[Big-Daddy]

If there is a solution with a certain concentration of SO₄^2- (K_b1=8.3 · 10^-13 M, K_b2≈0)  and NH₄⁺ (K_a=5.6 · 10^-10 M) of anywhere near the same concentration, the higher K_a of the conjugate acid than the K_b of the conjugate base tells us that the acidic action outweighs the basic action and so the solution is acidic.

What do we do to gain a pH estimate (or at least find whether the solution is acidic or basic) if one of the conjugate bases or acids is (properly) polyprotic? In this case if we see sulphuric acid as strong then the sulphate ion is a monoprotic base. But if we had the phosphate ion instead?

[magician4]

as initial remark:

concentration of SO₄^2- (K_b1=8.3 · 10^-13 M, K_b2≈0)

K_b2 (SO₄^2-) would be - 17 , approx.


with respect to your original question:

if we had a polyacidic system , combined with a pH-active cation (like in the hypothetical ammoniumphosphate *⁾), we would estimate the pH-outcome by looking for the most relevant species (here: of phosphoric acid) to be considered, and neglect the rest
i.e.: we would judge PO₄^3- vs. NH₄⁺ , meaning to compare pKb= 1.67 vs. pKa=9.75
  clearly , even with three times the concentration of ammonium around, phosphate will be the winner here, and the solution will be strongly basic

for more detailled calculations , or a situation where the differences pKa / pKb are not that clear or where there are amphotheric species involved (let's say (NH₄)(HCO₃) for example ) good estimations become a problem, and at times you hence can't circumvent charge balance equations based calculations


regards

Ingo



*⁾
solid tri-ammoniumphosphate is inexistant, as it will decay to diammoniumhydrogenphospohate + ammonia.
only the trihydrate (NH₄)₃(PO4) * 3 H₂O is stable and can be isolated
(if memory serves, this is (NH₄)₃(HPO4)(OH) * 2 H₂O in reality)

[Big-Daddy]

if we had a polyacidic system , combined with a pH-active cation (like in the hypothetical ammoniumphosphate *⁾), we would estimate the pH-outcome by looking for the most relevant species (here: of phosphoric acid) to be considered, and neglect the rest
i.e.: we would judge PO₄^3- vs. NH₄⁺ , meaning to compare pKb= 1.67 vs. pKa=9.75
  clearly , even with three times the concentration of ammonium around, phosphate will be the winner here, and the solution will be strongly basic

How do know what the most relevant species is? In this case, of course it may seem at first glance that only phosphate and ammonium are to be considered, but how are we discounting the action of the hydrogenphosphate ion for instance? On the basis that if the phosphate ion is winning then the solution must be basic? But if the phosphate ion is winning then surely by definition the reaction would be proceeding towards hydrogenphosphate ...

for more detailled calculations , or a situation where the differences pKa / pKb are not that clear or where there are amphotheric species involved (let's say (NH₄)(HCO₃) for example ) good estimations become a problem, and at times you hence can't circumvent charge balance equations based calculations

Ok, so then I will assume that unless the ions we are judging the acidic/basic action for can be regarded as monoprotic and non-amphoteric, it would be unreasonable to ask someone to make the estimate without a calculator.