When we have obtained a mixture of ethyl iodide and methyl iodide from the alcohols by treatment with phosphorus and iodine, the quantity of methyl iodide can be estimated by a method described by G. Reif.1 This depends upon the fact that methyl iodide reacts in the cold with methyl sulphide to form trimethylsulphine iodide, whilst ethyl iodide does not form a similar compound. The trimethylsulphine iodide behaves as a salt of hydriodic acid, and may be estimated volumetrically by titration with standard silver nitrate solution.

Formic acid method.2 - This process is based upon the oxidation of the methyl alcohol to formic acid by means of hydrogen peroxide, and the estimation of the formic acid by the mercurous chloride reaction.

The sample is cooled to 5°, made alkaline with sodium hydroxide solution, and treated with successive portions of 3 to 5 c.c. of a 1 per cent. solution of hydrogen peroxide. This reagent is added first at intervals of thirty minutes, and afterwards at intervals of forty-five to sixty minutes, until no more gas is evolved. The oxidised liquid is then allowed to stand for eight hours.

At the end of this period the excess of hydrogen peroxide is destroyed by means of sodium thiosulphate, the solution being then acidified and distilled with steam. The distillate is collected in a receiver containing a suspension of calcium carbonate in boiling water. About 700 c.c. of the distillate having been obtained, it is filtered hot, the filtrate evaporated to dryness, and the residue of calcium formate, etc., heated for an hour at 125° to 130°. It is then dissolved in 100 c.c. of water, and the solution extracted twice with 25 c.c. of ether. The aqueous layer is mixed with 2 grams of crystallised sodium acetate, slightly acidified with hydrochloric acid, and heated with 100 c.c. of a 5 per cent. solution of mercuric chloride for two hours in a boiling water-bath. The precipitated mercurous chloride is washed by decantation with hot water, filtered on a Gooch crucible, washed again with hot water and then with alcohol and with ether, and dried to constant weight.

1 Arbeit. Kais. Gesund., 1915, 50, 50-56; in Zeitsch. angvw. Chem., 1916, 29, Ref. 82.

2 R.Schmiedel, Pharm. Zentr.-h., 1913, 54, 709-716; Analyst, 1913, 38, 508.

Each gram of Hg2Cl2 thus obtained = 0.0875 gram of H.COOH == 0.0678 gram of CH3.OH.