In the presence of chloride ions, calomel formation takes place on the mercury thin film electrodes under open-circuit conditions, which leads to high base currents and erratic performance in ASV measurements. This study describes the effect of chloride concentration on the calomel formation on the surface by following the cathodic peak on the anodic scan. The effect of acetone on the surface reaction and baseline stability was studied. Application of the method with a sea water sample is also given.