Polyprotic Acids

[Rohita Thammineni 2D]

Hi! When it comes to polyprotic acids, does the weaker acid dissociating make the pH more acidic, or does it have a negligible effect? I wanted to clarify this because I know that strong acids dissociate much more than weak acids do, so I'm not sure if the weaker acid dissociating would significantly affect the solution...

[Anthony Tam]

So I'm pretty sure the weaker acid dissociating has a negligible effect on the pH because it is much harder to lose the H+ from a negatively charged ion. In the textbook in 6E, it says that when estimating the pH of a polyprotic acid we use only the first deprotonation equilibrium and assuming that further deprotonation is insignificant. I think the only exception to this is sulfuric acid (H2SO4) because it is a strong acid. Hope this helps.

[Bobak Pourrahimi 2L]

Assuming we are discussing weak acids, polyprotic weak acids almost always provide a considerable amount of H+ from their first deprotenation, but then it decreases considerably for the second deprotenation due to the significantly higher pKa of the negatively charged polyprotic weak acid in deprotenated form.