Precipitation may have for its object:

1st. To obtain the substance in as fine a powder as possible. Examples: Precipitated chalk or magnesium carbonate.

2d. To remove impurities. Iron is removed from calcium chloride by precipitation with calcium hydrate; from zinc chloride solution by precipitation with zinc oxide. An impure sodium acetate is freed from sulphuric acid by the careful addition of barium acetate, etc.

3d. To obtain new chemical compounds; for instance lead iodide and mercuric iodide, ammoniated mercury, bismuth subcarbonate and subnitrate, etc.

4th. The qualitative and quantitative determination of substances. In analytical chemistry, we identify and separate the various groups by means of different precipitating reagents. We also ascertain the quantitative composition of different bodies, by the precipitation and subsequent weighing of the various constituents.

As to their appearance, precipitates are designated as:

Crystalline

Amorphous

Granular

Curdy

Flocculent

Gelatinous

Magma

produced, for instance, by the cooling of hot saturated solutions of salts.

produced, for instance, by precipitation of ferric chloride by alkalies.

produced, for instance, by addition of alcohol to a concentrated aqueous solution of ferrous. sulphate.

produced, for instance, by addition of hydrochloric acid to a solution of silver nitrate. produced, for instance, by precipitation of albumen with alcohol.

produced, for instance, by addition of collodion. to carbolic acid.

-is a pasty mass resulting from the straining or filtering of an amorphous precipitate retaining water mechanically. Examples, ferric hy

drate, aluminum hydrate.

Hot, dense solutions, yield dense precipitates.

Cold, dilute solutions, yield light (diffusible) and often crystalline precipitates.

Dense precipitates are more easily washed, for they subside readily and admit of being washed by decantation.

Crystalline Precipitates.-In many operations, we aim to obtain the precipitated body in a so-called crystalline condition, for the purpose of facilitating its separation and subsequent purification.

For example, we cause calcium carbonate (precipitated chalk) and barium sulphate to be precipitated from hot solutions. This causes these compounds to form dense and crystalline precipitates, which are then easily removed and purified.

On mixing cold solutions, the precipitate formed will usually be crystalline and of greater purity; the solutions should be mixed slowly and with constant stirring. The precipitate will be denser, if it is allowed to stand at least twenty-four hours in the liquid. Example: On adding cold "magnesia mixture" to a cold, weak solution of a phosphate, a crystalline precipitate will gradually form, consisting of the double salt ammonium-magnesium phosphate. Light magnesia (MgO) is made from the light carbonate, which is made by reaction between cold solutions of sodium carbonate and magnesium sulphate. Heavy magnesia is made from the heavy carbonate, which is made by reaction between hot solutions of sodium carbonate and magnesium sulphate.

Amorphous Precipitates.-Amorphous precipitates (for instance, hydroxides or sulphides of the metals), are denser and separate more rapidly from saline solutions; for instance, when ammonium sulphydrate produces in solutions of ferrous salts only a green color of ferrous sulphide, the addition of ammonium chloride causes its immediate separation.

In order to obtain the hydroxides of aluminum (Al2(OH)) and iron (Fe2(OH)), the precipitation must take place in the cold, and in very dilute solutions, in order to insure a finely subdivided precipitate. Amorphous precipitates are much more easily washed in this finely subdivided condition. Moreover, in the case of the two examples quoted, precipitation must be effected in the cold, in order to obtain the precipitates in the hydrated condition and not as oxyhydrates (hot solutions). Upon these two conditions depends the solubility of ferric hydrate in solutions of the organic acids (preparation of scale salts).

In quantitative analysis many precipitates (metallic sulphides, hydroxides, etc.) are thrown down from boiling solutions, in order to render them as dense as possible. This will also cause them to settle rapidly, so that they may be washed by decantation; also this facilitates the solution of the soluble salt (impurity) formed by the reaction, which must afterward be removed by washing.

The order in which these solutions are added to one another, determines largely the purity of the product, and the ease with which it is afterwards washed. Hence, we should add the solution of ferric chloride to the diluted ammonia water, otherwise oxychlorides may be formed.

