My professor said that in order to have PI, both + and - charge have to exist, and therefore phosphoric acid cannot have a Pi. I am confused because I can't find any definition that states both + and - charge need to exist on text or online. Online, the definitions of PI indicates something similar to "at which the molecule carries no electrical charge." So why can't H3PO4 be PI if it the molecule has 0 charge?

  • $\begingroup$ Because there is also present $\ce{H2PO4-}$ It could have an IP if its dissiciation was compensated by existence of the same amount of $\ce{H4PO4+}$ in strong acid. $\endgroup$ – Poutnik Oct 5 '19 at 5:16
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    $\begingroup$ You can't have an isoelectric point with $\ce{H2PO4^-}$ and $\ce{H4PO4^+}$. The positive and negative charges need to be on the same molecule. Isoelectric points are most typically encountered for amino acids and proteins. $\endgroup$ – MaxW Oct 5 '19 at 8:20
  • $\begingroup$ en.wikipedia.org/wiki/Isoelectric_point "The isoelectric point (pI, pH(I), IEP), is the pH at which a molecule carries no net electrical charge or is electrically neutral in the statistical mean." I do agree the IEP is used mostly ( not exclusively ) for amino acids and proteins. $\endgroup$ – Poutnik Oct 5 '19 at 9:22
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    $\begingroup$ That means the nature of IEP is the molecule is statistically neutrall, with balanced charged forms. That applies to both amino acids and phosphoric acid in strong acid.(at some point ) Being a zwitterion is just one of ways, where all 3 forms are balanced. $\endgroup$ – Poutnik Oct 5 '19 at 11:51

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