=

double chloride can be ascertained by treatment with an excess of some reducing agent, whereby metallic gold is precipitated. Either ferrous sulphate or oxalic acid may be employed, the reaction occurring being illustrated by the following equations: 2AuCl, + 6(FeSO ̧ +7H,O) Au2 + 2(Fe2(SO2)3) + FeCl + 42H,O or 2AuCl, + 3(H2C2O, + 2H2O) Au2+6CO2+6HCl + 6H2O. From the second equation it is seen that 377.1 parts of crystallized oxalic acid can precipitate 393.4 parts of metallic gold; hence, in the official test, 0.15 Gm. of the metal will require 0.143 Gm. of the acid, thus insuring the necessary excess of the latter.

=

Gold chloride being readily reduced by contact with organic matter, all such mixtures should be avoided; and as the official preparation is chiefly used in pill-form, non-oxidizable excipients only should be employed (see also page 348).

THE COMPOUNDS OF SILVER.

Silver Cyanide. AgCN.-This salt may be prepared either by passing freshly diluted hydrocyanic acid into a solution of silver nitrate or by adding a solution of the latter salt to a solution of pure potassium cyanide as long as a precipitate continues to be formed. In either case the precipitate must be well washed with water and dried in a dark place.

Silver cyanide becomes discolored upon exposure to light, and must therefore be kept in dark bottles. It is used in pharmacy solely for the extemporaneous preparation of diluted hydrocyanie acid.

Silver Iodide. AgI. When a solution of silver nitrate is added slowly and with constant stirring to a solution of potassium iodide, a light-yellowish precipitate of silver iodide is formed by mutual decomposition, which, after being well washed with water, may be dried upon paper. Owing to the very slight solubility of silver iodide in ammonia-water, contamination with silver chloride or bromide can be readily detected by the pharmacopoeial tests. If absolutely pure, the salt remains unaltered by exposure to light, but the commercial article usually assumes a greenish tint.

The salt is scarcely ever used in medicine now, and its retention in the Pharmacopoeia appears unnecessary.

Silver Nitrate. AgNO,.-This salt is preferably made from pure silver, and in order to obtain a product free from acid the metal is dissolved in nitric acid, the solution evaporated to dryness, the residue fused and redissolved in water, the solution filtered and allowed to crystallize. The evaporation to dryness and fusion of the residue are for the purpose of expelling any uncombined acid present, which, if the first solution were allowed to crystallize would to some extent be retained mechanically within the crystals: a tem

perature exceeding 200° C. (392° F.) must, however, be avoided, lest some of the silver nitrate be reduced to nitrite.

Silver nitrate is easily decomposed by contact with organic matter, and when exposed to light gradually assumes a gray color; hence proper precautions must be observed in keeping and dispensing it.

The Pharmacopoeia requires absolute purity for crystallized silver nitrate, which is determined by titration with decinormal sodium chloride solution. The equation AgNO3 + NaCl AgCl + NaNO, shows that 169.55 parts of the silver salt require 58.37 parts of sodium chloride for complete precipitation; hence each Cc. NaCl solution corresponds to 0.016955 Gm. AgNO, and 0.34 Gm. of crystallized silver nitrate requires 20 Cc. of the decinormal solution, for 0.016955 X 20 = 0.33910. ×

N

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Diluted Silver Nitrate.-This preparation differs from the preceding in containing only 33.33 per cent. of pure silver nitrate, and being much milder in its action, is also known as mitigated caustic. It is made by fusing together 30 parts of silver nitrate and 60 parts of potassium nitrate, and, when a smooth, uniform mixture results, pouring the molten mass into suitable moulds, usually of a narrow cone shape.

N

The amount of pure silver nitrate present in any sample may be ascertained by means of decinormal sodium chloride solution, an excess of which is added and determined subsequently by retitration with decinormal silver nitrate solution, using potassium chromate as an indicator. The two solutions being of equal value volumetrically, the number of Cc. AgNO, solution required, after addition of 20 Cc. NaCl solution in the official test, to cause a permanent red precipitate of silver chromate, subtracted from 20 gives the exact number of Cc. NaCl solution necessary to precipitate all the silver from 1 Gm. of diluted silver nitrate; this number multiplied by 0.016955 and then by 100 gives the percentage of silver nitrate present in the sample.

N

Moulded Silver Nitrate.-Under this name the Pharmacopoeia recognizes a mixture of silver nitrate and chloride, containing 5 per cent. of the latter salt, and prepared by adding 1 part of hydrochloric acid to 25 parts of pure silver nitrate, melting the mixture at as low a temperature as possible and casting the mass in moulds. The object of converting a part of the silver nitrate into chloride is to render the resulting mass less brittle.

