The chemical compound \(\text{H}_3\text{PO}_4\), known commonly as phosphoric acid or orthophosphoric acid, is classified as an acid. Understanding its classification requires looking into the fundamental definitions of chemical behavior in water.
Defining Acids and Bases
Chemists rely on specific models to define the characteristics of acids and bases. The earliest is the Arrhenius definition, which focuses on the behavior of a substance when dissolved in an aqueous solution. Under this concept, an acid increases the concentration of hydrogen ions (\(\text{H}^+\)) in water, while a base increases the concentration of hydroxide ions (\(\text{OH}^-\)).
A more encompassing and widely used concept is the Brønsted-Lowry theory, based on the transfer of protons. In this framework, an acid is any species capable of donating a proton (\(\text{H}^+\)). Conversely, a base is any species that can accept a proton. This definition is not limited to water-based solutions and better explains the reactions of many compounds.
Why Phosphoric Acid is Classified as an Acid
Phosphoric acid’s structure is key to understanding its acidic properties. The molecule is composed of a central phosphorus atom bonded to one oxygen atom with a double bond and three hydroxyl (\(\text{OH}\)) groups. The three hydrogen atoms in the \(\text{H}_3\text{PO}_4\) formula are attached to highly electronegative oxygen atoms.
When dissolved in water, the polarity of the \(\text{O-H}\) bonds causes these hydrogen atoms to be readily donated as protons (\(\text{H}^+\)) to water molecules. This proton donation aligns with the Brønsted-Lowry definition, classifying \(\text{H}_3\text{PO}_4\) as an acid. The donation of a proton forms a hydronium ion (\(\text{H}_3\text{O}^+\)), which is responsible for acidic properties.
Because the molecule has three available hydrogen atoms to donate, phosphoric acid is categorized as a polyprotic, or specifically, a triprotic acid. The ability to donate multiple protons distinguishes it from monoprotic acids like hydrochloric acid (\(\text{HCl}\)). The presence of these three dissociable protons dictates the chemical behavior and strength of the acid.
Stepwise Ionization and Relative Strength
Phosphoric acid is classified as a weak acid because it does not completely dissociate into ions when placed in water. The dissociation process is partial and occurs in three distinct, sequential steps, reflecting its triprotic nature. Each step involves the loss of one proton, and each subsequent step becomes less likely to occur.
The first ionization step is the most significant, where the neutral \(\text{H}_3\text{PO}_4\) molecule loses a proton to form the dihydrogen phosphate ion (\(\text{H}_2\text{PO}_4^-\)). The equilibrium constant (\(\text{K}_{a1}\)) is the largest, indicating this initial dissociation occurs to the greatest extent. This loss establishes the majority of the solution’s acidity.
The second ionization involves the dihydrogen phosphate ion losing a second proton to become the hydrogen phosphate ion (\(\text{HPO}_4^{2-}\)). The second acid dissociation constant (\(\text{K}_{a2}\)) is smaller than \(\text{K}_{a1}\). Removing a positively charged proton is more difficult from the already negatively charged ion (\(\text{H}_2\text{PO}_4^-\)) due to increasing electrostatic attraction.
Finally, the third ionization step occurs when the hydrogen phosphate ion loses its last proton to form the phosphate ion (\(\text{PO}_4^{3-}\)). The third dissociation constant (\(\text{K}_{a3}\)) is extremely small, indicating this step happens to a negligible degree. Acid strength decreases dramatically with each step because removing a proton from a highly charged negative ion requires overcoming stronger attractive forces.
Practical Applications of Phosphoric Acid
The chemical properties of phosphoric acid make it useful across various industries and consumer products. A large proportion of the acid produced globally is directed toward agriculture, where it is a precursor for phosphate fertilizers. These fertilizers supply plants with the phosphorus nutrient necessary for healthy growth.
In the food and beverage industry, phosphoric acid is used as an additive. It serves as an acidity regulator and provides a sharp, tart flavor, notably in soft drinks like colas. The acid also acts as a preservative, inhibiting the growth of mold and bacteria, which extends shelf life.
Another application is in metal treatment, particularly as a rust remover. When applied to rusted iron, the acid reacts with iron oxide (rust) to convert it into a hard, black compound called iron phosphate. This process is known as passivation and not only removes the rust but also creates a protective layer that helps prevent further corrosion of the metal surface.