1969 Volume 42 Issue 4 Pages 952-955
After 5 ml portions of 1×10−4 M methylthymolblue (MTB or H2Me4−) and mercury(II) nitrate solutions had been added to an iodide sample solution adjusted to pH 7.3–8.8, it was diluted to about 50 ml with water. This solution was titrated with a 1×10−2 M standard mercury(II) ethylenediaminetetraacetate (Hg-EDTA or HgY2−) solution until it turned blue from greenish yellow. The iodide-ion content was calculated by applying the volume of the standard Hg-EDTA solution on the calibration curve determined with solutions of known iodide-ion contents. The iodide ion reacts with HgY2− and with greenish-yellow HgMe4− to form colorless [HgY(I)]3− and blue [HgMe(I)]5− complexes respectively. The [HgMe(I)]5− complex reacts subsequently with the iodide ion to form mercury(II) iodide and greenish-yellow H2Me4−. It has been found that, except when an excess amount of the iodide ion existed, the [HgMe(I)]5− complex is the most stable among these mixed ligand complexes and mercury(II) iodide. Therefore, the iodide ion can be titrated directly with a Hg-EDTA solution by using HgMe4− as an indicator according to the following equation:
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