This part of ISO 12782 specifies the determination of the content of “reactive” iron in the form of crystralline iron oxides and hydroxides in soil and other materials by extraction with dithionite. Other materials also include waste. The content of “reactive” iron can be used as input in geochemical models to represent the content of crystalline iron (hydr)oxides. NOTE Although the ammonium oxalate/oxalic acid extraction (ISO 12782-3) is commonly used to estimate “reactive” iron in the form of iron oxides and hydroxides, this part of ISO 12782, in conjunction with ISO 12782-1, has been shown to be more specific for the extraction of crystalline and amorphous iron (hydr)oxides, respectively[7].