Experiments on pattern formation in electrochemical systems show qualitative differences in comparison with phenomena observed with other chemical systems. In this paper we derive a general model from the basic transport equations which takes into account the special aspects of electrochemical systems. Stepwise introduction of three approximations considerably simplifies the equations and demonstrates the dominant role the electric potential plays for pattern formation. The simplest form of the model contains migration as the only transport mechanism, and the importance of intrinsic global coupling for the dynamic behavior of electrochemical systems becomes apparent. This simple model already reproduces the experimentally observed front behavior we recently reported [G. Flatgen and K. Krischer, Phys. Rev. E 51, 3997 (1995)].