Analysis of group IV Radicals

Transcription

1 1 Analys of group IV Radicals Radicals : Zn 2+, Ni 2+, Co 2+ and Mn 2+ Group reagent : H2S in presence of NH4OH. Precipitate : Sulphides of Zn 2+, Ni 2+, Co 2+ or Mn 2+ Principle: In alkaline medium, H2S highly ionized and so concentration of S 2- high. When H2S passed, sulphides of Zn 2+, Ni 2+, Co 2+ and Mn 2+ are precipitated. They may be separated as follows: (i) ZnS and MnS are soluble in dil. HCl while NiS and CoS are not. (ii) NiS and CoS are soluble in aqua-regia and form chlorides. (iii) With cyanide, Co form stable complex while Ni (CN)4 converted into Ni(OH)2 when treated with Nr2/NaOH. Filtrate of gp-iii + NH 4 OH + H 2 S (ZnS, MnS, CoS & NiS dil. HCl/warm residue (NiS or CoS) aqua-regia soluble NiCl 2 adn CoCl 2 and test for Ni 2+ adn CO 2+ Filtrate (ZnCl 2, MnCl 2 ) pass H 2 S NaOH Residue (MnO 2.H 2 O) and add filter Filtrate (Na 2 ZnO 2 ) Procedure: Filtrate of gp-iii concentrated and NH4OH added to make it ammonical. If ppt. are formed on passing H2S, gp-iv present. Filter the ppt. and use to analyse cations of gp-iv. Filtrate used to analyse fifty group. Treat the ppt. obtained with dil. HCl and filter the solution.

2 2 Residue: Black ppt. may be of CoS or NiS. Dsolve them in aqua-regia and boil till brown vapour evolve. Now small amount of dtilled water. The diluted solution divided into several parts. Filtrate: It may contains ZnCl2 and MnCl2. Filtrate heated to boil off H2S and Br2/NaOH added. The ppt. obtained filtered. Part I: In th Part IV: Residue: It may Filtrate: It may part, solid Solution be of MnO2. contain sodium NH4CNS treated with H2O. The ppt. zincate. Divide added, followed NH4OH and washed with hot the solution by addition of dimethyl water and into two parts: amyl alcohol or glyoxime divided into tow Part I: White acetone. Organic rosy-red ppt. parts: fleshly ppt. layer turns blue obtained. Part I: In th obtained if H2S or green. Part V: On part conc. HNO3 passed Part II: Th adding KCN added through part acidified solution. A followed by solution. with CH3COOH ppt. addition of 1 g Part II: It and yellow ppt. obtained solid PbO2. acidified with obtained if which turns solution acetic acid and acidified solution black on heated and kept blue-white ppt. shaken with heating with for same time obtained on crystal of KNO2. NsaOH/Br2 violet-pink addition of Part III: Pelit s water. In colour appears. K4[Fe(CN)6]. test: IN the Pelit s test Zn 2+ confirm.

3 3 solution solid NaHCO3 added and then addition of Br2/H2O gives green colour which remains unchanged on boiling. Co 2+ confirm. black colour obtained. Ni 2+ confirm. Part II: ppt. fused with fusion mixture (Na2CO3 + KNO3). Fused green mass cooled and diluted with dil. H2SO4 pink solution obtained. Mn 2+ confirm NOTE: 1. Precipitation must be done in hot solution otherwe ZnS or MnS may enter in colloidal state and it will be difficult to filter them properly. 2. H2S should not be passed continuously, otherwe NiS may enter in colloidal state. 3. If mixture/o.s. white and black or yellow ppt. obtained, it may be of PbS or CdS respectively. 4. before passing H2S in solution. NH4Cl/NH4OH must be present, because gp-iv sulphides are completely precipitated in alkaline medium.

