sup>2-</sup>)''' | '''Micro-chemical test for carbonate (CO<sub>3</sub><sup>2-</sup>)''' | ||
For more information on the necessary equipment and materials see the Micro-chemistry article. | |||
== Procedure == | == Procedure == | ||
===Solubility and pH of carbonate salts === | ===Solubility and pH of carbonate salts === | ||
''''' | '''''Alkaline earth metal carbonates''''' | ||
Ca and Mg carbonates, | |||
Ca and Mg carbonates are hardly soluble in water; however, when hydrated,i.e., containing water in their crystalline structure, they are more soluble. | |||
Solutions of hydrated magnesium carbonate such as Lansfordit (MgCO<sub>3</sub>.5H<sub>2</sub>O), and Nesquehonit (MgCO<sub>3</sub>.3H<sub>2</sub>O), are alkaline, with a pH of about 10. | Solutions of hydrated magnesium carbonate such as Lansfordit (MgCO<sub>3</sub>.5H<sub>2</sub>O), and Nesquehonit (MgCO<sub>3</sub>.3H<sub>2</sub>O), are alkaline, with a pH of about 10. | ||
''''' | '''''Alkali carbonates''''' | ||
The most common ones are natron (Na<sub>2</sub>CO<sub>3</sub>.10H<sub>2</sub>O) or thermonatrite (Na<sub>2</sub>CO<sub>3</sub>.H<sub>2</sub>O). | |||
Solutions of | The most common ones are natron (Na<sub>2</sub>CO<sub>3</sub>.10H<sub>2</sub>O) or thermonatrite (Na<sub>2</sub>CO<sub>3</sub>.H<sub>2</sub>O). | ||
For mixed salts of alkali carbonates and hydrogen carbonates (e.g., | Solutions of these salts are alkaline with a pH of about 12. | ||
For mixed salts of alkali carbonates and hydrogen carbonates (e.g., trona (Na<sub>3</sub>H(CO<sub>3</sub>)<sub>2</sub>.2H<sub>2</sub>O) the pH is around 10. | |||
===Carbonate test using mineral acids=== | ===Carbonate test using mineral acids=== | ||
When adding a drop of diluted hydrochloric or nitric acid | When adding a drop of diluted (2M)hydrochloric or (2M) nitric acid to the test drop a bubbling will be visible from the CO<sub>2</sub> (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(CO<sub>3</sub>)<sub>2</sub>) and magnesite (MgCO<sub>3</sub>). These carbonates will only dissolve in more concentrated or heated acids, the reaction being far slower. | |||
===Carbonate test using calcium chloride solution === | ===Carbonate test using calcium chloride solution === | ||
Adding a drop of CaCl<sub>2</sub> solution (1% aqueous solution) to a sample solution containing | Adding a drop of CaCl<sub>2</sub> solution (1% aqueous solution) to a sample solution containing CO<sub>3</sub><sup>2-</sup> leads to an immediate precipitation of white, finely crystalline calcite (CaCO<sub>3</sub>). 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=== | ===Carbonate test using silver nitrate solution=== | ||
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== Literature == | == Literature == | ||
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Latest revision as of 22:21, 24 November 2017
Author: Christine Bläuer
back to Micro-chemical testing
Micro-chemical test for carbonate (CO32-)
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 are hardly soluble in water; however, when hydrated,i.e., containing water in their crystalline structure, they are more soluble. 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 these salts are alkaline with a 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 (2M)hydrochloric or (2M) nitric acid 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, the reaction being 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