For example, nitric acid or sulfuric acid in an equal-molar solution of perchloric acid. Won't perchloric protonate a significant concentration of nitrate ions?

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    $\begingroup$ You’re in aquaeous solution? Yes, as long as the overall concentration isn’t too high. $\endgroup$ – Jan Nov 27 '15 at 21:09
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    $\begingroup$ Nitrate ions, not nitrite ions. Nitric acid -> nitrate; nitrous acid -> nitrite. But what Jan said. Unless you make the acids really concentrated, it's not likely to happen. $\endgroup$ – orthocresol Nov 27 '15 at 21:21
  • $\begingroup$ What if the solution was highly concentrated acid? $\endgroup$ – user32134 Dec 2 '15 at 18:13
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    $\begingroup$ As far as solutions in highly concentrated acid, Nitric acid acts like a base in concentrated sulfuric acid (it forms the nitronium ion). $\endgroup$ – AlaskaRon Dec 3 '15 at 5:19

Suppose you have nitric acid (about Ka=20) and perchloric acid (Ka > 100,000).

$\frac{[A-][H+]}{[HA]} = K_a$

In 1M nitric acid alone, let x be [$\ce{NO3-}$], that is dissociated nitric acid

$\frac{x^2}{(1-x)} = 20$

x = 0.954M

the nitric acid would be about 95% dissociated.

If the solution is 1M nitric and 1M perchloric, with all the perchloric acid dissociating, the nitric acid equilibrium would be:

$\frac{x(1 + x)}{(1-x)} = 20$

x= 0.913

the nitric acid would be about 91% dissociated.

For 2M nitric acid alone:

$\frac{x^2}{(2-x)} = 20$

x= 1.83M

or 92% dissociated.

For 2M nitric and 2M perchloric:

$\frac{x(2 + x)}{(2-x)} = 20$

x = 1.69M

or 84% dissociated.

So yes it can make a difference and it is more apparent at higher concentration.

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    $\begingroup$ It might be worth mentioning the "leveling effect," e.g. something in concentrated sulfuric or acetic acid. $\endgroup$ – Geoff Hutchison Dec 2 '15 at 18:54

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