UNIT-7 SYSTEMATIC QUALITATIVE ANALYSIS ANALYSIS always does not mean breaking of substance into its ultimate constituents. Finding out the nature of substance and identity of its constituents is also analysis and is known as qualitative analysis. Qualitative analysis of inorganic salts means the identification of cations and anions present in the salt or a mixture of salts. Inorganic salts may be obtained by complete or partial neutralisation of acid with base or vice-versa. In the formation of a salt, the part contributed by the acid is called anion and the part contributed by the base is called cation. For example, in the salts CuSO4 and NaCl, Cu2+and Na+ ions are cations and SO2– and Cl– ions are anions. Qualitative4 analysis is carried out on various scales. Amount of substance employed in these is different. In macro analysis, 0.1 to 0.5 g of substance and about 20 mL of solution is used. For semimicro analysis, 0.05 g substance and 1 mL solution is needed while for micro analysis amount required is very small. Qualitative analysis is carried out through the reactions which are easily perceptible to our senses such as sight and smell. Such reactions involve: (a) Formation of a precipitate (b) Change in colour (c) Evolution of gas etc. Systematic analysis of an inorganic salt involves the following steps: (i) Preliminary examination of solid salt and its solution. (ii) Determination of anions by reactions carried out in solution (wet tests) and confirmatory tests. (iii) Determination of cations by reactions carried out in solution (wet tests) and confirmatory tests. Preliminary examination of a salt often furnishes important information, which simplifies further course of analysis. Although these tests are not conclusive but sometimes they give quite important clues for the presence of certain anions or cations. These tests can be performed within 10–15 minutes. These involve noting the general appearance and physical properties, such as colour, smell, solubility etc. of the salt. These are named as dry tests. Heating of dry salt, blow pipe test, flame tests, borax bead test, sodium carbonate bead test, charcoal cavity test etc. come under dry tests. Some of these tests are given later in this unit. Solubility of a salt in water and the pH of aqueous solutions give important information about the nature of ions present in the salt. If a solution of the salt is acidic or basic in nature, this means that it is being hydrolysed in water. If the solution is basic in nature then salt may be some carbonate or sulphide etc. If the solution shows acidic nature then it may be an acid salt or salt of weak base and strong acid. In this case it is best to neutralise the solution with sodium carbonate before testing it for anions. Gases evolved in the preliminary tests with dil. HSO/dil. HCl and conc. HSO2424 also give good indication about the presence of acid radicals (see Tables 7.1 and 7.3). Preliminary tests should always be performed before starting the confirmatory tests for the ions. Aim To detect one cation and one anion in the given salt from the following ions: Cations -Pb2+, Cu2+, As3+, Al3+, Fe3+, Mn2+, Ni2+, Zn2+, Co2+, Ca2+, Sr2+, Ba2+, Mg2+, NH+ 42– 2– 2––– 3– 2–Anions -CO , S2–, SO , SO ,NO , NO , Cl– , Br–, I–, PO , CO ,3 3 423 424 CHCOO–.3(Insoluble salts to be excluded) Theory Two basic principles of great use in the analysis are: (i) the Solubility product and (ii) the Common ion effect When ionic product of a salt exceeds its solubility product, precipitation takes place. Ionic product of salt is controlled by making use of common ion effect which you have studied in the textbook of chemistry. Material Required SYSTEMATIC ANALYSISOF ANIONS Step - I : Preliminary Test with Dilute Sulphuric Acid In this test the action of dilute sulphuric acid (procedure is given below) on the salt is noted at room temperature and on warming. 2– 2––Carbonate ( CO ), sulphide (S2–), sulphite ( SO ), nitrite (33 NO2) and acetate (CH3COO–) react with dilute sulphuric acid to evolve different gases. Study of the characteristics of the gases evolved gives information about the anions. Summary of characteristic properties of gases is given in Table 7.1 below. Procedure (a) 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 Fig. 7.1 : Testing a Gascontent of the test tube. If gas is evolved test it by using the apparatus shown in Fig.7.1 and identify the gas evolved (See Table 7.1). Table 7.1 : Preliminary test with dilute sulphuric acid Inference Observations Gas evolved Possible Anion A colourless, odourless gas is evolved CO2with brisk effervescence, which turns Carbonate (CO2–)3lime water milky. Colourless gas with the smell of rotten eggs is evolved which turns lead acetate H2S Sulphide (S2– ) paper black. Colourless gas with a pungent smell, like burning sulphur which turns acidified SO2 2–Sulphite ( SO3 )potassium dichromate solution green. Brown fumes which turn acidified potassium iodide solution containing NO2 Nitrite ( NO– )2starch solution blue. Colourless vapours with smell of vinegar. Vapours turn blue litmus red. CH3COOH vapours Acetate, (CH3COO –) 2-2–2-–Confirmatory tests for CO3 S, SO3 , 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 CO2– is done by using aqueous solution of3 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 is given below. 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. Confirmatory tests for acid radicals, which react with dilute sulphuric acid are given below in Table 7.2. 2–2– 2– –3 333Table 7.2 : Confirmatory tests for CO, S, SO, NO , CHCOO– Anion Confirmatory test Carbonate ( 2 3 -CO ) Take 0.1 g of salt in a test tube, add dilute sulphuric acid. CO 2 gas is evolved with brisk effervescence which turns lime water milky. On passing the gas for some more time, milkiness disappears. Sulphide (S2–) Take 1 mL of water extract and make it alkaline by adding ammonium hydroxide or sodium carbonate extract. Add a drop of sodium nitroprusside solution. Purple or violet colouration appears. *Sulphite ( 2– 3SO ) (a) 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. (b) Take the precipitate of step (a) in a test tube and add a few drops of potassium permanganate solution acidified with dil. H2SO4. Colour of potassium permanganate solution gets discharged. Nitrite ( – 2NO ) (a) Take 1 mL of water extract in a test tube. Add a few drops of potassium iodide solution and a few drops of starch solution, acidify with acetic acid. Blue colour appears. (b) 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. * Like CO2 sulphur dioxide also turns lime water milky. But CO2 is odourless gas and SO2 has a characteristic smell. ** Prepareation of neutral Ferric Chloride : Add dilute NaOH