> I wonder if you can advice about the stability constant used in the PHREEQC.dat database for hydroxyapatite (-3.421). In the data base MINTEQ.dat this constant is much lower (given as -44.333 which corresponds to -7.149 if reaction is written as in PHREEQC.dat). The constant of MINTEQ.dat is similar to the constant given by Lindsay (1979) which is -7.14 (using the same presentation of the dissolution reaction). Data of my research with soils seems to fit well with the constant used in PHREEQC.dat, but I had difficulties to point a reference for this constant. I would appreciate if you could inform me of a reference for this constant. The number comes from wateq4f (Ball and Nordstrom, U.S. Geological Survey Open-File Report 91-183), which lists Brown (1960) as the source of the data (with conversion of the reaction to H+ from OH-). Brown, W.E., 1960, Behaviour of slightly soluble calcium phosphates as revealed by phase-equilibrium calculations: Soil Science, V. 90, p. 51-57. David David Parkhurst (dlpark@xxxxxxxx) U.S. Geological Survey Box 25046, MS 413 Denver Federal Center Denver, CO 80225 Project web page: https://wwwbrr.cr.usgs.gov/projects/GWC_coupled
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