Using macroscopic cantilevers of evaporated Au(111) films on glass substrates in conjunction with scanning tunneling microscopy, we performed quantitative surface stress measurements of copper underpotential deposition in bromide and chloride acid solutions. Upon differentiation of the stress data with respect to potential, definite signatures of the corresponding voltammograms were obtained, in contrast to previous contentions of the applicability of the Lippmann equation to solid electrodes which predicts such signatures only for the second derivative of the stress.