When we do titrations (w/ phenolphthalein) in lab, I know it's fun to see how close we can get the solution to clear, but why does my teacher place so much emphasis on accuracy and criticize overtitrating by just one drop? In our calculations, we end up just rounding volume of titrant used to the nearest tenth of a mL.

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    $\begingroup$ Well you really shouldn't be rounding to the nearest ml... If you do that then of course there's no point in being precise. $\endgroup$ – orthocresol Apr 21 '17 at 20:28
  • $\begingroup$ I've edited: I think I meant to say nearest tenth, which is what most burets are calibrated to; I thought the implication was still clear. I still don't think I drop will affect calculations that much? $\endgroup$ – khajiit Apr 21 '17 at 23:48

Yes, one drop might not affect your calculations that much.

However, at the end of the day, it is simply good laboratory technique to be as accurate as possible. If you only care about your titrations in the context of a lab grade, or the concentration of some solution that does not affect your life, then yeah, it's not a big deal. Nobody really cares whether your concentration is 1 M or 1.005 M.

That said, in general, you should always aim to carry out experiments to the best of your ability (and not just for "fun"). If you can do something well and get accurate results, then it's simply not good practice to settle for less.

Fundamentally, chemistry is an experimental science, and we can only talk about all the things we do now because of people who were very careful with their experiments and measurements.

Whether your teacher is being too strict or not is not a question for us to answer. Every individual will have their own approach to teaching the subject. However, what I can see is that he/she clearly wants you to take the lab seriously. Their insistence on your accuracy is almost certainly because of this, rather than any technical reason why one extra drop might lead to disastrous consequences.

  • $\begingroup$ This makes sense--not sure why my post got so much hate, but it's a bit clearer now. $\endgroup$ – khajiit Apr 22 '17 at 21:17

Because, near end point in titration pH shoots very fast with every single drop. To get the exact value of the concentration of your titrant, it is a MUST. 2 drops will be ~ 0.1 mL

I belive you ought to be rounding it off to the first decimal place, and not to the nearest mL. If you do so there is no point of skipping the end point by 20 DROPS!!!

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    $\begingroup$ You are correct in saying that near the end-point, the pH change will be very large even for a small volume of titrant. However, the change in pH is not relevant if the sole purpose is to determine the concentration. If the correct value is $\pu{20 cm^3}$, and you measure $\pu{20.05 cm^3}$, the final calculated concentration will be off by 0.25%; it does not matter whether the extra drop leads to a pH change of 0.01 or 1. $\endgroup$ – orthocresol Apr 21 '17 at 22:02
  • $\begingroup$ This was my point, thanks. I'm trying to clarify the difference between change in pH (certainly not a linear relationship with respect to volume) and what you're trying to find--typically something related to number of moles of base added, which IS linear with respect with number of MILLILITERS of base added. So being off by a drop shouldn't matter much. So I guess my question now becomes--is the purpose of being really accurate just for when you're recording pH rather than recording volume of titrant? $\endgroup$ – khajiit Apr 21 '17 at 23:04

I remember my professor back then saying the in the case of overtitration always back tirate and adjust accordingly. And I get what your teacher is trying to do, since you're still at the university (?) level or still in school, they're supposed to guide you in applying accurate laboratory techniques. Though when you're doing routinary work in industry, you get so used to doing titrations (at least for a few people I know) that you start to get a feel of how to adjust in such a way that your results are still pretty spot on. At the school level, you're going to have to live with the horror that comes with overtitration.


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