Systematic Analysis of Anions - Practice Questions & MCQ

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When a solution of mixture having two inorganic salts was treated with freshly prepared ferrous sulphate in acidic medium, a dark brown ring was formed whereas on treatment with neutral $FeCl_{3}$ . it gave deep red colour which disappeared on boiling and a brown red ppt was formed. The mixture contains:

$\mathrm{C}_2 \mathrm{O}_4^{2-} \& \mathrm{NO}_3^{-}$

$\mathrm{SO}_3^{2-} \& \mathrm{C}_2 \mathrm{O}_4^{2-}$

$\mathrm{CH}_3 \mathrm{COO}^{-} \& \mathrm{NO}_3^{-}$

$\mathrm{SO}_3^{2-} \& \mathrm{CH}_3 \mathrm{COO}^{-}$

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Concepts Covered - 3

Preliminary Test with Dilute Sulphuric Acid (Systematic Analysis of Anions):

In this test, the action of dilute sulphuric acid on the salt is noted at room temperature and on warming. Carbonate $(CO_3^{2-} )$ , sulphide $(S^{2-} )$ , sulphite $(SO_3^{2-} )$ , nitrite $(NO_2^{-} )$ and acetate $(CH_3COO^{-} )$ react with dilute sulphuric acid to evolve different gases. Study of the characteristics of the gases evolved gives information about the anions.

Procedure:
Take 0.1 g of the salt in a test tube and add 1–2 mL of dilute sulphuric acid. Observe the change, if any, at room temperature. If no gas is evolved, warm the content of the test tube. If gas is evolved test it

Observation	Inference
	Gas Evolved	Possible Anion
A colourless, odourless gas is evolved with brisk effervescence, which turns lime water milky	CO₂	Carbonate $\left ( CO{_{3}}^{2-} \right )$
Colourless gas with the smell of rotten egg is evolved which turns lead acetate paper black.	H₂S	Sulphide $\left ( S^{2-} \right )$
Colourless gas with a pungent smell, like burning sulphur which turns acidified potassium dichromate solution green.	SO₂	Sulphite $\left ( SO{_{3}}^{2-} \right )$
Brown fumes which turn acidified potassium lodide solution containing starch solution blue.	NO₂	Nitrite $\left ( NO{_{2}}^{-} \right )$
Colourless vapours with smell of vinegar turn blue litmus red	CH₃COOH Vapours	Acetate $( CH_3COO^{-} )$

Confirmatory Test of Anions (CO32-, S2-, SO32-, NO2-and CH3COO-)

Confirmatory (wet) tests for anions are performed by using water extract when salt is soluble in water and by using sodium carbonate extract when salt is insoluble in water. Confirmation of $CO_3^{2-}$ is done by using an aqueous solution of the salt or by using solid salt as such because sodium carbonate extract contains carbonate ions. Water extract is made by dissolving salt in water.

Preparation of sodium carbonate extract: Take 1 g of salt in a porcelain dish or boiling tube. Mix about 3 g of solid sodium carbonate and add 15 mL of distilled water to it. Stir and boil the content for about 10 minutes. Cool, filter, and collect the filtrate in a test tube and label it as sodium carbonate extract.

Anion	Cnfirmatory Test
Carbonate $(CO{_{3}}^{2-})$	Take 0.1 g of salt in a test tube, add dilute sulphuric acid CO₂ gas is evolved with brisk effervescence which turns lime water milky. On passing the gas for some more time, milkiness disappears.
Sulphide $\left ( S^{2-} \right )$	Take 1 mL of water extract and make it alkaline by adding ammonium hydroxide or sodium carbonate extract. Add a drop of sodium carbonate extract. Add a drop of sodium nitroprusside solution. Purple or violet colouration appears
Sulphite $\left ( SO{_{3}}^{2-} \right )$	Take 1 mL. of water extract or sodium carbonate extract in a test tube and add barium chloride solution. A white precipitate is formed which dissolves in dilute hydrochloric acid and sulphur dioxide gas is also evolved. Take the precipitate of step (a) in a test tube and add a few drops of potassium permanganate solution acidified with dil. $H_2SO_4$ . Colour of potassium permanganate solution gets discharged
Nitrite $\left ( NO{_{2}}^{-} \right )$	Take 1 mL. of water extract in a test tube. Add few drops of potassium iodide solution and a few drops of starch solution, acidify with acetic acid. Blue colour appears. Acidify 1 mL. of water extract with acetic acid. Add 2-3 drops of sulphanilic acid solution followed by 2-3 drops of 1- naphthylamine reagent. Appearance of red colour indicates the presence of nitrite ion.
Acetate $\left ( CH_3COO^{-} \right )$	Take 0.1 g of salt in a China dish. Add 1 mL. of ethanol and 0.2 mL. conc. $H_2SO_4$ and heat. Fruity odour confirms the presence of acetate ion. Take 0.1 g of salt in a test tube, add 1-2 mL distilled water, shake well, filter if necessary. Add 1 to 2 mL neutral ferric chloride solution to the filtrate. Deep red colour appears on boiling and a brown-red precipitate is formed.

