At the present day much of the iodoform produced is made by an electrolytic method, as described further on. Formerly, however, it was exclusively prepared by warming a mixture of alcohol, iodine, and sodium carbonate or hydroxide. Later, acetone often replaced the alcohol as raw material.

The earlier process, or one form of it, consisted in heating gently a mixture composed of alcohol (95 per cent.) 1 part, iodine 2 parts, potassium carbonate 2 parts, and water 5 parts, until the liquid became colourless. The bright yellow, scaly precipitate of iodoform thus obtained was rinsed upon a filter, washed with water, and dried in the air.

1 Wade and Finnemore, Trans. Ghem. Soc, 1904, 85, 946.

2 J. Pharm. Chim., 1891, 24, 243. 3 Schering, D.R.-P. 29771. 4 Zeitsch, Elekrochem., 1904, 10, 409.

The yield thus obtained, however, is only about 17 per cent. of that corresponding theoretically with the iodine used. Some five-sixths of the iodine remains unconverted into iodoform-mainly in the filtrate, though a little is lost in the form of ethyl iodide vapour.

In order to utilise as much of the iodine as possible, various methods have been devised. The filtrate contains the iodine in the form of potassium iodide and iodate. Addition of a mineral acid liberates the iodine in accordance with a well-known reaction; and, further, any excess of potassium iodide still remaining will yield its iodine on treatment with potassium bichromate and an acid, or with chlorine. On these principles R. Rother1 based the following method.

Thirty-two parts of iodine, 32 of potassium carbonate, 16 of alcohol (95 per cent.) and 80 of water are mixed in a comparatively large flask, and warmed in a water-bath until the solution is decolorised. The clear liquid is now decanted and the precipitate rinsed, with the addition of a little water, upon a capacious filter. The decanted liquid is now treated with hydrochloric acid, added gradually at first, an excess of 16 to 24 parts being employed, together with 2 to 3 parts of potassium bichromate, to liberate the iodine. After a few moments, potassium carbonate is carefully added till the solution is neutralised, and then a further quantity, 32 parts, of the carbonate, together with 6 parts of iodine and 16 of alcohol: this mixture is again digested in the water-bath, and when effervescence has ceased in the green solution, the supernatant liquid is poured off from the precipitate, and further treated as before. The separated iodoform is rinsed into the filter with the first product, and when the operations are finally discontinued the collected crystals are well washed with water and spread out in the open air to dry.

Thus the process may be continued indefinitely, or until the accumulation of salts becomes inconveniently large.

In another method, suggested by Filhol,2 chlorine is used to recover the combined iodine. A solution of 2 parts of soda-crystals in 10 parts of water is mixed with 1 part of alcohol, the mixture heated to 60-80°, and 1 part of iodine added gradually. The heating is continued until the solution becomes decolorised. After the precipitated iodoform has been filtered off, the filtrate is mixed with another charge of sodium carbonate and alcohol, heated up again to the temperature stated, and a stream of chlorine led in. This liberates more iodine, which again reacts with the alcohol to give a further precipitate of iodoform. The process is repeated until the product obtained represents approximately one-half of the original amount of iodine.

1 Pharm. J., 1874, [iii], 4, 593. 2 J. Pharm. Chim., 1845, [iii], 7, 267.

Ozone is employed in Otto's process for obtaining iodoform.1 A mixture of 100 kilos. of water, 300 of alcohol, 10 of sodium carbonate, and 55 of potassium iodide is warmed to 50°, and ozone passed in until the whole of the iodide is decomposed.

In the electrolytic process iodoform is produced by electrolysing a solution of an iodide in the,presence of alcohol. Acetone or aldehyde can also be employed. According to Elbs and Herz,2 a solution of 13 to 15 parts of sodium carbonate, 10 of potassium iodide, 20 of alcohol, and 100 of water is suitable; this is heated to 70° and electrolysed, using a platinum anode and a nickel cathode, and a current-density (anodic) of 1 ampere per square decimetre. The current is passed for two or three hours, and after several hours the resulting iodoform crystallises out. It is removed periodically, and the solution made up again to its original composition by the addition of sodium carbonate, potassium iodide, and alcohol. A porous cell is used as diaphragm round the anode. According to Forster and Mewes,3 the reduction of the iodoform by the hydrogen generated in the electrolysis is insignificant. Parchment paper round the kathode can replace the usual earthenware cell with advantage: this decreases the resistance, and less electrical energy is consumed. It also helps to prevent the formation of iodate, by keeping the liberated caustic alkali from contact with the iodine set free at the anode. A stream of carbon dioxide should be passed through the solution to neutralise the caustic alkali formed, as the latter tends to prevent the formation of iodoform. A solution indicated as suitable in these circumstances is composed of 2 parts of calcined sodium carbonate, 2 of potassium iodide, 5 of alcohol, and 20 of water, the electrolysis being carried out at 50-70°. The current density at the platinum anode may be from 1 to 3 amperes per sq. dcm., and at the platinum kathode 4 to 8 amperes.

The most probable equation representing the formation of iodoform electrolytically is considered to be:-

C2H5 OH + I10 + H1O= CHI3 + C02 + 7HI.

1 D.R.-P, 109013, 1898. 2 Zeitsch. Elektrochem., 1897, 4, 113.

3 Ibid., 268; J. pr. Chem„ 1897, 58, 353.

The iodine arises from the action of nascent oxygen, liberated at the anode, upon the potassium iodide present.

Advantages claimed for the electrolytic process are that it utilises the iodine better than does the chemical process, and gives a purer product.

Iodoform is a yellow solid melting at 119°, and possessing a peculiar, characteristic odour. It is largely used in surgery as an antiseptic. In this country it is chiefly made from industrial methylated spirit.