Coloured impurities

Decolorisation by Animal Charcoal. It sometimes hap pens (particularly with aromatic and heterocyclic compounds) that a crude product may contain a coloured impurity, which on recrystallisation dissolves in the boiling solvent, but is then partly occluded by crystals as they form and grow in the cooling solution. Sometimes a very tenacious occlusion may thus occur, and repeated and very wasteful recrystallisation may be necessary to eliminate the impurity. Moreover, the amount of the impurity present may be so small that the melting-point and analytical values of the compound are not sensibly affected, yet the appearance of the sample is ruined. Such impurities can usually be readily removed by boiling the substance in solution with a small quantity of finely powdered animal charcoal for a short time, and then filtering the solution while hot. The animal charcoal adsorbs the coloured impurity, and the filtrate is usually almost free from extraneous colour and deposits therefore pure crystals. This decolorisation by animal charcoal occurs most readily in aqueous solution, but can be performed in almost any organic solvent. Care should be taken not to use an excessive quantity... 

The crude product of an organic reaction may contain a coloured impurity. Upon recrystallisation, this impurity dissolves in the boiling solvent and is partly adsorbed by the crystals as they separate upon... 

Tlie zinc reduces the coloured impurities in the aniline and also helps to prevent oxidation of the amine during the reaction. 

The common impurities found in amines are nitro compounds (if prepared by reduction), the corresponding halides (if prepared from them) and the corresponding carbamate salts. Amines are dissolved in aqueous acid, the pH of the solution being at least three units below the pKg value of the base to ensure almost complete formation of the cation. They are extracted with diethyl ether to remove neutral impurities and to decompose the carbamate salts. The solution is then made strongly alkaline and the amines that separate are extracted into a suitable solvent (ether or toluene) or steam distilled. The latter process removes coloured impurities. Note that chloroform cannot be used as a solvent for primary amines because, in the presence of alkali, poisonous carbylamines (isocyanides) are formed. However, chloroform is a useful solvent for the extraction of heterocyclic bases. In this case it has the added advantage that while the extract is being freed from the chloroform most of the moisture is removed with the solvent. 

Chlorotriethylamine hydrochloride [869-24-9] M 172.1, m 208-210 , pK , -8.6 (free base). Crystd from absolute MeOH (to remove highly coloured impurities). 

Hydroxypyridine (2-pyridone) [142-08-5] M 95.1, m 105-107 , b 181-185 /24mm, e293nm 5900 (H2O) pK l.25, pK 11.99. Distd under vacuum to remove coloured impurity, then crystd from... 

This step is almost always performed to get rid of the coloured impurities in the fermentation broth. The method is based on the fact that amino adds (esperially the non-aromatic amino acids) do not adsorb onto activated charcoal. Although the treatment is very effective, some of the amino acid is lost during this step. 

Care is necessary in the preparation, because if coloured impurities are present, the compound may polymerise violently and very exothermically at ambient temperature. 

In order to remove coloured impurities, which often adhere tenaciously to a colourless substance, the hot saturated solution is boiled for a short time with a few knife-points of animal charcoal or specially prepared wood charcoal. Since the air which escapes from the charcoal causes copious frothing the adsorbent must be added carefully and with shaking. On account of their colloidal nature the coloured impurities are most easily adsorbed from aqueous solutions. 

Froth flotation has proven to be an efficient method of removing titaniferous impurities (mainly iron-rich anatase) from kaolin clays. Fatty acid reagent, primarily tall oil, is used extensively in the reverse flotation of these impurities. This flotation collector typically requires divalent cations (usually Ca +) to activate the coloured impurities and enhance collector adsorption. This is not very selective since the tall oil can also absorb on the kaolinite particles. Alkyl hydroxamate collectors are relatively new in the kaolin industry but provide significant advantages. Hydroxamates do not require activators, substantially increase the removal of colored impurities and are very selective. 

Ammonium d-a-bromocamphor-n-sulphonate [14575-84-9] M 328.2, m 284-285°(dec), [oc]q +84.8 (c 4, H2O). Passage of a hot aqueous soln through an alumina column removed water-soluble coloured impurities which remained on the column when the ammonium salt was eluted with hot water. The salt was crystd from water and dried over CaCl2 [Craddock and Jones JACS 84 1098 1962]. 

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