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by subsidence, thymol is liberated by means of hydrochloric acid and purified by distillation and crystallization; if necessary, it is also decolorized by treatment with animal charcoal.

RESINS.

Comparatively little was known until recently regarding the chemical composition of resins which occur in plants either alone or in combination with volatile oils as oleoresins or with gums as gum resins. Investigations have been in progress for some years in the hands of Prof. Tschirch, of Berne, Switzerland, and his colaborers, and much light has already been shed upon this rather obscure subject. This much has already been established, that resins are largely composed of organic acid esters or compound ethers of certain alcohols, to which latter the general name resinol has been applied; some of these alcohols give reactions similar to those characteristic of the tannins, and have therefore been designated as resinotannols. Thus we have benzoresinol, storesinol, peruresinotannol, toluresinotannol, etc. Some resins have decidedly acid properties, while others are known to be anhydrides, as in the case of common pine resin or colophony, which is chiefly composed of abietic anhydride, CH2O4; one of the resins found in copaiba is a crystalline acid, called copaivic acid, having the elementary composition, C2H3O2; the resin obtained from guaiacum wood and officially recognized as guaiac, consists largely (70 per cent. and over) of guaiaconic acid, C, H2O, to which the well-known color reactions of guaiac with oxidizing agents are due.

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Resin of Scammony consists almost wholly of scammonin, C3H6O167 the anhydride of scammonic acid, which behaves like a glucoside. Jalap resin consists of two distinct resins which can be separated from each other by ether, the one insoluble in that menstruum, and constituting about 90 per cent. of the official resin, consists almost entirely of convolvulin, CH,O16, an anhydride possessing glucosidal properties and being colorless when pure. The official resin of podophyllum is a complex mixture, containing an acid called podophyllinic acid, insoluble in ether, and a substance to which the name podophyllotoxin has been given; the latter, which constitutes about 50 per cent. of the official product, is said to be the active purgative principle. Both these substances are soluble in chloroform, and may be separated by addition of ether to the chloroformic solution, which precipitates podophyllinic acid; upon evaporation of the ethereal solution podophyllotoxin is obtained.

CHAPTER LIX.

ORGANIC ACIDS.

OF the large number of compounds termed organic acids, only the few that are of special interest in pharmacy have been officially recognized. Organic acids are considered as derived from hydrocarbons or their alcohols, by replacement of hydrogen or hydroxyl by the univalent group carboxyl, COOH, and vary in their basicity as one, two, or three carboxyl groups may have been taken up, carrying with them one, two, or three atoms of replaceable hydrogen, as in the case of inorganic acids. The official organic acids are acetic acid, benzoic acid, citric acid, gallic acid, lactic acid, oleic acid, salicylic acid, stearic acid, tannic acid, and tartaric acid. Diluted hydrocyanic acid, although usually reckoned among the inorganic acids, is preferably considered at this point, since cyanogen is a carbon compound probably derived from hydrocarbons by substitution of nitrogen for hydrogen. Oxalic and valerianic acids, although not officially recognized, are both of interest to pharmacists, as is also meconic acid.

Acetic Acid, HC,H,O, or CH,COOH.-This acid has already been considered in connection with the derivatives of cellulose on page 616.

Benzoic Acid, HC,H,C, or CH,COOH.-Several methods are in use for obtaining this acid from benzoin, the balsamic resin from which it takes its name.

Both a dry and a wet process are employed for extracting the acid from the resin, in which it exists in a free state. The former is by sublimation, benzoin in coarse powder, which has been dried over lime, being heated in shallow iron pans covered with a porous diaphragm and connected with a suitable condenser, carefully regulated sand-bath heat being used so as to avoid contamination of the acid with other products, partly the results of decomposition, which volatilize at a temperature approaching 200° C. (392° F.). The yield of acid by this method ranges from 6 to 8 per cent. of the weight of benzoin used, the fused resin retaining a considerable portion which can be recovered by the wet method; sublimed acid is never chemically pure, being always accompanied by a volatile oil to which the peculiar odor of the acid is due.

