FLOW-INJECTION GRADIENT TECHNIQUE IN SPECTROPHOTOMETRIC DETERMINATIONOF FORMATION-CONSTANTS OF MICROMOLECULE-CYCLODEXTRIN COMPLEXES

An automated gradient now injection (FI) spectrophotometric technique for the determination of formation constants of complexes of micromole cules with cyclodextrins (CDs) is described. Only two injections are r equired for the determination of the CD-micromolecule formation consta nt. The concentration gradient is calibrated by injecting a dye soluti on in a buffer carrier. A mixed CD-micromolecule solution is injected afterward into a flowing stream of buffered micromolecule solution. By the use of a mixing chamber, a great number of transient ''mixed solu tions'' are created and their absorbance readings are used for the det ermination of the CD-micromolecule formation constant. Formation const ants for phenolphthalein with beta-CD (K = 2.4(+/-0.2) x 10(4) M(-1) i n carbonate buffer pH 10.5, Theta = 23.5 degrees C) and p-nitrophenola te with alpha-CD (K = 2.0(+/-0.2) x 10(3) M(-1) in carbonate buffer pH 11.0, Theta = 24.5 degrees C) were determined on the basis of spectra l changes upon complexation by CDs. Formation constants of the complex es of the buffer species CHES (K = 4.9(+/-0.2) x 10(2) M(-1) in NaOH p H 10.5 Theta = 27.0 degrees C) and CAPS (K = 4.8(+/-0.5) x 10(2) M(-1) in NaOH pH 11.5 Theta = 25.0 degrees C) and the drag naproxen (K = 6. 3(+/-0.5) x 10(2) M(-1) in 0.020 M Na2CO3 pH 10.5 Theta = 25.0 degrees C) with beta-CD were also determined by adapting the competitive spec trophotometric indicator approach to the FI gradient technique. Format ion constants determined with the proposed method are in reasonable ag reement with literature values. This technique utilizes the total info rmation contained in FI peaks, requires minimal amounts of CD, and pro vides accurate and fast complexation information in a wide range of co ncentration ratios.