CHAPTER XVII. DIALYSIS. DIALYSIS is the process of separating crystallizable substances from those which do not crystallize, by placing a mixture of their solutions upon a porous diaphragm which has its under surface in contact with water. This remarkable process was discovered by Graham in 1861. He termed the substances which had the power of passing through the septum or diaphragm crystalloids, because they always have the crystalline form, and those which remained upon the diaphragm colloids, from their resemblance to gelatin and because they never crystallize. He also pointed out the differences between the two classes. Crystalline bodies, or crystalloids, are diffusible, brittle, hard, with clearly-defined angles and flat faces: their solutions are free from viscosity, and their reactions energetic and quickly effected. (Example, sugar.) Colloidal bodies, on the other hand, are not diffusible they are tough, with more or less softness of texture. The mathematically-arranged angles and faces of crystals are replaced in colloids by rounded, irregular outlines, the fracture often being curved or conchoidal. Water of crystallization is replaced by water of gelatination, and the solution of colloids is, when concentrated, viscous or gummy, and their reactions are sluggish or inert. (Examples, gum, glue, starch, dextrin, gelatin, albumen, extractive matter.) A thin layer of gelatin interposed between two liquids offers no obstacle to the passage of the crystalloids from one to the other, while it completely prevents the passage of the colloids: this property belongs not only to gelatin, but to other substances having a similar molecular constitution, as bladder, parchment, etc., of which the most convenient is the texture known as parchment-paper, prepared by immersing unsized paper in a cold mixture of two measures of sulphuric acid and one of water, and subsequently washing it thoroughly to free it from acid. Upon the principles stated above Prof. Graham contrived a very simple apparatus which he called the dialyzer (see Fig. 299). It consists of two parts, one a circular glass recipient (b), about a foot in diameter and six inches deep, the other (a) a similar circular vessel, from six to ten inches in diameter and about two inches deep, the circumference of which consists of a band of gutta-percha and the bottom of a circular piece of parchment-paper, the edges of which are brought over the lower rim of the gutta-percha band nearly to the top, and fastened outside of it by a string or by a narrow hoop of gutta-percha. The first part, or circular basin, is to receive distilled water, and should contain from five to ten times the quantity of the liquid that may be introduced into the smaller vessel. The latter is to float upon the surface of the water in the former, and is to receive the liquid to be submitted to dialysis, which should not be more than half an inch deep on the paper bottom. It is important that the parchment-paper employed should have no rent or aperture, and should be brought well up and secured on the outside of the gutta-percha to prevent the liquid from passing between them. If any liquid containing a mixture of colloid and crystalloid matter be placed in the floating vessel, after some hours it will be found that a portion of the latter has passed through the parchment-paper, and is held in solution FIG. 301. by the distilled water of the larger vessel, while the colloid matter remains. The distilled water thus impregnated is called the diffusate. The parchment-paper, or any similar material used as the septum, is applicable to the dialysis of substances held in watery solution only, and will not answer for alcoholic or ethereal liquids. The hard-rubber sieve, which has been already noticed (see Fig. 300), makes an excellent dialyzer if floated upon pure water in an evaporating dish or beaker. Fig. 301 shows another form, in which the glass dialyzer has its upper edge expanded into a flat shoulder, which rests upon the upper edge of the round dish containing the water. This has the advantage of keeping the diffusate covered whilst the dialysis is progressing. It is obvious that very different arrangements might be made to accomplish the same ends. Thus, a bladder three-fourths filled with a mixture, suspended in a jar of distilled water, would yield similar results. Graham's apparatus is preferable to others only for its convenience. 1 Dialyzer. The following applications of the process of dialysis have been made: 1. It facilitates in many instances to a considerable extent the separation of the active matter of any artificial or natural mixture from the inert and useless, the former being very often crystalline and the latter colloidal. Thus, infusions or decoctions of medicines, such as opium, belladonna, aconite, etc., submitted to dialysis might give up more or less completely their crystalline principles, such as the salts of morphine, atropine, aconitine, etc., to the water, while the gummy, resinoid, extractive, and coloring matters, etc., might remain behind. In effecting 1 This vessel is sometimes called the "exarysator." the analysis of organic bodies, one of the most embarrassing problems is to get rid of the inert principles which interfere with the action of chemical reagents, and the process of dialysis may here often be brought to the aid of the operator. 2. In searching for poisons in organic mixtures, as in the contents of the stomach, in which the application of tests is often rendered abortive by the colloidal matter present, the problem of the presence of the poison may sometimes be solved by submitting the suspected matter to dialysis. The poison will often be found in the diffusate separated from the other matters, and may then be detected by the ordinary tests. 3. In pharmaceutical operations it often happens that salts and other crystallizable substances are thrown away as refuse matter because they would not repay the cost of time and material necessary for their recovery. It is possible that by this simple, inexpensive process these substances may be separated from the useless matters and thus saved. 