Which of the following species is amphoteric? A) NaOH, B) HNO₃, C) K₃PO₄, D) HPO₄ ²⁻, or E) SO₄ ²⁻.
A substance that’s amphoteric can react as an acid or as a base. Typically, we think about acids as being donators of hydrogen ions. And we think of bases as acceptors of hydrogen ions. We need to put our five candidates through two tests. One, do they donate hydrogen ions? And two, do they accept hydrogen ions? The one that’s the correct answer will do both and will be amphoteric.
NaOH is the symbol for sodium hydroxide, a well-known strong base. Sodium hydroxide does contain hydrogen. But sodium hydroxide doesn’t dissociate into the sodium oxide ion and the hydrogen ion. But it does dissociate into the sodium ion and the hydroxide ions. It’s the hydroxide ion that reacts with hydrogen ions forming water in an acid–base reaction. So, sodium hydroxide doesn’t pass our first test, although sodium hydroxide does except hydrogen ions forming sodium salts and water. Sodium hydroxide is basic, but not amphoteric. So, it’s not a correct answer.
HNO₃ is the formula for nitric acid, which, unsurprisingly, is an acid. Nitric acid passes our first test. It readily donates hydrogen ions, for instance, to bases like sodium hydroxide. However, under most circumstances, nitric acid will not accept a proton. There are exceptions, for instance, where nitric acid interacts with an even stronger acid. But generally speaking, we don’t describe nitric acid as amphoteric because these are exceptional circumstances. So, nitric acid fails our second test, and it’s not considered amphoteric.
Our next candidate is potassium sulphate, which is a basic salt. The conjugate acid of the phosphate anion is phosphoric acid. Since we know that phosphoric acid is not a strong acid, we know that the phosphate anion will be slightly basic. Therefore, potassium phosphate will react to acids to some degree. However, there’s a crippling problem that there isn’t a single hydrogen in potassium phosphate, so potassium phosphate cannot act as an acid in this way.
Next up, we have hydrogen phosphate. We can think about the hydrogen phosphate anion as one rung on the ladder between the phosphate anion and phosphoric acid. Phosphoric acid can donate one proton to a base and becomes dihydrogen phosphate. And dihydrogen phosphate can lose another hydrogen ion to form hydrogen phosphate. And if hydrogen phosphate reacts with a base, it releases its last proton forming the phosphate anion.
But we can also do this process in reverse, reacting the phosphate anion with an acid to produce hydrogen phosphate and repeat this all the way up the ladder. So, we can clearly see that the hydrogen phosphate anion can donate a proton to a base forming the phosphate anion. And it can accept a hydrogen ion forming dihydrogen phosphate. Therefore, hydrogen phosphate is an amphoteric anion.
But just to be safe, let’s have a look at sulphate, which is a basic anion. It automatically fails the first test since there are no hydrogen ions to donate in the first place. But actually, it is basic. Even though we think about the strong acid sulphuric acid when we think about sulphate, the intermediate acid hydrogen sulphate is actually a weak acid. So, sulphate anions are slightly basic because they will react with acids to produce hydrogen sulphate.
However, while interesting, it doesn’t change our answer. The only species out of the five we’ve been given that is generally considered amphoteric is hydrogen sulphate because it readily reacts with both acid and base in aqueous solution under normal conditions.