[microberry1]

Hello there 

I am doing a practical chemistry investigation in which I have heated samples of orange juice at different temperatures (at 40°C intervals, starting at 40°C and ending at 240°C). I obtained a decreasing trend in vitamin C concentration as the temperature heated at increased. What I am looking for now is an explicit explanation as to why this occured; I have looked at and read many scholarly articles on people doing the same experiment (albeit with much better equipment that I used), but they only talk about the trend, and I can't see anywhere, in the report or in the cited articles, about the actual degredation of vitamin C as it is heated, or why it is so heat sensitive.

So I ask, what is it that happens to vitamin C when it is heated that makes its concentration in solution decrease? I know that it oxidises to dehydroascorbic acid, but does the heating of the vitamin C relate to this? Does something happen to the bonds in the molecule that causes it to degrade somehow? I think I should add that I determined the vitamin C of the heated samples by titrating the samples with potassium iodate in the burette, and also in the presence of iodide, 1M hydrochloric acid and starch indicator (the iodate reacted with the iodide to produce iodine, which reacted with the vitamin C, and once all the vitamin C went, the iodine was free to react with the starch indicator, indicating the endpoint). I used commercial orange juice.

Also, could I possibly be linked to some scholarly article or other reliable document that states why the vitamin C degrades, for the purposes of the report?

Thanks so much!
microberry1

[Dan]

I know that it oxidises to dehydroascorbic acid, but does the heating of the vitamin C relate to this?

Yes, I think this is the main reaction. Other products of decomposition will probably also form, but in much smaller amounts. See for example: http://www.sciencedirect.com/science/article/pii/S0308814611006224

[microberry1]

I know that it oxidises to dehydroascorbic acid, but does the heating of the vitamin C relate to this?

Yes, I think this is the main reaction. Other products of decomposition will probably also form, but in much smaller amounts. See for example: http://www.sciencedirect.com/science/article/pii/S0308814611006224

Thankyou soooo much for the link, I will definitely include information from this study. If I have any more questions, I will ask them later.

Thanks
microberry1

[microberry1]

Right, so I have another question: Can dehydroascorbic acid be further oxidised by iodine? I wouldn't think so, but I just want to make sure about this so that I can make mention of it in my report that the dehydroascorbic acid present in my juice samples could not further be oxidised by the iodine present.

Thanks
microberry1

[AWK]

Almost all analytical reactions proceed with 100 % yield

[microberry1]

Right, so I have another question.

I am doing a practical chemistry investigation in which I have heated samples of orange juice at different temperatures (at 40°C intervals, starting at 40°C and ending at 240°C). I obtained a decreasing trend in vitamin C concentration as the temperature heated at increased. I have carried out my titrations and have obtained results. But I can see that my titres are relatively high compared to what I would expect; after calculation, juice from a bottle with 100mg/250ml vitamin C (written on the bottle) had a concentration of 108mg/250ml for my sample heated at 40 degrees Celcius, but there was a negative trend after this value for samples heated at higher temperatures, but still, all except the two samples heated at 200 and 240 degress Celcius had vitamin C concentrations above 100mg/250ml. What I think may be causing my higher titre values could be some reaction(s) occuring in the conical flask that causes some of the iodate or iodine to be used up. I have done some research and have found that orange juice contains reducing sugars like fructose and maltose, and I also think that the sucrose in the juice can be hydrolysed to these reducing sugars by the hydrocholric acid used in the titration. So my question is, would any of the substances used in the procedure react with these reducing sugars, or any other things, in the orange juice, to require more iodate from the burette and hence produce higher titre values? I know that iodate and iodine can oxidise other substances, so would these possibly react with the reducing sugars?
Here is a link to the procedure I am following: http://www.outreach.canterbury.ac.nz/chemistry/documents/vitaminc_iodate.pdf

Thanks
microberry1

[AWK]

Ascorbic acid decompose at melting point (below 200 C), hence heating over this point makes no sense.
Orange juice may contain some amounts of other reducing compounds that react with iodine.