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 :shifty:

[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 :sherlock: 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 :D
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<small>^®</small> 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<small>^®</small> 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<small>^®</small> 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<small>^®</small> 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:
<table border="1" bordercolor="#dddddd" cellspacing="2" height="51" width="465"><tbody><tr bgcolor="#dddddd"> <td width="33%">
Sample
</td> <td width="33%">
Reflectoquant
[mg/l]
</td> <td width="33%">
Titration
[mg/l]
</td> </tr> <tr> <td width="8%">
Blackcurrant juice 1
</td> <td align="center" width="8%">
104
</td> <td align="center" width="8%">
106
</td> </tr> <tr> <td width="8%">
Blackcurrant juice 2
</td> <td align="center" width="8%">
179
</td> <td align="center" width="8%">
200
</td> </tr> <tr> <td width="8%">
Grape juice (red) 1
</td> <td align="center" width="8%">
18
</td> <td align="center" width="8%">
14
</td> </tr> <tr> <td width="8%">
Grape juice (red) 2
</td> <td align="center" width="8%">
36
</td> <td align="center" width="8%">
37
</td> </tr> <tr> <td width="8%">
Grape juice (red) 3
</td> <td align="center" width="8%">
63
</td> <td align="center" width="8%">
66
</td> </tr> <tr> <td width="8%">
Cherry juice
</td> <td align="center" width="8%">
478
</td> <td align="center" width="8%">
446
</td></tr></tbody></table>


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 :nono:)

[flash]

:shit: that looks a bit outer my league :shit:

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]

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 ?

[alexthekidd]

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!

Ah men to dropping chemistry after 5th Form AJ :D Give me History anyday :lol:

Oh sorry for not contributing anything, i'll hang my head in shame now

[flash]

nah i like the real sciences more :P, bios a girls subject :D.
usually chems pretty easy, but i seem to always make it harder for myself.

so everyone, ive tried to make a sealed cap for my pineapple juice instead of the whole grape juice, ill tell the results tomorrow and say if the seal holds up.

and no you wouldn't get wasted on it, you would have a very clean stomach though :P

[Curious_AJ]

biology is actually one of the more hard subjects... i mean... just the other day in my anatomy lecture we were going through the complexity of muscle cells and structure etc... it just gets smaller and smaller!! memorising that is a bitch... but we DO get to disect cool things like pigs trotters... i enjoyed that.. i should be a surgeon instead of a vet nurse....

mmm well, how did you seal it? and ill tell you how itll hold up... im good at that sort of thing... lol

[flash]

yeah i know, i was only joking bout the bio. just my mates are doing it this year and have to grow 200 plants :P

i cut a hole in the cap, and wraped that split black ruber around the cut and hot glue guned around the edge, then ill be puting in a rubber cork that lets u suck out air with a pump. if that doesnt work, im bringing the hot glue gun with me and gluing thdamn thing in :P

[tri boy]

Will this get you pissed ?

Big splash of vodka will accelerate the chemical reactions. (at least in your head):done:

[Curious_AJ]

yeah i know, i was only joking bout the bio. just my mates are doing it this year and have to grow 200 plants :P

i cut a hole in the cap, and wraped that split black ruber around the cut and hot glue guned around the edge, then ill be puting in a rubber cork that lets u suck out air with a pump. if that doesnt work, im bringing the hot glue gun with me and gluing thdamn thing in :P

that seems solid enough to work actually.. it should be fine... as for the 200 plants.., thats REALLY GHEY! in my time in school and uni, ive done SOO much more exciting bio! lol...

[Delphinus]

I did the VitC test back a couple of years ago as well... just used various orange juices like freshup and just juice etc. Got some good results from that.

Can you put it into another bottle?

[MrMelon]

If you think chemistry sux, how can you get into biology? Biological processes don't happen by magic, it's all chemistry!

