Thermal decompositions of arsine and stibine: the Marsh test

  The thermal decomposition of AsH₃ and SbH₃ is the basis for the Marsh test, which provides a classic example of an analytical technique important in forensic science. The arsenic- or antimony containing material is first converted to AsH₃ or SbH₃ (e.g. by treatment with Zn and acid, liberating H₂ and the trihydride). Subsequent passage of the gaseous mixture through a heated tube causes the hydrides to decompose, forming brown-black deposits of the elements:

   If both As and Sb are present, the relative positions of the deposits establishes their identity; the lower thermal stability of SbH₃ means that it decomposes before AsH₃ in the tube.  Treatment of the residues with aqueous NaOCl also distinguishes them since only arsenic dissolves: