I've never done an experiment before. So, the methodology provided in this pdf is quite confusing. Link to the pdf: https://www.canterbury.ac.nz/media/documents/science-outreach/vitaminc_iodine.pdf

So, this pdf here details the methodology for conducting an iodometric titration to find ascorbic acid content in fruits/vegetables. On page 2 under "Method" section, its written how to make a 100 ml of sample solution with 100 g of the vegetable.

But in the titration experiment, we use only 20 ml of this sample solution. If we're taking only 20 ml for titration, wouldn't the tested ascorbic acid content be low? I mean, if you tested only 20 ml of the sample solution, then how is it representative of the amount of ascorbic acid in 100 g of that vegetable?

  • 1
    $\begingroup$ If you have prepared 100 mL of the solution to be analyzed, and if you only use 20 mL of it, it means that you are supposed to repeat this titration up to four times, in case of difficulty. $\endgroup$
    – Maurice
    Jul 4 at 18:26
  • $\begingroup$ Obviously also a 20 ml aliquot from 100ml of solution contains 1/5 of the total analyte. You typically use multiple samples to get a reasonable mean and a standard deviation for the analysis which will indicated good technique. $\endgroup$
    – MaxW
    Jul 5 at 9:22

There are (at least) two reasons to consider: accuracy and precision.

  • Accuracy as in preparing a stem solution of $\pu{100 mL}$ with a balance right to say $\pu{0.1 g}$ or a graduated cylinders right to the full $\pu{1 mL}$ is easier, than for $\pu{20 mL}$. Depending on the class, volumetric flasks may be better in terms of absolute / relative error than graduated cylinders; look this up.

  • Precision as eventually drawing an aliquot of $\pu{20 mL}$ of $\pu{100 mL}$ of a stem solution per individual measurement allows you to perform the analysis four times, to determine at least arithmetical mean value, standard deviation, interval of confidence and identify outliers. (Well, with four recordings this is not as solid as with e.g., 20 measurements, yet you may look up a $t$-value table and engage a Dixon, or a Grubbs test).

  • $\begingroup$ I understand the reasons about precision...but still, if I'm trying to find the amount of ascorbic acid in 100g of a particular vegetable, I don't get how using just 20ml of the solution is gonna give me that answer...also, if I'm taking only 20ml, then I would not even know how many grams (out of 100) have I tested out of the vegetable. I may get a precise answer with 4 repeats, but I wouldn't be able to say in "x" amount of vegetable, I found a particular amount of ascorbic acid, because I don't know the "x" in the first place i.e. the tested amount of vegetable. $\endgroup$
    – ZetaFox
    Jul 6 at 14:34
  • $\begingroup$ @ZetaFox Say you start with $\pu{100 g}$ of lemons to prepare $\pu{100 mL}$ solution. You assume all of the vitamin of the fruits now is in this stem solution. From this, you draw an aliquot of $\pu{20 mL}$, and determine this fifth of the $\pu{100 mL}$ stem solution contains $\pu{0.5 g}$ ascorbic acid. Because this reading is only about a fifth of the sample, you infer the total in $\pu{100 g}$ of the lemons was five times as much, i.e. $\pu{2.5 g}$. Ok? $\endgroup$
    – Buttonwood
    Jul 6 at 16:31

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