Iodine and iodate determination by a new spectrophotometric method using N,N-dimethyl-p-phenylenediamine, validated in veterinary supplements and table salt

Abstract

A new spectrophotometric method has been developed for the determination of iodate through the in situ production of iodine. The oxidation of the N,N-dimethyl-p-phenylenediamine (R) molecule produces a stable oxidation intermediate, semiquinondiimine (S⁺), which has a marked absorbance at 550 nm (ε = 10 000). Iodine can be released from an iodate sample by adding KI. The released iodine begins to react in situ with R, producing twice as many S⁺ molecules as a result of the reaction of quinondiimine (T⁺), the final oxidation product, through a rapid syn-proportionation reaction with the excess R present in the solution. The absorbance is stoichiometrically proportional to the concentration of iodine and, consequently, the iodide concentration can be calculated. The pH range is 3–7 and the ratio [R]/[Ox] is 20 : 1. Calibration curves can be constructed with an R = 0.9997. The proposed method was compared with the traditional volumetric method and applied to veterinary supplement capsule and table salt samples. The limits of detection and quantification for iodine by the proposed method were estimated to be 0.020 mM and 0.036 mM, respectively.