Colorimetric determination of Fe2+/Fe3+ ratio in radioactive glasses

In the vitrification of nuclear wastes, the Fe2+/Fe3+ ratio in the glass is a measure of the redox properties of the glass melt. It is necessary to measure this ratio to ensure that the melt redox properties are suitable for the glass melter. This paper discusses colorimetric method for measuring the Fe2+/Fe3+ ratio in highly radioactive glasses, the authors developed and tested remotely in a shielded cell. The first step of the method is dissolution of finely crushed glass with a hydrofluoric/sulfuric acid mixture with ammonium vanadate added to preserve the Fe3+ content of the glass during the dissolution. Boric acid is then added to complex fluoride and to destroy iron-fluoride complexes. After adjusting the solution to pH 5, FerroZine (trademark of the Hach Company, Loveland, CO) reagent is added to form a magenta-colored complex with Fe2+. The absorbance at 562 nm is measured by using a fiber optic-coupled photodiode array spectrophotometer. Ascorbic acid is then used to reduce all the iron in solution to Fe2+ and the absorbance is again measured. The difference in absorbance measurements corresponds to the Fe3+ in the sample and the Fe2+/Fe3+ ratio can be calculated. The radiation levels used in the tests had little or no effect on the colorimetric method for measuring Fe2+/Fe3+ ratios