solution to ferric chloride solution drop by drop with shaking until a small but permanent precipitate of ferric hydroxide is obtained. Filter the precipitate and use the filtrate for analysis. Chemistry of Confirmatory Tests – 1. Test for Carbonate ion [CO2 ]3 If there is effervescence with the evolution of a colourless and odourless gas on adding dil. H2SO4 to the solid salt, this indicates the presence of carbonate ion. The gas turns lime water milky due to the formation of CaCO (Fig. 7.1)3Na2CO3 + H2SO4 ⎯→ Na2SO4 + H2O +CO2 Ca(OH) + CO⎯→ CaCO + HO22 32If CO gas is passed in excess through lime water, the milkiness produced 2disappears due to the formation of calcium hydrogen carbonate which is soluble in water. CaCO3 + CO2 + H2O ⎯→ Ca (HCO3)2 Hydrogen sulphide 2. Test for Sulphide ion [S2–] (a) With warm dilute H2SO4 a sulphide gives hydrogen sulphide gas which smells like rotten eggs. A piece of filter paper dipped in lead acetate solution turns black on exposure to the gas due to the formation of lead sulphide which is black in colour. NaS + HSO⎯→ NaSO + HS224 242(CH3COO)2Pb + H2S ⎯→ PbS + 2CH3COOH Lead sulphide Black precipitate (b) If the salt is soluble in water, take the solution of salt in water make it alkaline with ammonium hydroxide and add sodium nitroprusside solution. If it is insoluble in water take sodium carbonate extract and add a few drops of sodium nitroprusside solution. Purple or violet colouration due to the formation of complex compound Na[Fe(CN)NOS]45confirms the presence of sulphide ion in the salt. NaS + Na [Fe(CN)NO] ⎯→ Na [Fe(CN)NOS]225 45Sodium nitroprusside Complex of Purple colour 3. Test for Sulphite ion [SO32–] (a) On treating sulphite with warm dil. H2SO4, SO2 gas is evolved which is suffocating with the smell of burning sulphur. Na2SO3 + H2SO4 ⎯→ Na2SO4 + H2O + SO2 The gas turns potassium dichromate paper acidified with dil. HSO, green.24Barium compounds KCrO + HSO + 3SO ⎯→ KSO+ Cr (SO) + HO22 724224 2432Chromium sulphate (green) Potassium (b) An aqueous solution or sodium carbonate extract of the salt produces a permanganate white precipitate of barium sulphite on addition of barium chloride solution. NaSO + BaCl⎯→ 2NaCl + BaSO232 3 This precipitate gives following tests. (i) This precipitate on treatment with dilute HCl, dissolves due to decomposition of sulphite by dilute HCl. Evolved SO2 gas can be tested. BaSO3 + 2HCl ⎯→ BaCl2 + H2O + SO2 (ii) Precipitate of sulphite decolourises acidified potassium permanganate solution. BaSO + HSO⎯→ BaSO + HO + SO3 24 422 2KMnO4 + 3H2SO4 ⎯→ K2SO4 + 2MnSO4 + 3H2O + 5 [O] SO2 + H2O + [O] ⎯→ H2SO4 4. Test for Nitrite ion [NO2–] (a) On treating a solid nitrite with dil. HSOandwarming,reddish brown 2 4 fumes of NO2 gas are evolved. Addition of potassium iodide solution to the salt solution followed by freshly prepared starch solution and acidification with acetic acid produces blue colour. Alternatively, a filter paper moistened with potassium iodide and starch solution and a few drops of acetic acid turns blue on exposure to the gas due to the interaction of liberated iodine with starch. (i) 2NaNO2 + H2SO4 ⎯→ Na2SO4 + 2HNO2 3HNO2 ⎯→ HNO3 + 2NO + H2O 2NO + O2 ⎯→ 2NO2Brown gas (ii) NO2– + CH3COOH ⎯→ HNO2 + CH3COO– 2HNO2 + 2KI + 2CH3COOH ⎯→ 2CH3 COOK + 2H2O + 2NO + I2 I2 + Starch ⎯→ Blue complex (b) Sulphanilic acid — 1-naphthylamine reagent test (Griss-Ilosvay test) On adding sulphanilic acid and 1-naphthylamine reagent to the water extract or acidified with acetic acid, sulphanilic acid is diazotised in the reaction by nitrous acid formed. Diazotised acid couples with 1-naphthylamine to form a red azo-dye. NO– + CHCOOH ⎯→ HNO + CHCOO– 23 23The test solution should be very dilute. In concentrated solutions reaction does not proceed beyond diazotisation. 5. Test for Acetate ion [CHCOO–]3(a) If the salt smells like vinegar on treatment with dil. H2SO4, this indicates the presence of acetate ions. Take 0.1 g of salt in a china dish and add 1 mL of ethanol. Then add about 0.2 mL of conc. HSOand heat. Fruity24 odour of ethyl acetate indicates the presence of CH3COO– ion. 2 CHCOONa + HSO⎯→ NaSO + 2 CHCOOH324 243H+ CH3COOH + C2H5OH ⎯→ CH3COOC2H5 + H2O Ethylacetate (Fruity odour) (b) Acetate gives deep red colour on reaction with neutral ferric chloride solution due to the formation of complex ion which decomposes on heating to give Iron (III) dihydroxyacetate as brown red precipitate. ]+6 CH3COO– + 3Fe3+ + 2H2O ⎯→ [Fe3(OH)2 (CH3COO)6 + 2H+ [Fe(OH) (CHCOO)]+ + 4HO ⎯→ 3[Fe (OH) (CHCOO)] + 3CHCOOH + H+32362 233Iron(III)dihydroxyacetate (Brown-red precipitate) Step-II : Preliminary Test with Concentrated Suphuric Acid If no positive result is obtained from dil. H SO test, take 0.1 g of salt in a test24tube and 3-4 drops of conc. H2SO4. Observe the change in the reaction mixture in cold and then warm it. Identify the gas evolved on heating (see Table 7.3). Table 7.3 : Preliminary examination with concentrated sulphuric acid Observations 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. Inference Gas/vapours evolved HCl Possible anions 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 MnO2 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 MnO2 to the reaction mixture. I2 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. NO2 Nitrate, ( – 3NO ) 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, ( 2– 2 4CO ) Confirmatory tests for the anions which react with concentrated sulphuric acid are given in Table 7.4. –Table 7.4 : Confirmatory tests for Cl–, Br–, I–, NO3 and C2O42– Anion Confirmatory test Chloride (Cl – ) (a) (b) (c) Take 0.1 g of 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. Take 1 mL of sodium carbonate extract in a test tube, acidfy it with dil. HNO3 or take water extract and add silver nitrate solution. A curdy white precipitate is obtained which is soluble in ammonium hydroxide solution. Take 0.1 g salt and a pinch of solid potassium dichromate in a test tube, add conc. H2SO4, 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 1 mL of amyl alcohol followed by 1 mL of 10% hydrogen peroxide. After gentle shaking the organic layer turns blue. Bromide (Br – ) (a) (b) (c ) Take 0.1 g of salt and a pinch of MnO2 in a test tube. Add 3-4 drops conc.sulphuric acid and heat. Intense brown fumes are evolved. Neutralise 1 mL of sodium carbonate extract with hydrochloric acid (or take the water extract). Add 1 mL carbon tetrachloride (CCl4)/chloroform (CHCl3)/ carbon disulphide. Now add an excess of chlorine water dropwise and shake the test tube. A brown colouration in the organic layer confirms the presence of bromide ion. Acidify 1 mL of sodium carbonate extract with dil. HNO3 (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) (b) 