Colorimetric determination of iron(II) and iron(III) in glass

Abstract

A simple method to determine relative amounts of Fe^II and Fe^III was developed for the analysis of glass containing nuclear waste. The pulverized glass sample was dissolved in a sulfuric acid–hydrofluoric acid mixture with ammonium metavanadate added to preserve the Fe^II content. The V^V–V^IV couple, which is strongly acid-dependent, caused the Fe^II to be oxidized to Fe^III in the highly acidic dissolution and then to be regenerated when the solution was adjusted to pH 5 for formation of the magenta Fe^II–Ferrozine complex. Boric acid was added to complex the fluoride and permit regeneration of Fe^II, otherwise Fe^III would be stabilized as a fluoride complex. The Fe^II content was determined from the absorbance at 562 nm, and total Fe was determined from the absorbance of the same solution after ascorbic acid had been added to reduce Fe^III to Fe^II. The relative standard deviation was 5%, and the range was 3–97% for Fe^II in total Fe. The method was validated by analysing a glass frit spiked with ammonium iron(II) sulfate and/or ammonium iron(III) sulfate solutions. The method is applicable to glasses (or minerals) that contain more than about 5% total Fe.