What is an amphiprotic species? Give an example and explain its effect on pH.

• A. A species that can only gain a proton; Example OH-; pH about 5
• B. A species that can lose or gain a proton; Example H2O; pH about 9
• C. A species that can gain or lose a proton; Example HCO3-; pH about 7
• D. A species that can gain or lose a proton; Example HCO3-; tends to yield pH around 7 in pure water

Answer: (D)

Amphiprotic species are substances that can both accept and donate a proton in water, so they can act as either acids or bases. This dual behavior lets them buffer solutions and sit somewhere between acidic and basic species. A classic example is the bicarbonate ion, HCO3−. It can lose a proton to become carbonate (CO3^2−) or gain a proton to become carbonic acid (H2CO3). In water, this amphiprotic nature means its pH tends to reflect the two pKa values of the carbonic acid system, roughly around the midpoint between them (about 8, typically quoted near 8.3). So the essential idea is that the species can both gain and lose a proton, with bicarbonate being a textbook example that gives a modestly basic pH in solution.