The pKa of potassium hydrogen phosphate (K₂HPO₄) is 7.21 at 20 °C.
Understanding Potassium Hydrogen Phosphate and its pKa
Potassium hydrogen phosphate, also known as dibasic potassium phosphate, is a key component in buffer systems due to its ability to donate or accept protons, thereby stabilizing pH. Its pKa value is crucial for applications ranging from biological research to industrial processes.
The pKa of 7.21 refers to the second dissociation constant (pKa2) of phosphoric acid. In the context of potassium hydrogen phosphate (K₂HPO₄), this pKa is associated with the equilibrium between the dihydrogen phosphate ion (H₂PO₄⁻) and the hydrogen phosphate ion (HPO₄²⁻):
H₂PO₄⁻ ⇌ HPO₄²⁻ + H⁺
Potassium hydrogen phosphate primarily provides the HPO₄²⁻ species in solution, which, along with KH₂PO₄ (potassium dihydrogen phosphate, providing H₂PO₄⁻), forms an effective buffer system around a neutral pH.
Relevant pKa Values for Potassium Phosphate Compounds
Here's a summary of relevant pKa values for common potassium phosphate compounds:
| Compound | Formula | pKa at 20 °C |
|---|---|---|
| Potassium Phosphate, monobasic | KH₂PO₄ | 7.21 |
| Potassium Phosphate, dibasic | K₂HPO₄ | 7.21 |
| Potassium Hydrogen Phthalate | C₈H₅KO₄ | 5.51 (25 °C) |
The pKa value of 7.21 for both monobasic and dibasic potassium phosphate indicates their role in the same buffer equilibrium (the second dissociation of phosphoric acid).
Practical Applications
Potassium hydrogen phosphate is widely used in:
- Biological Buffers: It is a common component in phosphate-buffered saline (PBS) and other biological media, essential for maintaining physiological pH for cell culture, enzyme reactions, and diagnostic assays.
- Food Additives: Used as a buffering agent, emulsifier, and nutrient supplement in various food products.
- Pharmaceutical Formulations: Employed in drug formulations to control pH and enhance stability.
- Analytical Chemistry: Utilized in chromatographic separations and titrations where precise pH control is necessary.
Its pKa near neutrality makes it particularly useful for applications requiring pH control in the physiological range.
