Thermal Analysis Study of Phase Transformations of Magnesium and Calcium Methanesulfonates

Article abstract of DOI:10.1134/S0036023620050125

Abstract: Anhydrous magnesium methanesulfonate (Mg(SO₃CH₃)₂) and calcium methanesulfonate (Ca(SO₃CH₃)₂) as well as hydrates Mg(SO₃CH₃)₂ · 2H₂O an

Full text of DOI:10.1134/S0036023620050125

ISSN 0036-0236, Russian Journal of Inorganic Chemistry, 2020, Vol. 65, No. 5, pp. 752–757. © Pleiades Publishing, Ltd., 2020.
Russian Text © The Author(s), 2020, published in Zhurnal Neorganicheskoi Khimii, 2020, Vol. 65, No. 5, pp. 679–685.
THERMODYNAMICS
OF INORGANIC COMPOUNDS
Thermal Analysis Study of Phase Transformations
of Magnesium and Calcium Methanesulfonates
D. A. Kosovaa, *, D. I. Provotorova, S. V. Kuzovchikova, and I. A. Uspenskayaa
aMoscow State University, Moscow, 119991 Russia
*e-mail: dakosova@gmail.com
Received November 15, 2019; revised December 4, 2019; accepted December 24, 2019
Abstract—Anhydrous magnesium methanesulfonate (Mg(SO3CH3)2) and calcium methanesulfonate
(Ca(SO3CH3)2) as well as hydrates Mg(SO3CH3)2 · 2H2O and Mg(SO3CH3)2 · 12H2O have been prepared
and identified. Thermal degradation of the salts in air has been studied by thermogravimetric analysis. Param-
eters of phase transformations of Ca(SO3CH3)2 and Mg(SO3CH3)2 · 2H2O observed at –62.7 and –119°С,
respectively, have been found by differential scanning calorimetry. Melting point of Mg(SO3CH3)2 · 12H2O
(45.4°С) synthesized immediately in DSC instrument because of compound instability in air has been deter-
mined. Incongruent melting has been shown for Mg(SO3CH3)2 · 12H2O.
Keywords: phase transition, melting, differential scanning calorimetry, thermogravimetry
DOI: 10.1134/S0036023620050125
INTRODUCTION
equilibria is to assess stability range for individual crys-
talline phases formed in the systems and to determine
the temperature and enthalpy of phase transitions of
the compounds.
To date, methanesulfonic acid production reaches
50000 tons per year and expected to grow because
environmentally benign and economic method for its
preparation in one stage in >99% yield was proposed
recently [1]. Therefore, it is of fundamental and prac-
tical interest to study the properties of methanesul-
fonic salts and systems on their basis. The correspond-
ing literature information is fragmentary [2–5] in spite
of the fact that methanesulfonic acid and its inorganic
derivatives are used in different areas [6–15]. For
example, the study of properties of magnesium and
calcium methanesulfonates and their hydrates is of
interest to solve the tasks of geochemistry and atmo-
spheric chemistry [16–22]. Methanesulfonic acid
results from oxidation of dimethyl sulfide with air oxy-
gen, which, in turn, is produced by phytoplankton
[23]. Methanesulfonic acid aerosols participate in
cloud formation [18, 19]. When deposited with atmo-
spheric precipitation on the Earth surface, methane-
sulfonates undergo degradation in soil under the
action of different microorganisms [20]. Recently,
researchers found [23] methanesulfonates and their
hydrates in ice cores in Antarctica, which contained
marked amounts of sodium, potassium, calcium, and
magnesium methanesulfonates. Therefore, it is urgent
task to study phase equilibria in the corresponding
water–salt systems because the knowledge of proper-
ties of ice cores may be used to reproduce climate
changes and ocean composition during formation of
different rocks, i.e., to solve the tasks of geothermoba-
The subjects of this study are magnesium methane-
sulfonate hydrate and anhydrous magnesium and cal-
cium methanesulfonates. The aim of this work is to
study thermochemical properties of the noted com-
pounds. It was intended to solve the following tasks: to
synthesize the compounds under study, to identify
prepared compounds, to study thermal stability of
anhydrous and hydrated compounds, and to deter-
mine parameters of phase transformations. The main
research methods are thermogravimetry (TGA) and
differential scanning calorimetry (DSC).
It is known from literature that calcium methane-
sulfonate produce no hydrates [25], while magnesium
methanesulfonate crystallizes from aqueous solutions
as Mg(SO3CH3)2 · 12H2O at ambient temperature [26]
and as Mg(SO3CH3)2 · 2H2O at 80–90°C [25], Only
structures of Mg(SO3CH3)2 · 12H2O [26, 27] and
Ca(CH3SO3)2 [28] are established unambiguously.
Methanesulfonate Mg(SO3CH3)2 · 12H2O crystallizes
in space group
and has trigonal unit cell with
R3
parameters a = 9.27 Å, b = 9.27 Å, c = 21.13 Å, γ =
120°; the data were obtained at 110 K [26, 27]. Com-
pound Ca(CH3SO3)2 has space group Pbca, orthor-
hombic unit cell with parameters a = 17.235 0.016 Å,
b = 10.08 0.01 Å, c = 9.17 0.01 Å [28].
Thermal behavior of anhydrous magnesium and
rometry [24]. The first stage in the study of phase calcium methanesulfonates in dynamic air atmo-
752

