Talcum, Talcum Purificatum.
Other tomes: Cook
Synonyms.—Creta Gallica; French Chalk.
Talc is a native hydrated silicate of magnesium, and is usually purified before use. It is official in the U.S.P. It occurs as a white or greyish-white powder or greyish-green irregular masses of waxy lustre, without taste or odour, and permanent in air. Rubbed upon the skin it imparts a feeling of greasiness. Specific gravity, 2.2 to 2.8. If 1 gramme be boiled with 25 mils of diluted hydrochloric acid for thirty minutes, water being added from time to time to replace that lost by evaporation, the filtrate should yield, when evaporated, ignited and quickly weighed, not more than 5 centigrams of residue.
Insoluble in water, and in dilute solutions of the acids and alkali hydroxides.
Uses.—Commercial native talc, although largely used for technical purposes, is not suitable for pharmaceutical work on account of the impurities present. If required for this purpose, purified talc should be employed.
Purified talc is prepared by the following method:—Mix 100 of talc in fine powder with about 500 of boiling water, gradually add 10 of hydrochloric acid, and boil the mixture for fifteen minutes, then set aside for fifteen minutes. Decant and reject the supernatant liquid containing the finer particles of talc in suspension, and again boil the residue with 500 of distilled water mixed with 5 of hydrochloric acid, set aside for fifteen minutes. Again decant and reject the supernatant liquid, and wash the residue with distilled water until quite free from chlorides. Transfer the magma to a linen strainer drain, and dry at a temperature of 110°. Purified talc is official in the U.S.P. It occurs as a white, tasteless and odourless powder, which is permanent in air. When subjected to ignition at a red heat it should lose not more than 5 per cent. of its weight. If 10 grammes be boiled with 50 mils of distilled water for thirty minutes, water being added from time to time to replace that lost by evaporation, the filtrate should be neutral to litmus, and 25 mils of this filtrate, when evaporated and dried at 110°, should yield not more than 5 milligrams of residue (limit of soluble substances). The remaining half of the filtrate, after slightly acidifying with hydrochloric acid, should not yield a blue colour on the addition of potassium ferrocyanide (absence of iron).
Uses.—Purified talc in the finest powder is used as a dusting powder to allay irritation and prevent chafing from friction; for this purpose it is often mixed with zinc oxide, boric acid, or starch, and suitably perfumed. It is also used as an aid in filtering turbid liquids containing finely-divided matters in suspension, which are apt to pass through the filter, or to stop up its pores. For this purpose the powder should not be too fine; a No. 60 powder, or perhaps No. 80 powder, is much superior to the finer bolted varieties used for dusting purposes.