Moisture-mediated mechanism of the decomposition of Na2SO4 center dot 7H(2)O and Na2SeO4 center dot 7.5H(2)O

摘要

The spontaneous decomposition of Na2SO4 center dot 7H(2)O and Na2SeO4 center dot 7.5H(2)O is investigated by thermal analysis and differential scanning calorimetry. It is found that their decomposition occurs when cooled, only in presence of moisture, in the temperature range from -8 degrees C to -14 degrees C. The decisive role of the presence of moisture on the Na2SO4 center dot 7H(2)O crystals is explained by the relative humidity-temperature diagram of the Na2SO4-H2O system. On cooling of well-dried Na2SO4 center dot 7H(2)O the relative humidity (RH) moves along the boundary Na2SO4 center dot 7H(2)O-anhydrous Na2SO4 and at t =-9.75 degrees C enters the stability zone of Na2SO4 center dot 7H(2)O. In the presence of moisture, respectively ice, on lowering the temperature, the relative humidity of the system is determined by that of ice and Na2SO4 center dot 7H(2)O remains in the metastable zone down to the triple point ice-Na2SO4 center dot 10H(2)O-Na2SO4 center dot 7H(2)O at t = -92.5 degrees C. In this case, the Na2SO4 center dot 7H(2)O crystals should decompose in the temperature range from -8 degrees C to -14 degrees C.