Cationic surfactants, such as cetylpyridinium bromide (CPB), sensitize the color reaction of Nb(V) with 1-(2-benzothiazolylazo)-2-hydroxy-3-naphthoic acid (Ia), 5-(benzothiazolylazo)2,5-naphthalenediol (Ib), 5-(2-benzothiazolylazo)8-hydroxyquinoline (Ic) and 4-(2- benzothiazolylazo)2,2̄-biphenyldiol (Id) reagents. The formation of a ternary complex of stoichiometric ratio 1:2:2 (Nb-R-CPB) is responsible for the observed enhancement in the molar absorptivity and the Sandell sensitivity of the formed complex, when a surfactant is present. The ternary complex exhibits absorption maxima at 649, 692, 661 and 612 nm, (ε=3.35×104, 3.59×104, 4.46×104 and 2.79×104 l mol−1 cm−1) on using reagent Ia, Ib, Ic, and Id, respectively. Beer’s law is obeyed between 0.05 and 2.50 μg ml−1, while applying the Ringbom method for more accurate results is in the range from 0.20 to 2.30 μg ml−1. Conditional formation constants in the presence and absence of CPB for niobium complexes have been calculated. On the basis of a detailed spectrophotometric study, the nature of the chromophoric reagent–surfactant interaction and the peculiar features of the sensitization by CPB are discussed. |
