Molarity

Hi gh1991,


The titration curve should look like this:



The titration curve for phosphoric acid. The chemical formulas show the prevailing ionic species present at various pH values. Phosphoric acid (H3PO4) has three titratable hydrogens and therefore three midpoints are seen: at pH 2.15 (pK1), pH 7.20 (pK2), and pH 12.4 (pK3).

This substance is a polyprotic acid, meaning it has more than one dissociable proton. Indeed, it has three, and thus three equivalents of OH- are required to neutralize it, as the figure shows. Note that the three dissociable H+ are lost in discrete steps, each dissociation showing a characteristic pKa. Note that pK1 occurs at pH = 2.15, and the concentrations of the acid H3PO4 and the conjugate base H2PO4- are equal. As the next dissociation is approached, H2PO4- is treated as the acid and HPO42- is its conjugate base. Their concentrations are equal at pH 7.20, so pK2 = 7.20. (Note that at this point, 1.5 equivalents of OH- have been added.) As more OH- is added, the last dissociable hydrogen is titrated, and pK3 occurs at pH = 12.4, where [HPO42-] = [PO43-].

A biologically important point is revealed by the basic shape of the titration curves of weak electrolytes: in the region of the pKa, pH remains relatively unaffected as increments of OH- (or H+) are added. The weak acid and its conjugate base are acting as a buffer.