General Principles of Inorganic Qualitative Analysis

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Monica Hensley
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1 Chapter General Principles of Inorganic Qualitative Analysis.1 The qualitative analysis of salt mixture Salt analysis is carried out by systematic method of analysis. Generally or conventionally the anions are first identified and then the cations. The anion tests are carried out on the dry salts as well as on solutions (sodium carbonate extract). Tests on cations are carried out using dry salt (charcoal test, flame test) as well as by using salt solutions. The anions tested are : carbonate, sulphate, chloride, bromide, iodide, acetate, nitrate, borate and phosphate. The cations tested are : lead, copper, iron, aluminum, zinc, manganese, calcium, strontium, barium, potassium and ammonium.. Basic two physico-chemical principles used in the analysis In the separation and identifications of cations, two basic physico-chemical principles are employed. These are : (a) Solubility product K sp principle of salts. (b) Common ion principle. (a) Solubility Product K sp In qualitative analysis of a salt mixture, for metal ions, the salt mixture is brought into solution form using water or dil. HCl. Then the metal ions are separated from the salt solution by using characteristic group reagents as precipitates. These precipitates are chlorides, sulphides and carbonates of the metals generally. A soluble metal ion in the solution is precipitated by adding a suitable anion (mostly Cl, S, CO ). The metal ion will be precipitated as a salt of the above mentioned anions if only the product of the metal ion concentration in the solution and the anion concentration added exceeds the solubility product M x X y of the salt (metal ion + added anion) ie. + 1 x M X XM + yx x y (s) (aq.) (aq.) nt x m y K sp = [ M ] [ X ] Solubility Product of a Salt The product of concentrations of metal ions and the anions in a saturated solution of the salt is called solubility product (K sp )

2 Ex am ple : BaCl, [ Ba ][ Cl ], Al ( SO ), [ Al ] [ SO ] + + The solubility products of chlorides, sulphides and carbonates of some metal ions are given in the table.1 Table.1 Solubility products of some salts Substance PbS CdS CuS HgS MnS ZnS NiS CoS Fe(OH) Al(OH) Zn(OH) Mn(OH) CaCO BaCO SrCO K sp at 5 C AgCl [Unit are not given] Generally the salt is represented as M x A y. The solubility of the salt is S. Then the solubility product K sp is written as follows : If sol u bil ity of the ppt. is S. Some ex am ples General Principles of Inorganic Qualitative Analysis 1 M A xm + ya x y y+ x sp = y+ x x y K [ xm ] [ ya ] Ksp = [ x S ] [ y A ] x + AgCl Ag + Cl K = [ Ag ][ Cl ] = S S = S sp + y

3 Chemistry BaSO + BaSO Ba + SO K = [ Ba ][ SO ] = S S = S sp + Al S + Al S Al + S = S S Ksp = [ S ] [ S ] = S 7 S =108S 5 BaCl + BaCl Ba + Cl K = S (S) = S sp Prob lem : The solubility product of AgCl = mols /lit. The concentration of metal ion [ Ag + ] in the solution is 10 moles/lit. What minimum contraction of Cl is to be added to precipitate AgCl? + Answer : The prod uct of con cen tra tions of Ag and Cl in the so lu tion must ex - ceed the sol u bil ity prod uct k sp for AgCl to get pre cip i tated. + Hence [ Ag ][ Cl ] [ Cl ] 1 10 [ Cl ] = 10 Hence [ Cl ] must be equal or greater than 10 6 moles/lit. (b) Com mon ion ef fect A salt pres ent in a so lu tion can be pre cip i tated if its sol u bil ity is de creased by add - ing gen er ally an an ion of the salt. This an ion must be the same as the an ion of the salt re quired to be pre cip i tated. For ex am ple if NaCl is to be pre cip i tated as NaCl from its so lu tion in wa ter, Cl is added in ex cess. Due to the in crease in the con cen tra tion of the com mon ion ie Cl, the sol u bil ity of NaCl in the so lu tion de creases and is pre cip i - tated. This Com mon ion ef fect is very im por tant in the anal y sis of metal ions. For ex am ple Cu + as well as Zn + will be pre cip i tated as their sulphides CuS, ZnS. But the sol u bil ity prod uct of CuS and ZnS are dif fer ent. Sol u bil ity prod uct of + + CuS is less then than that of ZnS. Hence if to a so lu tion of Cu + Zn ions, S ions are added CuS will be pre cip i tated first at lower con cen tra tion of added S. Gen er ally S ions are added in qual i ta tive anal y sis as H S. Hence if H S is added in pres ence of HCl. Due to com mon ion [ H + ] ef fect, S con cen tra tion de creases. Hence [ Cu + ][ S ] ex ceeds K of CuS but [ ++ ][ Zn S ] does not ex ceed K sp sp of ZnS. Hence from the mix ture CuS is pre cip i tated but not ZnS. 6

