Conceptual Models > Database selection and modification

Potentially incorrect mole weight in distributed datasets

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BF:
As I understand it, the last column in SOLUTION_MASTER_SPECIES is the mole weight for the element described in the first column, not for the master species in the second column, or a formula you might type in the fourth column. So in phreeqc.dat, the elements C, N, and S, respectively, have mole weights of 12, 14, and 32. Thus secondary master species like CH4, NH4+, and HS- will have mole weights of 16, 18, and 33, which we can figure by adding the appropriate number of protons defined for the master species. And the "element" Amm (used in Amm.dat) is really unreactive ammonium, N + 3 H, with a mole weight of 17, so that the master species AmmH+, representing ammonium, is 18.


--- Code: ---SOLUTION_MASTER_SPECIES
#
#element   species   alk   gfw_formula   element_gfw
C      CO3-2   2.0   HCO3      12.0111
C(+4)      CO3-2   2.0   HCO3
C(-4)      CH4   0   CH4
Alkalinity   CO3-2   1.0   Ca0.5(CO3)0.5   50.05
S      SO4-2   0   SO4      32.064
S(6)      SO4-2   0   SO4
S(-2)      HS-   1.0   S
N      NO3-   0   N      14.0067
N(+5)      NO3-   0   N
N(+3)      NO2-   0   N
N(0)      N2   0   N
N(-3)           NH4+   0   N      14.0067
#Amm      AmmH+   0   AmmH      17.031

--- End code ---

By the same logic, then, it seems there is an issue with the definition of Sg. If the master species H2Sg is 2H + Sg, then since we know H2S should have a mole weight of 34, shouldn't the mole weight in the last column be 32? In other words, "Sg" is supposed to represent sulfur only, not the entire molecule ?



--- Code: ---
# redox-uncoupled gases
Hdg Hdg 0 Hdg 2.016 # H2 gas
Oxg Oxg 0 Oxg 32 # O2 gas
Mtg Mtg 0 Mtg 16.032 # CH4 gas
Sg H2Sg 1.0 H2Sg 34.08
Ntg Ntg 0 Ntg 28.0134 # N2 gas

--- End code ---
Am I interpreting this incorrectly?

Thanks,
Brian

dlparkhurst:
First, the fourth column of gram formula weights is used mostly to convert mass concentration units in SOLUTION to moles.

Now, these gases are defined to be inert (except for H2Sg). There can be no decomposition to other species (except by kinetics). So, I think the values for the 4th column are probably okay to convert mass to moles (except for H2Sg, see below). I don't often define these gases in mass units, but if the conversion factor (g/mol) is not correct for your situation, change (1) the database (not recommended), (2) define SOLUTION_MASTER_SPECIES in the input file with the different value, or (3) use the options in SOLUTION to change the GFW.

H2Sg is incorrect, and I will change it. It should be 32 in the 5th column, so that H2Sg is calculated correctly. Also, H2Sg should not contribute to the alkalinity.

BF:
Thank you for confirming! BTW, I think there's a similar issue in the redox uncoupled database linked below. I think Methane/MethaneH4 should have 12 in the last column instead of 16.

https://phreeqcusers.org/index.php/topic,2023.msg7264.html#msg7264

dlparkhurst:
OK. I'll get that one too.

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