This section is from the book "Materia Medica Pharmacy, Pharmacology And Therapeutics", by W. Hale White. Also available from Amazon: Materia Medica Pharmacy, Pharmacology And Therapeutics..
This occurs when the mixture of the substances will not form a clear solution; e.g., insoluble powders and oils will not mix with water, the addition of which, to some spirits and all resinous tinctures, and to fluid extract of male fern causes a precipitate; if an acid mixture is flavored with liquorice, the acid precipitates glycyrrhizin; an alcoholic solution added to chloral hydrate causes all the chloral to rise to the top.
In such cases the aqueous solution may be thickened so that the precipitate is suspended in it to form an emulsion, but even then the mixture must be shaken before a dose is taken. Mucilage of acacia, freshly made, is the best emulsifying agent. The substances incompatible with it are mentioned on p. 22. It should be made perfectly fresh. The addition of a little almond oil improves its appearance.
1 pt. of most fixed oils requires of acacia 3/4 pt., water 1 pt. 1 pt. of balsam of Peru " 2 " 1 1/2
1 pt. of oil of turpentine " I "I
Tragacanth because its preparations keep better is often used to form an emulsion or a suspension, and sometimes yolk of egg or milk are employed. Liquor Potassae much facilitates the admixture of fixed oils and water although it often acts chemically on the ingredients of the prescription. Tincture of senega aids the emulsification of any oil, even in small quantities,
x; .60 c.c., being sufficient for an ounce; 30. c.c., of a fixed oil. Extractum quillajae, one grain; .06 gm., dissolved in one ounce; 30. c.c., of water, will make a tolerably permanent emulsion with one ounce; 30. c.c. of fixed oil, or one drachm; 4. c.c., of oleoresin. Magnesium carbonate is employed to aid the diffusion of an oil in water through which air is to be inhaled. Resinous tinctures require an emulsifying agent; an equal part of mucilage of acacia is the best. The suspension of oil of turpentine in mucilage of acacia is a very common non-official example of an emulsion.
(c) Pharmacological Incompatibility; e.g., the combination of purgatives with astringents. Sometimes this is intentional, as in the occasional addition of atropine to a hypodermatic solution of morphine. After the description of each drug, those that are incompatible with it will be enumerated.