Second Limit of Hydrogen–Oxygen Mixtures by the Withdrawal Method

Abstract

DURING the course of preparations for the study of the inhibiting action of hydrocarbons on the second limit of hydrogen–oxygen mixtures, an unexpected influence of rate of evacuation has been found when using the withdrawal method in clean ‘Pyrex’ vessels. This effect is particularly marked at higher oxygen concentrations, and with a pre-mixed H₂–2O₂ mixture, no explosion occurs if the evacuation-rate is reduced below a critical value of the order of 1 mm./sec. The same critical influence is observed if the oxygen concentration is reduced by replacement with nitrogen, though here the pumping speed required is lower, and the second limit is reduced rather than entirely eliminated. Typical results are shown in the accompanying graph. The evacuation-rate was controlled by interchangeable capillaries, and the abscissa represents the time for the capillary to reduce the pressure from 110 mm. to 90 mm. mercury, this forming a convenient method of characterizing the capillary. Similar results are obtained in various-diameter clean ‘Pyrex’ vessels (13–40 mm.) and between 460° and 500° C, but the effect is completely absent in vessels coated with potassium chloride. It is hoped to publish a full account of the results later.