contact with the liquid. In many cases the ratio of solubility is not the same for equal increments of heat. Density of Solutions.-The effect of dissolving a solid body, specifically heavier than the solvent, is always to increase the density of the liquid in which the solid is dissolved. The specific gravity of water is 1.000: if five per cent. of sugar is dissolved in it, the specific gravity is 1.021; if ten per cent., 1.070; if twenty per cent., 1.088, etc. This fact is capable of optical proof; for if a piece of sugar is suspended near the top of some water in a beaker, the downward currents of the solution can readily be noticed if viewed by transmitted light. Solubility of Substances in Saturated Solutions.-Whilst a saturated solution is one which is incapable of dissolving any more of the substance which was dissolved in the liquid, it must not be assumed that the saturated solution will not dissolve other solids. For example, if granulated potassium nitrate be mixed with two per cent. of copper sulphate, and then placed in a funnel having a plug of cotton in the throat, it will be found that the copper sulphate can be very easily removed by pouring on the potassium nitrate mixture a saturated solution of potassium nitrate. The potassium nitrate cannot suffer loss, because the liquid passing through is a saturated solution of the same substance; but copper sulphate is soluble in a saturated solution of potassium nitrate, and it is thus washed out. Reduction in Temperature caused by Rapid Solution.-When solids dissolve rapidly in liquids without chemical action, a reduction in temperature always takes place, and cold is produced, in accordance with the well-known law governing the conversion of solids into liquids, whereby sensible heat is converted into latent heat. The so-called freezing mixtures are produced in this way: thus, if five parts of potassium sulphocyanide are quickly mixed with four parts of cold water, the temperature of the solution falls to -20° C. (-4° F.); thirty-two parts of sodium chloride, if mixed with one hundred parts of snow, will produce a brine having a freezing-point of -23° C. (-9.4° F.). Equal parts of crystallized calcium chloride and snow, when well mixed, will have a temperature as low as -45° C. (—49° F.). Elevation of Temperature produced by Solution accompanied by Chemical Action.—If chemical action takes place whilst solution is progressing, the opposite effect, or elevation of temperature, is frequently produced, as in dissolving anhydrous salts. The same fact is noticed when the solution of an alkaline oxide made by calcination is effected by treating it with an acid, as when calcined magnesia is dissolved in a solution of citric acid. Modes of Effecting Solutions of Solids.-The method usually employed by the pharmacist is by the use of the solution mortar and pestle (see page 190). The ordinary practice is to crush the substance into fragments in the mortar with the pestle, and then pour upon it the solvent, meanwhile stirring with the pestle until solution is effected. If definite quantities are used, and the whole of the solvent is required to dissolve the given weight of the salt, a portion only of the solvent should be added at first, and when this is saturated the solution is poured off, and a fresh portion of solvent added: this operation is repeated until the solid is entirely dissolved; the solutions are then mixed. Other methods of effecting solution are to shake the solid with the liquid in a bottle or flask, or to apply heat to the substances in a suitable vessel. FIG. 222. = Circulatory Solution.-A very excellent mode of dissolving substances, particularly where the solid is not very soluble or the relative proportion of liquid is small, is to suspend the solid near the top of the liquid upon a porous diaphragm or a suitable sieve, or tied up in a gauze bag if its nature will admit of this treatment. The parts immediately in contact with the solvent are dissolved, and the solution descends, its place being supplied by fresh portions of the solvent: a circulation is thus created and solution facilitated (see Fig. 222). Solvents used in Pharmacy.—Water. -The most useful of all solvents is water. It has a more extensive range than any other liquid, and the aqueous solutions are among the most important preparations of pharmacy. They are especially treated of in chapters, under the head of Liquores, Aqua, Syrupi, etc. Circulatory solution. Alcohol as a solvent is next in importance to water. It has an important advantage over water in the fact that preparations made with it keep almost indefinitely, whilst most aqueous solutions of organic substances soon decompose or become worthless. Resins, volatile oils, alkaloids, glucosides, salts, etc., are dissolved by alcohol, whilst many inert principles, like gum, albumen, and starch, are insoluble in it, so that it has also great usefulness in its negative character. Glycerin is an excellent solvent, although its range is not so extensive as either of the preceding: it has in its concentrated state antiseptic qualities of a high order, but has not the valuable negative qualities of alcohol. It dissolves the fixed alkalies, some of the alkaline earths, a large number of neutral salts, and vegetable acids, pepsin, tannin, etc., but it also dissolves gum, albumen, starch, etc., and thus its solutions are generally loaded with inert constituents. (See Glycerites.) Ether is a good solvent for special purposes. Oils, fats, resins, and some of the alkaloids and neutral principles are dissolved by it. Benzin is very similar in its solvent properties to ether. Chloroform resembles ether and benzin as a solvent. It has an advantage over both, however, in not being inflammable, although its costliness prevents any extended application. Carbon Disulphide is an excellent solvent for rubber, phosphorus, etc. Its range is limited, however, and its odor and inflammability detract from its usefulness. Acids, either strong or diluted, are used as solvents; as in vinegars. Oils are also used in this way in liniments, etc. |