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I am looking to add caffeine powder in pure form to water that has a pH level of 7 or higher. What would the pH level be if I do this? Is Caffeine itself acidic?

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    $\begingroup$ Possible duplicate of Best consumption-safe liquid for caffeine solubility? $\endgroup$ – Mithoron Oct 27 '15 at 13:33
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    $\begingroup$ The possible duplicate does not ask the same question at all. $\endgroup$ – Jan Oct 27 '15 at 14:22
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    $\begingroup$ Question is the diffrent but the answer is broad enough to answer also this. $\endgroup$ – Mithoron Oct 27 '15 at 19:07
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Caffeine is not acidic. In fact, it is basic. It will raise the pH of water to some degree. It is a weak base, so the free base will exist in equilibrium with its protonated counterpart.

Due to its basicity, it will be more soluble in low pH aqueous solutions than in high pH solutions. This is because in low pH solutions, caffeine would exist completely as a protonated salt, making it more hydrophilic. Despite this, caffeine is very polar and is still quite soluble in water at higher pH levels.

The six-membered pyrimidinedione can exist in an aromatic, zwitterionic form where both amide nitrogens have formed double bonds to respective adjacent carbonyls. This six-membered ring had been observed as being planar. Therefore, it has been concluded that caffeine does indeed exist primarily as its zwitterionic resonance form, with two positive charges and two negative charges. The negative charges exist on the two carbonyl oxygens. These may be slightly basic, too, but significantly less so than a structurally similar hydroxyphenol (e.g. resorcinol) due the electronegative nature of the attached aromatic ring. Therefore, the basic imidazole nitrogen likely has largest effect on the pH.

The fact that there is possibly more that one weakly basic atom on the caffeine molecule complicates calculations as far as determining a theoretical pH. Also, the calculation would require the quantity of caffeine you will add. If you need an exact number, you will have to measure it. You can be sure, however, that the pH of the water will rise as caffeine is added.

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  • $\begingroup$ I was previously corrected about caffiene being a zwitterion. Apparently it is not and due to there being no removeable proton it does not form a trivial anion. $\endgroup$ – Technetium Dec 28 '15 at 1:37
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    $\begingroup$ There are ways for things to be zwitterionic without a proton migration -- in this case, it is through resonance. $\endgroup$ – SendersReagent Dec 28 '15 at 1:57
  • $\begingroup$ I appreciate you explaining that @user23444 I will research further. I originally stated it had zwitterionic properties in my answer below but if you read the comments I was corrected that it was not a zwitterion. I had used my notes from caffeine isolation from tea leaves as the resource but I couldn't find anything to support my zwitterion ion comment directly so i altered the answer and stood corrected. My notes were correct in the first place. Should I change my answer back to what it was ? $\endgroup$ – Technetium Dec 28 '15 at 2:48
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    $\begingroup$ I personally think that would be reasonable. I should put a picture in showing the aromaticity and the dominant resonance form. I will when I get back to my desktop. $\endgroup$ – SendersReagent Dec 28 '15 at 3:10
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    $\begingroup$ Amide bond isn't zwitterionic! There are only partial charges. Also mesomeric forms don't exist, but are a way to describe molecule when one lewis structure is bad. $\endgroup$ – Mithoron Dec 28 '15 at 18:00
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Caffeine (1,3,7 trimethylxanthine) is not acidic by itself in its freebase form it is an alkaloid which behaves as a base, although it can be ionic or non-ionic. Resonance or mesomerism allow resonance structures to form due to delocalized electrons within the compound, because of this caffeine can be a zwitterion.

In water ph 7》caffeine will be in its freebase form so the solution will remain alkaline .

Use a ph indicator to determine the pH before and after the addition of the caffeine and analyze the change in pH.

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    $\begingroup$ Caffeine is not zwitterionic. $\endgroup$ – Mithoron Oct 27 '15 at 11:17
  • $\begingroup$ @Mithoron , in the answer above it is stated that caffeine can form an an anion without proton migration through resonance? What are your thoughts on this? $\endgroup$ – Technetium Dec 28 '15 at 2:54
  • $\begingroup$ @Jan , what are your thoughts on the above statement also? $\endgroup$ – Technetium Dec 28 '15 at 2:55
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    $\begingroup$ Caffeine cannot form a macroscopic anion, since there is no proton that can be abstracted. What the other answer meant (and was badly phrased) is that a resonance structure where two nitrogens each bear a formal positive charge and two exocyclic oxygens a negative charge each (for an overall neutral molecule) may be the better description of caffeine. However, that would be a formal zwitterion, not an anion. (CC @Mith ). $\endgroup$ – Jan Dec 28 '15 at 21:24
  • $\begingroup$ @Jan , so is my answer ok the way it is I have become confused?! $\endgroup$ – Technetium Dec 29 '15 at 0:11

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