Total mercury (Hg-T) determination requires the addition of concentrated hydrochloric acid solution (>= 10 mol L-1 HCl) in relatively high amounts to preserve the samples and to prepare reagent solutions. A method for the preparation of concentrated HCl with HgT concentration of lower than 5 ng L-1 is described in this article. It is based on the well-known chemical reaction: 2 NH4Cl+H2SO4 ->(NH4)(2)SO4+2 HCl. This method is validated thanks to the US Environmental Protection Agency method 1631 and standard reference materials BCR-579 (mercury in coastal seawater).