A study of the mechanism of gaseous transfer across an air-water interface using carbon dioxide

Abstract

The rate constant for interphase molecular exchange was determined as a function of the relative velocity between gaseous and liquid phases. The experiment monitored the rate of dissolved ¹�CO₂ depletion from a thermally regulated water container in a variable speed wind tunnel. The results provide substantial evidence for the existence of a laminar layer of non-turbulent flow on the surface of natural bodies of water. This pseudo membrane, which prevents turbulent mixing at the solution surface and restricts gaseous movement to molecular diffusion, was found to vary in thickness inversely with the air speed in the tunnel. The experimental routine used to verify the existence of this surface membrane simultaneously demonstrated that carbon dioxide interphase exchange is uniquely enhanced by a pH dependent molecular-ionic species interconversion within this layer. The 230 x 30 x 25 cm experimental water vessel was filled with distilled water, adjusted for temperature and pH, enriched with tracer quantities of ¹�CO₂, and its surface swept lengthwise with a carefully controlled air stream. The depletion rate for the radioactive species was monitored for a spectrum of air speeds between 1.0 and 7.0 meters/second, for water temperatures of 20° and 25° C , and for pH regions of 3.0 and 6.5. For any established air speed and temperature, the gaseous interphase transfer rate was found to be significantly greater at the higher pH region. ...