Colorimetric flow-injection analysis of dissolved iron in high DOC waters

An iron flow-injection analysis system has been optimized for the analysis of iron in waters high in dissolved organic carbon. The method detects either dissolved iron(II) or total dissolved iron with a detection limit of 10 nM, precision of 0.65% at 1 microM, and a dynamic range of four orders of magnitude. There are minimal interferences (< 1%) from other metals at environmental concentrations. The iron(II) method measures iron(II) in the presence of excess iron(III) with less than 1% interference. When used with pre-acidified samples, the total dissolved iron method agrees well with electrothermal atomic absorption spectrometry for a variety of natural waters with a range of dissolved organic carbon (3-36 mg C/L) and iron (1-28 microM) concentrations. When used with samples at their ambient pH, the total dissolved iron method detects dissolved iron, but not colloidal iron (size fraction 0.05-0.45 micron).