Competitive indicator methods for the analysis of citrate using colorimetric assays

Abstract

A competitive indicator method for studying host–guest complexes is extended to systems forming I:H, I:H 2, and A:H complexes of indicator (I), host (H) and competing analyte (A). Methods are applied to complexes of a synthetic organic host ( 1 ) with commercially available indicators xylenol orange ( 4 ) and methylthymol blue ( 5 ). Utilizing complexometric indicators results in an observable shift in λ max 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 4 and 1 are K 1=7.05×10 3 M −1 and K 2=2.08×10 3 M −1 and between 5 and 1 the binding constants are K 1=3.95×10 3 M −1 and K 2=1.10×10 2 M −1. Competition assays using UV–Vis spectroscopy and each host–indicator ensemble allowed for the determination of citrate concentration in various beverages.