The masses within an isobaric multiplet are related by the general model-independent mass formula M(A,T,Tz)=a(A,T)+b(A,T)Tz+c(A,T)Tz2. The values of the coefficients b(A,T) and c(A,T) are extracted from experimental data function of mass number for several triplets ranging from A=6 to A=32. Theoretical calculations are made for these coefficients on the basis of Coulomb interaction alone after accounting for the neutron-proton mass difference, and the residual shifts are attributed to charge dependent effects in the nuclear forces. A phenomenological effective charge-dependent potential is introduced and its parameters are determined by a least-square analysis.