PhreeqcUsers Discussion Forum

Beginners => PHREEQC basics => Topic started by: Zhaoyang on January 25, 2021, 04:03:50 PM

Title: C indicates CO3-2 or HCO3-
Post by: Zhaoyang on January 25, 2021, 04:03:50 PM
Dear all,
Code: [Select]
SOLUTION 1
   units mg/l
   Ca 15
   Mg 4.1
   Na 5.3
   K 2.3
   C 60
   Cl 8.8
   S 11
   Si 15.1
   Fe 0.67
   Al 0.07
   pH 7 charge

SAVE SOLUTION 1 # This SOLUTION will be used later

# The pH is 7.363

END

I am confused about C. Does it indicate CO3-2 or HCO3- in the code above? if I change "C 60" to "C 60 as CO3-2", I will get different pH. Any help will be appreciated.

Best regards,
Zhaoyang
Title: Re: C indicates CO3-2 or HCO3-
Post by: dlparkhurst on January 25, 2021, 09:22:14 PM
What you are really asking is what is the gram formula weight used to convert C(in SOLUTION) in mg/L to mol/kgw.

The answer depends on the SOLUTION_MASTER_SPECIES data block in the database that you are using. In phreeqc.dat, the definition is as follows:

Code: [Select]
SOLUTION_MASTER_SPECIES
C CO3-2 2 HCO3 12.0111

The fourth field defines the gram formula weight that will be used. In this case it is the gfw of HCO3, or ~61. If you uses "as CO3", gfw of ~60 will be used. If you use "as C", 12.0111 will be used.

So when using C in SOLUTION, you need to use the gfw that applies to your analysis to obtain the correct mol/kgw of total dissolved inorganic carbon in your solution.