Tests for carbonate ion, compounds, reactions, precipitates

The carbonate ion is the anion of carbonic acid which is a weak acid. Some compounds of carbonate ion forms precipitates and some metal carbonates are soluble in water. Also some metal carbonates have colours in solid state and aqueous state.

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 CO₃^2- ion.

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 CO₃^2- 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.

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

Heating bicarbonate compounds

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

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.

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.

AgNO₃ and aqueous carbonate solution

Add AgNO₃ to CO₃^2-_(aq) solution. A white precipitate Ag₂CO₃ is given. Ag₂CO₃ dissolves in dilute HNO₃ acid. HCl converts it into AgCl

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 MgSO_4(aq) solution to both CO₃^2-_(aq) and HCO₃^-_(aq) solutions.
• CO₃^2-_(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.
  
  
  

Heating magnesium bicarbonate (MgHCO₃)

OH^- ions and bicarbonate ions reaction

Questions

how to test potassium carbonate?

Potassium carbonate is soluble in water and a colourles solution. To test potassium ion, you can do flame test. Potassium will give purple colour to flame test.

how to test for carbonate in water?

Add barium chloride (BaCl₂) solution to carbonate ion solution. Barium carbonate (BaCO₃), white precipitate is given. Separate the precipitate from the solution. Then add dilute HCl acid solution to the precipitate. You can see white precipitate dissolve in water and a gas is emmitted from the solution.

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 CO₃^2- and HCO₃^-

• CO₃^2- → 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).

pH of dilute solution of carbonate solution

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.

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.