We report a general concept based on a selective electrochemical ion pump used for creating concentration perturbations in thin layer samples (~ 40 mL). As a first example, hydrogen ions are released from a selective polymeric membrane (proton pump) and the resulting pH is assessed potentiometrically with a second membrane placed directly opposite. By applying a constant potential modulation for 30 s, an induced proton concentration of up to 350 mm may be realized. This concept may become an attractive tool for in situ titrations without the need for sampling, because the thin layer eventually re-equilibrates with the contacting bulk sample. Acid–base titrations of NaOH and Na2CO3 are demonstrated. The determination of total alkalinity in a river water sample is carried out, giving levels (23.1 mm) comparable to that obtained by standard methods (23.6 mm). The concept may be easily extended to other ions (cations, anions, polyions) and may become attractive for environmental and clinical applications.