Development of flow injection spectrophotometric methods for the determination of free available chlorine and total available chlorine: comparative study

A comparative study on the development of flow-injection (FI) spectrophotometric methods for the determination of free available chlorine (FAC) and total available chlorine (TAC) is described. The use of o-dianisidine (ODS), 2,2-azino-bis-3-ethylbenzothiazoline-6-sulfonate (ABTS), 4-nitrophenylhydrazine (4-NPH), methyl orange (MO), syringaldazine (SYR) and a new reagent, indigo carmine (IC), in a flow-injection set-up was optimized for common FI operating parameters such as carrier composition, flow-rate, pH, length of reaction coil and injection volume. The FI conditions were designed by the choice of acid at suitable concentration in the carrier stream such that response from FAC is maximized while at the same time minimally interfered by combined available chlorine (CAC). Analytical characteristics of the FI method using these reagents such as detection limits, linear range, selectivity and their relative stability were compared and contrasted. The reagent that yielded the most sensitive signal is ABTS and is least interfered by Mn2+, Zn2+ and Cu2+ but interference from CAC was the most severe, while SYR and 4-NPH were least affected by CAC. IC exhibited the best selectivity characteristics amongst all the reagents studied. TAC can be conveniently determined by adding NaBr to the carrier stream. Finally, some of the promising reagents were applied to the analysis of a few commercial disinfecting formulations. Good agreement on the TAC (FAC + CAC) content was found between the proposed FIA and the manual iodimetric methods.