When dissolved in water, phosphoric acid exists as one of four distinct chemical species. The specific species present—phosphoric acid (H₃PO₄), dihydrogen phosphate (H₂PO₄⁻), hydrogen phosphate (HPO₄²⁻), and phosphate (PO₄³⁻)—is determined by the solution’s pH. This relationship determines when a phosphate species has a concentration so low that it can be considered negligible.
Phosphoric Acid and Its Dissociation Constants
Phosphoric acid is a weak, triprotic acid, meaning it donates three protons (H⁺ ions) in a stepwise manner. Each donation is a distinct chemical equilibrium, and the transition between species is governed by the acid dissociation constant (Ka), which measures acid strength. For convenience, chemists use the logarithmic form of this constant, called pKa.
The first proton is lost from phosphoric acid (H₃PO₄) to form dihydrogen phosphate (H₂PO₄⁻) with a pKa1 of approximately 2.15, making H₃PO₄ the strongest acid form. The second proton is donated from dihydrogen phosphate (H₂PO₄⁻) to yield hydrogen phosphate (HPO₄²⁻), with a pKa2 of about 7.20. The final proton is removed from hydrogen phosphate (HPO₄²⁻) to produce the phosphate ion (PO₄³⁻), with a pKa3 of around 12.35.
Each pKa value signifies the pH at which the concentrations of the acid and its conjugate base are equal. For instance, at a pH of 7.20, the amounts of H₂PO₄⁻ and HPO₄²⁻ are identical.
Determining the Dominant Species at a Given pH
The pKa values are reference points for identifying which phosphate species is most abundant at a specific pH. A comparison between the solution’s pH and the three pKa values reveals the dominant form. These relationships establish clear pH zones where one species prevails over the others:
- Phosphoric acid (H₃PO₄): Dominant in highly acidic solutions where the pH is below 2.15.
- Dihydrogen phosphate (H₂PO₄⁻): Most prevalent when the pH is between pKa1 and pKa2 (roughly 3 to 6).
- Hydrogen phosphate (HPO₄²⁻): The dominant form when the pH is between pKa2 and pKa3 (approximately 8 to 11).
- Phosphate (PO₄³⁻): Most abundant in very alkaline conditions where the pH is above 12.35.
Conditions for Negligible Concentrations
A species’ concentration is considered negligible when it is too low to impact chemical calculations. This occurs when its concentration is less than 1% of the dominant species, or several orders of magnitude smaller. A common rule is that a species’ concentration becomes insignificant when the solution’s pH is at least two units away from the pKa value governing its formation or dissociation.
For example, at a physiological pH of 7.4, which is close to pKa2 (7.20), both dihydrogen phosphate (H₂PO₄⁻) and hydrogen phosphate (HPO₄²⁻) are present in significant quantities. This pH is over five units from pKa1 (2.15) and nearly five units from pKa3 (12.35). This large difference means the concentrations of phosphoric acid (H₃PO₄) and the phosphate ion (PO₄³⁻) are negligible.
In an acidic solution with a pH of 4.0, dihydrogen phosphate (H₂PO₄⁻) is the dominant species. Because the pH is not far from pKa1, phosphoric acid (H₃PO₄) is also present in a notable amount. However, pH 4.0 is more than three units below pKa2 and over eight units below pKa3, so the concentrations of hydrogen phosphate (HPO₄²⁻) and the phosphate ion (PO₄³⁻) are negligible.
Visualizing Speciation with a Distribution Diagram
A speciation diagram, also called an alpha plot, visualizes how the relative amounts of the four phosphate species change across the entire pH scale. On this graph, the x-axis represents pH from 0 to 14, and the y-axis shows the fraction of each species relative to the total phosphate concentration. Each of the four species (H₃PO₄, H₂PO₄⁻, HPO₄²⁻, and PO₄³⁻) is represented by its own curve.
The peak of each curve corresponds to the pH range where that species is dominant. For instance, the curve for H₃PO₄ is highest at very low pH values and drops to nearly zero as the pH approaches 4. Similarly, the curve for the phosphate ion (PO₄³⁻) only begins to rise at a high pH. At any given pH, one or at most two species have significant concentrations, while the others are so low their curves are flat against the x-axis, confirming their negligible presence.