Substances, colourless removable

CH2CI2. A colourless liquid with a chloroform-like odour b.p. 4I°C. Prepared by heating chloroform with zinc, alcohol and hydrochloric acid manufactured by the direct chlorination of methane. Decomposed by water at 200°C to give methanoic and hydrochloric acids. Largely used as a solvent for polar and non-polar substances, particularly for paint removal (30%), dissolving cellulose acetate and degreasing (10%). It is more stable than carbon tetrachloride or chloroform especially towards moisture or alkali. It is somewhat toxic. U.S. production 1981 280000 tonnes. 

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. 

Schmidlin s experiment here described shows very clearly the equilibrium between hexaphenylethane and triphenylmethyl. The disappearance of the colour on shaking the substance with air indicates that the yellow radicle, present in equilibrium, is removed as (colourless) peroxide. The re-establishment of the equilibrium by renewed dissociation of (colourless) hexaphenylethane proceeds so slowly that the formation of the yellow radicle in the decolorised solution can be observed without difficulty. 

As is well known, animal charcoal possesses the property of being able to remove the colour from certain solutions for this reason it is frequently employed in the laboratory to free a colourless substance from coloured impurities. If it is to be used to remove the colour of a solid substance, the latter is first dissolved in a suitable solvent, then boiled with the animal charcoal and filtered. Before treating a hot solution, it is allowed to cool somewhat, since when animal charcoal comes in contact with liquids heated nearly to the boiling-point a violent ebullition is frequently caused, and an overflowing of the liquid may easily take place. When a. solvent not miscible with water is used, the ani-... 

Thin-layer chromatography (TLC) is a variation of column chromatography. A strip of glass is coated on one side with a thin layer of adsorbent (e.g. alumina). The substance to be tested is spotted near one end of the plate and the plate is put into a developing Jar with a small amount of the solvent (to a level below the spot) and a cap put on the jar (Fig. 19.20). The solvent rises up the plate, separating the sample into its components. At the end of the experiment, a spot for each component is seen on the plate. When the solvent is near the top of the plate, the position the solvent has reached is marked with apencil and the plate removed and dried. If the components are colourless, a number of techniques can be used to show them up ... 

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