The main source of iodine is

Options

A. `NaIO_(3)`
B. `NaIO_(4)`
C. `NaOl`
D. `Nal`

Answer

1

Explanation

Iodides occur only in minute concentrations in sea water but certain species of seaweed can use these minute amounts and concentrate the iodine in themselves as oranic compounds. This source, together with the brine well(in California) supply some of the world's iodine but the main source of this element is sodium iodate `(Na^(+)IO_(3)^(-))` which occurs in sodium nitrate deposits in chile. The mother liquor containing sodium iodate (i.e. the liquid remaining after crystallisation of `NaNO_(3)` is treated with sodium hydrogen sulphite when the iodate is reduced to iodine:
`2IO_(3)^(-)(aq.)+5HSO_(3)^(-)(aq.)rarr3HSO_(4)^(-)(aq.)+2SO_(4)^(2-)(aq.)+I_(2)(s)+H_(2)O(l)`
The reaction involves three stages, first of all the iodate is slowly reduced to iodide:
`IO_(3)^(-)(aq.)+3HSO_(3)^(-)(aq.) overset("slow")rarrI^(-)(aq.)+3HSO_(4)^(-)(aq.)`
The iodide then rapidly reacts with unchanged iodate in the presence of hydrogen ions,
`IO_(3)^(-)(aq.)+5I^(-)(aq.)+6H^(+)(aq.)overset("fast")rarr3I_(2)(s)+3H_(2)O(l)`
A reaction occurs between the iodine and unchanged hydrogen sulphite, and iodine is only precipitated once all the hydrogen sulphite has been consumed:
`I_(2)(s)+HSO_(3)^(-)(aq.)+H_(2)O(l)rarr2I^(-)(aq.)+HSO_(4)^(-)(aq.)+2H^(+)(aq.)`
Crude iodine may be contaminated with iodine monochloride `(ICI)` and iodine monobromide `(IBr)`. It is purified by sublimation in the presence of potassium iodide which removes these impurities:
`K^(+)I^(-)(s)+ICI(l)rarrK^(+)CI^(-)(s)+I_(2)(g)`