Your opinion on a class of ODE problems

[Namir]

I am using various numerical methods to solve a system of differential equations. These equations model chemical reactions. To be specific, the ODEs are sets of equations for monitoring the change in the concentration of chemical that participate in reactions -- chained irreversible reactions and chained consecutive equilibrium reactions. Due to rounding and inherit errors in the methods, I discovered that the solutions violate the "conservation of matter law" because the sum of the concentrations (or weights to be more exact) of the chemicals change with time!

My first fix to this problem is to readjust the concentration of one of the chemicals (say the main chemical that starts the reaction) to preserve the conservation of matter law. The conservation of matter law represents a constraint on the ODE solutions, which admittedly is unusual.

Your opinion? and suggestions? Is any fix corrupting the "math" and rendering the numerical solution invalid?

Namir