In terms of toxicity, trivalent arsenicals are more toxic and mobile than pentavalent ones, which is related to the speciation and its affinity with the -SH groups in biomolecules such as glutathione and cysteinyl residues in enzymes . In the first place, trivalent arsenicals are deprotonated above pH=9.2; then, trivalent arsenicals are neutrally charged at neutral and physiological pH . Besides, pentavalent methylarsenicals show acid-base equilibria in solution: