The compounding of mixtures brings all the dispenser's wits into full play. It is here that medicines have the opportunity of combining with each other, because mixtures are in the large majority of cases aqueous solutions of inorganic or organic substances, hence the conditions are the best for interaction, water being the great promoter of chemical change.

In dispensing a mixture- as, indeed, in every other phase of the art of dispensing- the primary consideration should be to get a correct appreciation of the intention of the prescriber. This object should be kept in view particularly when dealing with a supposed error on the part of the prescriber, or with the necessity or otherwise of making any alteration in the prescription. Long experience, both in dispensing and of the habits of prescribers generally, alone confers the ability to decide a question of this kind; and no alteration should be made which will in any way change the therapeutic character of the medicine without first consulting the writer of the prescription. If this is impossible, the dispenser must be guided by his experience, following the line he would adopt if he purposed taking the medicine himself.

Having decided that the prescription is free from error, the next consideration is whether any chemical decomposition is likely to occur, and, if so, is it the preserver's intention that such should be favoured or retarded? This point being settled, the prescription should then be dispensed in such a manner that the dosage from first to last will be practically uniform. The general rule to adopt is that chemical action among the ingredients of a prescription should be prevented or retarded as far as possible, unless the reverse is clearly ordered or intended by the prescriber.

In a great many instances the ingredients of a prescription require to be mixed in some special order so as to produce the best result, and it is desirable in such cases to record the modus operandi in the copy, so that, when repeated, the same method shall be followed. In the absence of any particular indication of this kind it is best to commence by putting half the vehicle into the bottle, then to put in the soluble solids, triturate insoluble solids in a mortar with a little of the vehicle and transfer to the bottle, put in the fluids, fill up, cork the bottle, and shake thoroughly. If the salts to be dissolved are likely to contain specks of dust or insoluble matter, it saves time to dissolve them in a measure and strain the solution into the bottle: this saves pouring out of the bottle and back again.

Always see if the bottle used is of correct capacity, or make up to the quantity required in a measure before pouring into the bottle.

Typical Mixtures

A typical mixture is like a quadruped- it has four legs to stand on. According to Pereira, these are the basis, the auxiliary (adjuvans), the corrective (corrigens), and the vehicle (excipiens); but the tendency is now towards simplicity, so that either the auxiliary or the corrective, and, what is of more importance to the pocket of the pharmacist, the vehicle, are frequently omitted.

When the quantity of solid ordered is within the limit of its solubility in the prescribed vehicle, it should be completely dissolved before sending out. Many salts- e.g., silver nitrate and sodium sulphocarbolate- although very soluble, dissolve but slowly in the crystalline form; if convenient, a small quantity may be kept ready in fine powder, otherwise, if the solution is wanted quickly, the crystals must be crushed in a glass mortar. Less soluble salts-e.g., potassium chlorate, sodium sulphate and phosphate, etc.- should be dissolved in water slightly warmed. A good example is:

Salicin is soluble in 28 parts of water, and glycerin increases the solubility; therefore dissolve the salicin in 7 ounces of water slightly warmed, and add the glycerin and tincture when nearly cold.

In the following prescription gallic acid is present in excess of what the amount of water is capable of dissolving at the ordinary temperature:

Rub the gallic acid to a fine powder in a mortar, add the water and the sulphuric acid, and dispense with a 'shake, the bottle' label. In this case the gallic acid dissolves with readiness in hot water, and some dispensers are apt to use such water, but the acid crystallises out on cooling.

Although the water alone would retain the chlorate in solution, the large amount of spirit present would cause it to crystallise out; the chlorate should therefore be finely powdered, and the bottle sent out with a "shake' label.

There are other substances with which a similar plan must be followed if sufficient solvent is not prescribed with them - viz., rub to fine powder and mix with the vehicle. It has been observed that potassium chlorate, when present in mixtures in excess, slowly becomes crystalline, due to changes in temperature: when the temperature goes up a little more chlorate is dissolved, and when it goes down that little crystallises out again.

Chloroform lessens the solubility of the salts, for, although the phosphate dissolves in 6 parts of water, it crystallises out of this mixture if the temperature falls below 6o° F.; it should, therefore, be finely powdered, and no heat used to complete solution.