Activity-product constants of aragonite at 90° and 51°C

The activity-product constants of aragonite, K_A=[Ca²⁺][CO₃^2-] (where the brackets denote activities), were determined experimentally at 90°C and at 51°C. Results at 90°C were obtained from four separate dolomite dissolution runs, in which aragonite precipitated and came to equilibrium with the aqueous phase (from the direction of. supersaturation), and from two runs starting with aragonite suspended in dilute MgCl₂ solutions (from the direction of undersaturation). The average values of K_A from the six runs were in excellent agreement; for the negative logarithm of K_A, PK_A, they yield a value of 8.95±0.05. One run was made at 51° C starting with aragonite suspended in dilute MgCl₂ solution; the value of pK_A at 51°C is 8.61±0.05. No X-ray detectable calcite was found at the conclusion of any of the runs, including a dolomite dissolution run at 90°C (D-10) that was sampled over a period of 10,268 h and toward the end had a concentration of approximately 0.002 molal total dissolved magnesium.