Analysis of group III

Transcription

1 1 Analysis of group III Radicals Fe 3+, Al 3+ and Cr 3+ Group Reagent NH4OH in presence of NH4Cl Principle: Hydroxides of Fe 3+, Al 3+ and Cr 3+ are insoluble in water. In presence of NH4Cl, NH4OH is feebly ionized and due to low solubility product, only Fe(OH)3, Al(OH)3 or Cr(OH)3 are precipitated. Their separation is based on following principle: (i) In excess of NaOH, only Al(OH)3 dissolves to form NaAlO2 while Fe(OH)3 and Cr(OH)3 remain insoluble (ii) Cr(OH)3 is soluble in bromine water/naoh and forms Na2CrO4 while Fe(OH)3 is insoluble. Procedure: Filtrate obtained after removal of sulphides of gp-ii is boiled with few drops of conc. HNO3 to remove H2S. If yellow or white ppt. is obtained, that is filtered and filtrate is used to analyse gp-iii. Before testing gp-iii radicals, interfering radicals (if present) are to be removed. If interfering radicals are removed then add small amount of solid NH4Cl and then NH4OH. The ppt. formed is used to analyse gp- III. The filtrate is used to analyse gp-iv radicals. The colour of ppt. also gives idea of cation. Red-brown ppt. may be of Fe(OH)3 or MnO2.xH2O. green of Cr(OH)3 and white of Al(OH)3. The precipitates are dissolves in excess of NaOH and are filtered.

2 2 Filtrate: It may contain sodium metaaluminate. On adding solid NH4Cl and then heating, a white gelatinous ppt, is obtained Al 3+ conform Residue: Divide the ppt, into three parts: Part I: Dissolve ppt. into dil. HCl divide it in two parts: (i) A dark blue colour obtained adding K4[Fe(CN)6]. the ppt. in ppt. is add water, then heat and conc. HNO3 is on (ii) Dark red colour Part II: Dissolve the Part III: NaOH/bromine dissolved in filter. Yellow filtrate and is indicates chromium. heated with (i) Acidify the filtrate PbO2. with acetic acid and Appearance add lead acetate. A of purple yellow ppt. is colour after obtained. sometime is (ii) Residue is fused observed. is with fusion mixture Mn 2+ obtained when and dissolve the confirm. KCNS is mixture in water. A added. yellow solution is Fe 3+ confirm obtained which gives yellow ppt. on adding CH3COOH/ (CH3COO)2Pb; Cr 3+ confirm.

3 3 NOTE: (i) Mn 2+ is palced in gp. IV but due to its partial precipitation in gp- III also, it may tested here also. (ii) Before testing gp-iii radicals, H2S must be removed otherwise gp-iv sulphides will be precipitated as medium turns alkaline. (iii) The amount of NH4Cl added must be proper and it should be added before adding NH4OH. If NH4Cl is added in very small quantities, then Mn 2+ will get precipitated as MnO2.xH2O in gp- III. If it is added after addition of NH4OH, common ion effect will not be there and hydroxides of higher group will get precipitated. (iv) Before addition of NH4OH, solution must be hot and heating must be done before addition of NH4OH. In cold the hydroxides precipitated may convert into colloid and so can t filtered. Also in cold Cr(OH)3 reacts with NH4OH to form chromamine complex (pink). Cr(OH)3 + 6NH4OH [Cr)NH3)6](OH)3 + 6H2O (v) NH4OH must not be in excess otherwise Al 3+ is not precipitated completely. (vi) To remove H2S small amount of HNO3 must be used otherwise Mn 2+ is oxidized to Mn 3+ and it may get precipitated as Mn(OH)3. Reaction and Explanations: On adding NH4OH is presence of NH4Cl, hydroxides of gp-iii cations are precipitated. FeCl3 + 3NH4OH Fe(OH)3 + 3NH4Cl

