I know that some acids for sure and for certain are polyprotic. For example sulfuric acid $\ce{H2SO4}$ is diprotic giving you disulfate $\ce{HSO4-}$ and sulfate $\ce{SO4^2-}$

However not all acids with multiple hydrogens are polyprotic, even if 2 or more hydrogens are attached to very electronegative atoms. So if I am given an acid and it's chemical structure how would I know if it is polyprotic or not? Acid base reaction with something like sodium hydroxide $\ce{NaOH}$ and then distillation to separate the product/products from the water that is produced? Or is there a better way to prove whether or not an acid is polyprotic?

  • $\begingroup$ Could you give an example for "even if 2 or more hydrogens are attached to very electronegative atoms" which is not polyprotic? $\endgroup$ Jan 13 '16 at 4:50
  • $\begingroup$ I can't really find an example but I would think most of these would be of the form $\ce{HaNbXc}$ where X is a halogen and easily gives up a hydrogen and nitrogen(a very electronegative element) does not easily give up a hydrogen simply because it is more electronegative. This would require that the halogen be hypervalent of which the smallest is chlorine. $\endgroup$
    – Caters
    Jan 13 '16 at 5:06
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    $\begingroup$ I don't understand that. Why should chlorine be hypervalent - what does hypervalent even mean to you? Chlorine is a little more electronegative than nitrogen. Does such a compound as you propose even exist? Your question must have come from a place where you had a doubt about the polyprotic nature of a compound, I was just trying to find out about your confusion. $\endgroup$ Jan 13 '16 at 5:14
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    $\begingroup$ You may believe whatever you like, but that's just not how chemistry works. $\endgroup$ Jan 13 '16 at 5:42
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    $\begingroup$ Chlorine cannot have more than 8 electrons. That is just impossible. There are various reasons at play, why compounds like ClO or ClO3 exist, hypervalency was used until there was a better understanding. In these compounds, the chlorine is indeed very positive. $\endgroup$ Jan 13 '16 at 5:43

As discussed in your question and ensuing comments, there are ways to predict via theory whether a particular compound would be a polyprotic acid, but your last sentence asks how to prove that an acid is polyprotic. This pretty much limits us, as you proposed, to laboratory experiment, namely acid-base titration.

Although your suggestion "Acid base reaction with something like sodium hydroxide NaOH and then distillation to separate the product/products from the water that is produced" is on track, the experiment needn't be nearly so complex.

The experimental procedure is as simple as preparing a solution of the unkown acid, then titrating with a strong base solution while monitoring the pH.

The following is a titration curve for the diprotic oxalic acid, using sodium hydroxide as the strong base titrant. (Image taken from this boundless.com post):

diprotic acid titration

Note that there are two equivalence points, at 25 and 50 ml of added titrant. These equivalence points are the features to look for when titrating the unknown acid. A single equivalence point indicates a monoprotic acid, two indicate a diprotic acid, etc.

There are a couple drawbacks to this experimental procedure. First, the equivalence points are not always as well defined as that shown in the figure. For instance, two acidic moieties may have similar Ka values and thus may almost appear as one. Making a derivative plot of the titration curve can help to distinguish the equivalence points. In other words, you plot the change in pH vs volume of titrant added rather than just the pH of the titrant added.

The second drawback of this procedure is that it doesn't tell you which protons were titrated by the base. This would require a far more complex experiment, for instance by synthesizing the acid using deuterium in place of hydrogen at specific sites, then analyzing the compound after each equivalence point to see whether the deuterium was pulled off.

I hope I understood your question correctly and that my explanation of the titration procedure to determine polyprotic acids was clear. Don't hesitate to ask for clarification in the comments.


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