Colorimetric determination of iron(II) and iron(III) in glass
A simple method to determine relative amounts of FeII and FeIII 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 FeII content. The VV–VIV couple, which is strongly acid-dependent, caused the FeII to be oxidized to FeIII in the highly acidic dissolution and then to be regenerated when the solution was adjusted to pH 5 for formation of the magenta FeII–Ferrozine complex. Boric acid was added to complex the fluoride and permit regeneration of FeII, otherwise FeIII would be stabilized as a fluoride complex. The FeII 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 FeIII to FeII. The relative standard deviation was 5%, and the range was 3–97% for FeII 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.