Spectrofluorimetric study of the β-cyclodextrin–rubidate complex and determination of rubidate by β-CD-enhanced fluorimetry

The supramolecular interaction of rubidate (chemically 2,3-naphthalenedicarboxylic acid, 1,4-dihydroxy-diethyl ester) and β-cyclodextrin (β-CD) has been studied by spectrofluorimetry. The influence of temperature on the supramolecular system has also been investigated and the thermodynamic parameters were calculated. The results show that β-CD reacts with rubidate to form a 1:1 host–guest complex with an apparent association constant of 566±23 l mol−1. It was also demonstrated that the thermodynamics of β-CD–rubidate complex displayed a compensatory enthalpy–entropy relationship. Based on the significant enhancement of the fluorescence intensity of rubidate produced through complex formation, a spectrofluorimetric method for the determination of rubidate in bulk aqueous solution in the presence of β-CD was developed. The linear relationship between the fluorescence intensity and rubidate concentration was obtained in the range from 2.7×10−2 to 3.0 μg ml−1, with a correlation coefficient of 0.9988. The detection limit was 8.2 ng ml−1 and the relative standard deviation was 1.6%. There was no interference from the excipients normally used in tablet formulations. The application of the present method to the determination of rubidate in tablets gave satisfactory results and was compared with the reference method.