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Chemical clock

   Probably every aspiring chemist likes experiments in which the originally colorless solutions not shade. Colouring usually occurs immediately, but this may not always be. The solution mixture may still remain colorless while and then suddenly turns. This will be done in 5 or 10 seconds, according to our wishes - you these "chemical clock" can be set. First, prepare two solutions.

       I: 3.9 g of potassium iodate to 1 liter of water
II: 1 g of sodium sulfite, 0.94 g of concentrated sulfuric acid (carefully) and 3 ml of starch paste all calculated per 1 liter of water     

           Now we can proceed with the actual experiment. To 100 ml of solution II, gradually adding 100, 50, 25 ml of solution I (specify up to 100 ml with distilled water). Depending on the concentration of solution I occurs bluing solution in the range of about 6-24 p reaction is very dependent on the temperature (the lower the temperature, the longer the time)         

   In solution, these reactions take place

2KIO3 + H2SO4 --> 2HIO3 + K2SO4
Na2SO3 + H2SO4 --> H2SO3 + Na2SO4
HIO3 + 3H2SO3 --> 3H2SO4 + HI
HIO3 + 5HI --> 3I2 + 3H2O

   This sequence of reactions is released iodine, which colors starch wax blue. If he was given a course of these reactions, the solution would gradually tmavnul gradually, as would be liberated iodine. but in the container is another process - sulfurous acid (solution of hydrated dioxide in the water in equilibrium with sulfurous acid for those curious) reacts with free iodine and then turns it on hydroiodic acid

I2 + H2SO3 + H2O --> H2SO4 + 2HI

This reaction is faster and iodine stained starch does not have time. In these reactions, sulphurous acid consumed! Once all converted to sulfuric acid is to eliminate iodine, which stained starch.

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