Evaluation of K-H₃O jarosite as thermal witness material

K-H₃O jarosite undergoes distinct stepwise thermal decomposition involving parallel and sequential sub-reactions. From jarosite decomposed partially under unknown temperature-time conditions, it is possible in principle to recover these temperature-time conditions if the residual decomposition of the material is determined and if the kinetics of the decomposition are known. The present study evaluates the use of a synthetic K-H₃O jarosite as a temperature witness material. A flow furnace experiment was designed to expose aerosolized jarosite to temperatures upto 1,200 K for times of 10 s and less. After exposure, partially decomposed particles were recovered, and the residual decomposition was determined by TG. In parallel, the partial decomposition in the flow furnace as well as the residual decomposition was calculated based on a previously published kinetic decomposition model. Sensitivity analysis shows that the residual decomposition must be determined with a resolution better than 1.5 mass% in order to resolve a 100 K temperature difference in the a priori unknown exposure conditions. The sensitivity is greatest in the temperature regions where (OH)^- groups are lost, and diminishes rapidly at higher or lower temperatures. Considering properties of the decomposition model, this suggests that multiple parallel reactions are necessary to achieve reasonable temperature (and time) resolution in forensic analysis using thermal decomposition of materials.