The Choice of Solvent

THE CHOICE OF SOLVENT

The ideal solvent is cheap, nontoxic, and noninflammable. It is highly selective, dissolving only the wanted constituents of the solid. It should have a low vis-cosity, allowing easy movement through a bed of solids, and, if the resulting solution is to be concentrated by evaporation, a high vapor pressure. These factors greatly limit the number of solvents of commercial value. Water and alcohol, and mixtures of the two are widely used. Both, however, are non-selective leaching varying proportions of gums, mucilages, and other unwanted components. Most of the tinctures and liquid extracts used in pharmacy are simple, impure extracts made with water or mixtures of water and alcohol. Acidified or alkaline mixtures of water and alcohol are used to extract insulin from minced pancreas. A more selective extraction is given by petroleum sol-vents and benzene and related solvents. In the preparation of many pure alkaloids, the powdered material is moistened with an alkaline solution, packed into a bed, and leached with petroleum. Subsequent purification by fractional crystallization is facilitated by the absence of gums. Acetone and chlorinated hydrocarbons also find applications in leaching. In some cases, specific prop-erties of the wanted constituents may suggest a particular solvent. Eugenol, for example, can be readily extracted from cloves with a solution of potassium hydroxide.