What is Qualitative analysis?
Qualitative analyses are mainly used for the determination of anions and cations present in the given sample. This analysis is a type of analytical chemistry that is used to determine the elemental composition present in the inorganic compounds via different reagents.
Qualitative analysis of Anions
Before the qualitative analysis of anions, some preliminary tests should be done.
Smell and color
The physical examination of the given inorganic salt includes the review of smell, color, and density. The anions like sulfide and acetate could be determined with the help of their characteristic smell.
Dry heating test
In this test, a pinch of salt is heated in the dry test tube. On heating, these salts could undergo decomposition, and specific gases evolve. By noting the smell of gas, the anions could be identified.
Determination of Anions
The anions could be determined by carrying out the tests below.
Reaction with dilute Sulfuric Acid (H2SO4)
A pinch of salt is reacted with dil. H2SO4. The type of anions can be identified by observing the type of gas evolved.
Sulfide: Sulphides on reaction with dil. H2SO4 gives hydrogen sulfide (H2S) gas.
Carbonate: Carbonate when reacted with dil. H2SO4, carbon dioxide (CO2) gas evolved with brisk effervescence.
Sulfite: Sulfite on reaction with dil. H2SO4, sulfur dioxide (SO2) gas is liberated.
Nitrite: Nitrites on reaction with dil. H2SO4 gives nitrogen monoxide (NO) gas which will react with oxygen present in the air to form nitrogen dioxide (NO2). It is identified by the liberation of brown gas.
Reaction with Conc. H2SO4
When there is no effect on reaction with dil. H2SO4, then heat a pinch of salt with conc. H2SO4 and determine the gas evolved.
Bromides: Bromide will result in the formation of bromine gas on reaction with con. H2SO4.
Chlorides: Chloride gas is evolved on the treatment of chloride with con. H2SO4.
Iodides: Iodide on reaction with conc. H2SO4, produces the vapors of iodine.
Nitrates: Nitrate on reaction with con. H2SO4 forms NO2 gas.
Oxalate: Oxalate anions on reaction with conc. H2SO4 produces a mixture of carbon monoxide and carbon dioxide.
Acetate: Acetate anions on reaction with acetic acid forms the vapors of acetic acid.
Independent groups ( PO43- and SO42-) test
When the unknown sample does not react with the dilute and concentrated sulphuric acid, the sample is tested for sulfate and phosphate by doing their individual tests.
Potassium permanganate (KmnO4) test
This test could be carried out with the help of KMnO4 & sulfuric acid as reagents. This test is useful for the detection of anions including chloride, bromide, iodide, and oxalate.
Wet tests for the analysis of Anions
The confirmation reactions of anions could be described below.
Confirmation tests for Carbonate ions
Reaction with dilute Hydrochloric Acid (HCl)
Carbonate ion when reacted with dilute HCl gives carbon dioxide which reacts with lime water to produce a white precipitate of calcium carbonate that turns the lime water milky. This test is carried out with water if the carbonate is soluble. If it is insoluble carbonate, then the test is carried out in solid-state. The reaction can be represented as follows:
Magnesium sulphate test
The soluble carbonates can be characterized by using this test. Soluble carbonates on reaction with magnesium sulfate produce the white precipitate of magnesium carbonate. The reaction could be represented as follows:
Confirmation tests for sulfite ion
Barium chloride (BaCl2) test
Sulfite ions reacts with to produce the white precipitate (barium sulfite). The formed barium sulfite is dissolved in dilute hydrochloric acid to produce sulfur dioxide gas.
Potassium Dichromate test
Sulfites react with potassium dichromate to form a green color solution. The reaction can be represented as follows:
Confirmation test for sulphide
Sodium Nitroprusside test
When sulfide reacts with sodium nitroprusside solution, violet or purple color solution is formed. This is because of the formation of .
Lead acetate test
Sulfides on treatment with lead acetate produces black precipitate due to the formation of lead sulfide.
Cadmium carbonate (CdCO3) test
Sulfides when treated with the suspensions of , produce cadmium sulfide which is formed as a yellow precipitate.
