[Nirupama]

hey everyone

I have a situation where there are 5 kmoles of NaOH in solution and i am adding H3PO4 (total amount added is 142 kg) dropwise to this solution at the rate of 2 kg/min. I want to estimate the equilibrium concentrations of NaH2PO4, Na2HPO4 AND Na3PO4. The acid dissociation constants which are given below are applicable only for the dissociation of acid in water right? is it possible to estimate the equilibrium constants for the reactions given below?
NaOH + H3PO4 = NaH2PO4
NaH2PO4 + NaOH = Na2HPO4
Na2HPO4 + NaOH = Na3PO4    Are all of them reversible?

[Borek]

Not trivial, but perfectly doable. All you need are dissociation constants for the phosphoric acid.

http://www.chembuddy.com/?left=pH-calculation&right=pH-salt-solution

http://www.chembuddy.com/?left=pH-calculation&right=pH-polyprotic-acid-base (eqs. 9.11, 9.12, 9.13)

Or just use http://www.chembuddy.com/?left=BATE&right=pH-calculator

[Nirupama]

But isn't the equilibrium constant for dissociation of acid in pure water? Doesn't it change when i have a base where there is a larger concentration of OH- ions.

[Borek]

Concentration of OH^- doesn't matter, as it is taken into account when you use K_w.

You have never explained how concentrated these solutions are. If they are concentrated, you should take ionic strength of the solution into account and probably look for the corrected constants. That'll change the result. Depending on the concentration, change can be relatively small, or relatively large, hard to predict (especially not knowing accuracy of the amounts/concentrations given).

[Nirupama]

I have sodium aluminate solution with Na2O percentage- 20.5%, Al2O3 - 24.5%and rest water. I can get the density of sodal and hence the concentration of NaOH also in the reactor. How do i go about getting the corrected value of equilibrium constant? How do i account for the corrected value of OH- ions concentration?

[Nirupama]

can you share some link where i can get the corrected values of equilibrium constant?

[Borek]

No link, sorry. These are published in books, the one I own and use is a Handbook of Chemical Equilibria in Analytical Chemistry, Kotrly and Sucha, Ellis Horwood Ltd. 1985.

I told you you don't need to account separately for OH^- concentration, you have to do the full equilibrium calculation, as outlined here: http://www.chembuddy.com/?left=pH-calculation&right=general-pH-calculation