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. Take 1 mL of sodium carbonate extract acidify it with dil. HNO3 (or take water extract). Add, silver nitrate solution. A yellow precipitate insoluble in NH4 OH solution is obtained. Manganese dioxide Chemistry of Confirmatory Tests 1. Test for Chloride ion [Cl–] (a) If on treatment with warm conc. HSO the salt gives a colourless gas24with pungent smell or and if the gas which gives dense white fumes with ammonia solution, then the salt may contain Cl– ions and the following reaction occurs. NaCl + H2SO4 ⎯→ NaHSO4 + HCl Sodium Hydrogen chloride hydrogen sulphate Colourless gas HCl + NH3 ⎯→ NH4Cl Ammonium chlorideSilver nitrate White fumes (b) If a salt gives effervescence on heating with conc. HSO and MnO and242a light greenish yellow pungent gas is evolved, this indicates the presence of Cl–ions. MnO2 + 2NaCl + 2H2⎯→ Na2SO4 + MnSO4 + 2H2O + Cl2SO4 (c) Salt solution acidified with dilute HNO on addition of silver nitrate3solution gives a curdy white precipitate soluble in ammonium hydroxide solution. This indicates the presence of Cl– ions in the salt. NaCl + AgNO⎯→ NaNO + Ag Cl33Silver chloride (White precipitate) AgCl + 2NHOH ⎯→ [Ag(NH) ]Cl + 2HO4322Diammine silver (I) chloride * This test can also be performed by adding first ferrous sulphate solution and then conc. H2SO4. (d) Mix a little amount of salt and an equal amount of solid potassium dichromate (KCrO) in a test tube and add conc. HSO to it. Heat the227 24test tube and pass the evolved gas through sodium hydroxide solution. If a yellow solution is obtained, divide the solution into two parts. Acidify the first part with acetic acid and then add lead acetate solution. Formation of a yellow precipitate of lead chromate confirms the presence of chloride ions in the salt. This test is called chromyl chloride test.* 4NaCl + K2Cr2O7 + 6H2SO4 ⎯→ 2KHSO4 + 2CrO2Cl2 + 4NaHSO4 + 3H2O Chromyl (Chromyl chloride chloride) CrOCl + 4NaOH ⎯→ NaCrO + 2NaCl + 2HO22 242Lead (CHCOO)Pb + NaCrO ⎯→ PbCrO + 2CHCOONa chromate3224 43Sodium Lead chromate chromate (Yellow precipitate) Acidify the second part with dilute sulphuric acid and add small amounts of amyl alcohol and then 1 mL of 10% hydrogen peroxide solution. On gentle shaking –organic layer turns blue. CrO2 ion formed in the reaction of chromyl chloride4 with sodium hydroxide reacts with hydrogen peroxide to form chromium pentoxide (CrO) (See structure) which dissolves in amyl alcohol to give blue colour. 5CrO4 2– + 2H+ + 2H2O2 ⎯→ Cr O5 + 3H2O Chromium pentoxide 2. Test for Bromide ion (Br –) If on heating the salt with conc. H2SO4 reddish brown fumes of bromine are evolved in excess, this indicates the presence of Br–ions. The fumes get intensified on addition of MnO. Bromine vapours turn starch paper yellow.2Bromine 2NaBr + 2H2SO4 ⎯→ Br2 + SO2 + Na2SO4 + 2H2O very toxic by inhalation 2NaBr + MnO + 2HSO⎯→ NaSO + MnSO + 2HO + Brcorrosive224 24422 (a) Add 1 mL of carbon tetrachloride (CCl4)/chloroform (CHCl3)** and excess of freshly prepared chlorine water dropwise to the salt solution in water or sodium carbonate extract neutralised with dilute HCl. Shake the test tube vigorously. The appearance of an orange brown colouration in the organic layer due to the dissolution of bromine in it, confirms the presence of bromide ions. 2NaBr + Cl⎯→ 2NaCl + Br2 2 * Chromyl chloride test should be performed with minimum amount of substance to avoid pollution by Cr3+ ions. ** In place of carbon tetrachloride or chloroform, carbon disulphide or dichloromethane (CH2Cl2) can also be used. (b) Acidify the sodium carbonate extract of the salt with dil. HNO. Add silver nitrate (AgNO3) solution and shake the test tube. A pale yellow precipitate is obtained which dissolves in ammonium hydroxide with difficulty. NaBr + AgNO3 ⎯→ NaNO3 + AgBr Silver bromide Pale yellow precipitate 3. Test for Iodide ion ( I – ) (a) If on heating the salt with conc. H2SO4 , deep violet vapours with a pungent smell are evolved. These turns starch paper blue and a violet sublimate is formed on the sides of the test tube, it indicates the presence of I– ions. Some HI, sulphur dioxide, hydrogen sulphide, and sulphur are also formed due to the following reactions.Iodine, 2NaI + 2H2SO4 ⎯→ Na2SO4 + SO2 + 2H2O + I2harmful by inhalation and contact I + Starch ⎯→ Blue colour2with skin solution Chlorine, NaI + H2SO4 ⎯→ NaHSO4 + HI toxic by inhalation 2HI + HSO⎯→ 2HO + I + SO24 222 6NaI + 4H2SO4 ⎯→ 3I2 + 4H2O +S + 3Na2SO4Chloroform, harmful and toxic by 8NaI + 5 HSO⎯→ 4I + HS + 4NaSO + 4HO24 22242inhalation On adding MnO2 to the reaction mixture, the violet vapours become dense. 2NaI + MnO + 2HSO⎯→ I + MnSO + NaSO + 2HO224 24242(b) Add 1 mL of CHCl3 or CCl4 and chlorine water in excess to the salt solution in water or sodium carbonate extract neutralised with dil.HCl and shake the test tube vigorously. Presence of violet colouration in the organic layer confirms the presence of iodide ions. 2NaI + Cl2 ⎯→ 2NaCl + I2 Iodine dissolves in the organic solvent and the solution becomes violet. (c) Acidify sodium carbonate extract of the salt with dil.HNO and add3AgNO3 solution. Appearance of a yellow precipitate insoluble in excess of NH4OH confirms the presence of iodide ions. NaI + AgNO⎯→ AgI + NaNO3 3silver iodide (Yellow precipitate) 4. Test for Nitrate ion [NO –]3 (a) If on heating the salt with conc. H2SO4 light brown fumes are evolved then heat a small quantity of the given salt with few copper turnings or chips and conc. HSO. Evolution of excess of brown fumes indicates24the presence of nitrate ions. The solution turns blue due to the formation of copper sulphate. Copper sulphateNaNO3 + H2SO4 ⎯→ NaHSO4 + HNO3 4HNO3 ⎯→ 4NO2 + O2 + 2H2O 2NaNO + 4HSO + 3Cu ⎯→ 3 CuSO+ NaSO+ 4HO + 2NO324 4 242NitricCopper sulphate acid(Blue) 2NO + O2 ⎯→ 2NO2(Brown fumes) (b) Take 1 mL of an aqueous solution of the salt and add 2 mL conc. HSOOxalates24 slowly. Mix the solutions thoroughly and cool the test tube under the tap. Now, add freshly prepared ferrous sulphate solution along the sides of the test tube dropwise so that it forms a layer on the top of the liquid already present in the test tube. A dark brown ring is formed at the junction of the two solutions due to the formation of nitroso ferrous sulphate (Fig. 7.2). Alternatively first ferrous sulphate is added and then concentrated sulphuric acid is added. NaNO + HSO⎯→ Na HSO + HNO3 24 43 6 FeSO + 3HSO + 2HNO ⎯→ 3Fe (SO) + 4HO + 2NO42432432FeSO4 + NO ⎯→ [Fe(NO)]SO4 Nitroso ferrous sulphate (Brown) 5. Test for Oxalate ion [C2O24–] If carbon dioxide gas along with carbon monoxide gas is evolved in the preliminary examination with concentrated sulphuric acid, this gives indication about