This section is from the book "Practical Materia Medica And Prescription Writing", by Oscar W. Bethea. Also available from Amazon: Practical Materia Medica and Prescription Writing.
Incompatibility is a term used to express the condition where two or more agents when brought into contact result in a chemical decomposition, physical disassociation, or therapeutic opposition.
The various classes of incompatibility are grouped under three heads: chemical, physical (or pharmaceutical), and therapeutical (or physiological).
Excellent treatises on this important subject can be obtained. Some of them can hardly be improved upon for completeness, but the average student is apt to find difficulty in gleaning the actual necessities and becomes bewildered by the very completeness that to some is desirable.
As many of the phases of incompatibility are of minor interest to the prescriber, it will here only be treated under the following heads:
In Liquids.
I. Formation of a precipitate:
(a) The separation of a metal or insoluble metallic salt by chemical action.
(b) The separation of an alkaloid or insoluble alkaloidal salt by chemical action.
(c) Precipitation due to other causes. II. The evolution of a gas.
III. Separation of an immiscible liquid.
IV. Formation of a gelatinous mass. V. Incomplete solutions.
VI. Immiscibility.
In Solids.
I. Chemical changes. II. Undesirable pharmaceutical mixtures.
I. Formation of a Precipitate.
It may be broadly stated that when two or more substances in solution are brought together, if by an exchange of radicals an insoluble substance can be formed or a gas evolved-that exchange will take place. The matter resolves itself, therefore, largely into a study of solubilities.
In ordering two or more solids in solution the prescriber must reason,"Are all soluble in the vehicle ? Can an insoluble substance be formed from the bases and radicals used ?"
For example:
Zinci Sulphatis | |
Plumbi Acetatis.................................................. | gr. x |
Aquae....................................................................... | .q. s. |
Is zinc sulphate soluble in water? Yes. Is lead acetate soluble in water? Yes.
Can an insoluble substance be formed by an exchange of radicals? Yes; lead sulphate, which is insoluble, can be formed. Will the exchange take place ? Certainly.
Potassii Iodidi.................................................... | 3iv |
Sodii Bromidi....................................................... | 3iv |
Aquae.................. | q. s. |
Is potassium iodide soluble in water ? Yes.
Is sodium bromide soluble in water? Yes.
Can an insoluble substance be formed (or gas evolved) by an exchange of radicals? No. If potassium bromide and sodium iodide were formed they would still be soluble in water. Will an exchange take place? No.
A precipitate is not always undesirable, so this must be taken into consideration by the prescriber The precipitate resulting in the following mixture does not spoil the value of the remedy:
Plumbi Acetatis, | |
Tincturae Opii, | |
Aquae..................................................................... | q. s. |
The following would be undesirable:
Strychninae Sulphatis..................................... | gr. j |
Potassii Iodidi..................................................... | 3iv |
Aquae....................................................................... | q. s. |
The matter of precipitation resulting from chemical action is often made to appear complicated by giving a large amount of information as to the solubilities of chemicals without sufficiently emphasizing the small part that is of practical importance.
The salts of these are soluble, so need cause no fear that they will be precipitated.
Soluble except the oxide and carbonate, which are sparingly soluble, and the phosphate, which is insoluble.
Seldom prescribed in solution except the citrate and sulphate, which are freely soluble and usually not ordered with other chemicals.
The insoluble oxide is often ordered in suspension.
The acetate, chloride, citrate, iodide, nitrate, sulphate and sulphide are soluble; so care need be used only when prescribing with acids or salts that will form other combinations.
Not often used in solution except the soluble calcium chloride, this being unassociated with other chemicals, and strontium bromide and iodide, which are soluble and are usually prescribed alone or with other bromides or iodides.
The important soluble salts are the acetates, bromides, chlorides, citrates, iodides, nitrates, and sulphides; so care should be used as to other combinations.
Seldom prescribed in solution except as follows:
Zinc as the soluble acetate, chloride, or sulphate, which are used alone or with other acetates, chlorides, or sulphates.
Manganese as potassium permanganate, which is soluble and not often used with other chemicals.
Copper as copper sulphate, which is soluble and used alone or with other sulphates.
The soluble salts of interest are the acetates, bromides, chlorides, citrates, iodides, nitrates, and sulphates; so care should be used where other combinations might be formed.
Seldom prescribed in solution except the soluble corrosive chloride or the red mercuric iodide, which are often ordered with potassium iodide in excess, forming the soluble potas-siomercuric iodide. The corrosive chloride is also prescribed in solution with ammonium chloride or tartaric acid. Mercury is sometimes ordered in an insoluble form in water by prescribing the mild chloride or the corrosive chloride with lime-water.
Insoluble salts, as the salicylate, are sometimes used suspended in oil for hypodermic medication.
Mercuric acetate, bromide, chloride, citrate, nitrate, and sulphate are soluble. Mercurous salts are insoluble.
With a few exceptions, iron salts are not often ordered in solution with active chemicals. The principal source of trouble is prescribing ferric salts with vegetable drugs containing tannic acid, when a dark-colored precipitate is formed. Salicylates also give a strong color reaction.
The tincture of ferric chloride is frequently prescribed with potassium chlorate with an excess of water and with the solution of potassium arsenite. The tendency is and should be to prescribe iron in dry form or, when given in solution, to use it alone. Exceptions to this are the iron hypophosphite and glycerophosphate in the compound preparations, and the scale salts as in the elixir of iron, quinine and strychnine, wine of iron, etc. The insoluble reduced iron is sometimes ordered with alkaloidal quinine suspended in a heavy syrup.
 
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