Triprocid acids

The triprotic acids are those acids in which the molecule contains three acid functions, or three hydrogen atoms dissociated. The triprotic acids are uncommon, certainly less common acids and mono diprotic.

The most common triprotic acid is definitely the phosphoric acid (or phosphoric) H 3 PO 4. Another example is the ortoarsenic acid, H 3 AsO 4.

For triprotic acids can write three equilibrium dissociation.

Ex: H 3 PO 4

$\bg_white&space;\bg_white&space;H_3PO_4&space;+&space;H_2O&space;\rightleftharpoons&space;H_2PO_4^-&space;+&space;H_3O^+$

$\bg_white&space;\bg_white&space;\bg_white&space;H_2PO_4^-&space;+&space;H_2O&space;\rightleftharpoons&space;HPO_4^{2-}&space;+&space;H_3O^+$$\bg_white&space;HPO_4^{2-}&space;+&space;H_2O&space;\rightleftharpoons&space;PO_4^{3-}&space;+&space;H_3O^+$

The orthophosphoric acid (as well as the ortoarsenic acid) is a weak acid, therefore, as we have seen for diprotic acids, for the purposes of the calculation of the pH the concentration of H 3 O + provision by H 2 PO 4 - and HPO 4 2- is practically insignificant. See the respective acid dissociation constant:

$\bg_white&space;\bg_white&space;K_{a1}&space;=&space;\frac{[H_3O^+][H_2PO_4^-]}{[H_3PO_4]}&space;=&space;7,1&space;\cdot&space;10^{-3}$

$\bg_white&space;\bg_white&space;\bg_white&space;\bg_white&space;K_{a2}&space;=&space;\frac{[H_3O^+][HPO_4^{2-}]}{[H_2PO_4^-]}&space;=&space;6,3&space;\cdot&space;10^{-8}$

$\bg_white&space;\bg_white&space;\bg_white&space;\bg_white&space;\bg_white&space;\bg_white&space;K_{a3}&space;=&space;\frac{[H_3O^+][PO_4^{3-}]}{[HPO_4^{2-}]}&space;=&space;4,4&space;\cdot&space;10^{-13}$

Between the first constant and the second there are 5 orders of magnitude of difference. Remember that the Ka is our indicator of acid strength . The anion H 2 PO 4 - (which is the conjugate base of H 3 PO 4) has really a very weak acid character. The justification for this behavior is really quite easy. To behave as H 2 PO 4-  should donate a proton (H +) or losing a positive charge. It's a mere electrostatic question. The anion has a negative charge and the proton is itself a positive charge. From here the poor tendency to dissociate again. Even more so, the discourse is valid for the second ionization (which in fact takes place in tiny minura). An anion which has a double negative charge to disassociate yet to lose a positive charge. In this case, the Coulomb force exerted between the charges is even greater.