ABSTRACT

Thermodynamic condition necessary for the appearance of negative Ce anomaly has been calculated with the simplest assumption that cerianite (CeO₂) is a unique species of Ce(IV) in water system and that hardly soluble nature of CeO₂ is the direct cause of the Ce anomaly. Only negative Ce anomalies possess straightforward information on the chemical conditions responsible for the anomalies. The extent of Ce anomaly has been calculated on the basis that the maximum dissolved concentration of Ce is limited by the maximum of activity of Ce³⁺ (a_Ce3+) allowed by the formation of cerianite, which is determined mainly by pH and E_H. In the calculation the activities of Ce in any solid phases are assumed to be unity. The Ce anomaly line exhibiting a certain magnitude of negative anomaly was invented and drawn in the E_H-pH diagram. We then applied the Ce anomaly line to some natural samples (seawater, ground water, iron formation, etc.) showing negative Ce anomalies and found that the Ce anomalies fairly well reflect the E_H-pH condition of the aqueous phase. In applying the Cc anomaly line to natural systems, the key parameter is the activity of Ce^∗3+, not the concentration of Ce^∗ (hypothetical Ce without Ce(IV) states). Too much credence, however, should not be given to the simple calculation, owing to the complication of the natural system and the lack of appropriate thermodynamic data.