  State space truncation methods are widely used to approximate solutions of
the chemical master equation. While most methods of this kind focus on
truncating the state space directly, in this work we propose modifying the
underlying chemical reaction network by introducing \emph{slack reactants} that
indirectly truncate the state space. More specifically, slack reactants
introduce an expanded chemical reaction network and impose a truncation scheme
based on user defined properties, such as mass-conservation. This network
structure allows us to prove inheritance of special properties of the original
model, such as irreducibility and complex balancing.
  We use the network structure imposed by slack reactants to prove the
convergence of the stationary distribution and first arrival times. We then
provide examples comparing our method with the stationary finite state
projection and finite buffer methods. Our slack reactant system appears to be
more robust than some competing methods with respect to calculating first
arrival times.
