Modification of the ferric chloride assay for the spectrophotometric determination of ferric and total iron in acidic solutions containing high concentrations of copper

During heap bioleaching for copper recovery, solution containing high concentrations of copper is recycled to the heaps. The commonly used 1,10-phenanthroline assay is not suitable in such situations; the presence of 6 g L−1 copper in a solution containing 50 mg L−1 ferrous iron resulted in the complete loss of the absorbance band associated with the tris(1,10-phenanthroline)iron(II) complex. The colorimetric determination of soluble ferric iron concentration using the ferric chloride assay is simple and rapid. However, it offers relatively low resolution and cannot be used to determine total iron concentrations. The detection wavelength was changed to improve resolution and the assay further modified to include an oxidation step using potassium persulfate. Therefore, both ferric and total iron concentrations can be determined rapidly using the same aliquot. There was minimal interference of copper (<5% deviation) on the modified assay with copper concentrations of 10 g L−1 and below. At greater copper concentrations the deviation increased. However, a linear relationship between absorbance and ferric iron concentration was maintained for all the solutions tested allowing correction through appropriate calibration.