A Modified Standard State for Biochemical Applications
We have just seen that the calculation of standard free-energy changes includes the stipulation that all substances be in standard states, which for solutes can be approximated as a concentration of 1 M. If the hydrogen-ion concentration of a solution is 1 M, the pH is zero. (Recall that the logarithm of 1 to any base is zero.) The interior of a living cell is, in many respects, an aqueous solution of the cellular components, and the pH of such a system is normally in the neutral range. Biochemical reactions in the laboratory are usually carried out in buffers that are also at or near neutral pH. For this reason, it is convenient to deÞne, for biochemical practice, a modiÞed standard state, one that differs from the original standard state only by the change in hydrogen-ion concentration from 1 M to 1 x 10-7M, implying a pH of 7.
When free-energy changes are calculated on the basis of this modified standard state, they are designated by the symbol ∆G°' (spoken Òdelta G zero primeÓ). The Biochemical Connections box describes other specific applica-tions of thermodynamics to living organisms.
The usual thermodynamic standard state implies that the system involved is at pH 0, which is seldom, if ever, found in living things. The modified standard state explicitly states that the system is at pH 7.