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As we have learned in school, generally speaking polar molecules can dissolve in solutions with other polar molecules. However in the case of glutamic acid, it appears that it is unable to dissolve in water (proven by experiment, which we have conducted in class). What is not clear to me is why it is unable to dissolve within water, as the glutamic acid in a solution with pH 7 should be in fact an anion with a formal charge of -1.

After we have added sodium hydroxide, eventually the glutamic acid dissolved. This process does make sense to me, as the hydroxide deprotonates the glutamic acid which then becomes an anion with a formal charge of -2. But it still does not explain why the anion with an formal charge of -1 did not already dissolve in water, as glutamic acid also contains several partially polar functional groups.

Thanks in advance, and I hope I was able articulate my question in an understandable way :)

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    $\begingroup$ It is incorrect to say it is insoluble. The measured solubility of Glutamic Acid in water is 8.57g/L $\endgroup$
    – Waylander
    Commented Feb 1 at 21:56
  • $\begingroup$ Ah, yes. Sorry for the misunderstanding. Indeed it dissolves to a certain degree, but $8.57g/L$ really is very less. $\endgroup$
    – Chem
    Commented Feb 2 at 11:57
  • $\begingroup$ You said the pH was 7. Did you adjust it to 7 after adding the glutamic acid or was that the pH of the water before you added the acid? If you did not adjust it to 7 after adding the acid, then it will be much lower and the charge will not be -1. $\endgroup$
    – Andrew
    Commented Feb 2 at 16:14
  • $\begingroup$ @Andrew Oh, that might indeed by the reason. As you assumed correctly I did not adjust the pH, it was the pH before adding the acid. Thanks for pointing that out. $\endgroup$
    – Chem
    Commented Feb 2 at 16:43

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Glutamic acid is a weak acid. It does not totally protonate (give H+ or hydronium ion H3O+), to form all glutamate ion with formal charge -1. (Only few of them.) An acid which totally protonates and dissolves in water should be a strong acid such as hydrochloric acid. Low solubility of glutamic acid is also because of it is an organic compound, with not high enough polarity.

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  • $\begingroup$ Ah, I see. So, basically the molecules that are present in water only consists partially of ions and most of them are just "normal" glutamic acid molecules? Also I don't fully get why being an organic compound leads to the low solubility? $\endgroup$
    – Chem
    Commented Feb 2 at 12:01
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    $\begingroup$ Organic compounds tend to have a lot of C-C and C-H bonds which create regions of hydrophobicity - the more of them the less likely to be water soluble $\endgroup$
    – Waylander
    Commented Feb 2 at 12:47
  • $\begingroup$ Ah, I see. I was just unaware that this is the meaning of the term (I am not an English student). I have taken this to consideration as well however what still does not make sense to me is that all the partially polar groups that should be able to create hydrogen bonds with water are not outweighing the "small" organic compound? $\endgroup$
    – Chem
    Commented Feb 2 at 17:05

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