Chemical instability of spent acids from nitration units has caused many severe accidents in the explosives industry. To cope with this hazard, safety charts have been constructed by various authors. In this work, evaluation of the three most commonly used safety charts found in the literature showed that they are not, in general, consistent with each other, due to the different storage temperatures considered. Mathematical modeling of experimental data was accomplished covering the whole range of their application. Based on this simulation a computer code was constructed and is presented in this article, and may be used as a subroutine in major safety computer programs.