Simple spectrophotometric determination of Fe in oxalate and HCl soil extracts

We describe a spectrophotometric method to determine the sum of Fe(II) plus Fe(III) in HCl and oxalate extracts. The principle of the method is to reduce Fe(III) by ascorbate in near-neutral solution and to sequester the Fe(II) formed as a tri-ferrozine complex, which is then determined photometrically at 562 nm. Because the complex is stable, the reaction is irreversible and complete. Fe(III) in HCl solution reacted very rapidly, whereas oxalate decelerated the overall reaction so that pseudo first-order kinetics with respect to Fe(III) was detected. However, when extractions were conducted at the recommended soil:solution ratio, the absorption reached 98% of its final value within a few minutes. To test the method, four soils differing considerably in texture and carbonate, organic matter, and Fe(III)(hydr)oxide contents were extracted with oxalate in the dark for amorphous (Feo), and with boiling oxalate for total Fe(III)(hydr)oxide (Febo). This newly developed spectrophotometric method showed excellent correspondence with the conventional atomic absorption spectroscopy (AAS) method. The method presented here can therefore be used as an alternative method to determine the Fe content of oxalate and hydrochloric acid extracts if AAS is not available. Oxalate extracts low in Fe content, which cannot be diluted, are easier to determine by the photometric method than by AAS.