Simple reaction RATES, KINETICS

[inhee]

Hello. I have a question about "RATES" and "KINETICS" functions.

Code: [Select]

TITLE Example 9


SOLUTION 1
pH 2


EQUILIBRIUM_PHASES 1
Calcite 1.0


RATES
calcite
-start
10  k = 8.9e-5
20  moles = k * ACT("H+") * TIME
200 SAVE moles
-end

KINETICS 1
calcite
        -formula  Calcite -1.0 H+  -1.0  Ca2+ 1.0 HCO3- 1.0
-steps 0.1 0.2 0.5 1



SELECTED_OUTPUT
-file ex9.sel
-reset false
USER_PUNCH
-headings Time(s)  H  pH
10 PUNCH SIM_TIME ACT("H+"), -LA("H+")
END

I have modeled simple calcite reaction depending on time. (CaCO3 + H+ = Ca2+ + HCO3-) So, I set initial H+ concentration is 1e-2 M (pH=2).

And calcite exists enough in the model (EQUILIBRIUM_PHASES 1 Calcite 1.0). The rate of reaction is defined that 8.9e-5 * ACT("H+").

Therefore, the mole of reaction depending on time is 8.9e-5 * ACT("H+") * TIME like RATES 20 part in my code.

The timestep is 0.1 s, 0.2 s, 0.5 s, and 1 s. However, the result of this code showed there is no change depending on time.

Only initial concentration of H+ is pH 2, but other concentrations are pH 7.26 consistently. I can't understand how this result occurs.

I expected that the concentration of H+ decreases due to the reaction with calcite as time goes.

Why H+ didn't decrease? Please help me.

Thanks.

[dlparkhurst]

By including EQUILIBRIUM_PHASES, the reaction solutions will always be in equilibrium with calcite, regardless of the KINETICS reaction of calcite.

If you remove EQUILIBRIUM_PHASES, you will see the results of your kinetic reaction. Your -formula is incorrect, you should simply add CaCO3 to the solution as calcite reacts. I've also increased the rate constant so that you can see the effects of the kinetic reaction over the time periods that you specify.

Code: [Select]

TITLE Example 9
SOLUTION 1
pH 2
RATES
calcite
-start
10  k = 1 #8.9e-5
20  moles = k * ACT("H+") * TIME
200 SAVE moles
-end

KINETICS 1
calcite
        -formula  CaCO3 1.0
-steps 0.1 0.2 0.5 1

END


[inhee]

Thank you for your answer.

I got a question. In your code, then isn't the concentration of calcite defined? I thought that "EQUILIBRIUM_PHASES" box determines the concentration of calcite.

Is it assumed that even if calcite is not defined, enough calcite exists to react to H+?

[dlparkhurst]

You do define the amount of an equilibrium-phases mineral in EQUILIBRIUM_PHASES, but it is not related to the amount of a kinetic reactant.

The amount of a kinetic reaction is defined in KINETICS. By default (as in your case), the amount of a kinetic react is 1 mole (-m 1).

If you want equilibrium with calcite, use EQUILIBRIUM_PHASES; if you want calcite to react kinetically, use KINETICS/RATES.