The carbonate ion is the anion of carbonic acid which is a weak acid. Therefore carbonates of all alkali metals have an alkaline reaction. Some compounds of carbonate ion forms precipitates and some are soluble in water.
In this tutorial, we study carbonate ion testing experiments, their reactions and forming precipitates or solutions. Later we discuss some examples and problems which are important in carbonate ion testing.
Carbonate ion is associated with metal ions and some of that compounds are soluble in water and some are not. In this tutorial, we are discussing how to identify carbonate ion which exists as a solid or in a solution.
It is good to know first, what are the precipitates and solutions of CO32- ion.
Some cations forms metal hydroxides and get precipitated by carbonate ion.
Al3+, Cr3+, Ti4+ ions are precipitated as the hydroxides by CO32- ion.
Carbonate compounds can be exist as solids or solutions. Na2CO3, K2CO3 are solutions. But CaCO3, BaCO3 are precipitates.
Carbonate ion in solid state can be identified by doing some experiments. This simple experiments are discussed below.
Solid metal carbonate compound reacts with dilute acids and releases carbon dioxide gas.
Add dilute HCl to solid carbonate. It will give colourless, odourless CO2 gas.
Send carbon dioxide gas into lime water. Then solution becomes milky and insoluble white precipitate (calcium carbonate) is given. When we send excess CO2, white precipitate becomes soluble calcium bicarbonate.
Heat calcium bicarbonate. It will give calcium carbonate, carbon dioxide and water as products.
calcium bicarbonate and NaOH react and give calcium carbonate white precipitate, sodium carbonate and water.
sodium carbonate - Na2CO3 is a alkaline solution.
A white carbonates BaCO3 or CaCO3 are given as products with Ba2+ or Ca2+ solutions respectively These two carbonates are precipitates. These precipitates dissolve in dilute acids.
Add AgNO3 to CO32-(aq) solution. A white precipitate Ag2CO3 is given. Ag2CO3 dissolves in dilute HNO3 acid. HCl converts it into AgCl
Li2CO3 is the only insoluble carbonate from alkali metals.
All alkali earth metals' carbonates are insoluble.
3d metal carbonates are also precipitates.
Both BaCO3 and BaSO4 are white precipitates Ba2+ ion is common and anion is different for both compounds. So we have to do some reactions which will affect differently, for carbonate and sulfate ion.
Solid BaCO3 dissolve in dilute HCl. But, BaSO4 is insoluble in dilute HCl and exists as the white precipitate.
We know all alkali earth metal carbonate compounds gives white precipitates in water.
First we add dilute HCl to MgCO3 and CaCO3. White precipitates dissolve and give colourless solutions.
Then add little amount of sulfuric acid ( H2SO4 ). It gives MgSO4 and SrSO4. SrSO4 is a white precipitate. So you can identify the two compounds magnesium carbonate and barium carbonate when forming final white precipitate is given.
Carboxylic acid reacts with bicarbonate ion to give carbon dioxide. But alcohols and phenols do not.
You are provided a white solid and one student has found there is calcium metal as the cation. Now another student says anion should be carbonate ion. Now you have do an experiment which will varyfy there is carbonate ion.
Take a piece of white solid and add dilute HCl to it. You can see a colurless gas emits. Send emitting gas to the lime solution. A white precipitate forms. When excess gas is sent, white precipitate dissolves.
Check pH value. Metal carbonate compounds are usually weak bases. So pH value is slightly over the 7. But this test is not enough to prove the presence of carbonate ion.
Therefore, add aqueous calcium chloride solution to the carbonate ion solution. You will see a white precipitate forms. (CaCO3).
Metal carbonate solutions are weak bases and have pH values just higher than 7 (like 8,9,10). Compare these pH values with pH of strong bases.
There are so many tests to identify carbonate ion.
Qualitative Analysis for anions and cationsEmission of gases Identify ammonium salts identify d block cations by ammonia solution Identify halides Identify carbonate ions identify chromium compounds Testing for thiosulfate (S2O32-) Testing for phosphate ion