Previous step values

[Laknesh]

Hello
I try to model a kinetic reaction using RATES and KINETIC keywords. I want to model the following equations in RATES.
A_t = A_t-1 + delta_t * B_t-1
B_t = X - A_t
where
A_t-1: value at time t-1 (previous time step value)
A_t: value at time t
B_t-1: value at time t-1 (previous time step value)
B_t: value at time t
delta_t: time step
X: a constant

How can I get previous step values in RATE Keyword to model this kinetic reaction.

Thank you and l look forward your valuable help and suggestions.

Regards
Laknesh

[dlparkhurst]

I do not think you should try to do this KINETICS. KINETICS uses a 5th order Runge-Kutta or an implicit method (cvode) to integrate the rate equations. It automatically uses sub time steps to maintain errors within the specified error tolerance.  The rate expression is evaluated multiple times per time interval, so it does not really make sense to try to use values at a previous time interval in the rate expression. You would need to recast you equations in terms of rates. For example, for first order, dA/dt = kA.

You could probably do what you suggested using REACTION under scripting control with IPhreeqc. y

[Laknesh]

Thank you for your reply. I will try this problem in IPhreeqc using REACTION.