Most of the theoretical and practical development of the ocean acidification (OA) phenomenon involves the open ocean, and not much is known of the significance of variation in pH and carbonate system in coastal environments and the effect, if any, of OA. Traditional potentiometric pH measurements are carried out on the NBS scale (pHNBS), developed for freshwaters, but for OA it is necessary to use the total scale (pHT), which includes the additional ions of seawater. Using a series of in-situ measurements of potential, carried out with a pHNBS electrode in the artificial coastal lagoon La Escollera in Santa Marta (Colombia), a methodology to calculate pHT was tested. For this, the equation pHT(X) = pHT(TRIS) - EX-ETRISR*T*ln10/F was used, which calculates pHT(X) of the sample from the pHT(TRIS) of the TRIS standard solution, the potentials E measured at temperature T, and the constants R and F. ETRIS was determined experimentally for the lagoon temperature range, and the linear regression showed a coefficient of determination (R2) of 0.9977. In a first qualitative analysis, it was verified that pH variations during the day-night cycle are closely associated with those of oxygen, from the production and consumption of CO2 by photosynthesis and respiration. These high-frequency variations are of greater magnitude than those of the open ocean, raising questions about the real effect of OA on coastal ecosystems.