CHAPTER XIII.

SEPARATION OF IMMISCIBLE LIQUIDS.

THE separation of liquids which are mutually soluble is usually effected by distillation, if one or both of the liquids are volatile (see page 148); but the separation of liquids which do not mix with each other is generally a simpler process, and the object of this chapter is to show the methods at present in use.

Use of the Pipette.-This simple instrument consists of a narrow tube with its lower end drawn out to a capillary orifice, and a bulb, either globular or elongated, blown in it near the top; the upper edge is usually surrounded by a smooth glass ring to strengthen it. It is used by dipping the lower orifice into the liquid that is to be separated and applying suction with the mouth at the upper end; the liquid rises, fills the bulb, and, if the end of the moistened forefinger is applied to the top, the liquid may be carried some distance without running out. Fig. 274 shows the ordinary form of the instrument. Fig. 275 can be readily made from a glass syringe-tube, a perforated cork, and a piece of tube, by one unskilled in glass-blowing. Fig. 276 shows a pipette which has a piece of sheet-rubber tied over the thistle-shaped top. If pressure is ap

FIG. 274. FIG. 275. FIG. 276. FIG. 277.

FIG. 278.

FIG. 279.

plied to the rubber, the air is partially exhausted and a slight vacuum produced; if the point of the pipette is then applied to the liquid and the pressure relaxed, the liquid will rise, and it may be transferred to another vessel. A modification of this, where a rubber bulb is attached to the pipette and used in a similar manner, is seen in Fig. 40.

Use of the Glass Syringe.-The glass ear-syringe, having a bulb

blown near the orifice, is very useful in collecting a small quantity of oil floating on top of a liquid contained in a beaker or open vessel. The piston of the syringe should be well soaked in warm water before attempting this simple but delicate operation.

Use of the Separating Funnel.-This instrument, as indicated by its name, is used to facilitate the separation of immiscible liquids. It is generally a funnel having a glass stop-cock in its neck (see Fig. 277). The mixed liquid is poured into the funnel, which is placed in position where it can come to rest, and the lower liquid is then allowed to run off by opening the stop-cock. With careful use, liquids can be separated with great accuracy in this way. Where the liquid is valuable or very volatile, as in some of the oils, the globe separator (see Fig. 278), which can be accurately closed and evaporation prevented, is preferred. Mitchell's Separator.-Dr. Mitchell devised a cheap but efficient substitute for the separating funnel (see Fig. 279). A good cork, C, is fitted into the throat of a funnel, E, and the end of a penholder-handle, H, or other suitable wooden rod, is whittled to a flat wedge, and this forced into the cork tightly. The lower portion of the holder is notched, and the upper part of the cork is tied securely to it, or a pin may be driven through it and the cork to fasten it: it is used by forcing the cork C into the neck of the funnel, pouring in the liquids, and, when they have separated, lifting the handle H carefully until the lower liquid has entirely escaped, and then pushing it down tightly to stop the flow.

FIG. 280.

FIG. 281.

Florentine Receiver. The separation of volatile oils from the water which usually accompanies them during distillation is a very important part of their process of manufacture. Where the volatile oil is lighter than water, the form shown in Fig. 280 is used. The mixed oil and water collect in the glass receiver during distillation, the oil floating on the top, whilst the water ascends the bent tube from the bottom; further addition of distillate causes an overflow, and the water from the bottom of the receiver is discharged through the tube into a suitable vessel; then the receiver becomes filled again gradually as distillation progresses. The process of

Florentine receiver.

Receiver for heavy and light oils.

separation is continued automatically. Where the oil is heavier than water, the method is reversed, and provision must be made for the escape of the water near the top of the receiving vessel. Labor may be saved by the use of a long-necked funnel, or by placing a funnel in a wide tube, so that the mixed oil and water may be conveyed at once to the bottom without the liability of some particles of oil being carried over through the lateral tube and necessitating a second separation.

Fig. 281 shows a receiver which can be used for either light or heavy oils, one or the other tube being stopped with a cork, as the case requires.

CHAPTER XIV.

PRECIPITATION.

PRECIPITATION is the process of separating solid particles from a solution by the action of heat, light, or chemical substances. The separated solid is termed a precipitate; the added liquid or substance which produces the precipitate, the precipitant; the liquid which remains in the vessel above the precipitate, the supernatant liquid.

The precipitate usually falls to the bottom of the vessel. It may, however, remain suspended or rise to the top.

The objects of precipitation in pharmacy are,

1. To obtain conveniently solid substances in the form of fine powder. Example, the precipitation of calcium carbonate.

2. To effect the purification of solids. Example, the precipitation of pepsin from its solution by sodium chloride.

3. To obtain through chemical reaction substances which are insoluble in the supernatant liquid. Example, the official preparation of red mercuric iodide.

Precipitation is largely used in testing, as it frequently affords the most ready means of recognizing chemical substances or of ascertaining their purity. A great many pharmacopoeial tests are based upon this process. The color, quantity, and character of the precipitate are all taken into account. The terms curdy, granular, flocculent, gelatinous, crystalline, bulky, and others, which are sufficiently distinctive, are used to define the peculiar form which the precipitate assumes when thrown out of solution. A magma is a thick, tenacious precipitate left after the liquid is decanted.

