The preparation of potassium iodate(V)

We saw in the previous experiment the reaction between halogens and cold alkali. At temperatures above 60 ºC a different reaction takes place. In this experiment we shall prepare some potassium iodate(V) in a boiling water bath where the following reaction takes place:

6KOH(aq) + 3I₂(aq) ⇒ 5KI(aq) + KIO₃ (aq) + 3H₂O(aq)

Great care needs to be taken in this experiment as the chemicals involved are particularly dangerous. The 4M potassium hydroxide solution is particularly corrosive and rapidly attacks skin tissue and wooden bench surfaces. The iodine causes brown stains on both skin and clothing. If you drop the solid iodine on the hot tripod it will sublime giving an attractive (but poisonous) purple vapour which can cause severe eye damage.

As soon as iodine is added to the potassium hydroxide solution a pale yellow solution is formed. Students often confuse this for the slight excess of iodine and consequently get very little product. Add iodine in small spatula measures until there is the clear brown colour of excess iodine. The addition of extra potassium hydroxide solution removes the excess iodine. Make sure you react all the excess iodine, particularly any which may be clinging to the upper parts of the test tube. If you do not do this your product will be contaminated with iodine. You should be able to calculate the maximum theoretical yield.

We use the principle of fractional crystallization to extract the potassium iodate(V) at this stage. We use suction filtration to remove the solid product from the solution. Use only a little cold water to wash any remaining solution from your crystals - if you use too much your crystals will dissolve leaving very little (or no!) product. Dry the crystals with clean, dry filter paper and then place in a desiccator.

Video - the preparation of potassium iodate(V)