Science Buddies: "Ask an Expert"

My experimentation has been completed for the question, "How much does the Vitamin C content in orange juice change with age?". My data is not consistant with the information I have researched. My hypothsis is that the amount of Vitamin C in orange juice would decrease with age. The amount of Vitamin C did go down, but not consistantly. Here are my results for the amount of vitamin C in orange juice:
week 1: 0.455mg/ml
week 2: 0.462mg/ml
week 3: 0.382mg/ml
week 4: 0.372mg/ml
week 5: 0.385mg/ml

I completed the iodine titration with an eyedropper, but I think that is why my data is not consistant. Is this what the titration should have done? Was it because of the eyedropper not being consistant? I would appreiate any help or information. Thanks!

MNK123 - Sorry to hear that you are having problems with your experiment. I am not an expert in food or nutrition, but I would agree with your hypothesis that vitamin C levels would degrade over time in an orange juice solution. I think your suspicion that the titration process control led to inconsistent readings is correct. Titration is a precise method of doing critical measurement of solutions. I reviewed the procedure in the experiment (it is quite complex) and was impressed with the required detail used in preparing the orange juice sample, the titrating solution and the actual measurement. There is a lot of very precise handling and preparation of the various agents and reagents used here. I don't know if the numbers you showed on your email are consistent with the actual concentrations of vitamin C in mg/ml. The troubleshooting FAQ for the experiment underscores some of the variables that can effect your results and the need for precise control of your testing reagents. The numbers you reported (although not monotonic over the time period) show a 20% variance from max to min value over a 4 week period. Perhaps you can check your work and procedure and salvage the experiment by hypothesizing what might have gone wrong in your control of the variables, although suggesting that your hypothesis was, in fact, correct. It appears that to re-do the experiment would take several more weeks. Hope this helps.

https://www.sciencebuddies.org/science- ... p044.shtml

Rick Marz

I have the Lugol's solution 2%. I got it from amazon. How much does it need to be diluted to get accurate results? When dropping the iodine solution into the flask with the starch and vitamin C solution I did over 60 drops and the color never changed. I only saw a liitle black swirl but then it went away. The instructions said it may only need 1 drop so I know I must have done something wrong because I did over 60 and it never changed completely. When I read the trouble shooting page it said one cause could be the iodine is too diluted. I spilled the iodine that came with the kit so I ordered more from amazon. I am now guessing it is NOT the same iodine. I think my is already diluted so how much more does it need to be diluted? or do I not need to dilute it at all???

It isn't uncommon to have chemicals diluted - indeed "Lugol's solution 2%" means "only 2% of this is Iodine", so you'd need 50 drops to be the same as 1 drop of fully concentrated iodine - and even then, you're adding the equivalent of 49 drops of pure water into your solution, so you are still going to probably be unhappy with the results if your test-tube isn't a lot larger than the 50 drops of liquid.

I'm not sure you can buy very concentrated iodine easily - the forms I've seen are crystals. But if I was to try to understand your results, I'd look at the label of the spilled bottle that came with the kit, and see if you can buy something with the same % iodine and try again.