This section is from the book "A Text-Book Of Materia Medica And Pharmacy For Medical Students", by Velyien Ewart Henderson. Also available from Amazon: A Text-Book of Materia Medica and Pharmacy for Medical Students.
Drugs may be broadly classified as of (1) inorganic origin, (2) organic origin. They may also be divided into two classes, (1) pure chemicals, (2) galenicals. The pure chemicals are now prepared by neither pharmacist nor physician and in consequence the latter ordinarily needs to know nothing more about the methods of their preparation than what he has acquired as a student of chemistry. In regard to the methods of galenical pharmacy he must be better informed, as a knowledge of some of the methods and terms are essential in order that he may write prescriptions intelligently. In consequence some of the terms and methods are defined below.
(e.g. Castor Oil, *01ive Oil), - fluid esters of the higher fatty acids with glycerol (glycerin C3H5(OH)3) obtained by expression from fruits, seeds, etc. They cannot be distilled without decomposition. They are freely soluble in ether, chloroform, carbon bi-sulphide, and benzene, slightly soluble in alcohol, but insoluble in water.
Fats, (e.g. Lard) - are solid esters of higher fatty acids and glycerol and are soluble in the same reagents as the oils. They are usually mixtures.
Waxes, (e.g. cera flava) - are usually mixtures of higher fatty acids and glycerol and higher alcohols.
Volatile or Essential Oils, (e.g. Oil of Cloves, Turpentine) - are usually mixtures of hydro-carbons chiefly fluids terpenes associated with more highly oxidized members stearoptenes which may be obtained in a solid state, (e.g. Camphor). They are usually isolated from plants by distillation. They are all soluble in ether, chloroform, carbon bi-sulphide, and benzene, fairly soluble in alcohol, slightly in water.
Resins, (e.g. Scammony Resin) - solid preparations obtained from oils by oxidation. The pharmacopoeial resins are usually mixtures of resins as defined above and other bodies many of which are weakly acid. They are insoluble in water but soluble in alkaline solutions, alcohol, and ether.
Oleo-resins, (e.g. Copaiba) natural mixtures of volatile oils and resins semi-liquid in consistency.
Balsams, (e.g. Benzoin, Balsam of Tolu) resins or oleo-resins either liquid or solid which contain benzoic or cinnamic acids or both.
Gums, (e.g. Acacia and Tragacanth) solid or semi-solid exudations of plants which dissolve either partially or completely in water, forming a mucilage or an adhesive jelly, and are precipitated by alcohol. They are complex hydro-carbons yielding pentoses on hydrolysis.
Gum-resins, (e.g. Myrrh) mixtures of gums and resins.
*N.B. Students are strongly advised, when reading over this chapter, to look up in Chap. V the examples cited.
Glucosides, (the important pharmacopoeial examples are, - Digitalin, Salicin, Santonin) active principles which may be readily broken up by acids or alkalies in the presence of water setting free glucose.
Alkaloids, (e.g. Morphine and Strychnine) nitrogenous organic bases usually pyridine derivatives which are generally crystalline though some are liquid. They are usually sparingly soluble in water, but readily in alcohol, chloroform, benzene, and ether. Like alkalies they form salts with acids. Those with inorganic acids are usually soluble in water, those with organic acids much less so.
Tannins or Tannic Acids, - These are weak acids containing a benzene ring, astringent in taste, freely soluble in alcohol and water. They occur very commonly in barks and roots and hence in pharmaceutical preparations of these they give precipitates with iron salts and some alkaloids. Their presence must be remembered when such preparations are prescribed.
In the following paragraphs the methods of preparation employed in galenical pharmacy are defined.
Solution, - the physico-chemical process by which a solid or fluid (the solute) disappears in a liquid (the solvent). The solute can usually be re-obtained chemically unchanged by any process which will remove the solvent.
There are a few solutions in the pharmacopoeia in which a solid undergoes a chemical change by the action of the solvent during the process of solution, e.g. iron wire is dissolved in dilute acid to produce the Solution of the Perchloride of Iron.
Extraction, - the process by which a solvent (or menstruum) removes from a drug one or more of its soluble constituents. Four types of extraction are made use of by the pharmacist.
(i) Infusion - In this process a suitably finally divided drug is treated with either hot or cold water for a certain length of time, after which the fluid portion is strained off and retained and the solid portion rejected.
In this process the active principle is extracted by boiling in water.
In this process the drug is placed in a vessel, the solvent poured upon it, and left to stand for a suitable length of time with occasional agitation. The fluid is then filtered off; the marc or solid portion pressed out, the fluid thus obtained being added to the filtrate and the marc rejected.
In this process the drug is packed in a conical vessel (a percolator) with a small outlet at its lower end and moistened with the solvent which is added from time to time, and allowed to run off slowly from the lower outlet until a certain quantity of solvent has passed through. The marc is usually pressed out and the fluid obtained added to the percolate.
In this process the drug is subjected to pressure and thus its juices are obtained.
In this process solids are separated from fluids by allowing the latter to pass out through a porous diaphragm.
In this process the watery constituents of drugs are got rid of by the aid of currents of either hot or cold air.
In this process volatile substances are separated from non-volatile or less volatile by the aid of heat. The volatile substances are passed over a cooled surface on which they condense and are collected.
Pulverization. By this process the drug is reduced to a very finely divided condition (or powder). The degree of fineness is determined by the number of meshes to the linear inch of the finest sieve through which the powder can pass. The sieves used contain 20, 40, 60, 80, 100 meshes to an inch. The simplest method of pulverizing the drug is by means of a mortar and pestle but in large pharmaceutical houses this end is usually obtained by means of a mill.
This term may be used as synonymous with pulverization, but more commonly refers to an intimate mixing and powdering of two drugs by means of a mortar and pestle or of a spatula.