pH estimation without Alkalinity measurement

[lisaR]

Hello,
I would like to estimate the pH of a solution of known composition by using the Amm.dat database. The composition (mg/L) is the following:
Na   12000
Cl   22500
SO4   3000
Ca   500
Mg   1500
K   500
Sr   10
B   1
N-NH4   70
P-PO4   2
TIC   50
Temperature 17 Celsius.
Is it possible to do it without Alkalinity information? Thank you.

[dlparkhurst]

Looks like a formation water. If you know the partial pressure of CO2, you could use that to define a pH.

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pH 7 CO2(g) 0.7 10 # 5 atm P(CO2)

Lacking PCO2 information, you could assume the water is in equilibrium with calcite.

Code: [Select]

SOLUTION
pH 7 calcite 0
-temp 17
Na   12000
Cl   22500
S(6)   3000
Ca   500
Mg   1500
K   500
Sr   10
B   1
N   70 as NH4
P   2 as PO4
C   50
END
This calculation gives a pH of about 6.

[lisaR]

Perfect! thank you, it actually works much better. Just one last question. Since I'm using the Amm.dat database, I added TIC as total inorganic carbon (mgC/L), Amm as mgNH4/L and N(+5) and N(+3) as respectively mg NO3/L and mgNO2/L. I have just doubt about the fact that the last three should be put in input all as mgN/L.

[dlparkhurst]

You will have to look at the original analysis to determine the units for each analyte. If necessary, you will have to contact the lab that did the analysis.

[lisaR]

Sorry I think I misexplained what I was meaning. I meant that the measurement made gave the concentration on mg NH4/L, mg NO3/L and mgNO2/L, I was wondering if as input using the Amm.dat I should convert these three measures each in the corresponding mgN/L or not. As example:
50 mg NH4/l -> 38.88888889 mg N/l
10 mg NO3/l -> 2.258064516 mg N/l
2 mg NO2/l-> 0.608695652 mg N/l
Thank you.
 

[dlparkhurst]

No, you should let PHREEQC do the conversions, just because the hand calculation is error prone, and it is easier to check your work when the original data are used.

So, I would use

Code: [Select]

SOLUTION
-units mg/L
50 as NH4
10 as NO3
2 as NO2

[lisaR]

And about the total inorganic carbon (measured as mg C/l), the input will just be:
C 50
without any correction to one of the species (HCO3 or CO3), right?
Thank you.

[dlparkhurst]

To be careful, you should probably use

Code: [Select]

C(4) 50 as C

assuming the C is total dissolved inorganic carbon (TDIC). The default gfw for each element and redox state is in the SOLUTION_MASTER_SPECIES data block. Often it is HCO3 for C, so explicit definition as C ensures that the concentration is converted correctly.