The synonym lunar caustic, given to this preparation in the U.S. Pharmacopoeia, does not correspond with the same term commercially, which is usually applied to pure silver nitrate moulded into sticks, as also indicated in the British Pharmacopoeia. The latter authority applies the name toughened caustic (argenti nitras induratus) to a mixture of 95 parts of silver nitrate and 5 parts of potassium nitrate.

The valuation of fused silver nitrate is made exactly as in the case of diluted silver nitrate. Like all silver salts, this one must also be protected from light to prevent discoloration.

Silver Oxide. Ag20.-This compound may be obtained by adding a solution of pure silver nitrate to a solution of potassa, soda, or lime, washing the resulting precipitate well with water, and finally drying the same on a water-bath. Ammonia-water is not suitable for the process, since it forms a soluble compound with the oxide, having the composition Ag2O + NH ̧.

When ignited in a porcelain crucible silver oxide should yield 93.1 per cent. of its weight of metallic silver. Like silver iodide, the oxide is very rarely employed in medicine at the present time. It is quickly decomposed by oxidizing agents, and should never be triturated with organic substances.

ORGANIC SUBSTANCES.

UNDER this head are classified those many compounds of carbon, hydrogen, and oxygen, frequently associated with nitrogen, sulphur, phosphorus, and other elements, which are chiefly derived from the vegetable kingdom; a few are obtained also from the animal kingdom, and some are produced synthetically.

Prior to 1828, when Woehler announced to the scientific world the successful synthetic production of urea, an excretory product of the animal economy, solely from inorganic material, thereby establishing the intimate relationship between organic and inorganic matter, the agency of a peculiar vitalizing force was considered essential for the formation of all so-called organic bodies. No elements unknown to the mineral kingdom have ever been found in organic bodies, and the one feature which serves to distinguish this very large class of chemical compounds from those commonly designated as inorganic substances is the invariable presence of carbon; the term carbon compounds is therefore most appropriately applied to them:

The simplest form of carbon compounds are the hydrocarbons, composed exclusively of carbon and hydrogen; of these, two, methane, CH,, and benzene, CH, may be said to be the source of all organic compounds, the constitution of which has thus far been studied and explained. The derivatives of these two hydrocarbons differ so widely in their properties that they have been conveniently grouped into two main classes, designated as fatty and aromatic compounds, respectively.

It is not within the scope of this book to enter into a detailed study of the so-called organic substances, and attention will be given only to those of pharmaceutical interest.

CHAPTER LIII.

CELLULOSE AND ITS DERIVATIVES.

ALL plants are made up of certain proximate principles, to which they owe their growth and value as nourishing or medicinal agents. The most widely diffused substance in the vegetable kingdom is cellulose or cell membrane, which goes to make up the body of all plants. During the growth and development of plants some of the cell membrane undergoes a change, becoming gradually hard and woody; to this modified form of cellulose the name lignin has been given, and the woody fibre of plants is assumed to be a combination of cellulose and lignin, called lignose. Cellulose and lignin being insoluble in all ordinary solvents, the chief object in pharmaceutical processes is to extract from them, by appropriate treatment, the many valuable principles they often enclose and upon which the medicinal value of vegetable drugs depends.

Lignin has not yet been obtained in a pure state, but pure cellulose has been isolated as a colorless, odorless, and tasteless gelatinous mass, which, upon drying, forms a horny substance, or may be obtained as a white powder. It is soluble in a solution of cupric hydroxide in ammonia-water, known as Schweitzer's reagent, forming a mucilaginous fluid which, after dilution, admits of filtration, and, upon addition of an acid, is again precipitated. The elementary composition of pure cellulose corresponds to the formula C,H,0%, or multiples thereof, as C12HO10 or C18H30O15

10

Cellulose is officially recognized in the form of gossypium, or cotton, and patent lint and paper are further examples of it. When heated with potassium or sodium hydroxide it is gradually converted into oxalic acid, alkali oxalates being formed, and, if boiled with diluted sulphuric acid, dextrin is produced, which is finally changed into dextrose, from which alcohol can be obtained by fermentation. Immersed in strong sulphuric acid, cellulose undergoes conversion into a substance called amyloid, upon which the preparation of parchment paper depends, the pores of the paper becoming filled with this modified cellulose, and thus made tough and impervious to water. Prolonged contact of the paper with strong sulphuric acid, however, is hurtful, the resulting product becoming friable; hence the best results are obtained if the paper be simply drawn through a mixture of two parts of concentrated sulphuric acid and one part of water, and then immediately well washed with water.