Estimation of Methoxyl or Ethoxyl Groups

All the usual determinations of these groups are based on the original method of Zeisel, which consists in decomposing the substance with hydriodic acid thus — 

The resulting methyl (or ethyl) iodide is converted into silver iodide by the action of alcoholic silver nitrate, and the number of methoxyl (or ethoxyl) groups calculated from the weight of silver iodide formed. 

The hydriodic acid used is purified as described on p. 506, selecting for the determination the fraction of constant B.P. 127° and D. T68. The alcoholic silver nitrate, which is prepared by dissolving 4 gms. of silver nitrate in 10 c.cs. of water and adding 90 c.cs. of absolute alcohol, is preserved in a well-stoppered bottle in the dark, and should be filtered and acidified with one drop of nitric acid immediately before use. 

A reliable modification of the process for the estimation of methoxyl is that of the British Pharmacopoeia, 1932. The apparatus is shown in Fig. 80. A Pyrex flask (A) of about 100 c.cs. capacity, having a bulb (B) of about 70 c.cs. capacity blown on the side tube, contains the mixture of substance (about 0-2 gm.) and hydriodic acid (10 c.cs.). The side tube is connected through a smaller bulbed tube (0) to a set of bulbs (D, see Fig. 65), immersed in a water bath at 60° (95° for ethoxyl), containing red phosphorus suspended in a 2% aqueous solution of cadmium sulphate. To this is attached two absorption flasks (E), each containing about 20 c.cs. of the above alcoholic silver nitrate. To the flask (A) is 

The contents of the two small flasks are diluted with water, allowed to stand for 5 minutes to ensure decomposition of the last traces of alkyl 

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