Sulfur Unit conversion

[Siggi]

Hello,

I am an absolute beginner with PHREEQC. By playing around a bit with it, I came across a problem with the conversion of the sulfur concentration from ppm to molality. The input file I used is the following:

SOLUTION 1
    temp      25
    pH        7
    pe        4
    redox     pe
    units     ppm
    density   1
    S         32000
    -water    1 # kg

-----------------------------Solution composition------------------------------

   Elements           Molality       Moles

   S                 3.441e-01   3.441e-01

However, by calculating the molality by my one, I come to a molality of 1.03mol/kgw, which is pretty intuitive since the molar mass of sulfur is 32.065 g/mol. If we have a solution with 32 g/kg sulfur and 1 kg water, the molality should be slightly about 1. By doing similar calculations with nitrogen or other elements, my calculation is in accordance with the results I got by using PHREEQC. I am pretty sure that I have some kind of error within my calculations and would like to ask you to double check it.

Thanks a lot!
Siggi

[dlparkhurst]

The default conversion from mass to moles, which you are using, is defined in SOLUTION_MASTER_SPECIES in the database that you choose. For most databases, the default is the gram formula weight of SO4. The definition in phreeqc.dat is included in the input below for your information.

However, you can change the gfw that is used several ways. (1) Simplest, is to add "as S" on the line that defines S in SOLUTION. (2) You can redefine S in SOLUTION_MASTER_SPECIES in the database, but I  think it is a good idea to leave the databases intact. (3) You can redefine S for SOLUTION_MASTER_SPECIES in you input file. Just change the SO4 to S in the attached file. (4) There is also an option to add "gfw 32.064" in the definition of S in SOLUTION.

Code: [Select]

SOLUTION_MASTER_SPECIES
S SO4-2 0 SO4 32.064
SOLUTION 1
    temp      25
    pH        7
    pe        4
    redox     pe
    units     ppm
    density   1
    S         32000 as S
    -water    1 # kg
END

[Siggi]

Thank you very much for your reply. I already thought that something like this is the reason. I was just confused since N(+5) and N(-3) is defined as N and not as NO3 or NH4, whereas S is defined as SO4. However, good to know where I can find the definition of each species! And thank you for the different ways to change the definition. I totally agree with you that any change within the database is not a good idea.