4 + + Sim i larly Al, Mn can both be pre cip i tated as their hy drox ides Al(OH), Mn(OH) by add ing NH OH. But K sp of Al(OH) is less than that of Mn(OH). So by add ing NH Cl to NH OH the con cen tra tion of OH in the so lu tion de creases due to + + com mon ion NH. Al is pre cip i tated but not Mn +. As men tioned ear lier OH ions are added in qual i ta tive anal y sis in the form of NH OH. So if NH Cl is added to NH OH so lu tion, due to the com mon ion NH +, the OH con cen tra tion de creases. Therefore Al(OH) is pre cip i tated but not Mn(OH).. Analysis and identification of anions The anions to be identified are : 1. carbonate ( CO ) 6. acetate ( CH COO ). sulphate ( SO ) 1 7. nitrate ( NO ). chloride ( Cl ) 8. borate ( BO ). bromide ( Br ) 9. phosphate ( ) 5. iodide ( I ) General Principles of Inorganic Qualitative Analysis PO These ions are classified into groups : 1. The first group ion reacts with dil HCl or dil HSO and liberate gas which can be identified. This gas liberated is CO and the ion is carbonate.. The second group ion react with conc. HSO alone or in presence of MnO, Cu turnings, or dichromate etc. and give characteristic gases. These can be identified by colour, smell and characteristic chemical tests. Chloride Cl (in presence of MnO ) Bromide Br (in presence of MnO ) Iodide I (in presence of MnO ) Nitrate NO (In presence of Cu turnings) CHCOOH CHCOOH vapours. Borate Ethylborate (in presence of CH5 OH). The third group ions do not react with dil. HCl or conc. HSO. Hence special tests are used. These are sulphate and phosphate. Based on the above facts, analysis of salts for anions is carried out by dry and wet tests. These are given in the next section.. Systematic analysis for anions The test are conducted in ways : (i) Heating of the sample of the salt mixture in a dry test tube. (ii) Action of dil HCl on the sample of the salt. (iii) Action of conc. HSO or conc. HSO + MnO, conc.hso + Cu turnings conc. HSO + CH5OH, on the mixture (iv) Some special tests.

5 Chemistry Prep a ra tion of orig i nal So lu tion : The salt solution prepared for identification of anions is known as original solution. It may be prepared by dissolving pinch of salt in water and filtered. (heated if necessary) Generally sodium carbonate extract is prepared for salts which are insoluble in water. 1 : ratio of sample salt and sodium carbonate is mixed and added with water. This solution is boiled and filtered. The supernatant solution (filtrate) is called sodium carbonate extract. In this process anion part is protected and cation part is replaced by sodium which allow the anions to dissolve in water. Example : Pb (NO ) water NaNo (s) + PbCo 1 : + Na Co filtered filterate (extract solution) Metal carbonate (Solid) The systematic procedure is presented below : Experiment Observation Inference 1. Heating of dry substance in a dry test tube Reddish brown vapours of NO. White sublimate. Action of dil HCl 1. Brisk effevescence with evaluation of colurless gas. The gas gives white precipitate on passing into solution of lime water.. Action of conc. H SO 1. Colurless gas gives dense white fumes with rod dipped in NH aq.. Action of conc. H SO + MnO. Colurless gas which does not give white fumes with NH aq. Smell of vinegar.. Colourless gas + reddish brown gas. Fumes in moist air.. Violet vapours, turns starch paper blue. Heavy metal nitrate salts NH salt gas is CO, may be carbonate gas is HCl may be chloride. gas is CH COOH, may be acetate. gas is ( HBr + Br ) may be bromide. gas is I, may be iodide. 1. Greenish yellow gas. gas is Cl, may be chloride. Reddish brown vapour gas is Br may be bromide.. Violet vapours gas is I may be iodide. 5. Action of conc. 1. Reddish brown gas gas is NO, may be nitrate. HSO Cu

