Recently I was studying salts and my professor provided a list of anions which are considered neutral in salts (that is, they do not change the $\mathrm{pH}$ of an aqueous solution when added as a salt): $\ce{Cl-}$, $\ce{Br-}$, $\ce{I-}$, $\ce{NO_3^-}$, $\ce{ClO_4^-}$.

Now I'm looking at this and I realized the list seems to be almost identical to a list of strong acids: $\ce{HCl}$, $\ce{HBr}$, $\ce{HI}$, $\ce{HNO_3}$, $\ce{HClO_4}$, $\ce{H_2SO_4}$.

Is this a coincidence? If it's not, what's the reason, and why is $\ce{SO_4^2-}$ not on the list of neutral ions?

  • $\begingroup$ The conjugate acid of a weak base is a strong acid, and the anions you have mentioned are weak bases. $\endgroup$ – a-cyclohexane-molecule Mar 18 '18 at 6:21

Though couched in different terms, the first part this question is essentially the same as this other one:

Does a strong acid's conjugate base act as Brønsted-Lowry base?

No, this is not a coincidence. The conjugate base of a strong acid will always be a weak base. The 'neutral salt anions' of your professor's nomenclature are just weak inorganic Brönsted bases.

The reason why $\ce{SO4^2-}$ doesn't appear in this list of 'neutral salt anions' is because it actually can noticeably affect the $\mathbf{pH}$ of a solution when added as a salt. $\ce{H2SO4}$ is often treated as though it's a diprotic strong acid, but in reality only the first deprotonation is strongly acidic. This can be seen by examining its acid dissociation constants; from Wikipedia:

$\mathrm pK_{\mathrm a1} = -3$
$\mathrm pK_{\mathrm a2} = 1.99$

This $\mathrm pK_{\mathrm a2}$ is about the same as the first $\mathrm pK_{\mathrm a1}$ of $\ce{H3PO4}$ $($$2.148$$)$, and is actually somewhat less acidic than nitroacetic acid $($$1.68$$)$.

So, in some cases sulfate may act more or less as a 'neutral salt anion', when the $\mathrm{pH}$ is sufficiently high that it will all stay doubly deprotonated, but in others it can definitely influence the solution $\mathrm{pH}$. I've actually used sulfate/bisulfate before as an acidic buffer for various projects at work—works great!


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