Abstract
The thermomineral waters from Massif Central are iron rich waters of sodium bicarbonate type. The aim of this work is to provide more information about the geochemistry of iron, which is clearly related to the redox potential in this type of water. Thermodynamic calculations were used, and the distribution of iron was found to be controlled by the solubility of siderite. The oxidation potential that can be measured with calomel and platinum electrodes corresponds to an electrochemical equilibrium which involves a ferric hydroxide or oxyhydroxide. Finally E Pt values measured in the field are in good agreement with values predicted from the equilibrium between siderite and amorphous Fe(OH) 3, the association of which acts as a redox buffer. Dissolved oxygen is not in equilibrium with the ferrous-ferric couple and does not affect the measured potential. Even when of local theoretical significance as a thermodynamic variable, this redox potential represents only partial equilibrium conditions.
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