Competitive indicator methods for the analysis of citrate using colorimetric assays

A competitive indicator method for studying host–guest complexes is extended to systems forming I:H, I:H2, and A:H complexes of indicator (I), host (H) and competing analyte (A). Methods are applied to complexes of a synthetic organic host () with commercially available indicators xylenol orange () and methylthymol blue (). Utilizing complexometric indicators results in an observable shift in λmax upon binding of the analyte and, subsequently may enhance the sensitivity of these competition assays due to a larger dynamic range in the change of absorption. The binding constants for the association between and are K1=7.05×103 M−1 and K2=2.08×103 M−1 and between and the binding constants are K1=3.95×103 M−1 and K2=1.10×102 M−1. Competition assays using UV–Vis spectroscopy and each host–indicator ensemble allowed for the determination of citrate concentration in various beverages.