碳同位素

[Mr.white]

The following are my input file and output file. In the output file, why has the 13C in the Isotopes section increased? I have set the parameters for C1 and 13C the same in the database, so why hasn't the concentration of C1 changed while 13C has increased? Also, should I add the conversion reaction equation between C1 and 13C to the database if I want to observe the fractionation of carbon isotopes?
SOLUTION 0
    temp      64
    pH        6.05
    pe        8.41
    redox     pe
    units     mol/kgw
    density   1
    C         0.0597
    Ca        0.00462
    Cl        0.309
    K         0.05728
    Mg        0.01031
    N         0.002191
    Na        0.2273
    S         0.005945
    [13C1]    0.000629
    [C1]      0.0631
    [C2]      0.00161
    [C3]      0.000236
    -water    1 # kg

Initial solution 0.   

-----------------------------------Isotopes------------------------------------

   Isotope       Molality          Moles          Ratio          Units

      [C1]    6.23988e-02    6.23988e-02
    [13C1]    7.01188e-04    7.01188e-04    6.29000e-04         permil

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

   Elements           Molality       Moles

   C                 5.970e-02   5.970e-02
   Ca                4.620e-03   4.620e-03
   Cl                3.090e-01   3.090e-01
   K                 5.728e-02   5.728e-02
   Mg                1.031e-02   1.031e-02
   N                 2.191e-03   2.191e-03
   Na                2.273e-01   2.273e-01
   S                 5.945e-03   5.945e-03
   [C1]              6.310e-02   6.310e-02
   [C2]              1.610e-03   1.610e-03
   [C3]              2.360e-04   2.360e-04

[dlparkhurst]

I would need to see all of your definitions to know what you have done. I suggest that you do not make changes to the database file iso.dat. Either include your new ISOTOPES, SOLUTION_MASTER_SPECIES, and other definitions in the input file, or put them in a separate file so that you can use INCLUDE$ to add the definitions to the input file.

If you use iso.dat and the following input

Code: [Select]

SOLUTION
-units mmol/kgw
C 1
[13C] 0 permil
END

you will note that C (total dissolved carbon) is defined as mmol/kgw, but [13C] is defined in permil. In the output for the initial solution calculation, only C is presented in the Solution composition and Distribution of Species sections of the output, and at this point C represents the total of all isotopes. The individual total molalities of 12C and 13C are calculated from the initial solution calculation and are given in the Isotopes section. The sum of the two is equal to the value for C in the distribution of species.

If you then use the solution in any reaction calculation, you will see that both C (12C) and [13C] are included in both the Solution composition and the Distribution of species. So, in the initial solution calculation, PHREEQC is doing preliminary calculations to determine the molalities of C and its isotopes. From that point onward, isotopes are treated as separate elements.