The Dissociation of Water in the Glow Discharge

Abstract

The theory that ionization in the dark space increases exponentially with distance from the cathode, is applied to the dissociation of water vapor. Experimental results are given, which it is shown can be explained quantitatively if the total energy of the electrons generated in the dark space is taken to be $\int 0^{s}e(V_1-V)dn$ where $S$ is the width of the dark space, $V_1$ is the total cathode potential drop, $V$ is the space potential, and $n$ is the number of electrons per element of volume, the exponential increase of which is given by Townsend's equation. The experimental data include measurements of the rate of dissociation for currents from 1 to 28 ma, electrode spacings from 1 to 9.75 cm, and a pressure of 0.75 mm of mercury, also measurements of the cathode potential drop and the width of the Crookes dark space. Possible reaction mechanisms are discussed. The net energy per dissociation has an almost constant value of 11 volts, whereas the amount of dissociation ranges from 4.78 to 5.90 molecules per electron in the dark space and negative glow, depending upon the magnitude of the cathode drop. Probe measurements in the positive column give an electron density of 7.95 × ${10}^7$ electrons per ${\mathrm{cm}}^3$, and an average energy of 3.71 volts per electron. The probability of dissociation by an electron in the positive column having an energy greater than 7.6 volts, is computed to be 0.00115.