How to Analyse the Ions in a Mixture of Copper II tetraoxosulphate VI and Ammonium trioxocarbonate IV

       Copper sulphate(copper II tetraoxosulphate VI) is one of the soluble compounds of copper, it is a blue crystal solid with five molecules of water of crystallization. It can lose its water of crystallization on strong heating to form anhydrous salt which is white in colour. For more detailed information on the analysis of copper sulphate, click Here.

     Ammonium carbonate (ammonium trioxocarbonate IV) is a white crystal solid and it is very soluble in water, Click here for more information.

 A mixture of ammonium Carbonate and copper sulphate will undergo double decomposition to give ammonium sulphate and copper carbonate. Double decomposition is a type of reaction in which two soluble compounds react by the exchange of radicals to form one soluble compound and one insoluble compound.

CuSO₄ + (NH₄)₂CO₃ ------- CuCO₃ + (NH₄)₂SO₄

Copper carbonate is a green powder and insoluble while ammonium sulphate is a white crystal solid and soluble in water.

How to Identify the Two Cations in the Mixture

To the mixture, add about 10 ml of distilled water, stir thoroughly and filter keeping both residue and the filtrate. Since the mixture is a product of double decomposition, the cations are in both filtrate and residue. The analysis must be carried out on both residue and filtrate in other to identify each of the ions.

Firstly the filtrate, divide the filtrate into two portions, and to one of the portions, add sodium hydroxide, The appearance of a pale blue precipitate or deep blue colour may be a result of copper sulphate as an impurity in the filtrate, Heat the mixture and dip a glass rod into potassium dichromate VI solution let it make contact with the gas given off, if the gas has a pungent smell and changes the orange colour of potassium dichromate VI to light green, it means that the gas is a reducing agent and it may be sulphur IV oxide or ammonium gas. Then bring damp litmus paper close to the gas, if it turns litmus blue then the gas is ammonia gas. Another test is to dip a glass rod into dilute hydrochloric acid and let it make contact with the gas evolving; dense white formed at the point of contact confirms ammonium ion or ammonia gas.

   Secondly, the residue is divided into two, To one portion of the residue, add dilute hydrochloric acid then aqueous ammonia solution in drops then excess pale blue gelatinous precipitate which is soluble in excess to form a deep solution confirms copper ion. Sodium hydroxide can be used but in this case, the pale blue gelatinous precipitate remains insoluble in excess sodium hydroxide solution. For more information on copper ion analysis, click here.

How to Identify the Anion in the Mixture

Again add the filtrate in the test tube, then add barium chloride solution, if a white precipitate is formed, it might be trioxocarbonate IV ions, trioxosulphate IV ion or tetraoxosulphate VI because all these radicals can give white precipitate barium chloride. Add dilute hydrochloric acid, if the precipitate remains, it means that the ion is tetraoxosulphate VI ions.

Finally, put the residue into a test tube, and add dilute hydrochloric acid, a colourless, odourless gas which turns damp litmus paper red and also turns lime water milky confirms trioxocarbonate IV ion

Example Question

H is a mixture of two inorganic compounds. Carry out the following exercise on H. Record your observations and identify any gases that evolved. State the conclusion you draw from the result of each test.

	Test	Observation	Inference
a(I)	H plus water stir thoroughly and filter, then divide the filtrate into two portions. Keep the residue	Green residue and clear filtrate.
II	To the first portion, add dilute NaOH and heat		NH3, SO2, and HCl gases are likely to be present
III	Dip a glass rod into acidified potassium dichromate VI solutions and bring in contact with the gas given off from (II)above	The orange colour turns to light green
IV	Dip a glass rod into dilute HCl and bring it closer to the gas then damp litmus paper		The gas is NH3 , NH4+  present or confirmed
V	To the second portion of the filtrate, add BaCl2 + dilute HCl	White precipitate insoluble in dilute HCl
B(I)	To the residue, add dilute HCl, test with litmus paper then bubble into lime water		CO2, gas, CO32- present
(II)	To the mixture above b(I)add aqueous ammonia solution in drops and in excess	Pale blue gelatinous precipitate soluble in excess to form a deep blue solution

 

 Answer

	Test	Observation	Inference
a(I)	H plus water stir thoroughly and filter, then divide the filtrate into two portions. Keep the residue	Green residue and clear filtrate.	H is a mixture of soluble and insoluble compounds, and transition ions are likely to be present.
II	To the first portion, add dilute NaOH and heat	Gas with a pungent smell	NH3, SO2, and HCl gases are likely to be present
III	Dip a glass rod into acidified potassium dichromate VI solution and bring in contact with the gas given off from (II)above	The orange colour turns to light green	The gas is a  reducing agent, SO2, and NH3 are likely to be present
IV	Dip a glass rod into dilute HCl and bring it closer to the gas then damp litmus paper	Dense white fumes formed, and red litmus paper turned blue	The gas is NH3 , NH4+  present or confirmed
V	To the second portion of the filtrate, add BaCl2 + dilute HCl	White precipitate insoluble in dilute HCl	SO42-  present or confirmed
B(I)	To the residue, add dilute HCl, test with litmus paper then bubble into lime water	Blue litmus turns red and lime water turns milky	CO2, gas, CO32- present
(II)	To the mixture above b(I)add aqueous ammonia solution in drops and in excess	Pale blue gelatinous precipitate soluble in excess to form a deep blue solution	Cu2+ confirmed