text: The acid dissociation constant for an acid is a direct consequence of the underlying thermodynamics of the dissociation reaction; the pK value is directly proportional to the standard Gibbs free energy change for the reaction. The value of the pK changes with temperature and can be understood qualitatively based on Le Châteliers principle: when the reaction is endothermic, K increases and pK' decreases with increasing temperature; the opposite is true for exothermic reactions.
  The value of pK also depends on molecular structure of the acid in many ways. For example, Pauling proposed two rules: one for successive pK of polyprotic acids (see Polyprotic acids below), and one to estimate the pK of oxyacids based on the number of =O and −OH groups (see Factors that affect pK values below). Other structural factors that influence the magnitude of the acid dissociation constant include inductive effects, mesomeric effects, and hydrogen bonding. Hammett type equations have frequently been applied to the estimation of pK.
  The quantitative behaviour of acids and bases in solution can be understood only if their pK values are known. In particular, the pH of a solution can be predicted when the analytical concentration and pK values of all acids and bases are known; conversely, it is possible to calculate the equilibrium concentration of the acids and bases in solution when the pH is known. These calculations find application in many different areas of chemistry, biology, medicine, and geology. For example, many compounds used for medication are weak acids or bases, and a knowledge of the pK values, together with the octanol-water partition coefficient, can be used for estimating the extent to which the compound enters the blood stream. Acid dissociation constants are also essential in aquatic chemistry and chemical oceanography, where the acidity of water plays a fundamental role. In living organisms, acid–base homeostasis and enzyme kinetics are dependent on the pK values of the many acids and bases present in the cell and in the body. In chemistry, a knowledge of pK values is necessary for the preparation of buffer solutions and is also a prerequisite for a quantitative understanding of the interaction between acids or bases and metal ions to form complexes. Experimentally, pK values can be determined by potentiometric (pH) titration, but for values of pK less than about 2 or more than about 11, spectrophotometric or NMR measurements may be required due to practical difficulties with pH measurements.