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Vitamin C in kiwifruit

Iodometric back titration; redox equations from descriptions; validity

Vitamin C (ascorbic acid,

\ce{C6H8O6}

, M = 176.0 g mol

^{-1}

) is oxidised by iodine to dehydroascorbic acid,

\ce{C6H6O6}

. 20.0 g of kiwifruit was blended and the extract made up to 100.0 mL. To a 25.00 mL aliquot, 10.00 mL of 0.00200 mol L

^{-1}

\ce{KIO3}

, excess KI and dilute

\ce{H2SO4}

were added; the iodate reacts with iodide in acid to form iodine and water. The remaining iodine was titrated with 0.00500 mol L

^{-1}

sodium thiosulfate,

\ce{Na2S2O3}

, which is oxidised to tetrathionate,

\ce{S4O6^2-}

; starch was added near the end point. Average titre: 13.80 mL. Reduction potentials (V):

\ce{C6H6O6/C6H8O6}

+0.06 (value at pH 7, the conditions in fruit);

\ce{I2/I-}

+0.54;

\ce{O2/H2O}

+1.23.

Facts you may need

redox half-equations
E°cell
back titration
starch–iodine indicator

(a)

Write half-equations and the overall equation for the oxidation of ascorbic acid by iodine, and use

E°

values to show that it is spontaneous.

(b)

Write balanced equations for (i) the formation of iodine from iodate and iodide, (ii) the reaction of iodine with thiosulfate.

(c)

Calculate the vitamin C content in mg per 100 g of kiwifruit.

(d)

Justify (i) generating the iodine in the flask rather than adding a standard iodine solution, and (ii) adding starch only near the end point. State the colour change at the end point.

(e)

A student left the kiwifruit extract standing open for two hours before analysing it. Evaluate the effect on the result, using

E°

values, and suggest one change to the procedure to improve validity.