the presence of oxalate ion. (COONa)+ Conc. HSO⎯→ NaSO + HO + CO↑ + CO ↑2 24 2422 Oxalate is confirmed by the following tests: (a) Acidify sodium carbonate extract with acetic acid and add calcium chloride solution. A white precipitate of calcium oxalate, insoluble in ammonium oxalate and oxalic acid solution indicates the presence of oxalate ion. CaCl + NaCO ⎯→ CaCO + 2NaCl2 22424Calcium oxalate (White precipitate) (b) KMnO test4Filter the precipitate from test (a). Add dil. H2SO4 to it followed by dilute KMnO solution and warm. Pink colour of KMnO is discharged:4 4CaC2O4 + H2SO4 ⎯→ CaSO4 + H2C2O4 Calcium sulphate Oxalic acid 2 KMnO + 3HSO + 5HCO⎯→2MnSO + KSO + 8HO + 10CO4 24224 42422 Pass the gas evolved through lime water. A white precipitate is formed which dissolves on passing the gas for some more time. Step-III : Test for Sulphate and Phosphate If no positive test is obtained in Steps-I and II, then tests for the presence of sulphate and phosphate ions are performed. These tests are summarised in Table 7.5. Chemistry of Confirmatory Tests 1. Test of Sulphate ions [SO42–] (a) Aqueous solution or sodium carbonate extract of the salt acidified with acetic acid on addition of barium chloride gives a white precipitate of barium sulphate insoluble in conc. HCl or conc. HNO3. Na2SO4 + BaCl2 ⎯→ BaSO4 + 2NaClBarium sulphate(White precipitate) (b) Sulphate ions give white precipitate of lead sulphate when aqueous solution or sodium carbonate extract neutralised with acetic acid is treated with lead acetate solution. Na2SO4 + (CH3COO)2Pb ⎯→ PbSO4 + 2CH3COONa Lead sulphate (White precipitate) – 2. Test for Phosphate ion [PO43 ] (a) Add conc. HNO3 and ammonium molybdate solution to the test solution containing phosphate ions and boil. A yellow colouration in solution or a canary yellow precipitate of ammonium-phosphomolybdate, (NH4[P (Mo3] is formed. Each oxygen of phosphate has been)3O10)4replaced by MoO group.310NaHPO + 12(NH) MoO + 23 HNO⎯→ (NH)[P (MoO)] + 2NaNO+ 21NHNO + 12HO2 44243 4331043 432Canary yellow precipitate The tests for cations may be carried out according to the following scheme. Step - I : Preliminary Examination of the Salt for Identification of Cation 1. Colour Test Observe the colour of the salt carefully, which may provide useful information about the cations. Table 7.6 gives the characteristic colours of the salts of some cations. 2. Dry Heating Test (i) Take about 0.1 g of the dry salt in a clean and dry test tube. (ii) Heat the above test tube for about one minute and observe the colour of the residue when it is hot and also when it becomes cold. Observation of changes gives indications about the presence of cations, which may not be taken as conclusive evidence (see Table 7.7). 3. Flame Test The chlorides of several metals impart characteristic colour to the flame because they are volatile in non-luminous flame. This test is performed with the help of a platinum wire as follows : (i) Make a tiny loop at one end of a platinum wire. (ii) To clean the loop dip it into concentrated hydrochloric acid and hold it in a non-luminous flame (Fig. 7.3). (iii) Repeat step (ii) until the wire imparts no colour to the flame. (iv) Put 2-3 drops of concentrated hydrochloric acid on a clean watch glass and make a paste of a small quantity of the salt in it. (v) Dip the clean loop of the platinum wire in this paste and introduce the loop in the non-luminous (oxidising) flame (Fig. 7.3). (vi) Observe the colour of the flame first with the naked eye and then through a blue glass and identify the metal ion with the help of Table 7.8. Fig.7.3 : Performing flame test Table 7.8 : Inference from the flame test Colour of the flame Colour of the flame Inferenceobserved by naked eye observed through blue glass Cu2+Green flame with Same colour as observed blue centre without glass Sr2+Crimson red Purple Ba2+Apple green Bluish green Ca2+Brick red Green 4. Borax Bead Test This test is employed only for coloured salts because borax reacts with metal salts to form metal borates or metals, which have characteristic colours. (i) To perform this test make a loop at the end of the platinum wire and heat it in a flame till it is red hot. (b)(a) (ii) Dip the hot loop into borax powder and heat it again until borax forms a colourless transparent bead on the loop. Fig. 7.4 : Borax bead test Before dipping the borax bead in the test salt or mixture, (a) Heating in reducing confirm that the bead is transparent and colourless. If it flame (b) Heating in is coloured this means that, the platinum wire is not clean. oxidising flame Then make a fresh bead after cleaning the wire. (iii) Dip the bead in a small quantity of the dry salt and again hold it in the flame. (iv) Observe the colour imparted to the bead in the non - luminous flame as well as in the luminous flame while it is hot and when it is cold (Fig. 7.4). (v) To remove the bead from the platinum wire, heat it to redness and tap the platinum wire with your finger. (Fig. 7.5). On heating, borax loses its water of crystallisation and decomposes to give sodium metaborate and boric anhydride. NaBO .10HO ⎯→ NaBO + 10HO24722472Borax NaBO⎯→2NaBO+ BO247 223Sodium metaborate Boric anhydride On treatment with metal salt, boric anhydride forms metaborate of the metal which gives different colours in oxidising and reducing flame. For example, in the case of copper sulphate, following reactions occur. Non -luminous flameCuSO+ BO⎯⎯⎯ ⎯⎯⎯⎯⎯⎯→ Cu(BO) + SO4 23 223 Cupric metaborate Blue-green Two reactions may take place in the reducing flame: (i) The blue Cu (BO) is reduced to colourless cuprous metaborate as follows:22Luminous flame2Cu(BO ) + 2NaBO⎯⎯⎯⎯⎯⎯⎯⎯ 2CuBONaBO +CO+C→ +22 2 2247or (ii) Cupric metaborate may be reduced to metallic copper and the bead appears red and opaque. Luminous flame2Cu(BO ) + 4NaBO + 2C ⎯⎯⎯⎯⎯⎯⎯⎯2Cu + 2Na BO→ + 2CO222 247 The preliminary identification of metal ion can be made from Table 7.9. Table 7.9 : Inference from the borax bead test Heating in oxidising (non-luminous) flame Heating in reducing (luminous) flame Colour of the salt bead Colour of the salt bead In cold Blue Reddish brown Light violet Yellow In hot Green Violet Light violet Yellowish brown In cold Red opaque Grey Colourless Green In hot Colourless Grey Colourless Green Inference Cu2+ Ni2+ Mn2+ Fe3+ 5. Charcoal Cavity Test Metallic carbonate when heated in a charcoal cavity decomposes to give corresponding oxide. The oxide appears as a coloured residue in the cavity. Sometimes oxide may be reduced to metal by the carbon of the charcoal cavity. The test may be performed as follows: (i) Make a small cavity in a charcoal block with the help of a charcoal borer. Do not apply pressure otherwise it will crack [Fig.7.6 (a)]. (ii) Fill the cavity with about 0.2 g of the salt and about 0.5 g of anhydrous sodium carbonate. (b)(a) Fig. 7.6 : (a) Making charcoal cavity (b) Heating salt in the cavity (iii) Moisten the salt in the cavity with one or two drops of water, otherwise salt/mixture will blow away. (iv) Use a blowpipe to heat the salt in a luminous (reducing) flame and observe the colour of oxide/ metallic bead formed in the cavity both when hot and cold [ Fig. (7.6 b)]. Obtain oxidising and reducing flame as shown in Fig. 7.7 a and b. (v) Always bore a fresh cavity for testing the new salt. Note : • To obtain oxidising flame hold the nozzle of the blowpipe about one third within the flame. • To obtain reducing flame place nozzle of the blowpipe just outside the flame. (a) (b) Fig. 7.7 : Obtaining oxidising and reducing flame (a) Oxidising flame (b) Reducing flame When test is performed with CuSO4, the following change occurs. Heat CuSO +Na CO ⎯⎯⎯⎯CuCO +Na SO →423 324 Heat CuCO 3 ⎯⎯⎯⎯CuO+CO 2→ Heat CuO + C ⎯⎯⎯⎯Cu +CO→ Red colour In case of ZnSO4 : Heat ZnSO +Na CO ⎯⎯⎯⎯ZnCO + Na → SO 423 324 Heat ZnCO 3 ⎯⎯⎯⎯ZnO + CO 2→Yellow when hot, White when cold The metal ion can be inferred from Table 7.10. 6. Cobalt Nitrate Test If the residue in the charcoal cavity is white, cobalt nitrate test is performed. (i) Treat the residue with two or three drops of cobalt nitrate solution. (ii) Heat it strongly in non-luminous flame with the help of a blow pipe and observe the colour of the residue. On heating, cobalt nitrate decomposes into cobalt (II) oxide, which gives a characteristic colour with metal oxide present in the cavity. Thus, with ZnO, Al2O3 and MgO, the following reactions occur. Heat 2Co(NO ) ⎯⎯⎯⎯2CoO+4NO +O →32 22 CoO + ZnO ⎯→ CoO.ZnOGreen CoO + MgO ⎯→ CoO. MgOPinkCoO + Al2O3 ⎯→ CoO. Al2O3Blue Step-II : Wet Tests for Identification of Cations The cations indicated by the preliminary tests given above are confirmed by systematic analysis given below. The first essential step is to prepare a clear and transparent solution of the salt. This is called original solution. It is prepared as follows: Preparation of Original Solution (O.S.) To prepare the original solution, following steps are followed one after the other in a systematic order. In case the salt does not dissolve in a particular solvent even on heating, try the next solvent. The following solvents are tried: 1. Take a little amount of the salt in a clean boiling tube and add a few mL of distilled water and shake it. If the salt does not dissolved, heat the content of the boiling tube till the salt completely dissolves. 2. If the salt is insoluble in water as detailed above, take fresh salt in a clean boiling tube and add a few mL of dil.HCl to it. If the salt is insoluble in cold, heat the boiling tube till the salt is completely dissolved. 3. If the salt does not dissolve either in water or in dilute HCl even on heating, try to dissolve it in a few mL of conc. HCl by heating. 4. If salt does not dissolve in conc. HCl, then dissolve it in dilute nitric acid. 5. If salt does not dissolve even in nitric acid then a mixture of conc. HCl and conc. HNO3 in the ratio 3:1 is tried. This mixture is called aqua regia. A salt not soluble in aqua regia is considered to be an insoluble salt. Group Analysis (I) Analysis of Zero group cation (NH+ ion)4 (a) Take 0.1 g of salt in a test tube and add 1-2 mL of NaOH solution to it and heat. If there is a smell of ammonia, this indicates the presence of ammonium ions. Bring a glass rod dipped in hydrochloric acid near the mouth of the test tube. White fumes are observed. (b) Pass the gas through Nessler’s reagent. Brown precipitate is obtained. Chemistry of Confirmatory Tests for NH4+ ion (a) Ammonia gas evolved by the action of sodium hydroxide on ammonium salts reacts with hydrochloric acid to give ammonium chloride, which is visible as dense white fume. (NH) SO + 2NaOH ⎯→ NaSO + 2NH + 2HO42 42432NH3 + HCl → NH4Cl Mercury Salts On passing the gas through Nessler’s reagent, a brown colouration or a precipitate of basic mercury(II) amido-iodine is formed. 2KHgI + NH + 3KOH ⎯→ HgO.Hg(NH)I + 7KI + 2HO24322Basic mercury (II) amido-iodine (Brown precipitate) For the analysis of cations belonging to groups I-VI, the cations are precipitated from the original solution by using the group reagents (see Table 7.11) according to the scheme shown in the flow chart given below: The separation of all the six groups is represented as below: * This flow chart is for the detection of one cation only. For detection of more than one cation modification will be required. (II) Analysis of Group-I cations Take a small amount of original solution ( if prepared in hot conc. HCl) in a test tube and add cold water to it and cool the test tube under tap water. If a white precipitate appears, this indicates the presence of Pb2+ ions in group –I. On the other hand, if the original solution is prepared in water and on addition of dil. HCl, a white precipitate appears, this may also be Pb2+. Confirmatory tests are described below in Table 7.12. Chemistry of the Confirmatory Tests of Pb2+ ions Lead is precipitated as lead chloride in the first group. The precipitate is soluble in hot water. 1. On adding potassium iodide (KI) solution, a yellow precipitate of lead iodide is obtained which confirms the presence of Pb2+ ions. PbCl2 + 2KI ⎯→ PbI2 + 2KCl (Hot solution) Yellow precipitate * Here only those cations are given which are in the syllabus. This yellow precipitate (PbI2) is soluble in boiling water and reappears on cooling as shining crystals. 2. On addition of potassium chromate (KCrO) solution a yellow precipitate24of lead chromate is obtained. This confirms the presence of Pb2+ ions. PbCl + KCrO ⎯→ PbCrO + 2KCl2244(Hot solution) Lead chromate (Yellow precipitate) The yellow precipitate (PbCrO) is soluble in hot NaOH solution.4PbCrO + 4NaOH � Na[Pb(OH)] + NaCrO42424Sodium tetrahydroxoplumbate (II) 3. A white precipitate of lead sulphate (PbSO) is formed on addition of4alcohol followed by dil. H2SO4. PbCl2 + H2⎯→ PbSO4 + 2 HClSO4 Lead sulphate (White precipitate) Lead sulphate is soluble in ammonium acetate solution due to the formation of tetraacetoplumbate(II) ions. This reaction may be promoted by addition of few drops of acetic acid. Hydrogen PbSO + 4 CHCOONH⎯→ (NH) [Pb(CHCOO)] + (NH)SO434 4234424sulphide Ammonium tetraacetoplumbate(II) (III) Analysis of Group–II cations If group-I is absent, add excess of water to the same test tube. Warm the solution and pass HS gas for 1-2 minutes (Fig. 7.6). Shake the test tube. If a precipitate2appears, this indicates the presence of group-II cations. Pass more H2S gas through the solution to ensure complete precipitation and