Incompatibility between two substances may be said to exist when their admixture brings about physical or chemical change other than simple solution. Such a change - (1) may be desired in a prescription, (2) may make little, if any, difference, or (3) may be undesirable. A chemic reaction may result in a precipitate, may show merely in an alteration of color, or may make no visible change at all. But the physician should know in what form his remedies are when the patient takes them.

"Incompatibility" is a bugaboo raised for the alarm of the prospective prescription writer, arid it is an unnecessary alarm. For, though a great many incompatibles for almost any active chemical may be found in the laboratory, yet but few of these are ever likely to be encountered in a prescription; and of those few, the result not infrequently makes no practical change in the medicinal value, or is deliberately desired.

The following are those most likely to be encountered in the practical use of drugs:

I. Incompatibility Depending On Change Of Solvent

(A) Precipitate When Added To Aqueous Liquids

Substances in alcoholic solution and insoluble in water; as in spirits, fluid-extracts, and tinctures, especially resinous ones, like tincture of cannabis, benzoin, myrrh.

(B) Precipitate When Added To Alcoholic Liquids

Substances in aqueous solution and insoluble in alcohol; as solutions of many salts (sodium sulphate, ammonium chloride), ichthyol, and mucilage of acacia. Mere insolubility, as of oils or bismuth subnitrate in water, makes these really incompatible with the solvent; but such are considered under the head of "solubility."

II. Chemic Incompatibility. - Rule 1: Acids and salts of acid reaction are incompatible with alkalies and salts of alkaline reaction and the halogen salts, as hydrochloric acid or potassium bitartrate with sodium bicarbonate or magnesia.

. Rule 2: Highly oxidized substances, like chromium trioxide (chromic acid), potassium permanganate, and potassium chlorate are decomposed by organic matter. Potassium permanganate in solution turns brown; dry potassium permanganate or chromic acid may take fire or explode. Potassium chlorate, when rubbed with sulphur, hypophosphites, ammonium chloride, tannic acid or other organic substance, will explode violently.

Rule 3: Silver nitrate is incompatible with organic material and turns to black oxide or black metallic silver. With chlorides or hydrochloric acid it forms the insoluble silver chloride.

Rule 4: Mild mercurous chloride (calomel) is incompatible with sodium carbonate and lime-water. With the latter it makes a black precipitate of mercurous hydroxide, and forms "black wash," sometimes employed as an application to venereal sores.

Calomel is insoluble in water or alcohol, comparatively inert chemically, and bland to tissues.

Rule 5: Corrosive mercuric chloride (corrosive sublimate) is incompatible with iodides, many metallic salts, alkaloidal salts, tannic acid, lime-water, and albumin.

With excess of lime-water it makes a yellow precipitate of mercuric oxide, and forms "yellow wash," employed as an application to venereal sores. When the mercury salt is in excess, the precipitate is red oxychloride.

With soap, as on the surgeon's hands, its antiseptic power is destroyed.

With potassium iodide it forms mercuric biniodide - 2 KI + HgCl2 = 2 Hc1 + Hgl2. The iodide is of a brilliant scarlet and dissolves in excess of the potassium iodide. These two salts are often prescribed together to form the biniodide.

In albumin, as in white of egg or milk, we have the antidote when the drug is swallowed.

Rule 6: Lead acetate decomposes alum and other sulphates and the iodides, and tends to precipitate many organic substances, e. g., glucosides, from their solution.

The admixture with alum makes Burow's solution. The precipitate of lead sulphate should be filtered off. The precipitate with the iodide is lead iodide of a brilliant yellow.

Rule 7: Ferric salts - (a) make "ink" with tannic acid; (b) make blue to reddish or purple colors with compounds of the phenol group, such as phenol, resorcin, salicylates, etc.; (c) make a red color with acetates, and (d) form a dirty-brown precipitate with alkalies or alkaline salts.

Rule 8: Tannic acid is incompatible with alkaloidal salts, dry potassium chlorate (explodes), metallic salts, gelatin, and albumin. With ferric salts it makes "ink." For salts of alkaloids and antimony it is the local antidote.

It occurs in many vegetable drugs, and preparations of these may not only precipitate alkaloidal salts, but may change the gelatin coating of a pill or a gelatin capsule to a tough, leathery, insoluble substance. Alcohol, as in tinctures, may prevent the precipitation of alkaloidal salts by tannic acid.

Rule 9: Chloral hydrate decomposes to chloroform under the influence of strong alkalies; and when mixed with camphor, menthol, thymol, and similar substances, undergoes a physical change to a liquid.

Rule 10: Alkaloidal salts are incompatible with -

(a) Alkalies - the precipitate is the pure alkaloid.

(b) Tannic acid - the precipitate is the insoluble tannate.

(c) Iodine, iodides and bromides - the precipitate is the iodide or bromide.

(d) Mercuric bichloride - the precipitate is an insoluble double salt.

Quinine in addition is especially precipitated by salicylates and benzoates.

All these precipitates are more soluble in alcohol than water, so may not show in tinctures and other alcoholic liquids.

Rule 11: Glucosides are incompatible for the most part with lead acetate and tannic acid, and are decomposed by the mineral acids.