New method for the direct determination of dissolved Fe(III) concentrationin acid mine waters

A new method for direct determination of dissolved Fe(III) in acid mine wat er has been developed. In most present methods, Fe(III) is determined by co mputing the difference between total dissolved Fe and dissolved Fe(II). For acid mine waters, frequently Fe(II)>> Fe(III); thus, accuracy and precisio n are considerably improved by determining Fe(III) concentration directly. The new method utilizes two selective ligands to stabilize Fe(III) and Fe(I I), thereby preventing changes in Fe reduction-oxidation distribution. Comp lexed Fe(II) is cleanly removed using a silica-based, reversed-phase adsorb ent, yielding excellent isolation of the Fe(III) complex. Iron(III) concent ration is measured colorimetrically or by graphite furnace atomic absorptio n spectrometry (GFAAS). The method requires inexpensive commercial reagents and simple procedures that can be used in the field. Calcium(II), Ni(II), Pb(II), Al(III), Zn(II), and Cd(II) cause insignificant colorimetric interf erences for most acid mine waters. Waters containing >20 mg of Cu/L could c ause a colorimetric interference and should be measured by GFAAS. Cobalt(II ) and Cr(III) interfere if their molar ratios to Fe(III) exceed 24 and 5, r espectively. Iron(II)interferes when its concentration exceeds the capacity of the complexing ligand (14 mg/L). Because of the GFAAS elemental specifi city, only Fe(II) is a potential interferent in the GFAAS technique. The me thod detection limit is 2 mu g/L (40 nM) using GFAAS and 20 mu g/L (0.4 mu M) by colorimetry.