separate the precipitate. If the colour of the precipitate is black, it indicates the presence of Cu2+ or Pb2+ ions. If it is yellow in colour, then presence of As3+ ions is indicated. Take the precipitate of group-II in a test tube and add excess of yellow ammonium sulphide solution to it. Shake the test tube. If the precipitate is insoluble, group II-A (copper group) is present. If the precipitate is soluble, this indicates the presence of group-II B (arsenic group). Confirmatory tests for the groups II A and II B are given in Table 7.13. Table 7.13 : Confirmatory tests for Group-II A and II B cations Black precipitate of Group II A ions (Pb2+, Cu2+) insoluble in yellow ammonium sulphide is formed. If a yellow precipitate soluble in yellow ammonium sulphide is formed then As3+ ion is present. Boil the precipitate of Group II A with dilute nitric acid and add a few drops of alcohol and dil. H2SO4. Acidify this solution with dilute HCl. A yellow precipitate is formed. Heat the precipitate with concentrated nitric acid and add ammonium molybdate solution. A canary yellow precipitate is formed. If no precipitate is formed, add excess of ammonium hydroxide solution. A blue solution is obtained, acidify it with acetic acid and add potassium ferrocyanide solution. A chocolate brown precipitate is formed. White precipitate confirms the presence of Pb2+ ions. Dissolve the precipitate in ammonium acetate solution. Acidify with acetic acid and divide the solution into two parts. (i) To the first part add potassium chromate solution, a yellow precipitate is formed. (ii) To the second part, add potassium iodide solution, a yellow precipitate is formed. Group-II A (Copper Group) Alcohol Chemistry of confirmatory tests of Group-II A cations 1. Test for Lead ion (Pb2+) Lead sulphide precipitate dissolves in dilute HNO3. On adding dil. H2SO4 and a few drops of alcohol to this solution a white precipitate of lead sulphate appears. This indicates the presence of lead ions. 3PbS + 8HNO3 ⎯→ 3Pb (NO3)2 + 2NO + 4H2O + 3S Pb(NO) + HSO⎯→ PbSO + 2HNO3224 43 The white precipitate dissolves in ammonium acetate solution on boiling. When this solution is acidified with acetic acid and potassium chromate solution is added, a yellow precipitate of PbCrO is formed. On adding potassium iodide4solution, a yellow precipitate of lead iodide is formed. PbSO + 4CHCOONH⎯→ (NH)[Pb (CHCOO)] + (NH)SO434 4234424 (NH4)2 [Pb(CH3COO)4] + K2CrO4 ⎯→ PbCrO4 + 2CH3COOK + 2(NH4)2SO4 Ammonium Yellow tetraacetoplumbate(II) precipitate 2. Test for Copper ion (Cu2+) (a) Copper sulphide dissolves in nitric acid due to the formation of copper nitrate. 3CuS + 8HNO3 ⎯→ 3Cu(NO3)2 + 2NO + 3S + 4H2O On heating the reaction mixture for long time, sulphur is oxidised to sulphate and copper sulphate is formed and the solution turns blue. A small amount of NH4OH precipitates basic copper sulphate which is soluble in excess of ammonium hydroxide due to the formation of tetraamminecopper (II) complex. S + 2HNO⎯→ HSO + 2NO3 242Cu2++ SO2− +2NH+2HO ⎯→ Cu(OH). CuSO+ 2NH+ 432 244 Cu(OH)2.CuSO4 + 8NH3 ⎯→ 2 [Cu(NH3)4]SO4 + 2OH – + SO2− 4Tetraamminecopper (II) sulphate (Deep blue) (b) The blue solution on acidification with acetic acid and then adding potassium ferrocyanide [K4Fe(CN)6 ] solution gives a chocolate colouration due to the formation of copper ferrocyanide i.e.Cu2[Fe(CN)6]. [Cu(NH)] SO + 4CHCOOH ⎯→ CuSO + 4CHCOONH3443 434 2CuSO4 + K4[Fe(CN)6] ⎯→ Cu2[Fe(CN)6] + 2K2SO4 Potassium Copper hexacyanoferrate (II) hexacyanoferrate (II) (Chocolate brown precipitate) Group-II B (Arsenic Group) If group- II precipitate dissolves in yellow ammonium sulphide and the colour of the solution is yellow, this indicates the presence of As3+ ions. Ammonium thioarsenide formed on dissolution of AsS decomposes with dil. HCl, and a23yellow precipitate of arsenic (V) sulphide is formed which dissolves in concentrated nitric acid on heating due to the formation of arsenic acid. On adding ammonium molybdate solution to the reaction mixture and heating, a canary yellow precipitate is formed. This confirms the presence of As3+ ions. As2S3 + 3 (NH4)2S2 ⎯→ 2 (NH4)3As S4 + S Yellow ammonium sulphide 2(NH4)3AsS4+ 6HCl ⎯→ As2S5 + 3H2S + 6NH4Cl 3AsS + 10HNO + 4HO ⎯→ 6HAsO + 10NO + 15S253234Arsenic acid H3AsO4 + 12(NH4)2 MoO4 + 21HNO3 ⎯→ (NH4)3[As (Mo3 )4] + 21NH4+12H2OO10NO3 Arsenic acid Ammonium Ammoniummolybdate arsinomolybdate(yellow precipitate) (IV) Analysis of Group–III cations If group-II is absent, take original solution and add 2-3 drops of conc. HNO3 to oxidise Fe2+ ions to Fe3+ ions. Heat the solution for a few minutes. After cooling add a small amount of solid ammonium chloride (NHCl) and an excess of ammonium4hydroxide (NH4OH) solution till it smells of ammonia. Shake the test tube. If a brown or white precipitate is formed, this indicates the presence of group-III cations. Confirmatory tests of group-III cations are summarised in Table 7.14. Observe the colour and the nature of the precipitate. A gelatinous white precipitate indicates the presence of aluminium ion (A13+). If the precipitate is brown in colour, this indicates the presence of ferric ions (Fe3+). Chemistry of confirmatory tests of Group-III cations When original solution is heated with concentrated nitric acid, ferrous ions are oxidised to ferric ions. 2FeCl + 2HCl + [O] ⎯→ 2FeCl + HO2 32Third group cations are precipitated as their hydroxides, which dissolve in dilute hydrochloric acid due to the formation of corresponding chlorides. 1. Test for Aluminium ions (A13+) (a) When the solution containing aluminium chloride is treated with sodium hydroxide, a white gelatinous precipitate of aluminium hydroxide is formed which is soluble in excess of sodium hydroxide solution due to the formation of sodium aluminate. AlCl + 3NaOH ⎯→ Al(OH) + 3NaCl3 3Al(OH)3 + NaOH ⎯→ NaAlO2 + 2H2O White gelatinous Sodium precipitate aluminate (b) In the second test when blue litmus is added to the solution, a red colouration is obtained due to the acidic nature of the solution. On addition of NH4OH solution drop by drop the solution becomes alkaline and aluminium hydroxide is precipitated. Aluminium hydroxide adsorbs blue colour from the solution and forms insoluble adsorption complex named ‘lake’. Thus a blue mass floating in the colourless solution is obtained. The test is therefore called lake test. 2. Test for ferric ions (Fe3+) Reddish brown precipitate of ferric hydroxide dissolves in hydrochloric acid and ferric chloride is formed. Fe(OH)3 + 3HCl ⎯→ FeCl3 + 3H2O (a) When the solution containing ferric chloride is treated with potassium ferrocyanide solution a blue precipitate/colouration is obtained. The colour of the precipitate is Prussian blue. It is ferric