This paper deals on the studies of the mannitol quantity necessary for the titration of boric acid with standardized solution of NaOH. The mannitol quantity is influenced by the pH of the end point of the titration. The results allowed to conclude that there is the possibility to reduce from 20.0 g to 5.0 g the quantity of mannitol necessary for titration of boron soluble in acid in fertilizers, provided the pH 8 be used as the end point of titration and proceeding the elimination of the ion NH+4 .