Twenty-two grams of coarsely powdered iodine and 5 c.c. of distilled water are placed in a small flask and cooled by immersion in ice-cold water. Then 5 c.c. of the wood spirit (at 60 o.p. strength) are added, the flask corked, the contents gently shaken, and allowed to remain in the ice-cold bath for ten to fifteen minutes.

When well cooled, 2 grams of red phosphorus are added to the mixture of spirit and iodine in the flask, and the latter is immediately attached to a reflux condenser.

The reaction soon commences, and must be moderated by dipping the flask into a cold water-bath. (Spirit may be lost if the reaction is too violent.) After about fifteen to twenty minutes, when all action appears to have ceased, the water-bath under the flask is the end of the operation only. The end of the condenser dips into water in a measuring tube, and the iodide is collected under water and measured at a temperature of 15.5°.

X gradually heated to a temperature of about 75°, and the flask being occasionally shaken is allowed to remain at this temperature for fifteen to twenty minutes. The source of heat is then removed, and the apparatus left for an hour till it has cooled, when the condenser is reversed, and the methyl iodide slowly distilled off - first at a low temperature, the bath being allowed to boil towards

Fig. 35. - battery of distilling apparatus. For estimation of methyl alcohol in wood-naphtha by the methyl iodide method.

The percentage (by volume) is found from the formula : c.c. methyl iodide found x 0647 x 100 / c.c. wood spirit taken

= percentage of methyl alcohol.

Or when 5 c.c. of spirit are taken: c.c. methyl iodide X 12.94 = percentage of methyl alcohol.

Esters and acetals also yield methyl iodide by this process, and from the percentage of methyl alcohol calculated as above, an amount equivalent to the percentage of these substances present must be deducted. Practically, however, methyl acetate is the only compound usually found in quantity sufficient materially to affect the result. The number of grams of methyl acetate per 100 c.c. of the spirit multiplied by 05465 gives the equivalent of methyl alcohol to be deducted from the total percentage by volume calculated from the methyl iodide found.

The method, it will be seen, depends upon the conversion of methyl alcohol into methyl iodide by the well-known phosphorus and iodine reaction: -

3CH3OH + P + I3 = 3CH3I + H3PO3.

The volume of the methyl alcohol is then deduced from that of the iodide produced. There are some small errors in the process, which, however, tend to compensate one another. Thus a little of the acetone may distil over and be partly dissolved by the methyl iodide, but on the other hand traces of the iodide are dissolved by the water, and traces of vapour of methyl iodide remain in the flask; the effects of these disturbing influences upon the volume of the iodide tend in opposite directions, and the net result is small. The degree of accuracy with which the volume of methyl iodide can be measured in an ordinary measuring tube is of more importance. Though not very great, it suffices for the purpose, and allows of the relatively simple method described being used instead of longer and more complicated processes.

A shortened form of the Zeisel process has been described by Hewitt and Jones as applicable to the determination of methyl alcohol in crude wood naphtha.1