ferro-cyanide. The reaction takes place as follows: 4FeCl3 + 3K4[Fe(CN)6] ⎯→ Fe4[Fe(CN)6]3 + 12KCl Potassium Prussian blue ferrocyanide precipitate If potassium hexacyanoferrate (II) (i.e. potassium ferrocyanide) is added in excess then a product of composition KFe[Fe(CN)] is formed. This tends to6form a colloidal solution (‘soluble Prussian blue’) and cannot be filtered. FeCl + K[Fe(CN)] ⎯→ KFe[Fe(CN)] + 3KCl346 6(Soluble prussian blue) (b) To the second part of the solution, add potassium thiocyanate (potassium sulphocyanide) solution. The appearance of a blood red colouration confirms the presence of Fe3+ ions. –Fe3+ + SCN ⎯→ [Fe(SCN)]2+Blood red colour (V) Analysis of group-IV cations If group-III is absent, pass HS gas in the solution of group-III for a few 2minutes. If a precipitate appears (white, black or flesh coloured), this indicates the presence of group-IV cations. Table 7.15 gives a summary of confirmatory tests of group-IV cations. Chemistry of confirmatory tests of Group–IV cations Fourth group cations are precipitated as their sulphides. Observe the colour of the precipitate. A white colour of the precipitate indicates the presence of zinc ions, a flesh colour indicates the presence of manganese ions and a black colour indicates the presence of Ni2+ or Co2+ ions. 1. Test for Zinc ion (Zn2+) Zinc sulphide dissolves in hydrochloric acid to form zinc chloride. ZnS + 2HCl ⎯→ ZnCl2 + H2S (a) On addition of sodium hydroxide solution it gives a white precipitate of zinc hydroxide, which is soluble in excess of NaOH solution on heating. This confirms the presence of Zn2+ ions. ZnCl2 + 2NaOH ⎯→ Zn(OH)2 + 2NaCl Zn(OH)+ 2NaOH ⎯→ NaZnO + 2HO2 222Sodium zincate (b) When potassium ferrocyanide K4Fe(CN)6 solution is added to the solution after neutralisation by NHOH solution, a white or a bluish white precipitate4of zinc ferrocyanide appears. 2 ZnCl + K [Fe(CN)] ⎯→ Zn [Fe(CN)] + 4 KCl24626Zinc ferrocyanide 2. Test for Manganese ion (Mn2+) Manganese sulphide precipitate dissolves in dil. HCl on boiling. On addition of NaOH solution in excess, a white precipitate of manganese hydroxide is formed which turns brown due to atmospheric oxidation into hydrated manganese dioxide. MnS + 2HCl ⎯→ Mn Cl2 + H2S MnCl + 2NaOH ⎯→ Mn(OH) + 2NaCl2 2(White precipitate) Mn (OH) + [O] → MnO(OH)2 2 Hydrated manganese dioxide (Brown colour) 3. Test for Nickel ion (Ni2+) The black precipitate of nickel sulphide dissolves in aqua regia and the reaction takes place as follows: 3NiS + 2HNO + 6HCI ⎯→ 3NiCl + 2NO + 3S + 4HO3 22After treatment with aqua regia nickel-chloride is obtained which is soluble in water. When dimethyl glyoxime is added to the aqueous solution of nickel chloride, made alkaline, by adding NH4OH solution, a brilliant red precipitate is obtained. Complex of red colour (Stable form of complex) 4. Test for Cobalt ion (Co2+) Cobalt sulphide dissolves in aqua regia in the same manner as nickel sulphide. When the aqueous solution of the residue obtained after treatment with aqua regia is treated with a strong solution of potassium nitrite after neutralisation with ammonium hydroxide and the solution is acidified with dil. acetic acid, a yellow precipitate of the complex of cobalt named potassium hexanitritocobaltate (III) is formed. CoS + HNO + 3HCl ⎯→ CoCl + NOCl + S + 2HO322CoCl2 + 7KNO2 + 2CH3COOH⎯→ K3 [Co(NO2)6] + 2KCl + 2CH3COOK + NO + H2OPotassiumhexanitritocobaltate(III) (Yellow precipitate) (VI) Analysis of Group–V cations If group-IV is absent then take original solution and add a small amount of solid NHCl and an excess of NHOH solution followed by solid ammonium carbonate44(NH4)2CO3. If a white precipitate appears, this indicates the presence of group–V cations. Dissolve the white precipitate by boiling with dilute acetic acid and divide the solution into three parts one each for Ba2+, Sr2+ and Ca2+ ions. Preserve a small amount of the precipitate for flame test. Summary of confirmatory tests is given in Table 7.16. Dissolve the precipitate by boiling with dilute acetic acid and divide the solution into three parts one each for Ba2+, Sr2+ and Ca2+ ions Table 7.16 : Confirmatory test for Group–V cations Ba2+ ions Sr2+ ions Ca2+ ions (a) To the first part add potassium chromate solution. A yellow precipitate appears. (b) Perform the flame test with the preserved precipitate. A grassy green flame is obtained. (a) If barium is absent, take second part of the solution and add ammonium sulphate solution. Heat and scratch the sides of the test tube with a glass rod and cool. A white precipitate is formed. (b) Perform the flame test with the preserved precipitate. A crimson-red flame confirms the presence of Sr2+ ions. (a) If both barium and strontium are absent, take the third part of the solution. Add ammonium oxalate solution and shake well. A white precipitate of calcium oxalate is obtained. (b) Perform the flame test with the preserved precipitate. A brick red flame, which looks greenish-yellow through blue glass, confirms the presence of Ca2+ ions. Chemistry of Confirmatory Tests of Group–V cations The Group–V cations are precipitated as their carbonates which dissolve in acetic acid due to the formation of corresponding acetates. 1. Test for Barium ion (Ba2+) (a) Potassium chromate (K2CrO4) solution gives a yellow precipitate of barium chromate when the solution of fifth group precipitate in acetic acid is treated with it. BaCO3 + 2CH3COOH ⎯→ (CH3COO)2 Ba + H2O + CO2 (CHCOO)Ba + KCrO⎯→ BaCrO + 2CHCOOK3224 43Barium chromate (yellow precipitate )(b) Flame test – Take a platinum wire and dip it in conc. HCl. Heat it strongly until the wire does not impart any colour to the non-luminous flame. Now dip the wire in the paste of the (Group–V) precipitate in conc. HCl. Heat it in the flame. A grassy green colour of the flame confirms the presence of Ba2+ ions. 2. Test for Strontium ion (Sr2+) (a) Solution of V group precipitate in acetic acid gives a white precipitate of strontium sulphate with ammonium sulphate, (NH)SO, solution on42 4heating and scratching the sides of the test tube with a glass rod. SrCO + 2CHCOOH ⎯→ (CHCOO) Sr + HO + CO33 3222 (CHCOO) Sr + (NH)SO ⎯→ SrSO + 2CHCOONH32424434Strontiumsulphate(White precipitate) (b) Flame test – Perform the flame test as given in the case of Ba2+. A crimson red flame confirms the presence of Sr2+ ions. 3. Test for Calcium ion (Ca2+) (a) Solution of the fifth group precipitate in acetic acid gives a white precipitate with ammonium oxalate solution. CaCO + 2CHCOOH ⎯→ (CHCOO) Ca + HO + CO33 3222 (CHCOO)Ca + (NH)CO ⎯→ (COO)Ca + 2CHCOONH324224 234Ammonium Calcium oxalate oxalate (White precipitate) (b) Flame test – Perform the flame test as mentioned above. Calcium imparts brick red