[StonyBear]

I'm doing the standard back titration of orange juice with excess triiodide using sodium thiosulfate.

For one of my variables I have decided to determine the effect of EDTA on vitamin C retention over a fixed period of time. The EDTA will work to bind metal ions which catalyse the oxidation reaction of vitamin C's vitamer of ascorbic acid into dehydroascorbic acid. As such I wanted to introduce metal ions into one sample of orange juice to outline one extremity of vitamin C loss.

To do this, I have decided to introduce copper sulfate into one sample of juice. I was just wondering if any of you guys would know whether this copper sulfate will effect the titration process at all? As in, will it react with thiosulfate, iodine, starch any acids in the juice that may effect the overall outcome of the experiment?

And secondly, would any of you guys know a rough estimate of the concentration of metal ions in natural orange juice (fresh squeezed) and commercial orange juice?

Thanks in advance.

[Hunter2]

Copperions are not good idea, they will react with iodide.  2 Cu²⁺ + 4 I^- => 2 CuI(s) + I₂ . Your titration result will have an error, because excess Iodine created.

[StonyBear]

Thanks for your reply,

I thought copper ions were originally in the orange juice in the first place, albeit a small amount. I'm only thinking about adding 20mL 1.0M CuSO4 to 500mL of orange juice stock and I read somewhere that approximately 32mg/100mL of Cu ions exist in natural orange juice. So assuming that I only add a small amount, will the error be somewhat negligible?

If not, would you happen to know which metal ions would be a good idea?

Regards,

[Hunter2]

I would use Magenesium or Calcium.

[opsomath]

Mg/Ca are not as good choices, they are not transition metals and as such they don't have an accessible redox couple. What Cu(II) is doing is serving as a catalyst by being reduced to Cu(I) then re-oxidized to Cu(II) by atmospheric oxygen. As long as the amount of added Cu(II) is small relative to the ascorbate present, you should be fine. Iron would also be interesting to see the effects.

It looks like Cd and Pb are the most common heavy metal contaminants in OJ, as well, if you want to try those out.

http://www.sciencedirect.com/science/article/pii/030881468790166X

http://www.dep.state.fl.us/waste/quick_topics/publications/shw/solid_waste/BackgroundConcFLSoilsRep99-7.pdf

[StonyBear]

Thanks! That helps alot.

However, I tried that today. Adding 10mL 1.0M CuSO4 to 500mL orange juice and the solution turned green.

Now the problem is that since we're doing a thiosulfate back titration with starch, the end point is green. I was wondering if the solution turning green with the addition of CuSO4 is 'supposed' to happen.

Additionally when EDTA was added to a solution with CuSO4 introduced, I found that the resulting orange juice had two layers. The supernatant being lighter than the dark green (identical to CuSO4 + Orange juice only) of the bottom layer.

Are these things 'meant' to happen?

I was assuming that the top layer might be the EDTA and cation complex. But after some researching, it doesn't seem like the complex is meant to have any colour.

Thanks.