TROCHISCATION

FIG. 204.

Consists in forming conical masses of levigated substances. Insoluble powders, such as chalk, bismuth salts, or bole, while still in a pasty condition, are placed into a tin cone (d, Fig. 204). This is then grasped by the handle (c), and the leg of the frame is tapped gently upon a slab of chalk-stone, or other absorbing surface, each tap causing a conical drop to fall out. The porous block absorbs the moisture quickly, after which the cones may be dried by heat, if necessary.

Mould for Levigated Chalk, Etc.

CHAPTER X.

SOLUTION.

Solution is a "molecular" subdivision of a body (solid, liquid or gas), in a liquid (solvent), the result being a clear homogeneous fluid.

If, on bringing a solid in contact with a liquid, the whole or a portion of the solid disappears, and a clear liquid results or can be separated from the mixture, then we say that the solid is wholly or partially soluble in the liquid.

Formerly, the solution of a solid in a liquid, such as that of salt in water, was designated as "simple" or "direct" solution, and where a change in the chemical nature took place, as when bases were dissolved in acids, as "indirect" or "chemical" solution.

Such a distinction is not necessary, and moreover incorrect, for we must consider, that "solution" in such a case is only apparent, there being new bodies of a different chemical constitution produced, which, as such, are dissolved in the excess of the solvent. The chemical action which takes place first, has nothing to do with the phenomena of solution.

The rate or ratio, according to which a substance is dissolved by any particular liquid, is called its "solubility." The solvent powers of a liquid for different bodies are very different. If none of the substance is taken up by the solvent, it is said to be “insoluble;" if comparatively little is dissolved, it is designated as difficulty" or "slightly soluble" (example, calcium hydrate); when large amounts are taken up readily, as very soluble" (example, potassium iodide).

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When a solvent has taken up as much of the substance as it is capable of at a certain temperature, it is said to be "saturated." If there be an insufficient amount of the body present, it will be entirely dissolved and an "unsaturated" solution will result.

The solubility of a solid depends upon several conditions: 1st. The Nature of the Substance (form, density); hence we pulverize bodies to facilitate contact with the solvent.

2d. The Nature of the Solvent.

3d. Temperature.

4th. Presence of other dissolved Solids.

The Nature of the Substance must be considered. The solubility of a solid depends primarily upon its nature, for we find, for instance, that barium sulphate requires 250,000 parts of water for solution,* while on the other hand, sodium thiosulphate is soluble in all proportions in water (at 45° C.).

* Silver bromide at 20° C., is soluble 1 in 1,971,650 parts of water.

The Nature of the Solvent plays a very important part in solution. We find the solvent powers of water entirely different from those of alcohol, ether or glycerin.

The chief solvents employed in pharmacy are water, alcohol, glycerin, ether, chloroform, acids and oils.

Water is a general solvent of wide application. It is employed in the preparation of the medicated waters, syrups, solutions (liquores), etc. When it is used in the preparation of silver solutions, eye-washes, hypodermic solutions, etc., it should be distilled and fulfill all the requirements of the Pharmacopoeia. Water dissolves most inorganic salts; from vegetable drugs it dissolves gums, starch, sugar, neutral principles, certain alkaloids, tannins, extractive and many coloring matters.

Alcohol is of the greatest importance pharmaceutically, because of its excellent solvent properties and the stability of the preparations made with it. It is a solvent for resins, volatile oils, alkaloids, and neutral principles, while it does not take up such inert principles as starch and gummy matter.

Glycerin. The solvent properties of this lie between that of alcohol and water, and when added to either of these, it insures greater stability of the finished preparation. It dissolves tannin, starch, many inorganic salts, pepsin, organic acids, etc. (See Glycerites.)

Ether dissolves principally fixed and volatile oils, fats, resins, and most alkaloids (not their salts).

Chloroform possesses about the same solvent properties as ether, but it has the advantage that it is not inflammable.

Acids. Acetic acid in a diluted form, is employed as a solvent in the preparation of the official vinegars. Glacial acetic acid and hydrochloric acid are solvents of many organic compounds.

Oils are employed as solvents of gums and resins in the preparation of liniments.

