Should acid ammonium oxalate replace hydroxylammonium chloride in step 2 of the revised BCR sequential extraction protocol for soil and sediment?

The revised, four-step BCR sequential extraction for soil or sediment has been compared with an alternative procedure in which 0.2 mol l⁻¹ ammonium oxalate (pH 3) replaced 0.5 mol l⁻¹ hydroxylammonium chloride (pH 1.5) in step 2, the reducible step. A variety of substrates were studied: BCR CRM601, a sewage sludge amended soil, two industrial soils, and a steel manufacturing by-product (basic oxygen furnace filter cake). Greater amounts of iron were recovered in step 2 when acid ammonium oxalate was used, for all substrates. Similar trends were observed for copper. Manganese and zinc were not strongly affected by the procedural modification, except for zinc in the two industrial soils, where oxalate extraction proved more efficient than use of hydroxylammonium chloride. A large proportion of the calcium and lead isolated in step 2 of the BCR procedure was not released until step 3 when the alternative procedure with oxalate in step 2 was used. This is probably due to rapid precipitation of analyte oxalates from solution. Thus, whilst oxalate offers superior dissolution of iron-containing matrix components, it should not be used if calcium or lead concentrations are to be measured. Selection of the most appropriated sequential extraction protocol for use in a particular study must always be carried out on the basis of “fitness for purpose” criteria. However, the revised BCR protocol, involving use of 0.5 mol l⁻¹ NH₂OH·HCl in the reducible step, appears to be more generally applicable than procedures involving acid ammonium oxalate.