[fobbz]

I have an upcoming titration lab for my first year chemistry class at uni. We have to design a titration lab for finding the mg of Vit C in some sort of juice, fruit, or vegetable.

I have chosen to use Orange Juice by minute maid, which contains 84mg of Vit C per 450mL. I calculated this based on Daily Values on the nutrition label with was 140% daily value Vit C in the bottle. Daily values say 60mg should be consumed every day.

Now in the titration we are using KIO3 among HCl and KI for other reasons. This is not what I am concerned about however. Based on my calculations for mg Vit C in 25ml which is the volume we will be titrating I have found there to be 4.667mg Vit C. When using molar ratios and finding the mass of KIO3 required for the titration I have found it to be 0.001889g. This is problematic as weighing out this much KIO3 will not be accurate on the analytical balance (despite carrying out 6 titrations, so 6x0.001889g).

What can I do do increase the amount of KIO3 needed for the reaction to occur? I was thinking about grinding up Vit C tablets and adding them into the mix. The lab manual also recommends that 0.01-0.4g KIO3 should be weighed out but my calculations are way off for that. Plus anything less than 0.1g will have only 3 significant figures and this will be problematic in the titration.

[Arkcon]

Briefly, you're not figuring the procedure for an analytical redox titration correctly.  Its good that you've developed a ballpark figure for what you titration will need,  But titration isn't done with adding a stoichiometric amount of the titrant.  You'll instead prepare a standardized solution, and add it gradually, and from the volume back calculate the moles.

http://en.wikipedia.org/wiki/Titration

[fobbz]

Briefly, you're not figuring the procedure for an analytical redox titration correctly.  Its good that you've developed a ballpark figure for what you titration will need,  But titration isn't done with adding a stoichiometric amount of the titrant.  You'll instead prepare a standardized solution, and add it gradually, and from the volume back calculate the moles.

http://en.wikipedia.org/wiki/Titration

I have done titrations before. I have to do some stoichiometric calculations in order to determine how much KIO₃ I am going to need. The problem is, there is not enough Vitamin C for me to weigh out and amount of KIO₃ that will maintain 4 significant digit accuracy. However if I am calculating incorrectly this could be why my values seem so far off. The ratios are 3 moles vit C for 1 mole KIO₃ are they not?

[fobbz]

Okay so I think I have found a solution to my problem.

What if I took, say 0.2000g KIO₃. I then dilute it with 250mL water.

If I use 75mL of OJ (inside the erlenmeyer obviously), that will have 14.00mg of Vit C in it.

Now based on my calculations and the 3:1 molar ratio of Vit C:KIO3, i should reach the end point of my titration after adding about 21.27mL of my KIO3 diluted solution.

[Borek]

Using larger sample of OJ seems to be a correct approach.

You will be not titrating with iodate. You will be titrating using solution prepared from iodate, but that's not the same.

[fobbz]

Using larger sample of OJ seems to be a correct approach.

You will be not titrating with iodate. You will be titrating using solution prepared from iodate, but that's not the same.

No I'm not titrating with Iodate, I'm titrating with Potassium Iodate KIO₃. I don't understand what you mean...

[Borek]

When I wrote "iodate" I meant potassium iodate - but it doesn't matter much, as sodium or lithium iodate would do as well. We don't use them for other reasons.

However, it is not iodate (of whatever) that is the titrating agent. What is the reaction equation?

[fobbz]

When I wrote "iodate" I meant potassium iodate - but it doesn't matter much, as sodium or lithium iodate would do as well. We don't use them for other reasons.

However, it is not iodate (of whatever) that is the titrating agent. What is the reaction equation?

[Arkcon]

How will you know there is no more vitamin C left, or that you have added enough KI to consume all the Vit C?  For that matter, which of those will you be looking for?

[Borek]

OK I checked the page the image is taken from and it looks like you will be really titrating with iodate - that's strange if you ask me, as one of the prerequisites of reaction used for titration is it its perfectly known stoichiometry,and  I am not convinced adding iodate to the solution already containing a reducing agent other than I^- is a good idea, as side reactions can make the method irreproducible. Classic approach would be to add well known amount of iodate to excess iodides, then to add ascorbic acid, then to titrate excess iodine with thiosulfate, apparently you are expected to take a shortcut.

I have no choice but to believe it was tried and gives good results.