It's been years since I really did any titrations so take my advice with a grain of salt. With the titration could you not titrate the grape juice with DCPIP (dichlorophenolindophenol)? a 0.1% solution will react with ascorbic acid and form a blue solution when all the ascorbic acid's used up so you can tell how much ascorbic acid you had to begin with. The grape juice being coloured might make it difficult though. More info on it here anyway. http://www-saps.plantsci.cam.ac.uk/osmoweb/vitc.htm

[doc]

biology is actually one of the more hard subjects... i mean... just the other day in my anatomy lecture we were going through the complexity of muscle cells and structure etc... it just gets smaller and smaller!! memorising that is a bitch... but we DO get to disect cool things like pigs trotters... i enjoyed that.. i should be a surgeon instead of a vet nurse....

mmm well, how did you seal it? and ill tell you how itll hold up... im good at that sort of thing... lol
Did you know that if one was to grasp the Trachea of a sheeps lung and blow into it they inflate and turn nice and pink again. Plus if some one squeezes them at the same time you can make Bagpipe sounds. My Army Adv Med 101 experience.

[doc]

Big splash of vodka will accelerate the chemical reactions. (at least in your head):done:
Can I just try it with the vodka.

[Curious_AJ]

[QUOTE=doc;1039135]Did you know that if one was to grasp the Trachea of a sheeps lung and blow into it they inflate and turn nice and pink again. Plus if some one squeezes them at the same time you can make Bagpipe sounds. My Army Adv Med 101 experience [QUOTE]

yes i did actually! ive done it before... its quite fun... but weird, considering i dont eat meat coz i think its gross... lol

[tri boy]

Did you know that if one was to grasp the Trachea of a sheeps lung and blow into it they inflate and turn nice and pink again. Plus if some one squeezes them at the same time you can make Bagpipe sounds. My Army Adv Med 101 experience.

Sculling vodka, AND blowing sheep....Damn! :sick:

[Curious_AJ]

hahaha, you certainly have a weird way of interpreting things tri boy....

[flash]

your not allowed to use different brands now, they said it became too easy or something. so now you need the exact same juice but test it at different time periods. i didnt want to change bottles but if myy makeshift sealer wont work ill put them into the wine bottles

[flash]

If you think chemistry sux, how can you get into biology? Biological processes don't happen by magic, it's all chemistry!

It's been years since I really did any titrations so take my advice with a grain of salt. With the titration could you not titrate the grape juice with DCPIP (dichlorophenolindophenol)? a 0.1% solution will react with ascorbic acid and form a blue solution when all the ascorbic acid's used up so you can tell how much ascorbic acid you had to begin with. The grape juice being coloured might make it difficult though. More info on it here anyway. http://www-saps.plantsci.cam.ac.uk/osmoweb/vitc.htm
that makes the whole grape juic look sooo much easier. might print that and take it to the teacher. but ill also bring the pineapple juice becaseu i know something gona go wrong with one of them :P

[Ixion]

If you think chemistry sux, how can you get into biology? Biological processes don't happen by magic, it's all chemistry!

It's been years since I really did any titrations so take my advice with a grain of salt. With the titration could you not titrate the grape juice with DCPIP (dichlorophenolindophenol)? a 0.1% solution will react with ascorbic acid and form a blue solution when all the ascorbic acid's used up so you can tell how much ascorbic acid you had to begin with. The grape juice being coloured might make it difficult though. More info on it here anyway. http://www-saps.plantsci.cam.ac.uk/osmoweb/vitc.htm

I think this may be problematic with grape juice due to the presence of tannins and tannic acid (might be better with white grape juice ?). Tannins are also reducing agents an will decolourise the DCPIP. (The DCPIP is actually blue in its native state, the ascorbic acid decolourises it.)

But I also have not been involve this stuff for donkeys years.

[MrMelon]

We learned the mechanism of how LSD is cyclised from a reasonably simple precursor today! Who said chemistry lectures were all boring :D

[Curious_AJ]

oooh LSD!!!!!!

[doc]

oooh LSD!!!!!!
I have friends that in the 70's worked in institutions, where they called for volunteers to experiment in the use of LSD. But back to topic about this juice how's it going with the vodka suggestion.

[flash]

well the seal didnt work, and i had yesteray off school, so im really behind now :(, ill try fix it on monday though, even if i have to wag all my classes to do so :P