Standard State in Solution

The standard state has been expressed up to now in terms of a pure substance. The chemical potential then depends on mole fraction.

The mole fraction can be related to molarity or molality as:

Substituting this expression into the chemical potential we obtain:

The standard state for solutions is a 1 molar concentration of the reacting species. If we express the chemical potential for species j, the has the form:

where

Thus, if express the activity in the Henry's law standard state it is:

Biological Standard State

For solutions we have defined the standard state as a 1 molar concentration of a particular species. However, for living organisms a 1 M [H+] concentration is not applicable. Instead the standard state can be referenced to the hydronium ion concentration at pH 7.

A + nH⁺ (aq) ßà P

DG_rxn = m(P)^o - m(A)^o - n m(H⁺)

m(H⁺) = m(H⁺)^o + RT ln a(H⁺) = m(H⁺)^o - 2.303 RT pH

DG_rxn = m(P)^o - m(A)^o - n m(H⁺)^o + n2.303 RT pH

DG_rxn = DG^o + n2.303 RT pH

at pH = 7

DG_rxn = DG^o + n 16.12 RT