The following method of testing iodine that has been adulterated with water, is given in . the " Industrie Suisse." It is well known that the dearer a chemical becomes the more it is adulterated, and from the increasing rise in the price of iodine, the attempts to adulterate it are becoming more numerous. The greater part of the substances employed for this purpose being neither soluble in alcohol nor susceptible of volatilization by heat, it is easy to discover them and to determine the quantity by the ordinary processes. Adulteration by water is the most frequent, and, at the same time the most difficult to discover with certainty, amounting, according to some writers, to as much as 10 and 12 per cent. Pulverized iodine, in fact, absorbs a large quantity without appearing humid, and the means of determining the proportion in which this substance is adulterated with water are very defective. Although iodine requires a higher temperature than water to reach the boiling point, the latter does not evaporate without drawing off at the same time a noticeable part of iodine, which partakes, with other bodies, in the property of easily evaporating when exposed to the steam of water, although much less volatile. N. A. Chevallier advises placing a certain quantity of iodine that has been previously weighed, between some sheets of blotting paper, to press it firmly, and then weigh it over again. It is clear, without being necessary to make any attempt to prove it, that there must be lost, in this manner, a large quantity of iodine, and that the resulting proportion would not be exact. Another process consists in pulverizing the iodine with a double weight of chloride of calcium, to place the whole in a tubular retort and to heat to 180 (Celsius). The iodine will change to vapor, and the chlorine remain colorless with the water. Only it is very important to take precautions that the water itself should not evaporate, this method has, however, its difficulties, for it requires a long time to loosen the neck of the retort. The following method is short and simple, as well as tolerably exact : Weigh about 1 dwt. of iodine in a small open porcelain vessel, and after having done so, leave it on the scale. Add to this half an ounce of mercury, and afterwards place the pestle of a small agate mortar in like manner in the vessel, and determine the whole weight. Then take it, altogether, off the scale and pound up the iodine and mercury with the pestle; care must be taken to place the vessel on a sheet of white paper and to hold it with the left hand. Pound it up until the odor ot the iodine has completely disappeared, the co-,.lor that the mass assumes at first is a reddish brown, when it suddenly becomes thick, and exactly resembles an amalgam. It appears indispensable to obtain this latter result, that the mixture should be made in such a manner that the mercury be added to the iodine at least in._the proportion of 7 to 1. That is to say, that the quantity of mercury should be much greater than in the preparation- of yellow mercurialized iodine, where a similar phenomenon is not manifested. When these has been obtained a perfect union of the parts, the vessel is placed on a warm water bath, and after some time weighed again. The operation may be repeated without any evidence of a diminution of weight, especially if care- has been taken to leave the vessel in the bath for half an hour before the first weighing. In the course of pounding, the entire mass is somewhat heated, and there results from this the loss of a small quantity of water, whilst the iodine, on the contrary, is very little volatilized by the operation. It is known that after the uniting again of the iodine with the mercury in the water bath, the former is no longer volatile ; this fact is, moreover, proved by the piece of sized paper placed over the vessel as a cover. The inventor of the process, Dr. Bolley has made a course of experiments, which do not differ much in the results when the same iodine was used.
This article was originally published with the title "Process for Testing Iodine" in Scientific American 8, 19, 152 (January 1853)