Adding Struvite to Database

[demanderx]

Cheers to all, I am trying to add Struvite to Phreeqc.Dat database or WATEQ4F as it is not present in any of those.

I start by saying that I am not very good with databases, but I give it a shot
So I added this lines to the database Phreeqc for example

Struvite            390
   MgNH4PO4:6H2O = Mg+2 + NH4+ PO4-3 + 6H2O
   log_k           -13.26

When I set this modified database to Phreeqc it pops an error saying

ERROR: Elements in species have not been tabulated, NH4

ERROr: Reaction for species NH4 has not been defined

ERROR: Calculation terminating due to input errors.

For what I understand and for what I could read, NH4 is not defined is that it? Do I neeed to Define NH4 as Master Species? what do I have to do to fix this errors that appeared? I tried to find any kind of tutorial to modify the database without screwing it up, but I couldn't find any or maybe I totally missed an obvious one. 

Thank you so much for any help you can provide

[dlparkhurst]

You are missing a plus sign.

   MgNH4PO4:6H2O = Mg+2 + NH4+     +       PO4-3 + 6H2O

[demanderx]

You are missing a plus sign.

   MgNH4PO4:6H2O = Mg+2 + NH4+     +       PO4-3 + 6H2O

Oh thanks didn't even noticed the "+" was missing

now the error message changed to

ERROR: Elements in species have not been tabulated, NH4+

ERROR: Reaciton for species has not been defined, NH4+

I guess somewhere on the database I have to define NH4+, I saw for example in WATEQ4F NH3 is defined through NH4+, by the follwing lines

#NH3                 26
        NH4+ = NH3 + H+
        log_k           -9.252
        delta_h 12.48 kcal
        -analytic    0.6322    -0.001225    -2835.76
        -gamma  2.5     0.0

Do I have to do something like this for NH4+? for what I am guessing Log_k is the equilibrium cosntant at 25ºC, right?

[dlparkhurst]

You should not need to define NH4+ if you are using phreeqc.dat or wateq4f.dat. The following should work.

PHASES
Struvite            390
   MgNH4PO4:6H2O = Mg+2 + NH4+ + PO4-3 + 6H2O
   log_k           -13.26
END
SOLUTION
EQUILIBRIUM_PHASES
Struvite 0 10
END

It will not work with Amm.dat. Ah, Tony Appelo tends to use the name phreeqc.dat for Amm.dat. In that database, NH3 is named Amm. If you are using a database where Amm is used, you should use the following definition.

PHASES
Struvite            390
   MgAmmHPO4:6H2O = Mg+2 + AmmH+ + PO4-3 + 6H2O
   log_k           -13.26
END

[demanderx]

thank you once again, the errors in database stopped
I guess I might have chosen the wrong Log_k as struvite still appears as unsaturated. Have to look better, to see if i'm forgetting anything

[rose24kwu]

May I know how you added struvite to the database?
I am wondering whether the value of log_k in "solution species" and "PHASES" should be the same or not?

[dlparkhurst]

No. Use PHASES to define a mineral. Use SOLUTION_SPECIES to define a species that exists in aqueous solution, like NH4+, the ammonium ion.