6 General Principles of Inorganic Qualitative Analysis 5 6. Action of conc. H SO + FeSO 7. Action of conc. H SO + K Cr O 7 8. Action of conc. H SO + C H OH 5 Brown ring [ FeNO] ring may be nitrate. Reddish brown vapours gas burning with Chromyl chloride vapours may be chloride. gas is ethyl borate, may be borate..5 Chemical reactions underlying the tests Given in section 9. (1) Ac tion of dil HCl tests (a) Carbonate.5.1 CO gas is liberated when carbonates are treated with dilute acids. The liberated CO when passed into lime water ( Ca(OH) ) it will turns into milky white (CaCO ) + CO + H CO + H O + Ca + OH + CO CaCO white pre cip i tate + H O HCl + (b) Ba + CO BaCO BaCl + CO + H O Carbonates give barium carbonate ppt with Barium chloride solution which dissolves in acids. () Ac tion of Conc. HSO (a) Chlorides on treatment with conc. HSOgives colourless. Pungent gas (HCl Vapours). A glass rod dipped in NH solution when exposed to the above gas will give densed fumes of NH Cl. Cl + H SO HSO + HCl NH + HCl NHCl (dense white fumes) (b) When bromide reacts with conc. HSO reddish-brown vapous of bromine are evolved along with HBr. Br + H SO HSO + HBr HBr + HSO Br + SO + HO (Brown va pours) (c) Iodide salts on treatment with conc. H SO gives violet Vapours of iodine ( ) I + H SO HI + HSO HSO + HI I + SO + HO Vi o let Va pours (d) To the solid ni trite salt if conc. HSO is added, a red dish-brown va pours of NO are evolved. NO + H SO HNO + HSO HNO NO + O + HO Red dish Brown va pours I

7 6 Chemistry Ac tion of Conc. HSO + MnO (e) chloride salts when added with conc. HSO in presence of MnO, and heated a greenish colour chlorine gas is evolved. Cl + H SO HCl + HSO MnO + HCl MnCl + Cl + HO Green ish yel low gas (f) Mixture of bromide salt and MnO when reacts withe conc. H SO conditions gives reddish brown vapours of bromine. Br + H SO HSO + HBr in hot MnO + HBr MnBr + Br + HO Red dish brown va pours (g) Solid Iodide salt reacts with conc. HSO in presence of MnO to produce iodine (I ) + I + MnO + H SO I + Mn + H O + SO Ac tion of conc. HSO + KCrO7 (h) When conc. HSO is added to the mixture of solid chloride salt and solid potassium dichromate and gently heated, a reddish-brown vapours of chromyl chloride ( CrO Cl ) are formed Cr O Cl H SO CrO Cl HSO H O Reddish brown vapour Ac tion of conc. H SO + Cu turn ings (i) Solid nitrate salt when added with Cu turnings and heated, Reddish brown gas ( NO ) is evolved. + + NO + H + Cu Cu + NO + HO Brown vapours Ac tion of conc. H SO + FeSO (Brown ring test) (j) To the salt solution of nitrate when freshly prepared saturated ferrous sulphate solution is added and conc. HSO Solution is slowly added by the sides of the test tube a brown ring is [ Fe(NO) ] formed at junction of two layers of solution NO + Fe + H Fe + NO + H O + Fe + NO [FeNO] Brown ring Ac tion of conc. H SO + C H 5 OH (k) On warming Borate salt will react with con. HSO and Ethyl alcohol mixture to give Vapours which on burning gives green flame of Ethyl borate ( B(OC H 5 ) ). B O + H SO + 5H O H BO + SO 7 HBO + C H5OH B(OCH5 ) + HO green flame

8 .6 Test for PO and SO (a) PO With ammonium molyblate an yellow precipitate is formed. (b) SO General Principles of Inorganic Qualitative Analysis HPO + 1MoO + H + NH ( NH ) PO 1MoO + 1HO yellow precipitate With BaCl so lu tion, a white pre cip i tate in sol u ble in conc. HCl SO + BaCl BaSO + Cl White pre cip i tate Flame Test for Metal Ions The salt is mixed with conc. HCl. and the paste formed is in tro duced into Bun sen flame Brick red colour Ca + Green colour Ba + Lilac colour K + Golden yellow colour Na + Crimson red colour Sr +.7 Systematic analysis of meal ions The cat ions that are in cluded for test ing are lead, cop per iron alu mi num, zinc, man ga nese, cal cium, bar ium, stron tium, po tas sium and am mo nium. These ions are iden ti fied by car ry ing sys tem atic qual i ta tive anal y sis by pre cip i tat - ing these ions as chlo rides, sulphides, hy drox ides, car bon ates and phos phate. The ions on the ba sis of these pre cip i tates are di vided into 6 groups. For ev ery group a pre cip i - tat ing re agent is used I Group chlorides reagent : dil HCl II Group sulphides reagent : dil HCl + H S gas III Group hydroxides reagent : NHCl aq. + NHOH solid IV Group sulphides reagent : NHCl solid + NHOH aq. + HS gas V Group carbonates reagent : NHCl solid + NHOHaq. + ( NH ) CO VI Group phosphomolybdate aq. Ammonium molybdate solution First of all the groups in which the cations in the salt mixture analysed belong are identified. For this the group reagent, the cations in the group and the precipitates formed are given in the table.. Then systematic analysis group table is used. This is shown below : The cations in the syllabus only are considered.

