[orangesturningblue]

So, as the title says i'm a bit stumped about determining the ascorbic acid content of orange juice by titrating it against Na₂S₂O₃.

The first reaction I produce is:

IO₃^- + 8I^- + 6H⁺ ---> 3I₃^- + 3H₂O

Basically, I produce a known amount of I₃^-. I dump some orange juice in this mixture so that

C₆H₈O₆ + I₃^- ---> C₆H₆O₆ + 3I^- + 2H⁺

Get it? I have an excess I₃^- and some of it reacts with an unknown amount of C₆H₈O₆. In order to determine how much I₃^- actually reacted with C₆H₈O₆ I titrate it:

2S₂O₃^2- + I₃^- ---> 3I^- + S₄O₆^2-

I know the concentration of S₂O₃^2-. However, I don't know 'when' the reaction ends. I tried this with a vitamin c tablet and the titration goes from (red brown--->yellowish) then I add some starch, which turns it dark blue and then I continue titrating it till it goes colourless.

However, now that I'm using orange juice, the colour changes differently. It went from (red-brown--->yellow greenish) then I added starch and it turned dark bluish and after I continue titrating it, it turns to a cloudy light bluish greenish colour.

In summary, I don't know the colour indications when I titrate S₂O₃^2- to a mixture of orange juice and I₃^-. If you got an answer, it will be nice if you can source it as well Thanks.

[Hunter2]

I think its a matter of sample volume. Orange juice contain colour yellow what disturbs the other colour from iodine and starch. What is your sample amount? I would use only 1 ml or so in high dilution with water in the titration flask. I think its need some expierence and testing. Also molarity of Thiosulfat can be adjusted.