Methylene blue potentiometric sensor for selective determination of sulfide ions

A new poly(vinyl chloride) (PVC) membrane sensor for methylene blue cation (MB⁺) is prepared, characterized and used for determination of sulfide ions. The sensor shows a cationic response slope of 56.2±0.3mV per concentration decade and a linear response range of 6×10^-7 to 10^-4moll^-1 MB⁺ with a lower detection limit of 3.8×10^-7moll^-1 MB⁺ in Tris buffer of pH 8.7. The sensor is used for determination of sulfide ions after conversion into MB using Caro's reaction. Optimized reaction conditions result in a wide dynamic, linear response range (0.12-15μgml^-1 sulfide-sulfur), short reaction time (1-2min), low detection limit (0.01μgml^-1 sulfide-sulfur), high accuracy (>98% recovery) and remarkable selectivity. The method displays significant advantages over many other techniques used for quantification of sulfide ions. Determination of sulfide ions in real samples of complex matrices, e.g. wastewaters gives results that compare fairly well with those obtained with the standard methods. The sensor is also used to monitor titration of some complex anions such as reineckate, hexanitrocobaltate(III), tetraphenylborate and tetraiodomercurate with MB as a titrant.