have been developed for estimating micro-constant values. For example, the isomerization constant for -DOPA has been estimated to have a value of 0.9, so the micro-species L1H and L2H have almost equal concentrations at all pH values. pH considerations (Brønsted constants) pH is defined in terms of the activity of the hydrogen ion pH = −log10 {H+} In the approximation of ideal behaviour, activity is replaced by concentration. pH is measured by means of a glass electrode, a mixed equilibrium constant, also known as a Brønsted constant, may result. HL L + H; It all depends on whether the electrode is calibrated by reference to solutions of known activity or known concentration. In the latter case the equilibrium constant would be a concentration quotient. If the electrode is calibrated in terms of known hydrogen ion concentrations it would be better to write p[H] rather than pH, but this suggestion is not generally adopted. Hydrolysis constants In aqueous solution the concentration of the hydroxide ion is related to the concentration of the hydrogen ion by \mathit{K}_W =[H][OH] [OH]=\mathit{K}_W[H]^{-1} The first step in metal ion hydrolysis can be expressed in two different ways It follows that . Hydrolysis constants are usually reported in the β* form and therefore often have values much less than 1. For example, if and so that β* = 10−10. In general when the hydrolysis product contains n hydroxide groups Conditional constants Conditional constants, also known as apparent constants, are concentration quotients which are not true equilibrium constants but can be derived from them. A very common instance is where pH is fixed at a particular value. For example, in the case of iron(III) interacting with EDTA, a conditional constant could be defined by This conditional constant will vary with pH. It has a maximum at a certain pH. That is the pH where the ligand sequesters the metal most effectively. In biochemistry equilibrium constants are often measured at a pH fixed by means of a buffer solution. Such constants are, by definition, conditional and different values may be obtained when using different buffers. Gas-phase equilibria For equilibria in a gas phase, fugacity, f, is used in place of activity. However, fugacity has the dimension of pressure, so it must be divided by a standard pressure, usually 1 bar, in order to produce a dimensionless quantity, . An equilibrium constant is expressed in terms of the dimensionless quantity.