Iodine
Iodine is a real non-metallic element, although its
appearance is somewhat metal-like. It is a dark grey solid, with a metal-like
lustre. Iodine is quite volatile. When some solid iodine is stored in a
transparent glass bottle, then one can clearly see that the air above the solid
is light purple. This light purple color is due to the color of iodine vapor.
Iodine vapor at higher concentration is deep purple with a blue tinge.
The element iodine is quite reactive. It reacts readily with many metals, as demonstrated by an experiment with magnesium powder.
Elementary iodine is available on eBay frequently. It also can be purchased at many photography raw chemical suppliers. The element is fairly expensive, at a price tag of $0.15 - $1 per gram. The lower price only is for quantities of 500 grams and up. Solutions of elementary iodine in ethanol can be obtained as 'tincture of iodine', which is used for treating wounds.
Elementary iodine is fairly toxic. Its toxicity forms the basis of wound treatment, in which bacteria are killed by the toxic iodine. Most compounds of iodine only are marginally toxic.
Unfortunately, for US-citizens, the purchase and also the owning of iodine has become illegal. This is due to the 'war on drugs'. Iodine is really interesting for the home chemist, and it is sad that having such an interesting compound is forbidden, even if nothing illegal or bad is done with it. Even if iodine is available on eBay or other online sources, US-citizens should not order this if they do not want legal trouble. Simple inorganic salts, containing iodine (e.g. potassium iodide and potassium iodate), are not illegal, and iodine can be easily prepared from such compounds.
In its compounds, iodine can have oxidation states -1, +1, +3, +5 and +7. The most stable oxidation state is -1. In aqueous chemistry, oxidation states +1 and +3 hardly have any meaning. Compounds like ICl and ICl3 can be prepared easily, but in water these are hydrolysed quickly. Oxidation states +5 and +7 also are quite stable in the form of iodates, IO3–, and periodates, of which several forms exist (ortho periodates and meta periodates).
Compounds of iodine, available for the general public, are the following:
- potassium iodide, KI
- sodium iodide, NaI
- ammonium iodide, NH4I
- cadmium iodide, CdI2
- potassium iodate, KIO3
- potassium (meta)periodate, KIO4
Potassium iodide, sodium iodide and ammonium iodide all are white crystalline
solids, soluble very well in water. Sometimes, the iodide-salts are slightly
yellow. This is due to formation of a tiny amount of iodine, due to aerial
oxidation of iodide to iodine. Potassium iodide, ammonium iodide (and sometimes
sodium iodide) are available from photography raw chemical suppliers. The most
interesting of these compounds are potassium iodide or sodium iodide. These
allow experimenting with iodide, without possible interference of ammonium ion.
All iodides are fairly expensive, at a price tag, somewhat lower than that of
elementary iodine.
Cadmium iodide is a white or very light yellow solid, soluble in water. It is a
very toxic compound, but this is due to the cadmium content. If one wants to
experiment with iodine or its compounds, then cadmium iodide certainly is not
the compound to be used. Its properties are quite different from other iodides,
the cadmium content will interfere in most experiments.
Potassium iodate is a fairly strong oxidizer, but it is not as dangerous as
potassium chlorate and potassium bromate in dry chemistry experiments. Potassium
iodate is only moderately soluble in water. When this compound is used as
oxidizer, then it frequently is reduced to iodine, sometimes to iodide.
Potassium periodate is a very strong oxidizer. It even is capable of oxidizing
manganese(II) ions to permanganate quantitatively, albeit slowly. Also in dry
chemistry experiments, potassium periodate can give stunning results, when used
as oxidizer. The solid is only very sparingly soluble in water (appr. 0.4 gram
per 100 ml) and this severely limits the usefulness of this compound in aqueous
chemistry. A suspension of very finely powdered solid, however, can be used in
some experiments. Unfortunately, potassium periodate is quite expensive, and it
is not one of the chemicals to start with in a home lab.