An optical sensing film for the determination of Co(II) based on disodium-1-nitroso-2-naphthol-3,6-disulfonate immobilized in chitosan film

A single-use optical chemical sensing film based on immobilized disodium-1-nitroso-2-naphthol-3,6-disulfonate or nitroso-R salt (NRS) in a modified chitosan film was developed for the determination of Co(II) in aqueous solution. In the presence of Co(II), the color of the sensing film changed from yellow-green to reddish orange as a result of complex formation between Co(II) and NRS. The absorbance measurement of the complex was made at 525 nm because the background of the sensing membrane was minimal. An optimum response of the sensing film to Co(II) was obtained in a solution with a pH of 5. Two sample sizes were studied: 3 mL with a Co(II) concentration range of 1–60 ppm and 300 mL with a Co(II) concentration range of 35–400 ppb. Linear calibration curves were obtained from both sample sizes with limits of detection of 0.82 ppm and 12 ppb, respectively. The relative standard deviation for five separate films in 10 ppm Co(II) solution was 1.7% and that for the other five individual films in 40 ppm Co(II) solution was 0.41%. The sensing films exhibited good stability for 2 months and were applied for determination of Co(II) in water samples with satisfactory results.