4. An economical application has been made of the process to the restoration of salted meat to the fresh state. If some salt beef with its brine be enclosed in a bag of material suitable for dialysis, as of untanned leather, and the bag be immersed in sea-water, in the course of some days the beef and brine will have been rendered sufficiently fresh for use, the salt having passed out into the sea-water. B. F. McIntyre, of New York, gave a great deal of attention to the practical applications of dialysis, and introduced a class of preparations called Dialysates. These were prepared from various drugs by dialysis, and it was claimed that they contained the active crystallizable constituents in their original combination, deprived of inert colloidal substances. Dialyzed iron, or Ferrum Dialysatum, is a colloidal preparation made by placing a mixture of basic ferric chloride and ammonium chloride upon a septum: the crystalloids (ammonium chloride and ferric chloride), with any free acid, pass into the diffusate, leaving the neutral colloidal liquids upon the septum. QUESTIONS ON CHAPTERS XVI. AND XVII. GRANULATION, EXSICCATION, AND DIALYSIS. What is meant by granulation? How are granulated effervescent salts prepared? What is exsiccation, and what is its object? What is dialysis? What are crystalloids? Give an example. What are colloids? Give examples. What is the most convenient substance to use for a dialyzing medium? Describe Professor Graham's dialyzer. What is the liquid called in which crystalloid matter is dissolved, which has been passed through a dialyzer? What applications of the process of dialysis have been made? What preparations have been called dialysates? What is dialyzed iron? CHAPTER XVIII. EXTRACTION. UNDER this head are included those operations which have for their object the separation of the soluble principles from drugs by treating them with a liquid capable of dissolving them, which is called the menstruum. Extraction differs from solution in the fact that the presence of insoluble matter is implied in the former, and the soluble constituents must therefore be extracted or separated, by appropriate methods, from those which are insoluble. The principal modes of extraction employed in pharmacy at present are as follows: 1. Maceration and expression. 2. Percolation. 3. Digestion. 4. Infusion. 5. Decoction. Maceration and expression are old processes, and they will be considered first; percolation is much more important and useful, and will be treated of in a separate chapter; digestion is merely a modified form of maceration. Infusion and decoction are processes which are used in producing separate classes of official preparations: hence they will be considered in Part II. MACERATION. FIG. 302. The process of maceration, which is of ancient origin, consists simply in soaking the properly comminuted drug or substance in the menstruum until it is thoroughly penetrated and the soluble portions softened and dissolved. The usual method is to introduce the drug or substance into a bottle with the menstruum, cork it tightly, and agitate it occasionally for a period ranging from two to fourteen days; then to pour off the liquid, express the residue to avoid waste, and filter the mixed liquids. An advantage is sometimes gained by suspending the ground drug, tied in a bag, in the upper part of the menstruum (see Fig. 302): this is sometimes termed circulatory displacement. Maceration is the process directed by the German Pharmacopoeia exclusively in preparing tinctures: the drugs are ordered in all cases to be macerated in definite weights of alcohol Circulatory displacement. for a week in a closed bottle, in a shady place, with frequent agitation, at a temperature of about 15° C. The liquid is then separated by strain ing or expressing from the insoluble residue, and, after having been allowed to settle, it is filtered. Evaporation during filtration is to be avoided as much as possible. Maceration has no advantages over percolation in making the greater number of liquid preparations from drugs, except in the hands of the careless or unskilful. If an operator possesses no knowledge whatever of the process of percolation, it is safer to trust to maceration, for here no particular skill or judgment is necessary; the soaking process is completed in due time, and the separation of the absorbed liquid, whilst laborious and uncleanly, has at least the merit of leaving the tincture uniform in strength; if the process of expression is not thoroughly performed, pecuniary loss results, but the finished preparation is uniform. On the other hand, in percolation, if the operator has, through careless packing, failed to exhaust thoroughly the drug with the amount of menstruum used, a portion of the activity of the drug remains in the residue, which is thrown out, and the preparation is thus deficient in strength. In making tinctures by maceration, the practice of weighing liquids, as directed in the German Pharmacopoeias, is seen to the best advantage, although, of course, the principal objection to preparations made from weighed liquids still exists,-the necessity for a calculation to determine the dose as compared with that of the drug. The tincture must always be administered by measure, and hence a teaspoonful or a fluidrachm should bear a certain relation to the drug, which is readily ascertained without resorting to a problem in specific gravity. Digestion is that form of maceration which consists in the application of a gentle heat to the substance which is being treated. It is used in those cases where a moderately elevated temperature is unobjectionable, the heat increasing the solvent powers of the menstruum. QUESTIONS ON CHAPTER XVIII. MACERATION. What is meant by extraction? What is meant by menstruum? What are the principal modes of extraction employed in pharmacy? What is meant by circulatory displacement? How are all tinctures directed to be prepared by the German Pharmacopoeia ? Which is the better process in unskilled hands, maceration or percolation, and why? What is the principal objection to preparations made from weighed instead of measured liquids? Define digestion. |