4 4 5. NH4Cl prevents the dsociation of NH4OH and so hydroxides of IV, V or gp-vi are not precipitated here, 6. in the analys of Co 2+, NH4SCN must be added. KSCN can t be used in place of NH4SCN. If red colour obtained on addition of NH4SCN. Fe 3+ present. In such case NaF added before NH4SCN. 7. H2S should be removed before addition of NaOH in the test of Mn 2+ and Zn 2+ chlorides. Reaction and Explanations: On passing H2S, insoluble sulphides of Zn 2+, Mn 2+, Co 2+ and Ni 2+ are obtained. MCl2 + H2S MS + 2HCl (M = Zn, Mn, Co or Ni) MnS and ZnS are soluble in dil. HCl but Ni 2+ and Co 2+ sulphides are insoluble. MS + 2HCl MCl2 + H2S (M = Zn or Mn) CoS and NiS are insoluble in HCl but soluble in aqua-regia, 3MS + 6HCl + 2HNO3 3MCl2 + 2NO + 3S + 4H2O (a) Test for Co 2+ : (i) Reaction with NH4SCN: CoCl2 + 4NH4SCN (NH4)2[Co(SCN)4] + 2NH4Cl Ammonium tetrathiocyanato Cobaltate (II) (Blue complex)

5 5 (ii) Reaction with CH3COOH/KNO2: CoCl2+7KNO2+ 2CH3COOH K3[Co(NO2)6]+2KCl+ 2CH3COOK + NO + H2O Potassium cobalt nitrite (yellow) (iii) Pelit s test: CoCl2 + 2NaHCO3 CoCO3 + H2O + CO2+ 2NaCl CoCO3 + 4NaHCO3 Na4 [Co(CO3)3] + 2CO2 + 2H2O Sodium cobaltocarbonate Br2 + 2NaHCO3 NaOBr + NaBr + H2O + 2CO2 NaOBr NaBr + [O] 2Na4 [Co(CO3)3] + 2NaHCO3 + [O] 2Na3[Co(CO3)3] + 2Na2CO3 + H2O Sodium cobalticarbonate (Green colour) (b) Test for Ni 2+ : (i) Reaction with Dimethyl Glyoxime (DMG): CH 3 C N OH 2 + NiCl 2 + 2NH 4 OH CH 3 C N OH CH 3 C CH 3 C O H O N N C CH 3 N + 2NH 4 Cl + 2H 2 O N N C CH 3 O O H DMG-Ni Complex (Red)

6 6 (ii) Reaction with KCN/NaOH/bromine water: NiCl2 + 4KCN K2[Ni(CN)4] + 2KCl Potassium nickel cyanide (soluble) 2NaOH + Br2 NaOBr + NaBr + H2O 2K2[Ni(CN)4] + NaOBr + 4NaOH Ni2O3 + 4KCN + 4NaCN + NaBr + 2H2O Black ppt. (iii) Pelit s test: NiCl2 + 2NaHCO3 NiCO3 + 2NaCl + H2O + CO2 2NiCO3 + 6NaOH + Br2 Ni2O3 + 2Na2CO3 + 3H2O + 2NaBr Black ppt. (c) Test for Mn 2+ : MnS + 2HCl MnCl2 + H2S MnCl2. On heating with NaOH/bromine water gives MnO2.H2O MnCl2 + 2NaOH Mn(OH)2 + 2NaCl 2NaOH + Br2 NaOBr + NaBr + H2O NaOBr NaBr + [O] Mn(OH)2 + [O] MnO2.H2O Hydrated manganese dioxide (i) Fusion test: MnO2 + KNO3 + Na2CO3 Fusion KNO2 + CO2 + Na2MnO4 Green

7 7 3Na2MnO4 + 3H2SO4 2HMnO4 + MnO2 + 3Na2SO4 + 2H2O Permangnic acid (pink) (ii) Reaction with HNO3/PbO2: 2MnO2 + 4HNO3 2Mn(NO3)2 + 2H2O + O2 2Mn(NO3)2 + 5PbO2 + 6HNO3 2HMnO4 + 5Pb(NO3)2 + 2H2O Permangnic acid (violet pink) (D) Tests for Zn 2+ : ZnCl2 + 2NaOH Zn(OH)2 + 2NaCl Zn(OH)2 + 2NaOH Na2ZnO2 + 2H2O Sodium zincate (i) Reaction with H2S: Na2[ZnO2] + H2S ZnS + 2NaOH White ppt. (ii) Reaction with CH3COOH/K4[Fe(CN)6]: Na2ZnO2 + 4CH3COOH (CH3COO)2Zn + 2CH3COONa + 2H2O 2(CH3COO)2Zn + K4[Fe(CN)6] Zn2[Fe(CN)6] + 4CH3COOK 3Zn2[Fe(CN)6] + K4[Fe(CN)6] 2Zn3K2[Fe(CN)6] Zinc potassium hexacyanoferrate (II) (white ppt.)