polyprotic titration

[destroythemcat]

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in a polyprotic titration

H2X--> HX- + H+ (pka1)

HX- --> X- + H+ (pka2)

Pka1>>>Pka2

So why does it take the same amount of strong acid to fully titrate each part of the reaction?

 

[Czarcasm]

in a polyprotic titration

H2X--> HX- + H+ (pka1)

HX- --> X- + H+ (pka2)

Pka1>>>Pka2

So why does it take the same amount of strong acid to fully titrate each part of the reaction?

It's because in the presence of strong base, an equivalent amount of protons attached to a weak acid that is present in solution will neutralize regardless of it's strength. Whether you have 1M H3PO4 or 1M H2PO4-, both will react an equivalent mole H+ with an equivalent mole NaOH to produce an equivalent amount of neutral H2O molecules. This is just a property of neutralization reactions. However, because the conjugate bases of these weak acids effectively get stronger, each of their pKb values will rise; a higher pKb is associated with a lower pOH, which in turn is associated with a higher pH. This is why a polyprotic titration graph looks the way it does.