In this work, we described an image processing procedure for the measurement of surface tension of the air-liquid interface using isothermal capillary action. The experiment, designed for an undergraduate course, is based on the analysis of a series of solutions with diverse surfactant concentrations at different ionic strengths. The objective of the work is to illustrate some concepts such as Gibbs isotherm and surface excess for the case of ionic surfactants. To perform the experiment, it is necessary to use glassware commonly found in a chemistry lab along with an USB microscope and digital image processing program (available in the public domain). The experiment can be completed in a 4 h laboratory session.