Analysis of group III
Radicals – Fe3+, Al3+ and Cr3+
Group Reagent – NH4OH in presence of NH4Cl
Principle: Hydroxides of Fe3+, Al3+ and Cr3+ 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.
| Filtrate: It may contain sodium meta-aluminate. On adding solid NH4Cl and then heating, a white gelatinous ppt, is obtained Al3+ conform | Residue: Divide the ppt, into three parts: | ||
| Part – I: Dissolve the ppt. into dil. HCl add divide it in two parts:
(i) A dark blue colour is obtained on adding K4[Fe(CN)6]. (ii) Dark red colour is obtained when KCNS is added. Fe3+ confirm |
Part – II: Dissolve the ppt. in NaOH/bromine water, then heat and filter. Yellow filtrate indicates chromium.
(i) Acidify the filtrate with acetic acid and add lead acetate. A yellow ppt. is obtained. (ii) Residue is fused with fusion mixture and dissolve the mixture in water. A yellow solution is obtained which gives yellow ppt. on adding CH3COOH/ (CH3COO)2Pb; Cr3+ confirm. |
Part – III: ppt. is dissolved in conc. HNO3 and is heated with PbO2. Appearance of purple colour after sometime is observed.
Mn2+ confirm. |
|
NOTE:
(i) Mn2+ 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 Mn2+ 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 Al3+ is not precipitated completely.
(vi) To remove H2S small amount of HNO3 must be used otherwise Mn2+ is oxidized to Mn3+ 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
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 Al3+: In excess of NaOH
Al(OH)3 + NaOH → NaAlO2 + 2H2O
Sodium meta-aluminate (soluble)
NaAlO2 + H2O + NH4Cl → Al(OH)3 + NaCl + NH3
White gelatinous ppt.
(b) Test for Fe3+: 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 Cr3+:
(i) On boiling with NaOH/Br2, Cr(OH)3 gives sodium chromate which gives yellow ppt. of PbCrO4 with
(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–, BO33–, C2O42+ and PO43– 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 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 C2O42– : Filtrate of gp-II is concentrated by boiling 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 (BO33–) 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–, C2O42– and (BO33–) can be made directly from mixture.
(iii) Removal of phosphate: (a) Boil and filtrate of gp-II with conc. HNO3 to remove H2S and by adding K4[Fe(CN)6], Fe3+ 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 approx. 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 red-brown. 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 analysis gp-III and filtrate for gp-IV.
