Acids and bases react to form salts in a neutralization reaction. Monoprotic acids form one type of salt, while diprotic and polyprotic acids can form more than one. Partial neutralization results in an acid salt. Acidic salts, such as sodium bicarbonate, can be prepared by reacting a weak acid with a strong base. Acid salts have many applications, including as leavening agents in baking and in buffer solutions.
Salts are formed by the reaction of an acid and a base, known as a neutralization reaction. Acids can be monoprotic, diprotic or polyprotic, which means that an acid molecule can donate one, two or more 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 only 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 as it can yield one or two H+ ions, and can form two series of salts, hydrogen sulphides and sulphates. Where an acid, reacting with a base, donates less than the maximum number of protons available, we speak of 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 sulfide is first formed: H2SO4 + NaOH -> NaHSO4 + H2O. If more sodium hydroxide is added, the neutral sodium sulfate salt is formed: NaHSO4 + NaOH -> Na2SO4 + H2O. Orthophosphoric acid (H3PO4) is triprotic and therefore forms three series of salts: dihydrogen phosphates, hydrogen phosphates and phosphates, of which the first two are acid salts.
Acidic salts are not necessarily acidic. Sodium bicarbonate (NaHCO3), or sodium bicarbonate, can be thought of as the acidic salt of carbonic acid – a weak diprotic acid – and sodium hydroxide – a strong base – and can be prepared by bubbling carbon dioxide ( CO2) in a sodium hydroxide solution. Initially sodium carbonate is formed: 2NaOH + CO2 -> Na2CO3 + H2O. When more CO2 is added, it reacts with sodium carbonate to form sodium bicarbonate: Na2CO3 + CO2 + H2O -> 2NaHCO3. This acidic salt is slightly alkaline in solution, due 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 baking soda, or “baking soda,” as a leavening agent in baking is well known; 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 finishing metals, 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.
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