Any uncharged group in a molecular entity that is capable of dissociating by yielding an ion (usually an H⁺ ion) or an electron and itself becoming oppositely charged is an ionizable group. Every acidic or basic group on a molecule has a different “pK” (K is the dissociation constant) value. The dissociation constant is a quantity expressing the extent to which a particular substance in solution is dissociated into ions, equal to the product of the concentrations of the respective ions divided by the concentration of the undissociated molecule. The relationship between the pH of the solution it is in and the pK of the ionizable group will determine the predominant form of the ionizable group. Every acidic or basic group has an “acid form” (also known as the “protonated form”) and a “base form” (also known as the “deprotonated form”). For carboxylic acids, the protonated form (acid form) is –COOH and the deprotonated form (base form) is –COO^– . For amines, the protonated form (acid form) is –NH₃ ⁺ and the deprotonated form (base form) is –NH₂. If the pH of the solution equals the pK of the ionizable group, then the acid and base forms of that group will be present in equal amounts. If the pH of the solution is lower than the pK of the ionizable group, then the acid form of that group will be more abundant than the base form. (The predominant form is the protonated form.) If the pH of the solution is higher than the pK of the ionizable group, then the base form of that group will be more abundant than the acid form. (The predominant form is the deprotonated form.) Knowing the pH of the solution and using the previous three possible scenarios, then it is possible to determine if an ionizable group will be neutral, positively or negatively charged. It is important to note that pK values depend on temperature, ionic strength, and the microenvironment of the ionizable group.