Science Buddies: "Ask an Expert"

We are conducting the experiment "Do Oranges Lose or Gain Vitamin C After Being Picked?" and were wondering
why we need to titrate a new 250mg vitamin C tablet each time we are titrating a new batch of juice. Some days we are needing 12ml of iodine to titrate the vitamin C tablet and some days it's as over 14ml. Why couldn't we just titrate the vitamin C tablet once and use that as a basis from which to calculate the orange juice titration? Just seems like that is an added variable that doesn't need to exist. Could you explain why?
Thanks so much.

Hi,

The reason for calibrating your titration solution each time is to improve the accuracy of your analysis. Your results show a vaiation of over 17% variation in run to run calibrations. Assuming you‘re doing the calibrations carefully, you would probably be getting >15% more error scatter in your analysis of your samples if you only calibrated one time and used that calibration for all your analyses of juice samples. Doing a calibration for each analysis sample is a very standard practice among analytical chemists. It checks whether your analysis procedure is working well and allows you to estimate the reproducibility of your procedure over time. The alternative if you did not recalibrate each time would be to analyze a standard sample (a sample of known composition) each time you did an unknown sample analysis. The variation in the results of the standard sample analysis allows you to estimate the accuracy of the results when analyzing the unknown samples.

These pages from the Science Buddies Project Guide may give you more information relevant to your question:

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

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

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

In simple terms, this all about trying to test how accurately your experiment is giving you the answer to the question you are asking. This question can be quite confusing to beginning scientists, but makes more sense as you study science and math more.

I hope this answer helps.

Best wishes for a successful science project!

Best regards,

Barrett L. Tomlinson

cyndi,

I think the most important point is to make up the vitamin C standard (the tablet) fresh the day you are going to measure it. Vitamin C will oxidize when exposed to air and breakdown. So you want to dissolve up your tablets and then do the titration right away to prevent it from breaking down and affecting your results.

In terms of your data (12 ml vs 14 ml). Titrations based on color changes can be pretty tricky. To get the most accurate results, you should stop the titration the very moment you see a hint of color. If your colors are very bright then it means that you have gone past the end point and are overestimating the amount of vitamin C. This can be difficult with orange juice since it is already colored. The best thing to do is practice!! Once you get the hang of it I'm sure everything will come out fine.

tzforbes