CHANGES IN THE STANDARD GIBBS ENERGIES ASSOCIATED WITH THE FORMATION OF DOUBLE SELENATES (NH4)2M(SEO4)2.6H2O (M= MN, CO, NI OR ZN) AND (NH4)2CD(SEO4)2.2H2O

The energy changes associated with the formation of double selenates of the schoenite type were studied. By using the Filippov eutonic method, the DELTA-G-degrees values were obtained directly for the formation of the (NH4)2M(SeO4)2.mH2O salts with M = Mn, Co, Ni, Zn and Cd and m = 6 except for Cd, for which m = 2. The DELTA-G-degrees values are comparable with those for the analogous potassium salts and they lie within the range of -13.9 to -45.9 kJ mol-1. The fact that the final DELTA-G-degrees value is primarily affected by the solubilities of the components and of the compound forming was verified for the examined (NH4)2SeO4-M(II)SeO4-H2O systems as well.