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For an AP chem lab dealing with determining the amount of citric acid in orange juice by titrating with NaOH, one of the discussion questions goes like this:

Choose an amount of beverage to be titrated that will require at least 10 but less than 20 mL of titrant. Explain why this range of titrant is optimal.

This question makes no sense to me. Why would there be an "optimal range" of titrant? I would think that I should fill the buret to full capacity if enough titrant is available. Would it not be better to have excessive titrant than not enough of it?

We completed the lab, and the titration required 23.05 mL of 0.1 M NaOH solution to titrate 20.00 mL of orange juice. If we had followed the instructions, we would have added less orange juice, and used less NaOH, but what is the benefit in doing so? Budget cuts? If it was about not wasting solution, why would there be a minimal requirement of 10 mL of titrant?

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  • $\begingroup$ Excessive amount of titrant doesn't make it more precise, it's a waste. $\endgroup$ – Mithoron Aug 29 '15 at 23:47
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In a perfect world there would be no optimal range. We do however live in a world full of errors and practical considerations.

Why use over a minimum amount?

Let's assume for demonstration, that each drop has a volume of 0.05 mL. Of course this is not an accurate value as various factors would affect this.

The titration has reached its end point when one last drop changes the colour. If your titre value is very small e.g. 1 mL, then each drop is 5% of your titre.

If only part of the drop is needed to reach the end point then all the extra part of the drop is inaccuracy, which in this scenario could be up to 5%! If you have the misfortune of accidentally adding one drop too many you are introducing an even larger error. This will impact concentration calculations based on the experiment.

Why not just use 'a lot' then?

There are a couple of things here:

1) As noted by Mithoron, this would be a waste. The accuracy gained from requiring additional titrant will level off rather rapidly.

2) If you use more than one full burette's worth there is an error penalty.

3) Using extra titrant is a waste of your time. Consider a 50 mL burette, if it needs to be filled up after each titration, that would be inefficient. I'm assuming you're doing repeats so this has real merit.

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  • $\begingroup$ Thanks! The more of the test substance, the smaller the pH change each drop is worth, but too much of the test substance is a waste of solution and time. I assume for the same reason it would be more accurate to use a smaller molarity NaOH solution, so one drop is not excessive? $\endgroup$ – Isaac Liu Aug 30 '15 at 21:01
  • $\begingroup$ Also, because we were using an pH probe to determine the titration curve, instead of an indicator, the minimum amount of test sample is unnecessary, correct? $\endgroup$ – Isaac Liu Aug 30 '15 at 21:09
  • $\begingroup$ @IsaacLiu Provided you aren't diluting your initial solution to make it more dilute and thus gaining an error. The minimum amount still applies, probably more so with a pH probe as you need to put the probe somewhere. $\endgroup$ – Christopher Aug 30 '15 at 22:46

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