Salts are formed by the reaction of an acid and a base, known as a neutralization reaction. Acids can be monoprotic, diprotic or polyprotic, meaning that a molecule of the acid can donate one, two or multiple protons when it reacts with a base molecule — the protons usually take the form of a hydrogen ion (H+). Monoprotic acids such as hydrochloric acid (HCl) and nitric acid (HNO3) can form just one type of salt, chlorides and nitrates, respectively, but diprotic and polyprotic acids can form more than one. Sulfuric acid (H2SO4), for example, is diprotic since it can donate one or two H+ ions, and can form two series of salts, hydrogen sulfates and sulfates. Where an acid, in reacting with a base, donates less than the maximum number of protons available, this is known as partial neutralization and the result is an acid salt.
If sulfuric acid is neutralized by adding the base sodium hydroxide (NaOH), the acid salt sodium hydrogen sulfate is first formed: H2SO4 + NaOH -> NaHSO4 + H2O. If more sodium hydroxide is added, the neutral salt sodium sulfate is formed: NaHSO4 + NaOH -> Na2SO4 + H2O. Orthophosphoric acid (H3PO4) is triprotic and thus forms three series of salts: dihydrogen phosphates, hydrogen phosphates and phosphates, of which the first two are acid salts.
Acid salts are not necessarily acidic. Sodium hydrogen carbonate (NaHCO3), or sodium bicarbonate, can be regarded as the acid salt of carbonic acid — a weak diprotic acid — and sodium hydroxide — a strong base — and can be prepared by bubbling carbon dioxide (CO2) into a solution of sodium hydroxide. Initially, sodium carbonate is formed: 2NaOH + CO2 -> Na2CO3 + H2O. As more CO2 is added, it reacts with the sodium carbonate to form sodium hydrogen carbonate: Na2CO3 + CO2 + H2O -> 2NaHCO3. This acid salt is mildly alkaline in solution, owing to the fact that the HCO3- anion forms carbonic acid (H2CO3) and hydroxyl ions (OH-) in water, giving an excess of hydroxyl ions.
There are many applications for acid salts. The use of sodium bicarbonate, or “baking soda,” as a raising agent in baking is well known; it reacts readily with weak acids to release carbon dioxide gas, forming bubbles in a cake mix. Sodium hydrogen sulfate, also called sodium bisulfate, forms a strongly acidic solution in water, and is sometimes used as a substitute for sulfuric acid for a variety of purposes, such as metal finishing, lowering the pH in swimming pools, and in children’s chemistry sets. Acid salts, especially hydrogen phosphates and dihydrogen phosphates, are also widely used in buffer solutions.