Colorimetric determination of Fe(sup 2+)/Fe(sup 3+) ratio in radioactive glasses

摘要

In the vitrification of nuclear wastes, the Fe(sup 2+)/Fe(sup 3+) 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. A colorimetric method for measuring the Fe(sup 2+)/Fe(sup 3+) ratio in highly radioactive glasses was developed and tested remotely in a shielded cell. The tests were performed on glasses similar in composition and radioactivity to those that will be produced in the Savannah River Site Defense Waste Processing Facility. 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 Fe(sup 2+) 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(sup TM) (trademark of the Hach Company, Loveland, CO) reagent is added to form a magenta-colored complex with Fe(sup 2+). 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 Fe(sup 2+) and the absorbance is again measured. The difference in absorbance measurements corresponds to the Fe(sup 3+) in the sample and the Fe(sup 2+)/Fe(sup 3+) ratio can be calculated.