How to define a maximum value for a dependent variable?

I have been trying to develop a performance model for an electrochemical system.
However, I got some evaluation errors of powers or logarithms when starting
the solver. This is due to concentrations getting negative values at some
solver step. I need to assure that I do not evaluate negative values, and use a maximum concentration value for all species like max(1e-9, c).

How can I define it?

Thanks,

Ertan