Uranium frequently is encountered in the form of uranyl, UO22+, in the natural state. Many times the concentration of uranyl must be detected in a difficult sampling location, such as down a well or borehole. This measurement can be accomplished in situ with the use of a fiber-optic-based probe with a colorimetric indicator. Because the introduction of additional chemicals to the ground water is not desirable and often is not permitted, the colorimetric reagent should be bound to the probe. One such colorimetric reagent for uranyl that can be bound is arsenazo III.
L. C. Baylor and B. R. Buchanan, "Reflectance Probe for Uranium(VI) Based on the Colorimetric Indicator Arsenazo III," Appl. Spectrosc. 49, 679-681 (1995)