A salt is an ionic compound formed when an acid and a base chemically react in neutralization. When dissolved in water, a salt breaks apart into its constituent positive and negative ions. While common table salt produces a neutral solution, many other salts result in solutions that are acidic or basic. Determining the nature of a salt solution requires understanding the identity of the original acid and base. The acidity or basicity is measured using the pH scale: below 7 is acidic, above 7 is basic, and 7 is neutral.
Identifying the Parent Acid and Base
Every salt is formed from a “parent” acid and a “parent” base, and predicting its behavior begins with identifying these original components. A salt is split into its positive ion (cation) and its negative ion (anion). The cation originates from the base, and the anion originates from the acid. For example, sodium chloride (NaCl) separates into the sodium cation (\(\text{Na}^+\)) and the chloride anion (\(\text{Cl}^-\)).
The next step is classifying these parents as either strong or weak by adding the components of water back to the ions. For \(\text{NaCl}\), \(\text{Na}^+\) combines with \(\text{OH}^-\) to form \(\text{NaOH}\), and \(\text{Cl}^-\) combines with \(\text{H}^+\) to form \(\text{HCl}\). Since \(\text{HCl}\) is a Strong Acid (SA) and \(\text{NaOH}\) is a Strong Base (SB), \(\text{NaCl}\) is categorized as a Strong Acid/Strong Base salt.
Common Strong Parents
- Strong Acids: \(\text{HCl}\), \(\text{HNO}_3\), and \(\text{H}_2\text{SO}_4\).
- Strong Bases: \(\text{NaOH}\), \(\text{KOH}\), and \(\text{Ca}(\text{OH})_2\).
The Chemical Mechanism: Understanding Salt Hydrolysis
The reason a salt solution can deviate from a neutral pH is hydrolysis, a chemical reaction with water. When a salt dissolves, its ions may interact with water molecules, which are slightly dissociated into \(\text{H}^+\) and \(\text{OH}^-\) ions. This interaction determines the final pH of the solution.
Ions derived from a strong parent acid or base are stable in water and are referred to as spectator ions. They do not react with water, and thus do not contribute to changing the solution’s pH.
Conversely, ions derived from a weak parent acid or base are less stable and readily react with water. This hydrolysis pulls \(\text{H}^+\) or \(\text{OH}^-\) ions out of the water, disrupting the neutral balance. A cation from a weak base produces \(\text{H}^+\) ions, increasing acidity. An anion from a weak acid produces \(\text{OH}^-\) ions, increasing basicity.
Predicting Solution pH: The Four Salt Categories
The strength of the parent acid and base provides a framework for predicting the pH of any salt solution, sorting salts into four categories based on which ion will hydrolyze. The Strong Acid/Strong Base (SA/SB) salt, such as potassium nitrate (\(\text{KNO}_3\)), has ions that are both spectators. They leave the water’s \(\text{H}^+\) and \(\text{OH}^-\) concentrations undisturbed, resulting in a neutral solution with a \(\text{pH}\) of 7.
Strong Acid/Weak Base (SA/WB) salts, exemplified by ammonium chloride (\(\text{NH}_4\text{Cl}\)), have a spectator anion (\(\text{Cl}^-\)) and a cation (\(\text{NH}_4^+\)) derived from the weak base ammonia (\(\text{NH}_3\)). The \(\text{NH}_4^+\) ion hydrolyzes water, releasing \(\text{H}^+\) ions. This causes the solution to become acidic, with a \(\text{pH}\) less than 7.
Weak Acid/Strong Base (WA/SB) salts, such as sodium fluoride (\(\text{NaF}\)), have a spectator cation (\(\text{Na}^+\)) and an anion (\(\text{F}^-\)) derived from the weak acid hydrofluoric acid (\(\text{HF}\)). The \(\text{F}^-\) anion hydrolyzes the water, releasing \(\text{OH}^-\) ions. This makes the solution basic, resulting in a \(\text{pH}\) greater than 7.
The final category is the Weak Acid/Weak Base (WA/WB) salt, such as ammonium acetate (\(\text{NH}_4\text{CH}_3\text{COO}\)). Both the cation and the anion hydrolyze, introducing both \(\text{H}^+\) and \(\text{OH}^-\) ions. The resulting \(\text{pH}\) depends on the relative strengths of the parent acid and base, comparing their dissociation constants (\(\text{K}_a\) and \(\text{K}_b\)). If the weak acid is stronger (\(\text{K}_a > \text{K}_b\)), the solution will be slightly acidic; otherwise, it will be slightly basic.
Real-World Application and Testing
Understanding the acidic or basic nature of salts has numerous practical applications. Sodium chloride is a neutral SA/SB salt, suitable for human consumption and industrial processes without altering the \(\text{pH}\) balance. Baking soda (\(\text{NaHCO}_3\)) is a WA/SB salt that forms a basic solution, used as an antacid and as a leavening agent.
In agriculture, acidic salts like ammonium sulfate (\(\text{NH}_4)_2\text{SO}_4\)) are common fertilizers used to lower the \(\text{pH}\) of alkaline soils. Basic substances are added to raise the \(\text{pH}\) of overly acidic ground. The simplest method to test the \(\text{pH}\) of a salt solution at home is by using \(\text{pH}\) strips, which change color to indicate acidity or basicity. More accurate measurement can be achieved with a digital \(\text{pH}\) meter.