amphoteric species
Summary
An amphoteric species can act as an acid or as a base, depending on what it reacts with. Water, HCO₃⁻, H₂PO₄⁻ and Al(OH)₃ are examples. The testable point is that the role depends on the partner: it donates H⁺ to a stronger base and accepts H⁺ from a stronger acid.
Detail
Meaning: An amphoteric species reacts as an acid with a strong base and as a base with a strong acid. Two accounts overlap. In Brønsted–Lowry terms, an amphiprotic species can both donate and accept H⁺: H₂O, HCO₃⁻, H₂PO₄⁻, HPO₄²⁻ and amino acids. Metal hydroxides such as Al(OH)₃ and Zn(OH)₂ are also called amphoteric because they dissolve in both strong acid and strong base. Which role wins in water depends on whether its Ka or its Kb is larger.
Equation: pH ≈ ½(pKa1 + pKa2) for the amphiprotic intermediate of a diprotic acid, where pKa1 and pKa2 are −log Ka for the first and second dissociations (unitless).
On the MCAT: Expect questions that ask you to pick the amphoteric species from a list (it needs a removable H and a site that can accept H⁺), to write the conjugate acid and conjugate base of HCO₃⁻ (H₂CO₃ and CO₃²⁻), or to read a diprotic titration curve. Amino acids are tested in the same way: the zwitterion can give or take H⁺, and pI is the average of the two pKa values on either side of the neutral form.
Common confusion: Amphoteric does not mean the solution is neutral. NaHCO₃ is mildly basic because HCO₃⁻ has Kb greater than Ka. Also, every amphiprotic species is amphoteric, but amphoteric is broader, since Al(OH)₃ has no proton-donor and acceptor pair in the same sense.
Sources
- Chemistry: The Central Science
- Chemistry: Atoms First
- General Chemistry: Principles and Modern Applications
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.