Salts are not acids, but the question of how they affect a solution’s pH is a complex one. A salt is chemically defined as an ionic compound, typically formed from the reaction between an acid and a base. While some salts, like common table salt, dissolve in water to create a neutral solution with a pH of seven, many others can shift the pH significantly higher or lower. This pH change occurs because the ions that make up the salt can interact with water molecules, a process that can mimic the effect of adding an acid or a base.
Defining Acids, Bases, and Salts
Acids and bases are fundamentally defined by their ability to transfer a proton, which is a hydrogen ion (\(\text{H}^+\)). A Brønsted-Lowry acid is a substance that can donate a proton to another molecule. Conversely, a Brønsted-Lowry base is a substance capable of accepting a proton. This proton transfer is the core of acid-base chemistry.
The strength of an acid or base is determined by how readily it donates or accepts a proton. A strong acid, such as hydrochloric acid (\(\text{HCl}\)), gives up its proton completely in water, while a weak acid, like acetic acid (\(\text{CH}_3\text{COOH}\)), only partially dissociates. A salt is distinct from these two, as it is an ionic compound composed of the cation of a base and the anion of an acid.
How Salts Are Formed
Salts are typically created through a neutralization reaction, where an acid and a base react together. The general equation for this reaction is \(\text{Acid} + \text{Base} \rightarrow \text{Salt} + \text{Water}\). For example, the reaction between hydrochloric acid and sodium hydroxide produces sodium chloride (table salt) and water (\(\text{HCl} + \text{NaOH} \rightarrow \text{NaCl} + \text{H}_2\text{O}\)).
The remaining ions—the cation from the base and the anion from the acid—combine to form the salt. Traditionally, a neutralization reaction was thought to produce a perfectly neutral solution with a pH of seven. This assumption is only true when both the acid and the base used are strong, leading to the formation of a neutral salt. When one or both of the parent compounds are weak, the resulting salt can cause a shift in the solution’s pH.
The Mechanism of Salt Hydrolysis
The reason some salts change the pH of water is a process called salt hydrolysis, which is essentially the reverse of the neutralization reaction. When a salt dissolves in water, it dissociates into its constituent ions, which may then react with the water molecules. This reaction with water is the hydrolysis mechanism, where the ion acts as either an acid or a base.
The key factor is the strength of the original parent acid and base from which the salt was formed. If an ion originates from a strong parent, such as the sodium ion (\(\text{Na}^+\)) from the strong base sodium hydroxide, it is considered too weak to react significantly with water. These ions are often called spectator ions because they do not affect the solution’s pH. Conversely, if an ion comes from a weak parent, it is a relatively strong conjugate and will react with water.
For instance, the acetate ion (\(\text{CH}_3\text{COO}^-\)) from the weak acid acetic acid will accept a proton from water, which produces hydroxide ions (\(\text{OH}^-\)). The increase in hydroxide ions makes the solution basic, raising the pH above seven. Similarly, the ammonium ion (\(\text{NH}_4^+\)) from the weak base ammonia will donate a proton to water, producing hydronium ions (\(\text{H}_3\text{O}^+\)) and making the solution acidic.
Classifying Salts by Their Impact on Solution pH
Salts can be categorized into three main groups based on the strength of their parent acid and base and their resulting effect on the \(\text{pH}\) of an aqueous solution.
The neutral salt is formed from a strong acid and a strong base, like sodium chloride (\(\text{NaCl}\)). Since neither the sodium cation nor the chloride anion hydrolyzes, the solution remains neutral with a \(\text{pH}\) of approximately seven.
The acidic salt is formed from a strong acid and a weak base, such as ammonium chloride (\(\text{NH}_4\text{Cl}\)). The cation (\(\text{NH}_4^+\)) is the conjugate acid of a weak base and hydrolyzes to produce hydronium ions (\(\text{H}_3\text{O}^+\)), resulting in a solution with a \(\text{pH}\) less than seven.
The basic salt originates from a weak acid and a strong base, with sodium acetate (\(\text{NaCH}_3\text{COO}\)) as a common example. The anion (\(\text{CH}_3\text{COO}^-\)) is the conjugate base of a weak acid, and its hydrolysis generates hydroxide ions (\(\text{OH}^-\)), making the solution basic with a \(\text{pH}\) greater than seven.