Determination of humic acid and iron(III) by solid-state spectrophotometry to study their interactions

Abstract

A simple and sensitive method for the determination of humic acid has been developed. The method is based on the adsorptive enrichment of humic acid using a finely divided anion-exchange resin, collection of the resin on a membrane filter by filtration as a circular thin-layer, and direct measurement of the absorbance of the resultant thin-layer of resin by densitometry at 470 nm. Up to 80 µg of humic acid in 100 ml of sample solution can be determined, the limit of detection being 1.3 µg. The effect of iron(III) is masked with ethylenediaminetetraacetic acid (EDTA). Iron(III) is also determined by densitometry at 600 nm after enrichment on the thin-layer of resin as a complex with ammonium pyrrolidine dithiocarbamate (ammonium pyrrolidin-1-yldithioformate)(APDC), the limit of detection being 0.06 µg of iron(III). In the presence of humic acid, the blank value is obtained by masking the iron(III) as the EDTA complex, and calculating the net absorbance due to the APDC complex. The methods have been used to investigate the effect of humic acid on the formation of filterable iron(III) species, which can pass through a 0.45 µm membrane filter. The possibility of characterizing humic acid based on the formation of a complex with iron(III) has been shown.