Novel oxidative electrophilic coupling reactions of phenoxazine derivatives with MBTH and their applications to spectrophotometric determination of residual chlorine in drinking water and environmental water samples

Novel, sensitive and rapid spectrophotometric methods, using phenoxazine (PNZ), 2-chlorophe-noxazine (CPN) and 2-trifluoromethylphenoxazine (TPN) as chromogenic reagents for the determination of residual chlorine are proposed. The methods are based on the reduction of chlorine by an electrophilic coupling reagent, 3-methyl-2-benzothiazoline hydrazono hydrochloride hydrate (MBTH) in mild hydrochloric acid medium and subsequent coupling with PNZ, CPN or TPN. The blue color formed in the reaction showed maximum absorbance at 680–690 nm and obeyed Beer's law over the range 0.1–2.2 μg ml⁻¹. The molar absorptivity values with PNZ, CPN and TPN were 2.80 × 10⁴, 2.67 × 10⁴ and 1.91 × 10⁴ l mol⁻¹ cm⁻¹ and Sandell's sensitivity values were 0.028, 0.027 and 0.028 μg cm⁻² respectively. The proposed methods were successfully applied in the determination of residual chlorine in drinking water and environmental water samples. The performance of proposed methods was evaluated in terms of Student's t-test and variance ratio F-test which indicated the significance of proposed methods over the standard spectrophotometric method.