How to Analyse Ions in Ammonium ferrous Sulphate

 

Before we go ahead with this analysis, it will be necessary to note that ammonium ferrous sulphate ((NH₄)₂Fe(SO₄)₂.6H₂O) is a double salt which has ammonium ions, iron (II)ions and tetraoxosulphate VI ions. It has six molecules of water of crystallization and it is also a reducing agent.

Materials required for the analysis

The materials and regents we are going to use in this analysis are as follows:

·        Dilute hydrochloric acid

·        Dilute sodium hydroxide solution

·        Aqueous barium chloride

·        Red and blue litmus paper

·        Distilled water

·        Test tubes and boiling tubes

·        Acidified potassium dichromate solution

·        Aqueous ammonia

·        Dilute tetraoxosulphate VI acid

·        Source of heat

·        Glass rod

Analysis of Ammonium Ferrous Sulphate

To analyse the unknown sample, add some samples of the unknown compound into a test tube then add about 10cm³ of distilled water and shake vigorously. Note that the unknown sample is soluble in water and it gives a slightly yellow solution. The coloured solution is a characteristic of transition ions which means that transition compounds may be present as other soluble compounds like K⁺, and Na⁺, Divide the solution into four portions for comprehensive analysis.

   To the first portion, add about 3cm³ of dilute tetraoxosulphate (VI) acid then potassium dichromate (VI) solution, and the orange colour of the K₂Cr₂O₇ turns green. This means that the unknown sample is a reducing agent, Fe²⁺, I^-, and NH₄⁺, likely present.

To the second portion, add dilute sodium hydroxide solution, a dirty green gelatinous precipitate is formed, warm gently, effervescence occurs, a gas which has a pungent smell and colourless evolves and turns red litmus paper blue, indicates the presence of ammonium ions while the precipitate shows that iron (II) ions are likely to be present. 

Note: ammonium ion in ammonium ferrous sulphate can also be identified by heating a solid sample of ammonium ferrous sulphate, on heating, notice the water condensing on the cooler part of the boiling tube. This shows that it has water of crystallization. Test the gas giving off with red and blue litmus paper, the red litmus turns blue. It means the ammonium ion is present. The light brown residue shows that transition ions are likely to be present.

To the mixture above, dip a glass rod into dilute hydrochloric acid, and place it just above the rim of the test tube, if a dense white fume is formed, that means ammonium ions are confirmed.

 To the third portion, add aqueous ammonia in drops then in an excess, dirty green gelatinous precipitate which is insoluble in excess confirms that iron (II) ions are present.

   To the fourth portion, add aqueous barium chloride white precipitate formed is an indication that trioxocarbonate IV ions or trioxosulphate IV ions or tetraoxosulphate VI ions may be present. Add dilute hydrochloric acid, if the precipitate remains, then it means the tetraoxosulphate VI ions are confirmed.

Example Question

The table below is an incomplete test on an unknown sample G. Carry out the tests then copy and completes the table.

s/n	Test	Observation	inference
1	G plus water, shake vigorously and divide into four portions	Soluble, the solution has a slightly yellow colour
2	To the first portion, add dilute H2SO4 and K2Cr2O7 solution		G is a reducing agent, Fe2+, I-, NH4+ likely present
3	To the second portion, add dilute NaOH solution and warm gently	A dirty green gelatinous precipitate formed, on heating, a colourless gas with a pungent smell which turned damp red litmus paper blue
4	To the mixture above (3), dip a glass rod into dilute hydrochloric acid then place the glass rod just above the rim of the test tube		NH3, NH4+ confirmed
5	To the third portion, add aqueous ammonia in drops then excess	Dirty green gelatinous precipitate insoluble in excess
6	To the fourth portion, add aqueous barium chloride then dilute HCl acid		CO32-, SO32-, SO42- likely present SO42- confirmed

 

 

Answers

 

s/n	Test	Observation	inference
1	G plus water, shake vigorously and divide into four portions	Soluble, the solution has a slightly yellow colour	Soluble ions of K, Na, NH4 and transition ions are likely to be present
2	To the first portion, add dilute H2SO4 and K2Cr2O7 solution	The orange colour of K2Cr2O7 turns green	G is a reducing agent, Fe2+, I-, NH4+ likely present
3	To the second portion, add dilute NaOH solution and warm gently	A dirty green gelatinous precipitate formed, on heating, a colourless gas with a pungent smell which turned damp red litmus paper blue	Fe2+ likely present, NH4+, NH3 present
4	To the mixture above (3), dip a glass rod into dilute hydrochloric acid then place the glass rod just above the rim of the test tube	Dense white fumes formed	NH3, NH4+ confirmed
5	To the third portion, add aqueous ammonia in drops then excess	Dirty green gelatinous precipitate insoluble in excess	Fe2+ confirmed
6	To the fourth portion, add aqueous barium chloride then dilute HCl acid	White precipitate formed Insoluble in dilute HCl	CO32-, SO32-, SO42- likely present SO42- confirmed