Effect of Temperature on Solubility.-Rise of temperature usually increases the solubility of a solid. In some cases, we find that the solubility increases in a definite ratio with the temperature of the solvent, for example, potassium chloride, barium chloride, or magnesium sulphate. In another class (the majority), we find the ratio of solubility to the temperature irregular. In a small number of cases we find that the solubility steadily diminishes as the temperature rises, that is, the substance is more soluble in cold, than in hot water. As examples of this we have calcium hydrate, sodium chloride, strontium sulphate, calcium citrate, and paraldehyde.

The relationship of temperature to solubility is most clearly shown by the table of curves,* Fig. 205, where the lower line of figures express the degree of temperature, and the side figures, the quantity dissolved by 100 parts of the solvent.

* Ostwald, "Outlines of Inorganic Chemistry," p. 150.

In cooling a hot saturated solution, a portion of the dissolved body separates out in a solid (usually crystalline) form, while the remainder, according to its coefficient* of solubility for the given temperature, remains in solution. When bodies of different degrees of solubility are dissolved, the less soluble separate out first, while the inore soluble remain in solution.

The Presence of Dissolved Bodies in a solution often affects the solubility of other substances in this menstruum. Here we may distinguish several different cases:—

1st. The presence of one salt may increase the solubility of another. For instance, the presence of an alkaline chloride, such as sodium or ammonium chloride, increases the solubility of mercuric chloride in water. Iodine is practically insoluble in water, but by the use of an alkaline iodide, we can cause water to take

Table of Curves of Solubility for Nitrates.

up large amounts of it (example, Lugol's Solution). This is an example where it is possible for us to form a saturated solution (KI), which is capable of taking up a further quantity of a second substance (I).

2d. The presence of one salt may diminish the solubility of another. For example, we cannot dissolve potassium sulphate in a solution of ammonium sulphate, or potassium nitrate in a solution of ammonium nitrate. Potassium carbonate is a very soluble salt, but it is insoluble in concentrated water of ammonia.

3d. An alteration in the nature of the solvent may alter the solubility of the salt. The addition of alcohol to many saline solutions will cause the salt to be precipitated. For example, on adding alcohol to a saturated solution of ferrous sulphate, the iron salt will be precipitated as a granular powder

CONDITION OF CONTACT OF THE BODY WITH THE SOLVENT.We may facilitate the solution of solids by triturating or agitating them in powder form with the solvent; this facilitates the contact of the two, by continually exposing the surfaces of the particles of the powder to the contact of fresh portions of the solvent. This same principle is involved, when air or carbonic acid gas is passed rapidly through a mixture of the solid and fluid, for the purpose of producing brisk agitation.

CIRCULATORY SOLUTION.-When the quantity of matter to be dissolved is large, and the time permits, we may suspend the

* Coefficient is a number, expressing a certain arithmetical ratio. "Coefficient of solubility" is the number expressing either how many parts of a salt, etc, are soluble in 100 parts of a solvent, or how many parts of a solvent are required to dissolve 1 part of a salt.

substance, placed in a porous diaphragm (perforated funnel, sieve or muslin bag), just below the surface of the liquid solvent. A more or less saturated solution (resulting from the immediate contact of the solvent with particles of the solid), owing to its density, sinks to the bottom of the vessel, and its place is taken by fresh portions of the solvent displaced. This circulation continues until the liquid becomes saturated. This same procedure applied to the extraction of soluble matter from vegetable drugs

Apparatus for Generation, Washing, and Solution of Gases.

is known as "Circulatory Displacement," and is employed in the preparation of infusions, tinctures (by maceration), etc.

SUPERSATURATED SOLUTIONS.-A hot saturated solution of Glauber's salt (Na2SO, + 10H2O), when proper care is taken to protect it from jarring and dust, may be cooled, and still remain liquid, notwithstanding the solution contains a larger amount of the salt than the coefficient of solubility for the lower temperature allows. The slightest jar, or bringing it in contact with a solid body, causes its solidification at once. Such a solution is called "supersaturated."

SOLUTION OF GASES.-Gases are usually more soluble in alcohol