State two ways to make standard triiodide solution.

• Iodine is a chemical element with atomic number $53$and the symbol $\mathrm{I}$.

• At typical conditions, it exists as a semi-lustrous, non-metallic solid that melts to produce a deep violet liquid at${114}^{\mathrm{\pi }}\mathrm{C}$and boils to form a violet gas at ${184}^{\mathrm{\pi }}\mathrm{C}$

• Iodine is found in a variety of oxidation states, including iodide$\left(\mathrm{I}\right)$, iodate$\left({\mathrm{IO}}_3^{-}\right)$, and periodate anions.

• It is the least common of the stable halogens, occupying the sixty-first position in the periodic table.

• It is the heaviest nutrient in terms of necessary minerals.

One way to make standard triiodide solution is to mix together ${\mathrm{KIO}}_3\mathrm{and}{\mathrm{KI}}_{\mathrm{and}}$then add a strong acid such as $\mathrm{HCI}\mathrm{or}{\mathrm{H}}_2{\mathrm{SO}}_4$.

Triiodide is formed by a reverse disproportionation

${\mathrm{IO}}_3-8\mathrm{I}+6\mathrm{H}\xcancel{}3{\mathrm{I}}_3+3{\mathrm{H}}_2\mathrm{O}$

Thiosulfate can also be used for standardization of triiodide.

It reacts with triiodide and iodide and tetrathionate ions are formed:

$2{\mathrm{S}}_2{\mathrm{O}}_3^{2‐}+{\mathrm{I}}_3^{-}\xcancel{}3{\mathrm{I}}^{-}+{\mathrm{S}}_4{\mathrm{O}}_6^{2‐}$