Definition of a phase

[luicho]

Hi People,

I have two questions about the definition of phases in a database:

1-)   In an old post in the mail archive, I found a topic related on modeling a redox reaction,

http://wwwbrr.cr.usgs.gov/projects/GWC_coupled/phreeqc/mail/msg00127.html

Here, Biswajit  modified the phreeqc database and included all species of chromium. My question is if the definition of the phase of Cr_hydroxide was correct (in the archive attached to the post and as below) ?

     Cr_hydroxide
        Cr(OH)3 = Cr(OH)3
        log_k          -6.84

2-) In the llnl.dat I found a Phase of "Fe" and just to confirm, That is the phase of Fe elemental (cero valent), right? (because it was told me that it wasn't).

Thanks

[dlparkhurst]

(1) You will have to look in the literature. minteq.dat and minteq.v4.dat have values for an amorphous and crystalline form. You will need to rearrange the reactions to compare to this number.

(2) Yes. Fe is elemental Fe in the PHASES definitions. Note that it will produce very reducing conditions and decompose water.

[luicho]

Hello David,

Refering to the first Question, effectively, in the paper "Chromium (III) hydrolysis constants and solubility of chromium (III) hydroxide" by Rai, Saas and Moore, 1986, they found the log_k for Cr(OH)3(s) =  Cr(OH)3^0,  was -6.84

but, in the book "Chemestry" by Raimond Chang - ninth-edition, I found the ksp for Cr(OH)3 and was  "Cr(OH)3 = 3.0x10-29" giving a log_k= -28.52

So, what could I take for correct log_k for the Cr(OH)3? 

[dlparkhurst]

The balanced chemical reaction makes a difference. The log K for this reaction

Cr(OH)3(s) = Cr(OH)3(aq)

is different from

Cr(OH)3(s) = Cr+3 + 3OH-