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is it possible to find ka2 for h3po4 using the Henderson hasselabalch equation?

is it possible to find ka2 for h3po4 using the Henderson hasselabalch equation?

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Answer #1

Yes it is possible

you will need:

pH = pKa2 + log(HPO4-2 /H2PO4-)

this is the 2nd ionization point

we will need the equilbirium concentrations of HPO4-2 and H2PO4-

recall that

H2PO4- <--> H+ + HPO4-2

if you have pH and concentraitons, just substitute and solve for pKa:

pH = pKa2 + log(HPO4-2 /H2PO4-)

pKa2 = pH -  log(HPO4-2 /H2PO4-)

OR

you can do this via titration

titrate NaH2PO4 and Na2HPO4

when

NaH2PO4 = Na2HPO4

then

pKa2 = pH -  log(HPO4-2 /H2PO4-)

pH = pKa2 + log(1)

pH = pKA2

Ka = 10^-pKa2

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