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Animal Charcoal

Animal charcoal is produced from bones, horn, blood, etc. Bone-black, or bone-charcoal, is obtained by the destructive distillation of bones in iron retorts, the distillate constituting bone- oil or Dippel's oil. Bone-black contains about 80 per cent, of calcium phosphate, a little calcium carbonate, some combined nitrogen, and only about 10 per cent, of carbon. The following is an analysis of a good sample of dried bone-black:

Ca and Mg phosphates, CaF2, etc80.21
Alkali salts0.35

The carbon is diffused in a very finely divided state throughout the mineral skeleton, and consequently presents a very large surface which possesses adsorptive properties.

Blood-charcoal is made by evaporating blood with potassium carbonate, igniting, washing the charred residue with water and hydrochloric acid, and again igniting. The potassium carbonate furnishes a basis on which the carbonaceous matter is deposited and rendered porous.

An artificial form of charcoal, which simulates animal charcoal in porosity and adsorptive power, may be obtained by mixing sugar with bone-ash and igniting the mixture.

The special characteristic of animal charcoal, on which its use depends, is its power to remove substances from solution. For example, it is well known that a brown solution of raw sugar is decolorised when boiled with animal charcoal. Other substances that are similarly removed from solution are indigo, litmus, iodine, the colouring-matter of red wine, the brown matter of peaty water or sewage, astringent principles, and certain basic salts, as well as fusel oil from alcohol. It is upon this property that the use of animal charcoal for filtering water depends, and also its employment in the laboratory for purifying organic compounds by removing the tarry matter which is frequently formed with them. Animal charcoal is also employed technically for purifying paraffin and glycerine, and its dust for making ivory-black and blacking. When the power of the charcoal is exhausted it may berenewed by ignition or treatment with reagents.

All these phenomena are further examples of adsorption, which includes the fixation by solids of liquids or substances in solution, as well as of gases.

It is chiefly substances of large molecular weight, especially colloidal substances, which are adsorbed from solution by charcoal.

From a study of the decolorising action of various forms of charcoal Knecht supports the view that animal charcoal owes its decolorising action to the presence within it of organic compounds stable at red heat.

The theory of adsorption has been developed by Freundlich and Losev, whose conclusions, however, have been criticised by McBain. The process of adsorption of iodine by various forms of charcoal has been investigated by Davis, who has reached the following conclusions:

  1. Adsorption of iodine by charcoal consists of a surface condensation and a diffusion (solid solution) into the interior of the carbon.
  2. Surface condensation is rapid, and complete in some hours; diffusion continues for weeks or months.
  3. Surface condensation is independent of diffusion.
  4. Sugar and animal carbons possess, roughly, the same power of adsorption; coco-nut carbon is much less efficient, and its action is chiefly due to diffusion.
  5. The amount of adsorption is specific, and depends on the nature both of the solvent and of the adsorbing substance.
The adsorption of phenol, benzoic acid, and picric acid from alcoholic solutions by carbon has been studied by Gustafson, who finds that the adsorption of picric acid takes place in accordance with the theory of Freundlich.

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