In preparing yellow oxide of mercury, we pour the solution of mercuric chloride into the solution of potassa, for if the operation is reversed we obtain red oxychloride of mercury.

In analytical operations we should add, for instance, the barium solution to the solution of the sulphate or sulphuric acid, otherwise some of the barium salt may be carried down with the precipitated barium sulphate.

The nature of the precipitate depends on the conditions under which the two solutions come in contact, that is, which of the two

remains longest in excess. If we pour a solution of mercuric chloride into a solution of potassium iodide, so that the latter remains in excess, the solution remains clear, because as long as the latter remains in excess the mercuric iodide is dissolved as fast as formed. If we reverse the order, having the mercuric chloride in excess, the mercuric iodide is likewise dissolved. It is only by a careful regulation of the proportions that success is obtained.

Fractional Precipitation has for its object the purification of substances by partial precipitation. For this purpose the precipitant is added only in small portions at a time, each precipitate being removed, before a further portion of the precipitant is added. In these different fractions, the substance may be found in different degrees of purity. Thus carbolic acid is separated from the empyreumatic resins which accompany it, by adding to its alkaline solution an acid, in small portions at a time, the first precipi

tations consisting of these resinous impurities, while the later ones contain the acid in pure form. We may thus separate salicylic acid from its inert isomers by fractional precipitation with silver

nitrate.

In carrying on precipitation, it is necessary that the liquid be constantly stirred during the addition of the precipitant; this is for the purpose of facilitating the contact of the substances dissolved in the liquids, also to prevent the precipitate from forming lumps, which might enclose particles of the substance yet unacted upon. With the exception of some operations in analytical chemistry, it is always advisable that both solutions be well diluted before mixing.

In direct precipitation, the precipitant is added until no further precipitation takes place. This may be ascertained by allowing

the precipitate to settle away from near the surface and then adding to the clear supernatant liquid a drop or two of the precipitant, which should not produce any further turbidity.

Precipitation is carried on, in small operations, in deep beaker glasses (Fig. 239), or in the so-called "precipitating jar" (Fig. 240), the latter being a deep, heavy glass vessel, broad at the bottom and narrow at the top. This enables the precipitate to settle over a larger area of surface, thus assisting the operations of washing and decantation. On the large scale earthenware jars are usually employed, in which the supernatant liquid may be drawn off by means of spigots placed at different heights; wooden tanks are also constructed upon the same principle.

CHAPTER XVII.

DECANTATION.

Decantation is the act of removing the supernatant liquid from a precipitate or sediment.* We usually resort to decantation in removing the clear supernatant liquid from a precipitate which may be deposited by tinctures or fluid extracts on standing. It is an expeditious and accurate method of separating soluble from insoluble matter, when conducted with due care. Decantation is employed in the process of Elutriation, where the lighter are separated from the heavier particles of matter, the former being suspended in the upper layers of the mixture.

By "decantation with washing," is understood the removal of soluble from insoluble matter in precipitates, by the repeated affusion and withdrawal of water. It is employed in those cases where a large quantity of a heavy, insoluble precipitate is to be washed, requiring, necessarily, a large amount of water. The precipitate is first allowed

to settle; the supernatant liquid, after it has become clear, is drawn off; then hot or cold water is poured upon the precipitate, with thorough stirring. It is again allowed to settle and treated as before; this operation is continued, until all foreign soluble matter is removed.

In decanting off the supernatant fluid, it is necessary to use the guiding rod (Fig. 241), otherwise, owing to adhesion, the liquid will run down the side of the vessel (Fig. 242). If due care is taken, not a drop of the liquid need be lost. When the

FIG. 241.

A Sediment consists of solid matter, which is deposited by force of gravity, from solutions (in which it was suspended) on standing.

A Precipitate consists of solid matter (amorphous or crystalline), which separates from a solution, as the immediate result of a reaction, be it heat, light or chemical influence.

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