$6* PRACTICAL ORGANIC AND BIO-CHEMISTRY 



division and on simple drying by exposure the surface forms an impenetrable 

 skin, preventing evaporation of water. Mincing, chopping, grinding with sand 

 and other processes are used to prepare them for drying. 



It is essential that drying be carried out rapidly to prevent metabolic 

 changes going on during the process of drying. 



(a) Exposure to Air or Indifferent Gas. 



The finely ground material is spread over a large surface on glass plates, 

 or if vegetable on a metal mesh. It can be left exposed to air at the 

 ordinary temperature or in a room or box at 40 or higher, or better by 

 blowing a current of clean air, heated to 40, over the surface. To prevent 

 oxidation of the constituents during drying, the material is placed in a vacuum 

 desiccator or suitable chamber through which a current of carbon dioxide or 

 nitrogen can be blown instead of air. 



After exposure to air or indifferent gas, more water can generally be 

 removed by placing the material in a vacuum desiccator over dehydrating 

 agents. 



(b) Treatment with Alcohol, or Acetone. 



Since alcohol and acetone are miscible with water in all proportions, they 

 not only help to remove water, but also they disturb the conditions in the 

 tissue, which are necessary for metabolic changes, by precipitating proteins, 

 etc., and by disturbing the peculiar solubility of fatty and protein material. 

 The material is mixed with about an equal bulk of solvent or sufficient to 

 cover it completely and the mass is allowed to stand for 12-24 hours. The 

 material may be put into boiling solvent and heated for about an hour. 



The solvent is filteied or strained off and the tissue treated again with 

 the same solvent, or with ether, etc. Substances are soluble in the mixture of 

 alcohol or acetone and water separated from the tissue and this extract 

 requires examination. 



(c) Admixture with Neutral Dehydrating Agent. 



Animal tissues after grinding are sometimes mixed with anhydrous 

 sodium sulphate, or calcium sulphate, by grinding together equal parts in a 

 mortar. The mixture sets after some time to a cake which can be finely 

 ground and extracted with ether, acetone, etc. By boiling with alcohol 

 proteins are coagulated and the other constituents may be extracted with 

 water. 



ISOLATION OF SOLIDS FROM SOLUTION. 



The extracts obtained above, either directly or after concentration 

 by evaporation, evaporation in vacuo, distillation, etc. (if acid or alkaline 

 after neutralisation), or solutions of solid compounds whether natural 

 or obtained by chemical reactions between compounds, may be treated 

 in several ways in order to separate the solid compound. 



A. Precipitation. 



(i) By adding another solvent. 



(ii) By acidifying. Aqueous or alkaline aqueous extracts may 

 contain acids. On acidifying with mineral acid, the acid or acids, if 

 they are insoluble or soluble with difficulty in water, will be pre- 

 cipitated. 



