a Ca(II) concentration between 3 and 30 mM with a detection limit of 3 mM. Immobilization alters both the acid and chelation strength of the Indicator. Possible causes of these reactivity differences are discussed, based on Insights provided by Infrared internal reflection and photoacoustic spectroscopies. Equations to determine the metal complex formation constant and acid dissociation constants of the Immobilized agent from diffuse reflectance spectra are developed.