The surfactant-sensitized analytical reaction of niobium with some thiazolylazo compounds

Cationic surfactants, such as cetylpyridinium bromide (CPB), sensitize the color reaction of Nb(V) with 1-(2-benzothiazolylazo)-2-hydroxy-3-naphthoic acid (I_a), 5-(benzothiazolylazo)2,5-naphthalenediol (I_b), 5-(2-benzothiazolylazo)8-hydroxyquinoline (I_c) and 4-(2- benzothiazolylazo)2, -biphenyldiol (I_d) 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, (=3.35×10⁴, 3.59×10⁴, 4.46×10⁴ and 2.79×10⁴ l mol⁻¹ cm⁻¹) on using reagent I_a, I_b, I_c, and I_d, 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.