Recall that monoprotic acids are acids able to donate one proton per molecule during the process of dissociation (sometimes called ionization) as shown below (symbolized by HA):
Common examples of monoprotic acids in mineral acids include hydrochloric acid (HCl) and nitric acid (HNO3). On the other hand, for organic acids the term mainly indicates the presence of one carboxylic acid group and sometimes these acids are known as monocarboxylic acid.
Polyprotic acids, also known as polybasic acids, are able to donate more than one proton per acid molecule, in contrast to monoprotic acids that only donate one proton per molecule. Certain types of polyprotic acids have more specific names, such as diprotic acid (two potential protons to donate) and triprotic acid (three potential protons to donate).
Let's take a quick look at a diprotic acid, oxalic acid, also called ethanedioic acid. As stated, oxalic acid is diprotic, having two protons to donate Figure 1.
If a dilute solution of oxalic acid were titrated with a sodium hydroxide solution, the protons would react in a stepwise neutralization reaction Figure 2.
If the pH of this titration were recorded and plotted against the volume of NaOH added, a very clear picture of the stepwise neutralization emerges Figure 3. Note the very distinct equivalence points on the titration curves.
This oxalic acid is an example of an acid able to enter into a reaction with two available protons Figure 4.
An inorganic example of a triprotic acid is orthophosphoric acid (H3PO4), usually just called phosphoric acid. All three protons can be successively lost to yield H2PO4−, then HPO42-, and finally PO43-, the orthophosphate ion, usually just called phosphate. An organic example of a triprotic acid is citric acid, which can successively lose three protons to finally form the citrate ion. Even though the positions of the protons on the original molecule may be equivalent, the successive Ka values will differ since it is energetically less favorable to lose a proton if the conjugate base is more negatively charged.
Although the subsequent loss of each hydrogen ion is less favorable, all of the conjugate bases are present in solution. The fractional concentration, α (alpha), for each species can be calculated. For example, a generic diprotic acid will generate three species in solution: H2A, HA-, and A2-. The fractional concentrations can be calculated as below when given either the pH (which can be converted to the [H+]) or the concentrations of the acid with all its conjugate bases Figure 6.