The wet method consists in treating powdered benzoin for some time with warm milk of lime, and finally boiling the mixture and

filtering while hot. The filtrate is supersaturated with hydrochloric acid, the crude benzoic acid being allowed to crystallize and then purified by resolution in boiling water, with the addition of animal charcoal, filtered and again crystallized. In this process calcium benzoate, Ca(C,H,O,), is first formed and then decomposed with hydrochloric acid, whereby benzoic acid is liberated while calcium chloride remains in solution, thus, Ca(C,H,O2)2+2HCl=2HC,H2O2 +CaCl. Benzoic acid obtained by this method is of fine white appearance, and devoid of the peculiar aroma of sublimed acid.

Of late years synthetic benzoic acid has been extensively produced, and the Pharmacopoeia recognizes both the natural and synthetic products. The latter is made from toluene, CH,CH,, by passing chlorine gas into it while boiling until an increase in weight is no longer observed. Toluene is thereby converted into benzo-trichloride, CH,CC1, which liquid, when treated with water under pressure, is converted into benzoic and hydrochloric acids, thus, C,H,CCI, +2H2O = C2H,COOH + 3HC; the benzoic acid is separated by straining, and washed with cold water until free from hydrochloric acid. It is important in this process that the chlorine gas be passed into the boiling toluene in diffused daylight, to avoid the formation of other products.

Large quantities of benzoic acid are also made from the urine of cattle and horses, which contains hippuric acid, or benzoyl glycocoll. By boiling hippuric acid with strong hydrochloric acid, the former absorbs water and is split up into benzoic acid and glycocoll or amidoacetic acid, thus, CH,(NH) (C,H,CO)COOH + Ĥ,0 = CH,COOH + CH2(NH)COOH. Benzoic acid from this source is always accompanied by a fetid odor, which is removed by recrystallization and sublimation with benzoin.

Citric Acid, H,C,H,O,+H2O or C,H,OH(COOH), +H2O.-This acid belongs to the class known as fruit acids, and, although occurring in many plants, is obtained for use solely from lemons and limes. It is manufactured both in this country and Europe, on a large scale, from the juice of immature fruit, which contains from 6 to 8 per cent. of acid. The juice is first clarified by ebullition and then neutralized by addition of chalk, the resulting calcium citrate being washed with boiling water, in which it is sparingly soluble, and finally decomposed by means of diluted sulphuric acid; the newly formed calcium sulphate is removed by straining, the solution of citric acid being concentrated and allowed to crystallize in large wooden vats lined with lead. If necessary, the crystals of citric acid are redissolved in water, the solution being subsequently filtered through animal charcoal, to remove color, and recrystallized.

As citric acid crystallizes better from solutions containing a little sulphuric acid traces of the latter are generally found in the commercial article. Small particles of metal found adhering to the crystals and deposited in solutions thereof are lead, derived from

the crystallizing vats. Contamination with crystals of tartaric acid can be readily detected by placing some of the crystals in a small dish with a little solution of potassa; the crystals of citric acid slowly dissolve, while those of tartaric acid gradually become opaque, owing to the formation of acid potassium tartrate. The Pharmacopoeia requires absolute purity for citric acid, with the exception of very small traces of sulphuric acid. The official test for the presence of tartaric and oxalic acids depends upon the solubility of potassium citrate in acetic acid, in which the tartrate and oxalate are insoluble.

During the past few years citric acid has been prepared from dextrose (grape-sugar) by action of certain fungi, known as citromyces. The yield amounts to about 55 per cent. of the weight of dextrose used, and the resulting citric acid is in every respect identical with the natural acid. The manufacturer of artificial or synthetic citric acid is said to be already in progress on a commercial scale.

Solutions of citric acid gradually separate fungous growths; this can, however, be prevented by addition of 5 or 10 per cent. of

alcohol.

