# 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 chargeSAVE SOLUTION 1 # This SOLUTION will be used later# The pH is 7.363END`
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_SPECIESC 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.