Chemical name. Talc; talc [Mg₃H₂(SiO₃)₄]; CAS Reg. No. 14807-96-6.

Description. A white or almost white, very fine, homogeneous, unctuous powder; odourless.

Solubility. Practically insoluble in water, dilute solutions of acids and alkalis, ethanol (~750 g/l) TS, and ether R.

Category. Tablet and capsule lubricant; glidant; diluent.

Storage. Talc should be kept in a well-closed container.

Additional information. Talc adheres readily to the skin. Attention should be paid to its microbiological quality since it is of mineral origin.

Requirements

Definition. Talc is a powdered, natural hydrate of magnesium silicate that may contain variable proportions of aluminium and iron.

Identity tests

A. Transfer 0.5 g to a metal crucible, add 1 g of potassium nitrate R and 3 g of anhydrous sodium carbonate R, mix, and heat until melted. Allow to cool, add 20 ml of boiling water, mix, and filter. Wash the filter with 50 ml of water. Take up the residue with a mixture of 0.5 ml of hydrochloric acid (~420 g/l) TS and 5 ml of water and filter. To the filtrate add 1 ml of ammonia (~260 g/l) TS and 1 ml of ammonium chloride (100 g/l) TS and filter. To this filtrate add 1 ml of disodium hydrogen phosphate (100 g/l) TS; a white, crystalline precipitate is produced.

B. Using a copper wire mix 0.1 g with 10 mg of sodium fluoride R placed in a lead or platinum crucible, and add a few drops of sulfuric acid (~1760 g/l) TS to obtain a thin slurry. Cover the crucible with a thin, transparent, plastic plate from which a drop of water is suspended and warm gently; a white ring is produced around the drop of water within a short time.

Microscopic examination. Irregular plates, length up to 50 μm, are observed. Talc is free from microscopic asbestos fibres. A 1 mg/ml solution of methylthioninium chloride R in ethanol (~750 g/l) TS does not notably stain the particles.

Arsenic and heavy metals. Transfer 10 g to a 250-ml flask, add 50 ml of hydrochloric acid (0.5 mol/l) VS, attach a reflux condenser, and heat on a water-bath for 30 minutes. Cool, transfer the mixture to a beaker, and allow the undissolved material to settle. Decant the supernatant liquid through a thick, strong filter-paper of medium grade into a 100-ml volumetric flask, retaining as much as possible of the undissolved material in the beaker. Wash the beaker with three 10-ml portions of hot water, decanting each washing through the same filter. Finally, wash the filter with 15 ml of hot water, cool the filtrate and dilute to volume, and mix. Use this solution for the following tests:

- Use 10 ml and proceed as described under 2.2.5 Limit test for arsenic; the arsenic content is not more than 3 μg/g.

- Use 5 ml and proceed as described under 2.2.3 Limit test for heavy metals, Procedure 1; determine the heavy metals content according to Method A; not more than 40 μg/g.

Carbonates. Suspend 0.25 g in 40 ml of sulfuric acid (~100 g/l) TS; no effervescence is observed.

Acid-soluble substances. Digest 1.0 g with 20 ml of hydrochloric acid (~70 g/l) TS at 50 °C for 15 minutes, cool, add sufficient water to produce 50 ml, and filter. (If the filtrate is not clear, centrifuge it.) To 25 ml of the clear solution (keep the remaining solution for "Iron"), add 1 ml of sulfuric acid (~100 g/l) TS, and evaporate to dryness. Ignite the residue at 800 ± 25 °C and weigh; not more than 20 mg/g.

Iron. Acidify 10 ml of the filtrate obtained in the test above for "Acid-soluble substances" with hydrochloric acid (~70 g/l) TS and add 1 ml of potassium ferrocyanide (45 g/l) TS; no blue colour is observed.

Reaction and soluble substances. Boil 10 g with 50 ml of water for 30 minutes, adding water from time to time to maintain approximately the original volume, and filter; the filtrate is neutral to litmus paper R. Evaporate 20 ml of the filtrate to dryness and dry the residue at 105 °C for 1 hour; the mass of the residue does not exceed 4 mg (1 mg/g).

Loss on ignition. Ignite 1.0 g at 1000 °C to constant mass; it loses not more than 65 mg/g.