Diluted Hydrocyanic Acid.-The official preparation of this name is an aqueous solution of gaseous hydrocyanic acid, HCN, prepared by decomposing a solution of potassium ferrocyanide with sulphuric acid, in a flask or retort, and conducting the resulting vapors into distilled water. In this process the following reactions occur: 1. The formation of hydroferrocyanic acid, thus, K4Fe(CN)6 + 2H,SO =H,Fe(CN)+2K,SO,; 2. The decomposition of a further portion of potassium ferrocyanide by the newly formed acid in the presence of sulphuric acid, thus, K,Fe(CN)6 + H ̧Fe(CN)6 + H2SO, =6HCN+K,ŠO ̧+K ̧Fe(Fe(CN)), hydrocyanic acid being evolved, while potassium sulphate and potassio-ferrous ferrocyanide, or Everitt's salt, remain in the flask or retort; the latter salt is white at first, but gradually changes to blue. Aqueous vapor of course passes over with the vapor of the acid, both of which are usually condensed in a Liebig condenser interposed between the retort and receiver. The directions of the Pharmacopoeia are to continue distillation until the original volume of the mixture has been reduced to about one-half, and then to dilute the contents of the receiver with sufficient distilled water that the finished product shall contain 2 per cent. by weight of absolute hydrocyanic acid.

The quantity of water required for dilution of the distillate is readily ascertained by first determining volumetrically the amount of absolute HCN present; this is done by titrating a small weighed portion with decinormal silver nitrate solution, using potassium chromate as an indicator. Since silver chromate is soluble in both acid and alkaline liquids, it becomes necessary to neutralize the acid liquid, and for this purpose magnesium hydroxide is preferable to

soda or potassa, as a slight excess of it is not hurtful-in fact, serves to sharpen the end reaction by providing a white background, against which the red color is more plainly seen. The equation, HCN+AgNO3=AgCN+HNO,, shows that 26.98 parts of absolute hydrocyanic acid require 169.55 parts of silver nitrate for complete decomposition; hence each Cc. of AgNO, solution represents 0.002698 Gm. of HCN, and, as red silver chromate is not permanently formed until all hydrocyanic acid has been removed, the number of cubic centimeters of decinormal silver solution required to produce the permanent red color, multiplied by 0.002698, gives the total quantity of hydrocyanic acid present in the sample used for the assay, which, multiplied by 100 and divided by the weight of the sample, expresses the percentage of absolute acid. Thus, if 0.27 Gm. of the distillate requires 3.4 Cc. of AgNO, solution, 3.4 per cent. of absolute HCN is present, for 3.4×0.002698 0.0091732, which multiplied by 100 and divided by 0.27=3.39+or 3.4. The amount of water necessary for dilution of the distillate can now be found by simple calculation, according to the following wellknown rule multiply the total weight of the distillate by the percentage of absolute acid found and divide the product by the percentage required (which in this case is 2 per cent.); the quotient indicates the weight to which the distillate must be brought by the addition of distilled water. If, from the weight so found, the weight of the original distillate be subtracted, the remainder will indicate the weight of water to be added.

As the vapor of hydrocyanic acid is very poisonous, special care must be observed that all joints of the flask, tubing, etc., be secure, so as to prevent leakage; and to guard against the bumping of the liquid in the flask, frequently observed, a tin hood may be placed over it, or a spiral of glass or platinum be suspended in the liquid.

The alternative formula in the Pharmacopoeia for making the official acid is simple, and offers a convenient method for rapidly preparing small quantities. Six Gm. of silver cyanide will yield 1.21+ Gm. of absolute hydrocyanic acid, which, dissolved in 60 Cc. of fluid as directed in the formula, makes a 2 per cent. solution; if strictly official hydrochloric acid be used, a very slight excess of the latter will be present

Solutions of hydrocyanic acid are unstable, hence the official diluted acid is a very unsatisfactory preparation, even if carefully kept in small, tightly closed amber vials. Good sound corks are probably preferable to glass stoppers, as they fit more closely, as a rule. Various substances, such as sulphuric and hydrochloric acids, diluted alcohol, etc., have been suggested for the preservation of the diluted acid, but thus far none have proven strictly reliable.

A strong solution of hydrocyanic acid, known as Scheele's acid, contains 5 per cent. of absolute HCN, but is not used in this country for medicinal purposes.

The test with potassa, ferrous sulphate, and ferric chloride. men

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