Preliminary Test with Concentrated Sulphuric Acid

If no positive result is obtained from dil. H₂SO₄ test, take 0.1 g of salt in a test tube and 3-4 drops of conc. H₂SO₄. Observe the change in the reaction mixture in cold and then warm it.

Observation	Inference
	Gas	Possible
A colourless gas with pungent smell, which gives dense white fumes when a rod dipped in ammonium hydroxide is brought near the mouth of the test tube.	$HCl$	Chloride $(Cl^-)$
Reddish brown gas with a pungent odour is evolved. Intensity of reddish gas increases on heating the reaction mixture after addition of solid ${MnO}_{2}$ to the reaction mixture. Solution also acquires red colour.	$Br_2$ Vapours	Bromide $(Br^-)$
Violet vapours, which turn starch paper blue and a layer of violet sublimate is formed on the sides of the tube. Fumes become dense on adding ${MnO}_{2}$ to the reaction mixture.	$I_2$ Vapours	Iodide $(I^-)$
Brown fumes evolve which become dense upon heating the reaction mixture after addition of copper turnings and the solution acquires blue colour.	$NO_2$	Nitrate $(NO_3^-)$
Colourless, odourless gas is evolved which turns lime water milky and the gas coming out of lime water burns with a blue flame, if ignited	$CO$ and $CO_2$	Oxalate, $\left ( C_2O{_{4}}^{2-}\right )$

Confirmatory tests for the anions which react with concentrated sulphuric acid are given below in the Table:
Confirmatory tests for $Cl^-, Br^-, I^-, NO_3^-$ and $C_2O_4^{2-}$

Anion	Confirmatory Test
Chloride $(Cl^-)$	(a) Take 0.1 g salt in a test tube. Add a pinch of manganese dioxide and 3-4 drops of conc. Sulphuric acid, Heat the reaction mixture. Greenish yellow chlorine gas is evolved which is detected by its pungent odour and bleaching action. (b) Take 1 ml. of sodium carbonate extract in a test tube. Acidify it with dil. ${HNO}_{3}$ or take water extract and add silver nitrate solution. A curdy white precipitate is obtained which is soluble in ammonium hydroxide solution. (c) Take 0.1 g salt and a pinch of solid potassium dichromate in a test tube, add conc. ${H}_{2} {SO}_{4}$ , heat and pass the gas evolved through sodium hydroxide solution. It becomes yellow. Divide the solution into two parts. Acidify one part with acetic acid and add lead acetate solution. A yellow precipitate is formed. Acidify the second part with dilute sulphuric acid and add 1ml of amyl alcohol followed by 1 ml of 10 % hydrogen peroxide. After gentle shaking the organic layer turns blue.
Bromide $(Br^-)$	(a) Take 0.1 g of salt and a pinch of ${MnO}_{2}$ in a test tube. Add 3-4 drops conc. sulphuric acid and heat. Intense brown fumes are evolved. (b) Neutralise 1 ml. of sodium carbonate extract with hydrochloric acid (or take the water extract). Add 1 ml. carbon tetrachloride $\left({CCl}_{4}\right)$ /chloroform $\left({CHCl}_{3}\right)$ /carbon disulphide $(CS_2)$ . Now add an excess of chlorine water drop wise and shake the test tube. A brown colouration in the organic layer confirms the presence of bromide ion. (c) Acidify 1 ml of sodium carbonate extract with $dil. {HNO}_{3}$ (or take 1 mL water extract) and add silver nitrate solution. A pale yellow precipitate soluble with difficulty in ammonium hydroxide solution is obtained.
Iodide $(I^-)$	(a) Take 1 ml. of salt solution neutralised with $HCl$ and add 1 mL chloroform/carbon tetrachloride/carbon disulphide. Now add an excess of chlorine water drop wise and shake the test tube. A violet colour appears in the organic layer. (b) Take 1ml of sodium carbonate extract acidify it with dil. ${HNO}_{3}$ (or take water extract). Add, silver nitrate solution. A yellow precipitate insoluble in ${NH}_{4} {OH}$ solution is obtained.
Nitrate $({NO}_{3}^-)$	Take 1 mL of salt solution in water in a test tube. Add 2 ml of conc. ${H}_{2}{SO}_{4}$ and mix thoroughly. Cool the mixture under the tap. Add freshly prepared ferrous sulphate along the sides of the test tube without shaking. A dark brown ring is formed at the junction of the two solutions.
Oxalate $\left ( C_2O^{2-} _{4}\right )$	(a) Take 1 ml. of water extract or sodium carbonate extract acidified with acid and add calcium chloride solution. A white precipitate insoluble in ammonium oxalate and oxalic acid solution but soluble in dilute hydrochloric acid and dilute nitric acid is formed. (b) Take the precipitate from test (a) and dissolve it in dilute. ${H}_{2} {SO}_{4}$ . Add very dilute solution of ${KMnO}_{4}$ and warm. Colour of ${KMnO}_{4}$ solution is discharged. Pass the gas coming out through lime water. The lime water turns milky.