Evolved Gas Analyses (TG-FTIR and TG/DTA-MS) of
Magnesium Nitrate Hexahydrate in Air and Nitrogen
Péter Pál Varga
Department of Inorganic and Analytical Chemistry
Budapest University of Technology and Economics
H-1521 Budapest, Hungary
E-mail: pedro83@freemail.hu
Supervisor: Dr. János Madarász
Magnesium nitrate hexahydrate, widely used agent of burning organics to ash, had
been studied several times by the means of thermal analysis [1-4]. Thermogravimetric
(TG), differential thermal analysis (DTA), differential scanning calorimetric (DSC) studies
were mainly focused on the temperature (ca. 90°C) and heat of its congruent melting,
possible intermediate hydrated forms during the elongated dehydration process, and
opportunities for formation of anhydrous salt. Simultaneous TG/DTA studies [1-2] show
that dehydration and nitrate pyrolysis processes depending on the sample size, type of
sample holder, and heating rate cannot be separated at an anhydrous state. Under quasiisothermal-quasi isobaric conditions [2] in self-generated atmosphere the temperature of
nitrate decomposition is significantly lower than expected, proven also by simultaneous
thermogas-titrimetry (TGT) [2]. The decomposition products of anhydrous compound in
vacuum [3] contained small amounts of magnesium nitrite and nitrogen in addition to
magnesium oxide, nitrogen dioxide and oxygen as the major products. In argon flow both
NO gas and oxygen were released thermally from the title compound reported recently by
simultaneous evolved gas analysis with online quadruple mass spectrometer [4].
We have analyzed and monitored the thermally evolved gases released from
Mg(NO3)2·6H2O in flowing air and pure nitrogen atmosphere by simultaneous
thermogravimetry and differential thermal analysis coupled online with mass spectrometry
(TG/DTA-MS) and with FTIR spectroscopic gas cell (TG-FTIR) up to 700°C. The
components of released gaseous mixtures were monitored and identified mostly on the
basis of their FTIR and MS reference spectra available on world-wide web in the public
domain spectral libraries of NIST [5].
Figure 1: EGA-MS curves of ion fragments of NO2 and O2 released from
[Mg(H2O)6](NO3)2 in flowing nitrogen
Both in air and nitrogen the mass spectrometry (TG/DTA-MS) definitely proved
parallel evolution of O2 (m/z = 32, 16) and NO2 (m/z = 30, 46, 47, 48) between 300-500°C.
The m/z = 30 ion fragment with highest intensity among the fragments of nitrogen oxides
suggested parallel evolution of NO, anyhow according to the reference spectra [5] m/z =
30 is the most intense fragment of NO2. Meanwhile similar changes in intensities of the
m/z = 44 and 45 fragments might indicate evolution of N2O or CO2, as well. Anyhow, the
TG-FTIR spectroscopy showed only evolution of NO2 with two PR-bands positioned at
1616 and 2911 cm-1, and no bands of NO and N2O could be detected, furthermore the
parallel CO2 evolution could be confirmed, nevertheless in nitrogen, evolution of some
CO2 could not be excluded because of accidental furnace contaminations.
References
[1] Atlas of Thermoanalytical Curves, Vol.3., No. 137., G. Liptay (Ed.), Heyden & Sons /
Akadémiai Kiadó, Budapest (1974) and refs therein.
[2] F. Paulik, J. Paulik, M. Arnold, R. Naumann, J. Therm. Analysis 34 (1988) 627-35, and
refs therein.
[3] T. M. Oza, B. V. Mirza, Indian J. Chem., 3 (1965) 280-1.
[4] A. Migdal-Mikuli, E. Mikuli, R. Dziembaj, D. Majda, L. Hetmanczyk, Thermochim.
Acta, 419 (2004) 223-9.
[5] NIST Chemistry Webbook Standard Reference Database No. 69, June 2005 Release
(http://webbook.nist.gov/chemistry)