Re: Why didn't my starch-iodine reaction occur properly?

Area: Chemistry
Posted By: Artem Evdokimov, Grad student, Structural Biology (Chemistry)
Date: Tue Jul 8 15:26:46 1997
Area of science: Chemistry
ID: 868311116.Ch

Message:

Dear Craig,

I asume that you are asking about Briggs-Raucher reaction. General scheme is approximately as follows :

            Mn2+          Mn2+ -----------\           Mn2+
               \        /               H2O2 \        /
            H2O2 \     /                       \    /
                   Mn3+                         Mn3+
CH2(COOH)2 + HIO3 ----> ICH-COOH + CO2 + H2O ----> HI + CO2
       (colorless)   ^         (colorless)                                                 
                     | (colorless)
                     |           
                     |
                     | This is the main cycle.            
                     |                                                                                 
                     |
                       H2O2                                                                      
                                                              H2O2
                     |                                           |                                     
                     |                                      starch, I- v
                     |                                           |              
                     I  I -*I2*starch complex  <----------------------   I2
                    (blue)                                   (yellow)

Temperature will affect the rates of different reactions in different ways, but oxidation of iodine with peroxide would probably be affected the most. At high temperature, the rate of this reaction may be so high that it would take a lot of time for significant amounts of iodine to accumulate initially and produce color.

That's my suggestion, I could be wrong of course.

A.G. Evdokimov

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