9 8 Chemistry Group number Table. Group reagent, metal ions and metal precipitates Group reagent Cations in the group Nature of precipitate I dil HCl Pb + PbCl white II III + + dil HCl + H Sgas Cd, Cu CuS, black Cds yellow NH Cl solid + NH OH soluiton IV NH Cl solid + NH OH solution + H S(gas) V NH Cl solid + ( OH) solution + ( NH) CO saturated solution VI No specific reagent Nessler s reagent.8 Cation Analysis + + Al, Fe Al(OH) white Fe(OH) yellow Mn +, Zn + MnS Pink coloured ZnS white Ca +, Ba +, Sr + CaCO white BaCO white SrCO white NH +, K +, Mg + Reactions of Ammonium ion ( NH + ) : (a) Reaction with NaOH solution : To the salt solution NaOH solution is added and warned. A gas is liberated. The gas turns red litmus to blue. NH + OH NH + H O Confirmation Test for NH + ion : To the salt solution add 1 or drops of NaOH and add Nessler s Reagent ( KHgI ) is added. Reddish brown ppt. confirms the presence of NH Group Separation Table Test solution + dil HCl Precipitate Ist Filtrate : H S gas to passed. Solution is diluted and H S is passed. group ions Precipitate II Filtrate : H present. Ions S is boiled off then boiled with conc. group present : white HNO Cus black Precipitate NH Cl solid and NH OH solution are added. CdS yellow PbCl Filtrate : H S is passed. Precipitate III group ions present Fe(OH) brown Al (OH) white Precipitate IV group ions present ZnS white MnS Buff colour Filtrate : NH Cl solid + NH OH solution + ( NH) CO saturated solution V grapions present ion. Filtrate : No reagent CaCO white NH, K +, Mg + BaCO white SrCO white +

10 After establishing in which group a precipitate is obtained, the salt solution is taken and the individual metal ion is confirmed. Group I cations identification To salt solution dil HCl is added. If a white precipitate Pb + is present. Solution must be tested for this ion by using specific reagents Salt solution + KI solution Pb + yellow precipate Salt solution + CrO solution Pb + yellow precipate Group II cations identification The filtrate in I is boiled with conc. HSO and diluted. If a white precipitate is obtained Pb + is present. Filtrate may contain Cu +, NH OH solution is added and boiled. If blue solution is formed Cu + is present. Group III cations identification To the solution, NaOH, solution is added, yellow precipitate confirms Fe +. White precipitate confirms Al +. To the solution NH OH solution is added pale green precipitate confirms Fe +. White precipitate or solution confirms Al +. K Fe(CN) 6 is added to the solution blue solution confirms Fe, ( NH ) CNS solution-red precipitate confirms Fe +. General Principles of Inorganic Qualitative Analysis 9 Group IV cations identification Addition of NaOH solution in excess to the test solution white precipitate confirmsmn + white gelatinous precipitate confirmszn + Addition of NH OH solution to test solution. buff coloured precipitate confirmsmn +. Group V cations identification KCrO solution is added to test solution yellow precipitate soluble in CH COOH confirms Sr + ions no precipitate in dilute solution confirms Ca + ions yellow precipitate insoluble in CH COOH confirms Ba + ions Group VI cations identification Reactions of K + : To the test solution cobalti nitrate solution is added yellow precipitate confirms K +. Reactions of Mg + : (a) To the centrifugate add NH Cl, NH OH and sodium hydrogen phosphate (Na HPO ). and shake well. white ppt of Mg(NH )PO.6H O (magnesium Ammonium Phosphate) is formed. Confirmation Test : To the contrifugate add NH Cl, NH OH and 8 Hydroxy quinoline solution and keep the Test tube on water bath for - min. Formation of yellow ppt. confirms the presence of Mg +. Interfering anions Some anions interfere in the analysis of cations. Especially oxalate and phosphate ions. +

