Tests for carbonate ion, compounds, reactions, precipitates

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.


Solutions and precipitates of carbonate ion

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.

precipitate and soluble carbonates

The only soluble carbonates in the water are potassium, sodium, rubidium, caesium and ammonium.

All other cation carbonates are are precipitates and some of them have colours. Specially 3d metal cations.


Some cations forms metal hydroxides and get precipitated by carbonate ion.

Al3+, Cr3+, Ti4+ ions are precipitated as the hydroxides by CO32- ion.



Tests for carbonate ion

Carbonate compounds can be exist as solids or solutions. Na2CO3, K2CO3 are solutions. But CaCO3, BaCO3 are precipitates.


Identify carbonate ion in solid state

Carbonate ion in solid state can be identified by doing some experiments. This simple experiments are discussed below.


Carbonate ion and dilute acid reaction

Solid metal carbonate compound reacts with dilute acids and releases carbon dioxide gas.


Reaction of dilute HCl and solid metal carbonate

Add dilute HCl to solid carbonate. It will give colourless, odourless CO2 gas.

calcium carbonate and hydrochloric acid reaction


Identify carbon dioxide 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.

  • lime water - Ca(OH)2
  • calcium carbonate - CaCO3
  • calcium bicarbonate - Ca(HCO3)2

carbon dioxide lime water experiment

Heating bicarbonate compounds

Heat calcium bicarbonate. It will give calcium carbonate, carbon dioxide and water as products.

calcium bicarbnate heating

Calcium bicarbonate and sodium hydroxide

calcium bicarbonate and NaOH react and give calcium carbonate white precipitate, sodium carbonate and water.

sodium carbonate - Na2CO3 is a alkaline solution.

calcium bicarbnate and sodium hydroxide



Barium chloride or calcium chloride and aqueous carbonate 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.

BaCO3 and CaCO3 precipitates dissolve in diute acids


AgNO3 and aqueous carbonate solution

Add AgNO3 to CO32-(aq) solution. A white precipitate Ag2CO3 is given. Ag2CO3 dissolves in dilute HNO3 acid. HCl converts it into AgCl

Ag2CO3 dossolve in HNO3 acid



Metal carbonate precipitates

Li2CO3 is the only insoluble carbonate from alkali metals.

All alkali earth metals' carbonates are insoluble.

3d metal carbonates are also precipitates.



Separating carbonate and bicarbonate ions

  • Add dilute MgSO4(aq) solution to both CO32-(aq) and HCO3-(aq) solutions.
  • CO32-(aq) solution give a white precipitate MgCO3.
  • Mg(HCO3) is soluble in water. But when Mg(HCO3) solution is heated or added OH- ions, MgCO3 white precipitate is formed.
    NaHCO3 + MgSO4

Heating magnesium bicarbonate (MgHCO3)

Heating magnesium bicarbonate

OH- ions and bicarbonate ions reaction

OH- ions and HCO3- ions reaction

Questions


How to identify BaCO3 and BaSO4 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.

Add dilute HCl

Solid BaCO3 dissolve in dilute HCl. But, BaSO4 is insoluble in dilute HCl and exists as the white precipitate.



How to identify magnesium carbonate and barium carbonate

We know all alkali earth metal carbonate compounds gives white precipitates in water.

  • Magnesium carbonate - MgCO3
  • Barium carbonate - CaCO3

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.



Reactions of bicarbonate with organic compounds

Carboxylic acid reacts with bicarbonate ion to give carbon dioxide. But alcohols and phenols do not.



Testing for carbonate ions in calcium carbonate

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.

Answer

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.



Colours of phenolphthalein with CO32- and HCO3-

  • CO32- → pink
  • HCO3- → colourless


test the presence of Carbonate ions in water

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).



what test is done to identify carbonate ions

There are so many tests to identify carbonate ion.

  • Add dilute acid to the carbonate compound. carbon dioxide gas is emitted. Do tests for carbon dioxide gas.