What causes vitamin C concentration to increase in freshly squeezed orange juice?

Question

Hi.

In year 13 Chemistry we titrated an iodine solution with freshly squeezed orange juice which had been stored at different temperatures. We keep the orange juice solution in an incubator to reach the temperatures (20⁰C, 40⁰C, 60⁰C, 80⁰C). To get our orange juice to reach 100⁰C we used a water bath. We were expecting the vitamin C concentration to decrease as the temperature increased, but the vitamin C concentration increased. Does anyone know why this would happen?

These are my titration results: 20⁰C average titre volume - 9.2ml, 40⁰C average titre volume - 7.2ml, 60⁰C average titre volume - 6.5ml, 80⁰C average titre volume - 6.3, 100⁰C average titre volume - 7.1ml. 
These are my vitamin C concentration results: 20⁰C -0.0827mol/L, 40⁰C- 0.113 mol/L, 60⁰C - 0.125mol/L, 80⁰C - 0.129mol/L, 100⁰C - 0.114mol/L.

If you could help I would really appreciate it. Thank you.

Laura

Answer 1

You state that you titrated an iodine solution with orange juice, but my guess is that you titrated the orange juice with the iodine solution after adding starch to the orange juice.  This being the case, it appears from your titration results that as the temperature increases, the volume of of titrant (iodine solution) decreased.  That is, 9.2 ml at 20ºC to 6.3 ml at 80ºC.  Not sure what happened at 100ºC.  This indicates that the concentration of ascorbic acid DECREASED as the temperature increased, not the other way around as you have concluded.  I'm not sure how you did your calculations, so I can't comment on where your mistake may be.