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Using Alk and CO2(aq) as carbonate system constraints

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It's my understand that (in addition to the thermo data), you need two constraints (e.g., pCO2, CO2(aq), HCO3-, CO3--, pH, Alk) to solve the carbonate system. If I give React CO2(aq) and HCO3-, it works as expected. But I want to constrain the carbonate system with CO2(aq) and Alk (~ carbonate alkalinity). However, I get the error, "pH must be set to use alkalinity as a constraint" when I run the below model. Why do I need to set pH for this to work?


-- Adam


1 kg free H2O
T = 25 C

Na+ = 100e-3 mol/L
Cl- = 100e-3 mol/L
Mg++ = 1e-12 mol/L
Ca++ = 1e-12 mol/L
K+ = 1e-12 mol/L
Fe++ = 1e-12 mol/L
SO4-- = 1e-12 mol/L
SiO2(aq) = 1e-12 mol/L
Al+++ = 1e-12 mol/L
HPO4-- = 1e-12 mol/L
F- = 0.07 mmol/L
fix F-

swap CO2(aq) for H+
CO2(aq) = 1e-4 mol/L
fix activity CO2(aq)
HCO3- = 10 mg/l_as_CaCO3

react 60 g An10
react 25 g En50
react 15 g Fo

unsuppress all
suppress Dawsonite

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The program takes carbonate alkalinity as a concentration unit and not a basis species. Therefore it does not swap into the program as you would a regular basis species (e.g.CO2(g) for H+). The alkalinity alone cannot give a relationship to the carbonate speciation without the pH. Remember that the speciation of the carbonate bulk component is pH dependent.

Best regards,
Jia Wang
Aqueous Solutions LLC

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