In chemistry, pK is the logarithmic value of the dissociation constant, Ka, of a hydrogen atom present on a molecule. A molecule has a distinct pK for each hydrogen atom that can be deprotonated. The acidity of a specific hydrogen atom in a molecule is represented by the pK value.
In order for a molecule to give up a hydrogen atom, or deprotonate, the pH of the surrounding system must surpass a specific value, which is referred to as the pK. If a molecule is present in an environmental pH lower than the pK value, then it can exist in the protonated or acidic form. Conversely, if the molecule is in an environmental pH higher than the pK value, then it can exist in the deprotonated or conjugate base form.