Preparation of 2-nitro-5-thiobenzoate for the routine determination of reagent hypochlorous acid concentrations

Solutions of hypochlorous acid (HOCl) decay over time. This decay indicates the necessity for methods and reagents for the routine measurement of this oxidant. 2-Nitro-5-thiobenzoate is commonly used to measure HOCl concentrations. This article describes a method for the preparation of 2-nitro-5-thiobenzoate that is stable for at least 3 months. This method relies on the partial rather than full reduction of 5,5′-dithiobis-(2-nitrobenzoic acid) and the resulting equilibrium between the substrate and the product.