[elerock21227]

So for my lab I performed an iodometric titration of ascorbic acid to determine amount of vitamin c in a sample of orange juice. However, I was also required to investigate how presence of a transition metal affects the amount of vitamin C in a sample. I chose to use copper. However, my results don't make sense.

The more copper I added, the less titrant I needed. My control needed 5ml to endpoint. I added 2g of Cu (NO3)2 to my 25.00ml of orange juice-hcl-starch-KI solution and I only needed one drop of KIO3 to reach the endpoint. In my calculations, the concentration of vitamin C actually INCREASES?  

I investigated the possibility of a side reaction

2 CuI2 → 2 CuI + I2

Although, I did not observe any gas emitted... There was no CUI precipitate because KI was present

Any help is appreciated.

[AWK]

http://daphne.palomar.edu/ngeetha/210%20Stuff/Iodimetric%20Titration%20of%20Vitamin%20C.htm

You did completely wrong experiment. You cannot add any metal cation that oxidize iodide to iodine ie. Cu²⁺, Fe³⁺ Mn³⁺ etc.