Group 7: Halogens
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This is a question about experiment 6.8a in the Nuffield Chemistry students’ book. The aim is preparation of potassium iodate. It asks you to predict using a graph which will crystallise first: potassium iodate or potassium iodide. I really don't understand how to use this graph. Can you tell me which will form crystals first?
Igloo says
Have a good look at the graph on page 128 of the Nuffield Chemistry book. This shows how the solubilities in water of potassium iodide and potassium iodate vary with temperature. In hot solutions, e.g. at 80°C, 100 g of water will dissolve as much as about 180 g of potassium iodide but only about 15 g of potassium iodate can dissolve.
At lower temperatures, e.g. 20°C, the solubility of potassium iodide has dropped to about 140 g but that of potassium iodate has fallen to a very low value indeed, perhaps to about 5 g.
Now, although you are not using exactly 100 g of water in your experiment, the solubility figures still show that potassium iodate is about 30 times less soluble than potassium iodide, and is therefore the solid more likely to be incapable of remaining in the cooled solution. That is, it is the solid which will crystallise out.
The experimental conditions which you have followed in the instructions were designed to ensure that while all the potassium iodide remains dissolved, most of the potassium iodate crystallises.
This technique has helped to remove the potassium iodate from the mixture in solution, though it has to be admitted that the main impurity in your potassium iodate product will of course be potassium iodide.
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updated: 19 January 2006
