The chemical compound Potassium Nitrite (\(\text{KNO}_2\)) is used in various industrial and laboratory applications. When dissolved in water, it undergoes a process that results in the formation of a basic, or alkaline, solution. This means the solution possesses a \(\text{pH}\) value greater than 7.0.
Defining Salts and Electrolytes
Potassium Nitrite is classified as a salt, typically formed from the neutralization reaction between an acid and a base. When this solid salt dissolves in water, it rapidly separates into its constituent components, a process known as dissociation. \(\text{KNO}_2\) breaks apart into a positively charged potassium ion (\(\text{K}^+\)) and a negatively charged nitrite ion (\(\text{NO}_2^-\)). These dissolved, charged particles are known as electrolytes, which conduct electricity within the solution. The acid or base character of the resulting solution depends on how these individual ions interact with water molecules.
The Hydrolysis Mechanism
To understand the basic nature of the solution, we examine the parent compounds from which the salt was derived. Potassium Nitrite is the product of a reaction between Potassium Hydroxide (\(\text{KOH}\)), a strong base, and Nitrous Acid (\(\text{HNO}_2\)), a weak acid. In salt hydrolysis, the ion originating from the strong parent compound is chemically inert; thus, the potassium ion (\(\text{K}^+\)) does not react with water. The nitrite ion (\(\text{NO}_2^-\)), however, is the conjugate base of the weak acid and is chemically active.
This active nitrite ion reacts with water in a process called hydrolysis, where the ion pulls a proton (\(\text{H}^+\)) from a water molecule. This reaction generates a molecule of the weak acid, Nitrous Acid (\(\text{HNO}_2\)), and a hydroxide ion (\(\text{OH}^-\)). The chemical equation for this interaction is \(\text{NO}_2^- + \text{H}_2\text{O} \rightleftharpoons \text{HNO}_2 + \text{OH}^-\). The production of these hydroxide ions is the chemical reason that the solution becomes basic.
Contextualizing Basicity
The presence of hydroxide ions shifts the \(\text{pH}\) balance of the water to a value above 7.0, confirming the solution’s basicity. The concentration of dissolved \(\text{KNO}_2\) directly influences the degree of basicity, as a higher concentration means more nitrite ions are available to produce hydroxide ions. This basicity is quantified by the equilibrium constant, known as the \(\text{K}_b\) value for the nitrite ion. The \(\text{K}_b\) value is very small, approximately \(2.2 \times 10^{-11}\), indicating that the reaction with water is not extensive and only a small fraction of the nitrite ions undergo hydrolysis. This confirms that the solution is only weakly basic.