Methods of Effecting Precipitation.-Precipitation may be produced in many ways. If solutions containing albuminous matter be heated, a flocculent precipitate of coagulated albumen will be thrown down; whilst if solution of the silver salts be exposed to the light, precipitation is apt to take place. Precipitation will usually occur when a hot saturated solution of an amorphous substance is allowed to cool, as in the preparation of antimony oxysulphide, or when to a solution is added a liquid in which the dissolved substance is insoluble, as when strong alcohol is added to a small quantity of mucilage of acacia, or water to an alcoholic solution of resin.

Precipitation is most generally effected by the reaction of chemical substances, and some of the most interesting processes in pharmacy are the results of this method of producing precipitates. When acid solutions are brought in contact with alkaline solutions, insoluble precipitates are sometimes formed, as the solution of oxalic acid with lime water, form

ing calcium oxalate. By far the most common method is to mix a solution of one salt with a solution of another, thereby producing an insoluble precipitate, as in the official process for preparing mercuric iodide, where solution of mercuric chloride is added to solution of potassium iodide. The methods of producing precipitates are numerous, and will be noted in detail under the head of the respective substances.

FIG. 282.

Vessels used in Precipitation.-Precipitating vessels should be deep, comparatively narrow, and broader at the bottom than at the top (see Fig. 282). This construction permits the precipitate to occupy less height in the vessel, by causing it to spread out upon the bottom; thus the supernatant liquid can be more thoroughly decanted off, the particles of the precipitate will lie in closer contact, and a better opportunity is given for the escape of imprisoned air or gas, which frequently exercises a buoyant effect on the particles and prevents their subsiding rapidly and closely. Upon the large scale, cedar tanks, of the shape just described, Precipitating may be used: these may have wooden spigots introduced,

jar.

a foot or so apart, so that the supernatant liquid may be conveniently run off.

FIG. 283.

Manner of conducting the Process.-If two solutions are used, and it is known that they contain the exact quantity of solid substance to react mutually without leaving an excess of one or the other, the order in which the liquids are mixed is immaterial; but when this is not the case, and the precipitant is to be added until precipitation ceases, it is necessary to proceed with caution. The precipitant is then added gradually, and, where acid or alkaline solutions are used, litmus-paper is useful in indicating the approach of an excess. In other cases the precipitate may be allowed to subside, and the precipitant slowly dropped into the clear liquid above until it is noticed that further addition is without effect. If the precipitate is too bulky to subside quickly, the whole may be vigorously stirred until thoroughly mixed, a small portion transferred to a small plain filter, and the filtrate tested by a further addition of precipitant. If this small portion is weighed, or measured, or is a known proportion of the whole, a simple multiplication will determine about the quantity necessary to complete the precipitation. Ammonia water is one of the most useful of alkaline precipitants, because an excess is at once noticed by the odor. This is easily done by blowing the air from the surface of the liquid, thoroughly stirring up the mixture, and then noting whether it smells of ammonia.

Plain filter.

The Production of Heavy and Light Precipitates.-Hot, dense solutions usually produce heavy precipitates, and such precipitates are more readily washed from adherent contaminating salts than those which are light and bulky. An additional advantage is, that they oc

cupy less space, and consequently their dose is less bulky. A good example of this is found in the manufacture and use of heavy and light magnesium carbonate.

Collecting and Washing Precipitates. In small operations precipitates are collected upon plain filters (see Fig. 283): the special advantages of such filters in this respect have been already pointed out on page 208. On a larger scale muslin strainers are generally used. These are suspended on frames, as shown in Figs. 284 and 285. In washing precipitates which

FIG. 284.

S

FIG. 285.

0

Collecting a precipitate.

Frame and strainer for precipitates.

are placed on strainers of this kind, care must be observed to close up the fissures which usually appear in the magma after it has been allowed to stand a short time, by stirring the precipitate thoroughly before adding more water. (See Lotion, Decantation, pages 208, 210.)

QUESTIONS ON CHAPTERS XIII. AND XIV.

SEPARATION OF IMMISCIBLE LIQUIDS AND PRECIPI

TATION.

What is a pipette, and how is it used?

What is a separating funnel, and how is it used?

Describe Dr. Mitchell's separator.

What is a Florentine receiver?

What is precipitation?

What is the separated solid termed ?

What is the substance which produces the precipitate termed?

What is the liquid which remains in the vessel above the precipitate called?

What are the objects of precipitation in pharmacy?

Give examples of each of these objects.

How and why is precipitation used in testing?

What is meant by the term "magma"?

What various methods are there of effecting precipitation?

What is the best shape for precipitating vessels?

If two solutions are used to produce a precipitate, in what order should they be mixed ?

If an acid and an alkaline solution are mixed, how may it be determined when the mixture is neutral, or nearly so?

tates?

Why is ammonia the most useful of alkaline precipitates?

How are heavy precipitates formed?

Are they more or less easily washed from adherent salts than light precipi

What advantage have heavy precipitates over light ones?