Fugacity control and dissociation constants of HBr and HI

Two new hydrothermal acid-base buffers have been developed and calibrated. The Ag+AgBr and the Ag+AgI buffers in conjunction with an external hydrogen buffer control the fugacities of HBr and HI respectively at the pressure and temperature of the experiment. Both buffers are much less acid than the Ag+AgCl buffer of Frantz and Eugster (1973). For corresponding conditions, the molalities of HBr and HI are one and three orders of magnitudes smaller respectively than the molality of HC1.Both buffers have been calibrated using quench bromide, iodide and pH measurements. At higher temperatures HBr and HI are apparently not dissociated and the measured molalities agree closely with those calculated from thermochemical data. At lower temperatures the degree of dissociation increases greatly, and dissociation constants for HBr and HI at P and T have been calculated from the data.The Ag+AgBr and Ag+AgI buffers have been tested by measuring constants for the quartz-talc reaction in H-O-Br and H-O-I solutions respectively. These constants in conjunction with available thermochemical data can be used to calculate the free energies of formation for MgBr₂ and MgI₂.The Ag+AgBr and Ag+AgI buffers extend the pH range towards neutral solutions over which acid-base reactions in supercritical fluids can now be measured. This allows the use of less concentrated solutions and of minerals which are not stable in acid solutions.