4 4 AlCl3 + 3NH4OH Al(OH)3 + 3NH4Cl CrCl3 + 3NH4OH Cr(OH)3 + 3NH4Cl In excess of NaOH, Al(OH)3dissolves while other hydroxides remain insoluble. (a) Tests for Al 3+ : In excess of NaOH Al(OH)3 + NaOH NaAlO2 +2H2O Sodium meta-aluminate (soluble) NaAlO2 + H2O + NH4Cl Al(OH)3 White gelatinous + NaCl + NH3 ppt. (b) Test for Fe 3+ : Red brown Fe(OH)3 dissolves in dil. HCl as Fe(OH)3 + 3HCl FeCl3 + 3H2O (i) Reaction with K4[Fe(CN)6]: 4FeCl3 + 3K4(CN)6] Fe4[Fe(CN)6]3 + 12KCl Ferric ferrocyanide (Dark blue) (ii) Reaction with KCNS: FeCl3 + KSCN Fe(SCN)Cl2 + KCl Ferrithiocyanate (Dark red colour) (c) Tests for Cr 3+ : (i) On boiling with NaOH/Br2, Cr(OH)3 gives sodium chromate which gives yellow ppt. of PbCrO4 with

5 5 (CH3COO)2Pb/CH3COOH. 2NaOH + Br2 NaOBr + NaBr + H2O NaOBr NaBr + [O] 2Cr(OH)3 + 4NaOH + 3[O] 2Na2CrO4 + 5H2O Yellow solution Na2CrO4 + (CH3COO)2Pb PbCrO4 + 2CH3COONa Yellow ppt. (ii) Fusion test: Cr(OH)3 on fusion with Na2CO3/KNO3 (fusion mixture) gives Na2CrO4 which gives yellow ppt. of PbCrO4 with (CH3COO)2Pb. 2Cr(OH)3 + 2Na2CO3 + 3KNO3 2Na2CrO4 + 3KNO2 + 3H2O + 2CO2 Yellow solution Na2CrO4 + (CH3COO)2Pb PbCrO4 + 2CH3COONa Lead chromate Yellow ppt. Interfering radicals and their removal: Some acid radicals interfere in the normal analysis of basic radicals after gp-ii. These radicals are known as interfering radicals e.g., F, BO 3 3, and 3 4 C O, PO etc. These radicals must be removed before analysis of III and higher groups. Reason for interference: Interfering radicals are anions of weak acids. When original solution is prepared, in presence of HCl, these anion are converted into their corresponding acids

6 6 and remain unionized. But when medium turns alkaline after gp-ii they ionize and from insoluble slats with cations of IV, V and VI groups. So their removal is necessary before proceeding to gp-iii. (i) Removal of C O 2,: Filtrate of gp-ii is concentrated by boiling 2 4 and then is heated with conc. HNO3 to almost dryness. The process if repeated 3-4 times. Solid thus obtained is diluted with hot conc. HCl and is filtered if necessary. Na2C2O4 + 2HNO3 2NaNO3 + H2C2O4 2HNO3 H2O + 2NO2 + [O] H2C2O4 + [O] H2O + 2CO2 NOTE: After removal of oxalate. Sn must also be tested because in presence of oxalate. Sn forms soluble complex diaquatetraoxalato stannate in [Sn(C2O4)4.(H2O)2] 2-. (ii) Removal of borate BO3 3 and fluoride (F- ): Filtrate of gp-ii is concentrated by heating and is heated with conc. HCl to almost dryness. Residue obtained is diluted with dil. HCl and is filtered. Filtrate is used for analysis of gp-iii. Ca3(BO3)2 + 6HCl 3CaCl2 + 2H3BO3 CaF2 + 2HCl CaCl2 + 2HF If gp-i cations are absent, then removal of F -, C O 2, can be made directly from mixture. 2 4 and 3 3 BO

7 7 (iii) Removal of phosphate: (a) Boil and filtrate of gp-ii with conc. HNO3 to remove H2S and by adding K4[Fe(CN)6], Fe 3+ is tested. (b) Now NH4OH is added dropwise, till solution is slightly turbid. NH4OH should not be added in excess otherwise phosphates of III, IV, V and VI gp. will be precipitated. (c) Now add apprx. 1g CH3COONa or CH3coonh4 followed by addition of CH3COOH. If any ppt. is formed at this stage. Filter and discard it. (d) Now FeCl3 solution is added till colour of solution turns redbrown. The solution thus obtained is boiled for 5 minutes and filter in hot. The ppt. may be phosphate or basic acetate. These ppt. are used to analyse gp-iii and filtrate for gp-iv.