Fig. 118.—Type of Spray Nozzles.
For accurate work, it is necessary to work with a pure sample which is at least fairly volatile. Mixtures cannot be run by this method. In this case it is necessary to volatilize each separately, passing the vapors simultaneously into the mixing chamber E.
A spray method may also be used with satisfactory results. Types of sprays are shown in [Fig. 118].
Odors
Because of the great value in detecting low concentrations of gases in the field, it is important to know the smallest amount of a gas that can be detected by odor. In some cases, this test is more delicate than any chemical test yet devised.
Odors may be divided into two classes, true odors, and mild irritation. By true odor is meant a definite stimulation of the olfactory nerve, giving rise to a sensation which is more or less characteristic for each substance producing the stimulation. Mild irritation defines the sensation which is confused with the sense of smell by untrained observers, but which is really a gentle stimulation of the sensory nerve endings of the nose. This so-called odor of substances producing this effect is not characteristic. Higher concentrations of these compounds almost invariably cause a definite irritation of the nose.
Examples of true odors are the mercaptans, mustard gas, bromoacetone, acrolein, chlorine and ammonia. Substances which cause mild irritation are chloroacetone, methyl dichloroarsine, ethyl iodoacetate and chloropicrin.
In making the test for odor, the same apparatus is used as for lachrymators. The time of exposure is shortened to 30 seconds, as the subject always detects the odor at the first or second inhalation.
In this connection the recent work of Allison and Katz (J. Ind. Eng. Chem. 11, 336, [1919]) is of interest. They have designed an instrument, “the odorometer,” for measuring the intensity of odors in varying concentrations in air. It is based on the principle given above. A measured volume of air is passed through the liquid and then diluted to a given concentration. The mixture is then passed through a rubber tube with a glass funnel at the open end. Only one inhalation of the mixture is used to determine the intensity of the odor. The position of any strength of odor on the scale depends upon the sensitiveness and judgment of the operator, but with one person conducting the entire test, the results have been found quite satisfactory. ([See tables on pages 360 and 361].)