11 0 Chemistry.9 Interfering anions and some dry tests (a) Oxalate ion is eliminated before proceeding for the analysis of cation. The salt sample is taken in a china dish and heated strongly. Oxalate decomposes. The residue is dissolved in HCl and used in the analysis. (b) Phosphate is eliminated before proceeding to the III group. Phosphate was eliminated in earlier days by adding neutral FeCl solution to the filtrate in the II group of the main table. But presently this is eliminated by adding NH Cl solution + zirconyl chloride solution to the filtrate of II group. Phosphate is eliminated as zinconyl phosphate. (c) Charcoal test : This is conducted in the case of coloured salts. The salt is mixed with Na CO solid. The mixture is placed in the cavity of charcoal piece and heated. brown residue Ca + yellow residue Zn ++ (d) Borax Beed Test : Borax is taken on a platinum wire and heated to form a beed. This is then dipped in the salt and is again heated. The beed gets colour. From the colour, the metal ion is identified. Mn + Vi o let or red..10 Conclusions Thus salt mixture is analysed in a systematic way by the following types of tests. (1) Dry tests : (a) Action of heat NH +, heavy metal nitrates (b) Flame test Sr +, Ba +, Ca +, K + identified (c) Charcoal test Coloured salts, Mn + (d) Borax beed test Mn + () Wet Tests : with dil HCl Pb + with cone. H SO alone or in presence of MnO, K Cr O 7, Cu turnings, FeSO, C H 5 OH for Cl, Br, I, NO, borate respectively. () By systematic analysis of cations through using the group reagents as shown below I II group dil HCl group dil HCl + H S gas III group NH Cl solid + NH OH solution IV group NH C1 solid + NH OH solution + H S V group NH C1 solid + NH OH + (NH ) CO solution, sat. solution VI group NH + Nessler s reagent K HgI + NaOH solution K + Sodium cobalt nitrate solution

12 (S) : Sample analysis of ZnCo + KCl Experiment Observation Inference 1. physical appearance cryptalline + +. colour white No Cu, Mn. flame test lilac flame K borax beed test no colour Mn, Zn absent 5. charcoal test yellow when hot white when cold Zn + 6. action of heat yellow when hot white when cold Zn + 7. action of dil. HCl on solid effervascence colourless gas turns lime water milky 8. action of consc. H SO + MnO on solid 9. Action of conc. H SO + MnO on solid 10. Action of conc. H SO + K Cr O 7 solution + salt solution pungent colourless gas fumes with NH OH greenish yellow gas Brown red oily vapours 11. Salt + C H 5 OH (heated) vapours burning with green flame are absent CO Cl Cl Cl NO is absent 1. Salt + Cu turnings + conc. No brown vapours NO is absent H SO Hence, CO and Cl are present. To the sodium carbonate extract dil HNO and AgNO solution are added white precipitate solute in NH OH is obtained Cl is confirmed. Tests for cat ions General Principles of Inorganic Qualitative Analysis 1 1. To salt solution dil HCl is added. No precipitate Pb + is absent. To salt solution dil HCl + H S gas added No precipitate Cu + is absent. To salt solution NH Cl solid + NH OH. To salt solution NH Cl solid, white precipitate NH OH solution + H S gas are added. No precipitate Fe + Al + are absent Zn + is present To salt solution NH Cl solid + No precipitate Ca, Ba, Sr are absent NH OH solution + ( NH) CO solution are added 6. To salt solution + Nessler s reagent solution is added 7. To salt solution + solidum cobaltnitrite solution No precipitate Yellow precipitate formed NH + is absent K + is present

13 Chemistry Hence, Zn + and K + are present. The given salt mixture therefore contains CO, Cl Zn +, K + 1. What is common ion effect?. What is solubility product?. How are common ion effect and solubility product concepts used in qualitative analysis? Give examples.. How are CO, NO identified? 5. Given the tests for the identification of Cl, Br, I, NO. 6. How are borate and phosphate identified? 7. Name the colours obtained in the flame test for Ba ++, Ca +, Sr +, K What is charcoal test? Give examples. 9. What is Borax beed? How is it useful in the analysis. 10. What are elimination anions? How are they eliminated? 11. How are NH +, K + identified? 1. Give the main group table used in the identification of metal ions. 1. Write the test with all details and chemical reactions for Cl, BO and SO. 1. How are the following identified? Questions CO, Po. 15. Give the chemical reactions underlying in the identification of NO and I by action of conc. H SO on the salts containing these ions. 16. What are the group reagents for the identification of Al +, Sr Give the flame colours for the following metal ions Ba +, Sr +, K How is Na CO extract prepared and used for identification of SO ions. 19. What is the reason for the formation of a white sublimate when NH salt is heated. 0. What is brown ring test? +

Systematic Procedure for Inorganic Qualitative Analysis Systematic Analysis of Anion

Systematic Procedure for Inorganic Qualitative Analysis Systematic Analysis of Anion S.No Experiment Observation Inference 1 Preliminary Reactions Colourless Absence of Fe 2+, Fe 3+, Ni 2+, Co 2+. 2 Appearance