Formaldehyde

[Alexey]

I didn't find formaldehyde (CH2O) in any Phreeqc data base. However I can calculate and add its equilibrium constants from Supcrt slop07. My question is what reaction is better to use?

Schulte and Shock (1995) providing thermodynamic data for formaldehyde recommend to use (1) CO2,g + 2H2,g = CH2O,aq + H2O,g or (2) CO,g + H2,g = CH2O,aq. But I'm not sure that reactions with gases will be applicable in Phreeqc. All reactions here are with solution species. So I would use something like (3) HCO3- + H+ = H2CO,aq + O2,aq or (4) CH4,aq + H2O,aq = CH2O,aq + 2H2,aq. What reaction is better to use? Do exist some rules and limitations to construct those reactions?

[dlparkhurst]

You can use any reaction among aqueous species provided formaldehyde is the first species on the right-hand side of the equation, and PHREEQC is able to rewrite the equation in terms of secondary and master species (SOLUTION_MASTER_SPECIES).

However, I suspect that for almost any aqueous environment, formaldehyde will be unstable relative to other carbon aqueous species. Thus, if you put the reaction as part of the carbon system, I doubt that it would ever be present in significant quantities; equilibrium will distribute the carbon to other species, probably CH4 and HCO3-.

If I am right that formaldehyde is never present in significant quantities at equilibrium, alternatively, you can add a new "element", say "Formaldehyde", with SOLUTION_MASTER_SPECIES and SOLUTION_SPECIES, and then use REACTION or KINETICS to remove Formaldehyde and add CH2O to simulate formaldehyde decomposition.

[Alexey]

Yes, you are right, the content of formaldehyde is always very low (>1e-13 M)