Chemistry trouble

[flash]

hay everyone, stuck in a pickle for chem.
im doing titration's right, and i have to find the concentration of vitamin c in a juice right?. i decided to do sparkling grape juice to be different, sadly theres next to no asorbic acid in there and from my calculation's im gonna need 80ml+ to even get a good enough value.
im wanting to use preferably around 20ml, anyone know how to do this? i know i should but i didnt listen when we were.
im titrating 25ml of KIO3, 5ml of H2SO4, 15ml of KI and my sparkling grape juice with 0.1mol L-1 of sodium thiosulfate

[Mr. Peanut]

Just dip a vitamin C pill in there while no one's watching

[flash]

good idea, but now im quite interested how much is in there now :P

[Mr. Peanut]

Ixion is trained in this sort of thing I imagine you'd have to reduce the concentration of the reactants.

[Ixion]

Ascorbate is usually assayed using phosphomolybdic acid.

I assume you are relying on reduction of the iodate to free iodine and then titrating against thiosulphate in the usual way.

In theory , the amount of ascorbic acid is what matters. The extra water and other stuff *shouldn't* make any difference, except that it will be harder to see the endpoint in such dilute solution. Adding a little starch will help.

The method seems instinctively reversed I would have thought you'd have titrated the acid solution against a known quantity of thiosulphate and taken the persistence of iodine as an end point. But, whatever.

I'm wondering if this will work in something as complex as grape juice, with a lot of other possible acids (pectic, malic, tannic spring to mind) in there.

But anyway, your tutors should be looking for the LOGIC of what you do, not a right answer.

So, do the experiment, Then explain the results. Your results are your results, there's no "right" or "wrong" . You are the analyst. Do the analysis as prescribed. Then document and discuss what you find.

EDIT If ascorbate was heat stable you could concentrate the solution. Alas , it is not. Though it might be interesting to concentrate 80ml down to 20 ml and compare the two results.

[flash]

hay thanks alot Ixon
i chose sparkling grape juice because not only do we find the vitamin c, weve got to see if it deteriorates over time and one of my experiments are seeing if vacuum sealing the bottle will slow down the deterioration, seem im using a vacuum sealer made for wine, grape juice is the only bottle i know that is small enough for the cap to fit without changing bottles.

and yes i did do a blank tiration with no vitamin c in, to find the amount of iodine. my avarage was about 14.6mls of S2O3, but when i tried it with 20ml of grape juice added the results rose to around 16mls. i tried with 40ml and got about 15.1mls. the teacher wants it to be atleast 5ml less that the blank tiration so i can mius the new amount of iodine to the old

[Ixion]

Here is the standard method, probably beyond the resources of a school laboratory

Method: Reflectometric determination after reaction with molybdophosphoric acid to phosphomolybdenum blue

Reagents:
Reflectoquant^® Ascorbic Acid Test, Cat.No. 1.16981.0001
Reflectometer RQflex, Cat.No. 1.16970.0001 or RQflex plus
Cat. No. 1.16955.0001
Oxalic acid dihydrate, Cat.No. 1.00495.0500
Sulfuric acid 25%, Cat.No. 1.00716.1000
Polyvinylpolypyrrolidone Divergan^® RS, Cat.No. 1.07302.0100
Water GR, Cat. No. 1.16754.9010
1% oxalic acid solution
Dissolve 1 g of oxalic acid in 100 ml water GR.

Sample preparation:
Dilute juice 1:1 with oxalic acid solution 1% (10 g/l). Add approx. 500 mg polyvinylpolypyrrolidone Divergan^® RS to 10 ml of that diluted juice and adjust pH to <1 using sulfuric acid (add approx. 5 drops of sulfuric acid 25%). Mix thoroughly for approx. 1 minute and filter through a filter paper.

Analysis:
Press the reflectometer START key and simultaneously dip the analytical test strip into the sample
(5 - 30°C) for ca. 2 seconds, ensuring that both reaction zones are immersed. Remove all excess liquid from the strip.

Wait 15 seconds and then measure the strip in the reflectometer. The value [mg/l] will be stored automatically (Please refer to the RQflex operating instructions and the package insert of the Reflectoquant^® Ascorbic Acid Test).

NB:
Instead of filtering the solution, it is also possible to allow the polyvinylpolypyrrolidone to be deposited and to dip the analytical test strip into the supernatant solution.

Calculation:
Ascorbic acid content [mg/l] = Measured value [mg/l] x 2

Results - comparison with potentiometric titrition:

Sample	Reflectoquant [mg/l]	Titration [mg/l]
Blackcurrant juice 1	104	106
Blackcurrant juice 2	179	200
Grape juice (red) 1	18	14
Grape juice (red) 2	36	37
Grape juice (red) 3	63	66
Cherry juice	478	446

In industry, where it is normally a matter of checking for a specific level, rather than needing to know an actual concentration, enzymatic test strips are usually used. Dip the low strip if it changes colour that's good, if not add ascorbate, dip the high strip if it doesn't change colour thats good if it does add must (or water )

[flash]

that looks a bit outer my league

so thers no easier way? and i dobt the school will have all that, or ill have the time :P

[flash]

does anyone where i can buy some juice that comes packaged in something that has the same size and neck to a wine bottle?. i fear the grape juice may be too hard

[Oakie]

Pardon ?

[Skyryder]

Ask Glaxo Industries. The maker of Ribena. Vitamin C

Skyyrder

[Curious_AJ]

wow, i didnt even get as far as titration in chem... hah.. i quit after 5th form.. its mostly boringness to me... give me biology any day!

sorry i couldnt help mate!

[nodrog:Me]

geez that shit looks complicated, all they teachded us at school was how to make P.

[Curious_AJ]

hmm... P.. how interesting... all they taught us at school was "P makes you think you have things inside you" ...

[doc]

hay everyone, stuck in a pickle for chem.
im doing titration's right, and i have to find the concentration of vitamin c in a juice right?. i decided to do sparkling grape juice to be different, sadly theres next to no asorbic acid in there and from my calculation's im gonna need 80ml+ to even get a good enough value.
im wanting to use preferably around 20ml, anyone know how to do this? i know i should but i didnt listen when we were.
im titrating 25ml of KIO3, 5ml of H2SO4, 15ml of KI and my sparkling grape juice with 0.1mol L-1 of sodium thiosulfate

Will this get you pissed ?