Conditional in RATES

[javiera.aedo]

Hi, I'm trying to model the dissolution of goethite and I found that for a certain range of pH the specific rate is 10^-9.39 * [H+] ^ 1 and for another range it is 10^-14.6. I don't know how to write the command in RATES,  I try with:

Hematite   
-start
 10 REM parm(1)= A0 #m2 
 20 REM parm(2) = V #L
 30 if (pH > 1.3 and pH <= 5.6) THEN rate = 10^-9.39 * act("H+")^1*(1 - SR("Hematite")) * parm(1)/parm(2) *(m/m0)^0.67
 40 if (pH > 5.6 and pH <= 8.2) THEN rate = 10^-14.6 *(1 - SR("Hematite")) * parm(1)/parm(2) *(m/m0)^0.67
 50 moles = rate * time
 200 SAVE moles
-end

[dlparkhurst]

Your rate definition might work. A couple of issues.

(1) the rate is zero at pH less than 1.3 and greater than  8.2. I don't know if you want to remove the lower and upper bounds.

(2) the rate is discontinuous at pH 5.6. You would need -9.39 + -5.6 = -14.99 for the rate constant rather than -14.6 for the high-pH rate expression. Even so, the combined rate would be non-differentiable, which probably would work, but could cause problems with the integration method.

Presumably, you intended to use Goethite instead of Hematite in the SR definitions.

[javiera.aedo]

Why should I use Goethite instead of Hematite in the SR definitions if I am trying to remove Hematite?

[dlparkhurst]

You said "I'm trying to model the dissolution of goethite", but if you meant hematite, then you are right.

[javiera.aedo]

Yeah, sorry. Thank you.