Rapid Method for Detecting Small Amounts of Air Pollutants

In a number of technological processes, and also for purposes of safety engineering, it is essential to be able to register, in a timely and fail-safe manner, the appearance or change in concentration of certain impurities in air (or another gas). Such rapid monitoring presents considerable difficulties even in the case of high concentrations of an impurity. All the more important is the development of methods of rapid monitoring for impurities present in small quantities.

The author of the note under review*) has developed an ingenious and, apparently, effective method for solving this problem. The idea of the method is that the presence of an impurity changes the surface potential of a metallic plate blown by air, and consequently, from the technical point of view, the problem reduces to continuous and sufficiently accurate measurement of the surface potential. This is accomplished as follows. A metallic tongue, making harmonic oscillations, is placed above the surface of the metallic plate whose potential is measured. As a result, an alternating potential difference arises between the plate and the tongue; this is then fed through an amplifier and detector to a recording device. A change in the chemical composition of the gas mixture blowing over the plate is immediately reflected in the surface potential and thereby in the measured potential difference. As an example the authors give Fig. 1, which is a recording showing

Fig. 1.

the change in the surface potential of a steel plate when ethyl chloroacetate is added to the air blowing over it in an amount of 0.0002 percent by weight (the zero line corresponds to pure air). Attention is drawn not only to the considerable magnitude of the effect, but also to the rapidity with which it appears, and, most importantly, to the complete reversibility of the phenomenon: 5 seconds after the air has been purified of the impurity, the potential assumes its initial value.

The author points out that, when the impurities are polar gases of the type of ether, ethyl nitrite, or nitrobenzene, a change in surface potential corresponding to a signal-to-noise ratio equal to 5/1 is caused by relative concentrations of impurities

) G. Phillips, Nature 165*, 895 (1950).

on the order of \(10^{-7}\) by weight, and the smallest distinctly detectable concentrations are less than \(10^{-8}\) by weight. A considerably smaller, but quite measurable, effect is produced by nonpolar molecules such as benzene, octane, or carbon tetrachloride.

Depending on the kind of impurity, as well as on the nature and character of the surface treatment, the magnitude and sign of the change in surface potential may be different and, thus, conditions most favorable for solving a given specific problem can be selected.

Fig. 2.

A typical calibration curve for such a device is shown in Fig. 2.

Of substantial importance, of course, is the stability of the zero reading corresponding to clean air. The authors indicate that in their experiments there was a zero drift of \(0.1\) mV in 2 hours and \(7\) mV in a week. In other words, the device is sufficiently stable, but requires periodic checking of the zero position. It should be supposed that, after appropriate technical improvement, the described method will find practical application.

R. G.