colour to the flame which looks greenish-yellow through blue glass. (VII) Analysis of Group–VI cations If group–V is absent then perform the test for Mg2+ ions as given below. Chemistry of Confirmatory Tests of Group–VI cations Test for Magnesium ion (Mg2+) (a) If group–V is absent the solution may contain magnesium carbonate, which Note :is soluble in water in the presence of ammonium salts because the Some times equilibrium is shifted towards the right hand side. precipitate of magnesiumNH+ + CO2- NH + HCO – 43 ��33 ammonium phosphateThe concentration of carbonate ions required to produce a precipitate is appearsnot attained. When disodium hydrogenphosphate solution is added and after some the inner walls of the test tube are scratched with a glass rod, a white time. So crystalline precipitate of magnesium ammonium phosphate is formed which warm the solution andindicates the presence of Mg2+ ions. scrach the Mg2+ + Na2HPO4 ⎯→ Mg (NH4)PO4 + NH4OH + 2Na+ + H2O sides of test tube after addingMagnesium ammoniumsodium phosphate (White precipitate) hydrogen phosphateNote down the observations and the inferences of the qualitative analysis solution. in tabular form as given in the specimen record given in pages 114-115. Discussion Questions (i) What is the difference between a qualitative and a quantitative analysis? (ii) Can we use glass rod instead of platinum wire for performing the flame test? Explain your answer. (iii) Why is platinum metal preferred to other metals for the flame test? (iv) Name the anions detected with the help of dilute H2SO4? (v) Why is dilute H2SO4 preferred over dilute HCl while testing anions? (vi) Name the anions detected by conc. H2SO4. (vii) How is sodium carbonate extract prepared ? (viii) What is lime water and what happens on passing carbon dioxide gas through it? (ix) Carbon dioxide gas and sulphur dioxide gas both turn lime water milky. How will you distinguish between the two? (x) How will you test the presence of carbonate ion? (xi) What is the composition of dark brown ring which is formed at the junction of two layers in the ring test for nitrates? (xii) Name the radical confirmed by sodium nitroprusside test. (xiii) What is chromyl chloride test ? How do you justify that CrO2Cl2 is acidic in nature? (xiv) Why do bromides and iodides not give tests similar to chromyl chloride test? (xv) Describe the layer test for bromide and iodide ions. (xvi) Why is silver nitrate solution stored in dark coloured bottles? (xvii) How do you test the presence of sulphide ion? (xviii) Why does iodine give a blue colour with starch solution? (xix) What is Nessler’s reagent? (xx) Why is original solution for cations not prepared in conc. HNO or HSO?3 24(xxi) Why cannot conc. HCl be used as a group reagent in place of dil. HCl for the precipitation of Ist group cations? (xxii) How can one prevent the precipitation of Group–IV radicals, with the second group radicals? (xxiii) Why is it essential to boil off H2S gas before precipitation of radicals of group–III? (xxiv) Why is heating with conc. nitric acid done before precipitation of group–III? (xxv) Can we use ammonium sulphate instead of ammonium chloride in group–III? (xxvi) Why is NHOH added before (NH)CO solution while precipitating group–V cations?4 423(xxvii) Why do we sometimes get a white precipitate in group–VI even if the salt does not contain Mg2+ radical? (xxviii) What is aqua regia? (xxix) Name a cation, which is not obtained from a metal. (xxx) How can you test the presence of ammonium ion? (xxxi) Why are the group–V radicals tested in the order Ba2+, Sr2+ and Ca2+? (xxxii) Why does conc. HNO3 kept in a bottle turn yellow in colour? (xxxiii) Why should the solution be concentrated before proceeding to group–V? (xxxiv) Why is the reagent bottle containing sodium hydroxide solution never stoppered? (xxxv) What do you understand by the term common ion effect? (xxxvi) Why is zinc sulphide not precipitated in group–II? SPECIMEN RECORD OF SALT ANALYSIS Aim To analyse the given salt for one anion and one cation present in it. Material required Sl. No. 1. Experiment Noted the colour of the given salt. Observation White Inference Cu2+, Fe2+, Ni2+,Co2+ , Mn2+ are absent. 2. Noted the smell of the salt. No specific smell. S2–, SO3 2– , CH3COO – may be absent. 3. Heated 0.5 g of the salt in a dry test tube and noted the colour of the gas evolved and change in the colour of the residue on heating and cooling. (i) No gas was evolved. (ii) No particular change in colour of the residue is observed when heated and when cooled. (i) CO3 2– may be present, NO3 – , NO2 – , Br – may be absent. (ii) Zn2+ may be absent. 4. Prepared a paste of the salt with conc. HCl and performed the flame test. No distinct colour of the flame seen. Ca2+, Sr2+, Ba2+ Cu2+ may be absent. 5. Borax bead test was not performed as the salt was white in colour. — — 6. Treated 0.1 g of salt with 1 mL dil.H2SO4 and warmed. No effervescence and evolution of vapours. CO3 2–, SO3 2–, S2–, NO2 – , CH3COO – absent. 7. Heated 0.1 g of salt with 1 mL conc. H2SO4. No gas evolved. Cl – , Br – , I – , NO3 – , C2O4 – are absent. 8. Acidified 1mL of aqueous salt solution with conc. HNO 3 . Warmed the contents and then added 4-5 drops of ammonium No yellow precipitate PO4 3– absent. molybdate solution. 9. Acidified water extract of the salt with dil. HCl and then added 2mL of BaCl2 solution. A white ppt. is obtained which is insoluble in conc. HNO3 and conc. HCl. SO4 2– present. 10. Heated 0.1 g of salt with 2 mL NaOH solution. Ammonia gas is not evolved. NH4 + absent. 11. Attempted to prepare original solution of the salt by dissolving 1g of it in 20 mL water. Clear solution formed Water soluble salt is present. 12. To a small part of the above salt solution added 2 mL of dil. HCl. No white precipitate formed. Group–I absent. 13. Passed H2S gas through one portion of the solution of step 12. No precipitate formed. Group–II absent. 14. Since salt is white, heating with conc. HNO3 is not required. Added about 0.2 g of solid ammonium chloride and then added excess of ammonium hydroxide to the solution of step 12. No precipitate formed. Group–III absent. 15. Passed H2S gas through the above solution. No precipitate formed. Group–IV absent. 16. Added excess of ammonium hydroxide solution to the original solution and then added 0.5 g of ammonium carbonate. No precipitate formed. Group–V absent. 17. To the original solution of salt added ammonium hydroxide solution, followed by disodium hydrogen phosphate solution. Heated and scratched the sides of the test tube. White precipitate. Mg2+ confirmed. Result The given salt contains: 2–Anion : SO4 Cation : Mg2+

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