What is the equilibrium constant pK defined as?

The equilibrium constant pK is defined as a value equal to the pH at which equal concentrations of the acidic and basic forms of a substance are present. This is particularly relevant in the context of weak acids and their conjugate bases, where the pK (often referred to as pKa for acids) indicates the pH level at which half of the acid is dissociated into its ions.

When the pH of a solution is equal to the pKa, it signifies that the concentrations of the protonated form (acid) and the deprotonated form (base) are equal. This relationship is fundamental in acid-base chemistry, as it helps to determine the buffering capacity of a solution and the position of equilibrium in reactions involving weak acids and bases.

This concept is crucial for understanding how substances behave in different pH environments, and it plays a significant role in biological systems, pharmaceuticals, and various chemical applications. The pKa value can also be used to predict the dominant species of an acid or base in a given solution based on the pH.