Confirmation tests for Nitrite
Nitrite on treatement with KI in the presence of dilute. results in the formation of iodine. When iodine is treated with starch, it results in the formation of a blue-black complex. The reaction could be represented as follows:
Diphenlyl Amine test
Diphenylamine, when reacted with nitrites, gets oxidized to form blue coloration.
Ferrous sulphate test
Nitrites, on reaction with ferrous sulfate, forms black or brown color solution due to the formation of .
Confirmation tests for Nitrate
Nitrate on treatment with copper chips undergoes reduction to form reddish-brown NO2 gas.
In this test, the iron sulfate is mixed with the solution of nitrate and conc. sulphuric acid is slowly added to this mixture. As a result, a brown ring is produced at the junctions of the solutions. It shows the presence of nitrate.
Confirmation test for chloride
Chromyl Chloride test
Chloride on reaction with conc.sulphuric acid & potassium dichromate produces red fumes of chromyl chloride that will react with sodium hydroxide to produce sodium chromate (yellow solution). This solution when treated with dilute CH3COOH and lead acetate to produce lead chromate (yellow precipitate).
Confirmation tests for Bromide
Silver Nitrate test
The reaction of bromide with silver nitrate produces yellow precipitate (silver bromide) that is sparingly soluble in ammonium hydroxide (NH4OH).
Manganese dioxide test
Bromide salt on reaction with concentrated sulphuric acid and manganese dioxide liberates bromine.
Confirmation test for iodide
Silver nitrate test
In this test, a small unknown sample is taken in the test tube and silver nitrate solution is added to it. The formation of yellow precipitate shows the presence of iodide which is insoluble in NH4OH.
Manganese dioxide test
A small amount of the unknown sample is taken in the test tube. Concentrated sulphuric acid & manganese dioxide is added to the test tube. The liberation of iodine gas shows the presence of iodide.
Confirmation test for acetate
Oxalic acid test
A small amount of sample is taken in the test tube, oxalic acid is added to it. It forms acetic acid that is characterized by a vinegar-like smell.
Confirmation test for oxalate
Calcium Chloride test
A small amount of sample is taken in the test tube and calcium chloride is added to it. It forms calcium oxalate (white precipitate).
Confirmation test for sulfate
Barium Chloride test
When sulfate is reacted with barium chloride, white precipitate of barium sulfate is produced.
Confirmation test for phosphate
Magnesia mixture test
When phosphate is reacted with magnesia mixture, magnesium ammonium phosphate is produced. It is formed as a white precipitate.
Context and Applications
This topic is important for both undergraduate and postgraduate courses, particularly for Bachelors and Masters in Chemistry.
Question 1: A brown ring is produced in the test for __________.
Answer: Option 4 is correct.
Explanation: In the case of the nitrate test, a brown ring is formed at the junction of salt solution and the ferric sulfate solution with concentrated sulfuric acid.
Question 2: Among the given compounds, the compound which is insoluble in CH3COOH is _______.
- Calcium oxalate
- Calcium hydroxide
- Calcium carbonate
- Calcium oxide
Answer: Option 1 is correct.
Explanation: Calcium oxalate could not be dissolved in weak acid like acetic acid. It can only be dissolved in strong acids.
Question 3: The result of chromyl chloride test is _____________.
- A colourless solution will be formed
- A dark green solution will be formed
- A yellow precipitate will be formed
- A yellow solution will be formed
Answer: Option 3 is correct.
Explanation: The chromyl chloride vapor on react with NaOH, acetic acid & lead acetate will form lead chromate (yellow precipitate).
Question 4: The presence of effervescence is because of ______.
- Presence of carbonate
- Presence of chloride
- Absence of bicarbonate
- Absence of carbonates
Answer: Option 1 is correct.
Explanation: Carbonate on reaction with dil. H2SO4, carbon dioxide (CO2) gas evolved with brisk effervescence.
Question 5: Unknown sample on heated with dil. H2SO4 & the lead acetate paper turns black with the liberation of gas. The sample contains _______.
Answer: Option 1 is correct.
Explanation: Sulphide when reacted with dil. H2SO4, the lead acetate paper turns black with the liberation of gas. Hence, the mixture has a sulfide ion.
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