Here, VMS 0 is termed the standard half-cell potential. VMS is the p.d. between the metal and

the solution, relative to the solution. The symbol a(M) refers to the activity of metal ions in solution, which is directly proportional to the metal ion concentration [M]. We can write

a(M) = γ [M],

where γ is the activity coef‹cient of the ion in solution. In general, γ varies with [M], and tends toward 1, as [M] tends toward 0 (i.e., dilute solutions). Biological solutions can be treated as being dilute, even though γ has a value of around 0.5-0.8 for ions in cytoplasmic £uid. An approximate form for the half-cell potential equation is thus

V V ln M in mVMS MS= + 0 25 [ ]( ).