AN ANALYSIS OF METHODS FOR MEASURING AQUEOUS IRON REDOX SPECIES

Iron is highly reactive and often the most abundant cation in acid mine drainage (AMD). Consequently, accurate measurements of iron redox species for AMD are important. One method for measuring aqueous iron is a colorimetric method using FerroZine, an organic compound that forms a magenta colored species when it complexes with Fe(II). However, samples with high concentrations of Fe(III) compared to Fe(II), such as acid mine waters, can form a brick-red color that compromises the absorbance measurements of the Fe(II) concentrations. The objective of this study was to explore modifications to the current FerroZine method to improve the reliability of aqueous measurements with high concentrations of Fe(III). These modifications include: NaF addition (Herrera, et al., J. Chem Tech. Biotechnol. 1989, 44, 171-181), and the addition of HCl after color development to reduce the Fe(III) interference. This study found that the original FerroZine method became less reliable as Fe(III) concentrations neared 100 (mg/L). NaF addition improved the reliability of Fe(II) measurements in the presence of Fe(III) concentrations up to 100 (mg/L). The HCl addition reduced the brick-red color interference, but was also shown to degrade the ferrous color complex within hours. As a result the HCl addition is only useful when the samples can be analyzed immediately after this addition. NaCl was also found to have a potential as an Fe(III) complexing agent, however more investigation into its efficacy is required. As none of the modifications attempted in this study were viable options for measuring Fe(III) concentrations above 500 (mg/L), more work needs to be done to find a better alternative. The next step in our research is to attempt Fe(II) and Fe(III) separation using ion chromatography.