Page 1 of 1

### Unit of specific rate

Posted: Fri Dec 02, 2011 3:56 pm
Hello,

I have a little problem with the unit of the specific rate.
I modeled kinetically limitted gypsum dissolution in a 25 cm column in HP1.
Choosen units in HP1:
Length in [cm]
Time in [h]
Concentrations in [mmol/kgw]

My rate expressions is:

RATES
Gypsum
-start
10 A = parm(1) #surface area
20 V = parm(2) #volume
25 k = parm(3) #specific rate
30 sr_gypsum = SR("Gypsum")
40 rate = k * (A/V) *(1 - sr_gypsum) * m/m0
50 moles = rate * time
60 save moles
-end

I entered a value of 2*10^-5 for the specific rate (parm 3). Now I am not sure if the unit of k*(A/V) is [mmol/(cm?*h)] or [mmol/(L*h)] (L for liter)?

Does the information given suffice to answer the question?

Kind regards

Horst Schonsky

Posted: Mon Dec 05, 2011 7:47 am
The outcome of a kinetic reaction in HP1 has to be:<b> moles/dm? bulk soil</b>.

Consequently, your line 50 is in moles/dm? soil
and line 40 is moles/dm? soil / h

Thus, the units of k*(A/V) = moles/dm? soil / h

Typically, this will be: k = moles / h / L?
with L? related to the specfic reactive surface area

and A : L? / Amount (or volume)of reactant
and V: Amount (or volume) of reactant / dm? soil

Hope this is clear,
diederik

Posted: Mon Dec 05, 2011 9:22 am
Actually I applied the model to descripe the leachate of column experiments. As I did not measure specific surfaces for my reactants I lumped the specific rate and the specific surface into one parameter (k*(A/V)).

Posted: Mon Dec 05, 2011 9:28 am
Sorry I missed one point.
I specified the units for my input concentrations as [mmol/kgw] (in the "Definition of Solute Composition" window). Do these units also apply to the specific rate?

Posted: Mon Dec 05, 2011 10:05 am
The unit of mass in the GUI of Hydrus is not used by HP1.

Units of input concentrations are as you defined during the definitions of the solution compositions (default mmol/kgw).

All further calculations (thus, also the calculations in the RATES) and output are in moles (/kgw or moles/dm? soil depending on the variable). You can change the output to other units using e.g. user_punch.

Posted: Mon Dec 05, 2011 10:10 am
Thank you very much.