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The jarosite sample was subjected to characterization by dichromatometry, gravimetric analysis, XRD, AAS, ICP, SEM and EDS. The particles of this compound are spherical and made up of rhombohedral microcrystals. The density of potassium jarosite with arsenic was 2890 kg/m3. The synthesis study of this compound indicated that AsO4 replaced SO4, thus forming arsenic jarosite. Such replacement makes of jarosites potential compounds for the stabilization of arsenic. The S-shaped decomposition curve of potassium jarosite with arsenic in NaOH media showed a modest induction period, a progressive conversion period and a stabilization zone which indicates that the reaction has ended. Through alkaline reactivity studies we observed that SO42- and K+ diffuse from the arsenic jarosite particle into the solution, and at the same time OH- diffuses from the bulk of the solution into the core of arsenic jarosite particles. The result of this process is that Fe, As and O remain distributed throughout the particle, forming an amorphous gel of iron hydroxide with absorbed arsenic. This arsenic-rich gel does not evolve into crystal phases. Arsenic is retained in this amorphous compound. However, when it is exposed to temperatures in the order of 600 °C, it decomposes, giving Fe2O3(s) and As(g) as a result.

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