It is not improbable that in almost every mixture dispensed chemical change of some kind takes place, either immediately or after a few hours or days have elapsed. There may be a precipitate or change of colour to make the interchange of molecules visible to the eye, or a decomposition may occur in which the products have the same physical characteristics as the original compounds. Even the simple solution of a single salt may involve dissociation, or perhaps chemical combination with the solvent, or some change due to the presence of the ubiquitous microbe may arise. The dispenser should be aware of all these changes, and know in each case whether the prescribed intention is that the reaction should be accelerated or retarded.

In the great majority of cases involving chemical change it is desirable to retard or prevent the reaction as far as possible. This is usually accomplished by separately diluting the opposing ingredients and interposing any protective fluids {e.g., mucilage, glycerin, or syrup) that may be ordered. They often prevent the occurrence of a precipitate, and usually retard chemical change.

Mix the tincture and syrup with 2 ounces of the water, and pour into the remainder of the water in which the lead has been previously dissolved.

If the liquid extract be poured into a solution of the ammonium carbonate, a lumpy mixture results. The liquid extract should be stirred with the glycerin, and diluted to about 3 ounces; then the carbonate of ammonium, dissolved in the remainder of the water, should be added.

The following was a favourite prescription of a consulting physician:

If the old-fashioned plan of 'putting everything in first and then filling up' be followed, a thick mixture will result. If, however, the dialysed iron be diluted with 4 ounces of the water, and the liquor arsenicalis mixed with the remainder and added gradually to the former solution, a beautifully bright mixture results.

Some mixtures are clear at first, and gradually throw out a precipitate, e.g.:

This is clear at first, but slowly deposits bismuth carbonate. The presence of an excess of ammonium citrate retards the precipitation.

Some iron solutions deposit basic compounds on dilution:

This remains a clear solution for a time, then begins to deposit basic phosphate of iron; large excess of phosphoric acid completely prevents the change.

This remains bright for several days, and then a copious deposit of ferric hydrate gradually appears, while the solution loses the deep-red colour characteristic of ferric acetate. An excess of acetic acid prevents the change.

These examples sufficiently illustrate the importance of carefully considering the order of mixing. It is useful to note the following:

First, that where syrup, glycerin, honey, or mucilage is ordered along with fluids which decompose each other or which produce unsightly combinations, it is highly probable the pre-scriber has anticipated this result, and added this particular ingredient to avoid or mitigate the evil. Glycerin has in many cases a powerful influence in preventing decompositions, as well as in preventing depositions; syrup, less so; while honey and mucilage are favourable to fine division and suspension of insoluble salts and organic matter.

Second, that where any alteration takes place producing unsightly mixtures, as in the case of resinous solutions, the best result is obtained by pouring the tincture through a dry funnel into the bottle containing all the other ingredients.

Third, in no case should liberties be permitted in the shape of additions to or subtractions from prescriptions, with a view to producing what is called 'elegant pharmacy.' Cases where such expedients are necessary are very rare, and even in these the error is generally due to oversight on the part of the pre-scriber, and is so apparent that the dispenser cannot possibly have any difficulty in the matter.

A common case of chemical incompatibility is in prescriptions containing potassium iodide and spirit of nitrous ether:

This mixture cannot be dispensed without reaction between the potassium bitartrate and iodide and spirit of nitrous ether, iodine and nitric oxide being liberated, thus:

KHC4H406 + C2H3NO2 + KI = K2C4H4O6 + C2H5HO + NO + I.

If the dispenser can communicate with the prescriber, he should inform him that the mixture will contain free iodine, and also that it will not contain a particle of nitrous ether; if consultation is impossible, proceed as follows: Dissolve 1 drachm of cream of tartar and 8 grains of potassium iodide in 4 ounces of water contained in a mortar; add the spirit of nitrous ether, stir briskly, so that the gas may escape, and allow to stand for half an hour in order to get rid of the nitrous fumes entirely. Then make up the rest of the mixture and add it to the contents of the mortar. The object of this procedure is to limit the action of the nitrous ether, for while theoretically the 4 drachms will liberate the iodine from about 8 grains only of iodide, the liberated nitric oxide on coming into contact with air is changed to higher oxides, which are capable of decomposing iodide, so that if the mixture were made up in a 10-ounce bottle, iodine would continue to be liberated, until the whole of the iodide of potassium was decomposed.