The question of whether the dihydrogen phosphate ion (\(\text{H}_2\text{PO}_4^-\)) is a strong acid requires understanding chemical definitions. While this ion can donate a proton, its strength is tied to the behavior of its parent compound, phosphoric acid (\(\text{H}_3\text{PO}_4\)). Classifying its acidity depends entirely on the extent to which the substance releases a proton when dissolved in water, a process governed by equilibrium principles.
Defining Acid Strength
Acid strength in chemistry is defined by the tendency of a compound to release a proton, or hydrogen ion (\(\text{H}^+\)), when dissolved in an aqueous solution. This process is known as dissociation or ionization. Acids are broadly categorized into two groups based on the completeness of this dissociation.
Strong acids are those that completely ionize in water, meaning that essentially 100% of the acid molecules break apart to release their protons. Examples include hydrochloric acid and sulfuric acid, which leave virtually no intact acid molecules. This complete breakdown results in a high concentration of free hydrogen ions.
In contrast, weak acids only partially dissociate in water, establishing an equilibrium where a significant amount of the acid remains in its undissociated form. This partial breakdown releases fewer free protons compared to a strong acid of the same concentration. Most acids found in nature, such as acetic acid in vinegar, fall into this weak category.
The degree of dissociation is quantitatively measured by the acid dissociation constant, symbolized as \(K_a\). This constant is the equilibrium constant for the ionization reaction. A very large \(K_a\) value indicates a strong acid because the equilibrium lies far to the side of the dissociated ions. Conversely, a small \(K_a\) value points to a weak acid, confirming that the majority of the acid molecules remain together. Acid strength is inversely related to the stability of the conjugate base formed after the proton is lost.
Phosphoric Acid – The Parent Molecule
The dihydrogen phosphate ion (\(\text{H}_2\text{PO}_4^-\)) originates from phosphoric acid (\(\text{H}_3\text{PO}_4\)). Phosphoric acid is classified as a weak to moderately weak acid, not a strong acid like nitric or hydrochloric acid. This classification is made clear by examining its initial dissociation behavior in water.
Phosphoric acid is a triprotic acid, meaning that a single molecule possesses three hydrogen atoms capable of being donated sequentially. Each subsequent proton loss has its own acid dissociation constant, which measures the acidity of the remaining species. The first dissociation step is the loss of the first proton, where \(\text{H}_3\text{PO}_4\) becomes the dihydrogen phosphate ion, \(\text{H}_2\text{PO}_4^-\).
The acid dissociation constant for this first step (\(K_{a1}\)) is approximately \(7.1 \times 10^{-3}\). This value is significantly smaller than the \(K_a\) values of strong acids, which are typically much greater than one. The small \(K_{a1}\) confirms that the initial dissociation of phosphoric acid is incomplete, solidifying its categorization as a weak acid.
The first ionization is the most favorable of the three steps, but even this initial step does not meet the criteria for a strong acid. The remaining species after this initial dissociation, the dihydrogen phosphate ion, still retains two more potentially acidic protons.
The Specific Acidity of the Dihydrogen Phosphate Ion
The dihydrogen phosphate ion (\(\text{H}_2\text{PO}_4^-\)) is not a strong acid; it is significantly weaker than its parent molecule, \(\text{H}_3\text{PO}_4\). This ion is the product formed when phosphoric acid loses its first proton. When \(\text{H}_2\text{PO}_4^-\) acts as an acid, it donates its second proton to become the hydrogen phosphate ion (\(\text{HPO}_4^{2-}\)).
The acidity of the dihydrogen phosphate ion is measured by the second acid dissociation constant (\(K_{a2}\)) of the phosphoric acid system. The \(K_{a2}\) value for this second dissociation is substantially lower than the first, measured at approximately \(6.3 \times 10^{-8}\). This drastic decrease in the \(K_a\) value demonstrates a massive reduction in acid strength.
The smaller \(K_{a2}\) value indicates that the \(\text{H}_2\text{PO}_4^-\) ion has a much lower tendency to release its second proton than the original \(\text{H}_3\text{PO}_4\) molecule had to release its first. This is a common pattern in polyprotic acids, where the loss of each subsequent proton becomes progressively more difficult. The remaining negative charge on the ion makes it harder to separate the next positively charged proton.
The dihydrogen phosphate ion also exhibits amphoteric properties, meaning it can function as both an acid and a base. While it can donate a proton, as described by \(K_{a2}\), it can also accept a proton to reform the parent acid, \(\text{H}_3\text{PO}_4\). Its weak acid character, defined by its small \(K_{a2}\) value, places it firmly in the category of a weak acid.