Measurements of parameters expressed in terms of carbonic species, such as alkalinity and acidity of saline waters, do not analyse the influence of external titration parameters such as total free and bound carbonic species concentration, activity ratio, ion pair formation and residual liquid coupling potential in pH measurements. This paper shows the development of an F5BC titration function based on the titration developed by Grahn (1952) for the carbonate system of natural waters. For practical use, saline water samples from the Pocinhos Reservoir in Paraíba were titrated and linear regression analysis was performed. The results showed that F5BC implies the determination of the Grana functions F1x and F2x, respectively, for alkalinity and acidity calculations without knowing "a priori" the titration endpoint. F5BC also allows the determination of the first and second apparent dissociation constants of the carbonate system of saline and high ionic waters.