Simultaneous transfer across an air-water interface of gases that interact through acid-base reactions

Abstract

Abstract Theory is developed to describe how the transfer of gas across an air-water interface may be influenced by the simultaneous transfer of a different gas with which it interacts through acid-base reactions. The theory is applied to the problem of NH 3 loss from lowland rice-field flood water as influenced by the simultaneous loss fo CO 2 . It uses a two-thin-layer model of the interface and describes the diffusion and reaction of gases and dissolved species, and of acids and bases that move in response to the pH changes that result from gas diffusion and reaction. Reaction kinetics are allowed for. In the rice-field system, NH 3 loss tends to make the solution more acidic, whereas CO 2 loss makes it more alkaline. Over the range of conditions found in rice fields, NH 3 loss may be limited by diffusion in both air and solution; CO 2 loss is always limited by diffusion in solution. When the equilibration between CO 2 species is uncatalysed, acidification resulting from NH 3 loss dominates alkalinization resulting from CO 2 loss, and the pH at the air-water boundary may be as much as one unit lower than that in the bulk solution. But in the presence of carbonic anhydrase, the situation is reversed and the boundary pH may be considerably greater than that in the bulk, especially at high CO 2 pressures.