Re: Questions

["David L Parkhurst" <dlpark@xxxxxxxx>]

(1)   DATABASE C:\Programme\USGS\PhreeqcI2.3\my_phreeqc.dat
ERROR: DATABASE must be the first keyword in the input file.

Your input file may have two DATABASE data blocks. The first does not show
up in the PhreeqcI editor because PhreeqcI reads the DATABASE line and sets
the default database internally. If you can see a DATABASE data block in
your input file in PhreeqcI, delete it and one will be included
automatically. It also may be that the database file has a DATABASE data
block, which it should not. If it does, use WordPad to remove it. If you
edited the database file in PhreeqcI, it will have inserted a DATABASE data
block. This is probably a bug in PHREEQC. DATABASE data blocks should be
ignored as the database file is being read.

(2)  Checking a book (e.g. Werner Stumm, James J. Morgan, Aquatic Chemistry
an Introduction Emphasizing Chemical Equilibria in Natural Waters. John
Wiley &Sons, New York Chichester Brisbane Toronto 1981 P245) or some other
books, hydroxyapatite has a solubility constant of log K = -57 at 25°C,
I=0, i.e.

Ca5(PO4)3OH(s)    5Ca2+ + 3PO43- + OH-      log K=-57

But according to the calculation result of phreeqc, a datum of logK=-54
must have been used. If it is true, this of course can be understood. But I
am not sure whether my judgement is correct. Could you please tell me where
people got data when this phreeqc.dat database was written?

I think the log K does work out to be in the range of -54 for the same
reaction. To check, you need to take the reaction given in the database and
add (or subtract) the appropriate aqueous association reactions and their
log K's. There is a wide variation in log K for apatite in the literature.
The data in phreeqc.dat and wateq4f.dat according to the Wateq4f
documentation come from Brown, W.E., 1960, Behavior of slightly soluble
calcium phosphates as revealed by phase-equilibrium calculations: Soil
Science, v. 90, p. 51-57.


David Parkhurst (dlpark@xxxxxxxx)
U.S. Geological Survey
Box 25046, MS 413
Denver Federal Center
Denver, CO 80225