Analytical quantification of aqueous permanganate: Direct and indirect spectrophotometric determination for water treatment processes

Three spectrophotometric methods have been developed and compared for the quantification of low concentrations (0.03-63 mu M) of aqueous permanganate in neutral pH conditions. Although permanganate is a widely used oxidant in drinking water and wastewater treatment, no widely accepted method of quantification has been reported to date. While one method presented does not require the need for any reagent chemicals (direct spectrophotometric analysis), it yielded a relatively low molar absorption coefficient of 3340 M-1 cm(-1) at 525 nm and a level of detection (LOD) and quantification (LOQ) of 0.45 and 1.51 mu M, respectively. Some instability of permanganate species during direct quantification was found to occur over 60 min, with a total decrease of 0.002 (arbitrary units) of absorbance, equivalent to a decrease in concentration of 0.6 mu M. Beyond 60 min, no further degradation was observed. Indirect spectrophotometric analyses using 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) and sodium iodide (NaI) provided a significantly more sensitive method for permanganate quantification, yielding molar absorption coefficients of 140,030 and 61,130 M-1 cm(-1), respectively. The LOD and LOQ were determined to be 0.01 and 0.03 mu M for the ABTS method and 0.02 and 0.08 mu M for the NaI method, respectively. Although conservative and accurate limits of quantification for both the ABTS and NaI methods are presented, which should be sufficient of most practical applications, lower limits may be possible with further refinement of the methods. (C) 2020 Elsevier Ltd. All rights reserved.