Science Buddies: "Ask an Expert"

Hi,
I'm assisting my daughter in doing a project from the Science Buddies website entitled,"Which Orange Juice Has the Most Vitamin C?'"

We made the Iodine Titration Solution using Lugol's Iodine Solutiion we ordered from Home Science Tools. We made a 1:10 mixture
by mixing 30ml of Lugol's Solution in 300ml of distilled water.

Next we made a starch indicator solution. We mixed .25mg of cornstarch in 50ml of near boiling water. I noticed the instructions called
for water soluble starch. The cornstarch we used was 100 percent pure cornstarch. It appeared to dissolve in the water creating a milky
appearance. Maybe I used the wrong starceh. But let me continue.

Next we made a standard Vitamin C solution. I could only find 500mg tablets of Vitamin C. Also, all the Vitamin C tablets I saw on the shelves had some other ingredients mixed in. I bought the most pure Vitamin C tablets I could find. In addition to ascorbic acid, each tablet containec vegetable cellouse, croscamellose, vegetable magnesium stearate, silica, and vegetable stearic acid. We cut the tablet in half, and weighed out 250 mg on a digital scale. We crushed the half of tablet, dissolved it in 100ml of distilled water, and then added another 150ml of distilled water.

We measured 20ml of Vitamin C solution into a 50ml Ehrlenmeyer flask and added 10 drops of starch indicator solution, using an eyedropper.

We filled a burret with the iodine titration solution, placed the 50ml flask containing the Vitamin C solution and 10 drops of starch indicator solution under the burret. We let the titration solution drip into the flask, swishing after every drop. After dripping over 25ml of titration solution into the flask it NEVER CHANGED TO THE BLUE BLACK COLOR. Eventually the solution in the flask started taking on the color of the iodine solution, a bronze color. But it never ever changed to any shade of blue.

We next decided to titrate some orange juice. We poured 20ml of orange juice into flask and added 10 drops of the starch indicator solution. We began to titrate the solution of orange juice and the 10 drops of starch indicator solution. It NEVER turned to the grayish brown color indicated in the instructions.

I don't know what we are doing wrong. As I stated above, should I have used another starch other than cornstarch? I thought cornstarch woud suffice because I read instructions for another less precise way to measure for Vitamin C which indicated that cornstarch could be used.

We Need Help!!

Cy Grant
Windows XP and Windows 7
MS Internet Explorer (Version Unknown)

Hi,

I too am a bit puzzled as to your problem. The instructions call for a 1% starch solution, ie 1g/100ml, so your solution is much mofre dilute than specified. Also if the starch dissolves, the solution will be clear. I would start again,and proceed like you were making gravy.. Suspend the desired amount of starch in cold water to suspend it, then dilute the with the desired amount of water. Then heat the mixture until the solution turns clear. Test the resulting solution by adding 10 drops to the same size water sample as the OJ you will test. Now titrate with your Iodine solution. You should see the blue color develop almost immediately. If this does not work, then make your starch solution using wheat flour. If it still does not work then something is wrong with the iodine solution. The iodine starch complex is a very deep purple that is quite intense and very hard to miss.

Hope this helps!

Good luck,

Barrett L. Tomlinson

I noticed that you added the 30mL Lugol's solution to 300ml water, should you have instead added the Lugol's to enough distilled water to instead total 300ml? If so, then you may have over diluted your iodine solution.

Instructions on one of the experiments describes the titrate solution as follows:

Dilute the Lugol's solution 1:10 in distilled water to make your iodine titration solution. (Note: if you purchased the iodine solution for starch test, you can skip this step.)

1. Pour the 30 mL Lugol's solution into the 500 mL graduated cylinder.
2. Add enough distilled water to bring the total fluid volume to 300 mL and mix.
3. Store the solution in a clean, tightly covered glass jar that is clearly labeled. Store it in a location that is protected from light.

Hi judgecy, Did you ever resolve you dilemma with the vitamin C/starch solution and the OJ/starch solution not turning bluish-black when adding the iodine titration solution? My daughter is having the same problem with the same experiment. Rather than the issue being with the starch, I wonder if it's with the Lugol's solution. The label on the iodine bottle reads, 'Dilute Lugol's Solution (30 ml)', so does that mean that the Lugol's does not need to be diluted with the 300 ml distilled water and should be used from the bottle as the titration solution?

I also find that I cannot reach the endpoint in my experiment. I dilute the solution to 300ml and I could not reach the proper end point with over 25ml of iodine solution.

http://www.hometrainingtools.com/images ... IODINE.pdf
in the specifications for the lugol's solution it says that it is 98% water...

so has anyone tried it with only 30ml>?

We made the assumption that Lugol's is already diluted and did not dilute further. We also increased the starch ratio (using grocery store corn starch like what you use to thicken gravy). We're working on determining the best ratio of starch but these changes - non-dilution of Lugols and ramping up the starch - seem to have made the experiment successful. Now I need to find a local source for Lugol's because we'll go through it much quicker without diluting and the results are due next week!