THERMOGRAVIMETRICAL STUDY OF ZNSO4.7H(2)O - IS IT A PRIMARY STANDARD FOR COMPLEXOMETRIC TITRATIONS

A small number of substances has been recommended for the standardizat ion of EDTA solutions, among them the reagent ZnSO4 . 7H(2)O. However, it was observed that its composition is not stable at room temperatur e. Different treatments were carried out with samples of this salt and a different stable and defined composition was obtained, ZnSO4 . H2O. The thermal behaviour of the heptahydrated salt confirmed the existen ce of the monohydrated one when it was healed up to 152 degrees C. In order to evaluate the monohydrated salt as primary standard, some titr ations of ZnSO4 . H2O with EDTA solution were carried out and the resu lts were compared with those obtained by using the same kind of titrat ions with Ca2+ (CaCO3 - reference standard) and Zn2+ (Zn + HCl) soluti ons. The results obtained were in excellent agreement and showed that the monohydrated salt can be used more easily as primary standard.