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Quite often when reading through some instructions or documentation or an outline that I've found online (EG: acid-base extractions), theres a step that involves using a "dilute hydrochloric acid solution", but oddly enough, it often doesn't detail exactly how diluted the HCl should be. Heres one example (little past half way into the 3rd paragraph)

Dilute hydrochloric acid is often used in the extraction of basic substances from mixtures or in the removal of basic impurities. The dilute acid converts the base such as ammonia or an organic amine into water soluble chloride salt.

So my question is: How does one know exactly how diluted the HCl should be? Is there a commonly used concentrated amount of HCl that I just don't know about? Sometimes it'll suggest using Muriatic acid instead, which I believe is basically 31.45% concentrated HCl, is that a commonly used concentration?

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  • $\begingroup$ Muriatic acid is typically loaded with iron, hence its yellow color, so it is not used in chemistry. $\endgroup$ – MaxW Aug 20 '18 at 17:24
  • $\begingroup$ @MaxW - Thanks for the comment. I was actually wondering why it was yellow if all it supposedly contained was diluted HCl.. $\endgroup$ – Justin Aug 20 '18 at 18:02
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The exact concentration doesn't matter that much. The stock solution that I used to use was 1 M aqueous HCl (~3%). If your compound is acid-sensitive then you might want to try using even more dilute HCl (e.g. 0.01–0.1 M), or skip this step altogether and just purify it using a different method. The important thing is to make sure that you have enough HCl to protonate all the basic stuff you want to remove – a relatively simple stoichiometry exercise.

I would suggest not using concentrated HCl (12 M, or ~37%) as for a typical acid-base extraction it's simply not necessary. You should save that for other circumstances, such as when you want to adjust the pH of a solution without diluting it too much with water.

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  • $\begingroup$ When would one know that enough HCl has been added and the base is fully (but not overly) protonated? When the pH reaches ~7? $\endgroup$ – Justin Sep 4 '18 at 23:58
  • $\begingroup$ There's nearly never any harm in adding too much HCl, because any extra acid is retained in the aqueous layer, which you are going to remove anyway. For most purposes you therefore only need to do some maths: count how many moles of base you used, and hence the volume of HCl you need to add. If (for whatever reason) you really needed to monitor the pH, though, you could use pH paper to make sure the pH is around 7, as you suggested. $\endgroup$ – orthocresol Sep 5 '18 at 5:14
  • $\begingroup$ Interesting - Thanks for the input! Also, when you say "or skip this and purify using a different method", what other methods would yield similar results to A/B? (sorry if thats too general of a question) $\endgroup$ – Justin Sep 5 '18 at 23:54
  • $\begingroup$ Chromatography, distillation, or recrystallisation are the most common methods. Often even after an acid-base extraction you will still need to carry out one of these three purification methods. The acid-base extraction just makes it easier because you don’t have so many things left to separate. $\endgroup$ – orthocresol Sep 6 '18 at 2:36
  • $\begingroup$ Recrystallization is actually what I usually follow up with, actually. $\endgroup$ – Justin Sep 7 '18 at 15:24

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