Some people have an allergic reaction to the food preservative sulfite (SO 3 2-

Question

Some people have an allergic reaction to the food preservative sulfite (SO32- ), which can be measured by instrumental methods or by a redox titration: Initially, a solution of I3- was prepared by dissolving 1.100 g of KIO3, 7.000 g of KI, and 1.0 mL of 6 M H2SO4 in a 100 mL volumetric flask and diluting to the mark.

a)Write the reaction that occurs when the sulfuric acid is added to potassium iodate and potassium iodide.

b) Write a balanced equation for the reaction that occurs between the I3- and the sulfite in the wine.

c) 5.00 mL of the solution described in (a) is added to 50.00 mL of the wine and all of the sulfite reacts. The excess I3- is then titrated with 0.05111 M Na2S2O3 and 13.55 mL are required to reach a starch-indicator end point. What is the concentration of sulfite in the wine?

d) When should the starch-indicator be added to the titration solution – at the start or just before the equivalence point is reached? Justify your answer.

Explanation / Answer

a) the rection goes as :

5KI + KIO3 + 3H2SO4 3I2 + 3K2SO4 + 3H2O

D) STARCH SHOULD BE ADDED JUST BEFORE EQUIVALENCE POINT IS REACHED AS IF IT IS ADDED EARLIER IODINE TENDS TO REMAIN BOUND TO THE STARCH EVEN AFTER EQUIVALENCE POINT IS REACHED GIVING AN UNWANTED ERROR

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