A highly stable ternary complex of iron(III) with 5-(4-nitrophenylazo) salicylic acid (NPAS) and Eosin (Es) in acidic medium at room temperature gave a maximum absorption at 545 nm with a molar absorptivity 2.81×104 L mol−1 cm−1. A spectrophotometric method using these ligands was developed and optimized in terms of pH, stability of the complex, amount of reagent required, sensitivity, linearity, and the effect of various foreign ions was studied. The linear range for iron(III) determination is 0.18–6.0 mg L−1. The method was sensitive, accurate, and all the reagents were stable under the working conditions. Moreover, the method was easy to perform for the determination of iron in pharmaceutical and water samples.

A highly sensitive method is proposed to determine copper(II) ions by forming a stable complex through their interaction with 4-(2′-benzothiazolylazo)-salicylic acid (BTAS) at room temperature and pH of about 5.0. The complex gave a maximum absorption at λ = 485 nm with a molar absorptivity coefficient of 2.35⋅104 l/(mol⋅cm). The linear range for the copper determination is 0.63–5.04 mg/l. The method can be applied to determine copper ions in different biological specimens like some drugs and water samples.