THERMAL ANALYSIS STUDY OF PHASE TRANSFORMATIONS OF MAGNESIUM
753
Table 1. Purity and sources of chemicals used in the work
Formula
Source
Alfa Aesar
4MgCO3 · Mg(OH)2 · nH2O Komponent-reaktiv
Purity, wt %
Analysis method
Established by supplier
HSO3CH3
98.0
>99
>99
–
Established by supplier
Established by supplier
DSC
CaCO3
Komponent-reaktiv
Prepared in this work
Prepared in this work
Mg(CH3SO3)2 · 12H2O
Mg(CH3SO3)2 · 2H2O
99.9
Thermogravimetry, elemental analysis, complex-
ometric titration
Mg(CH3SO3)2
Ca(CH3SO3)2
Prepared in this work
Prepared in this work
99.9
99.9
Thermogravimetry, complexometric titration
Thermogravimetry, elemental analysis, complex-
ometric titration
sphere (20 mL/min) was studied by TGA at heating was added to the initial solid compounds, next, meth-
rate 20 K/min in the work [25]. Onset degradation anesulfonic acid was slowly added in minimal excess
temperatures are 476 and 474°С, weight fractions of with vigorous stirring, medium pH was monitored
solid residue are 21.85 and 41.87 wt % for magnesium using indicator paper. Carbon dioxide evolved during
and calcium salts, respectively. Solid residue after salt the reaction, heat evolved; therefore, reaction vessel
degradation was studied by X-ray powder diffraction. was placed in an ice bath to prevent boiling. After reac-
The residue was shown to consist of a mixture of the tion completion, the reaction mixture was evaporated
corresponding metal oxide and sulfate. The work [26] at 60°C, and allowed to stand at ambient temperature.
reported TGA curve for Mg(SO3CH3)2 · 12H2O The precipitated crystals were recrystallized twice
from water and collected by filtration on a sintered
glass filter under reduced pressure, washed with alco-
hol, and carefully grinded. The salts were stored in
tightly closed vessels for chemicals.
The obtained compounds were identified by ele-
mental and thermogravimetric analysis and by com-
plexometric titration with EDTA. The results of the
study are presented in Tables 2–5.
Elemental analysis was carried out on a CE1106
automated CHN analyzer (sample weight 0.8–1.0 mg,
burning temperature 1050°С) in the Nesmeyanov
Institute of Organoelement Compounds, Russian
Academy of Sciences. When sample placed into a tin
capsule flashed in oxygen, the temperature reached
1800°C. Burning proceeded in a helium flow as inert
carrier gas at flow rate 120 mL/min. Full time of anal-
ysis cycle is 10 min. Destruction products (nitrogen,
carbon dioxide, and water) were separated using a gas
chromatograph on a GC column in helium flow, using
katharometer as a detector. The content of C and H
obtained at heating rate 5 K/min. Dehydration occurs
in three stages, the stages are poorly separated, how-
ever, the authors supposed that Mg(SO3CH3)2
·
12H2O decomposes according to the following
scheme.
Mg(CH3SO3)2 ⋅12H2O
t
⎯⎯→Mg(CH3SO3)2 ⋅10H2O
t
→ Mg(CH3SO3)2 ⋅ 2H2O ⎯⎯→Mg(CH3SO3)2.
There are no information in the literature on the
other phase transformations of calcium and magne-
sium methanesulfonates and its hydrates.
EXPERIMENTAL
Synthesis. Magnesium and calcium methanesul-
fonates were obtained using methanesulfonic acid
HSO3CH3, mixed magnesium oxide-carbonate
4MgCO3 · Mg(OH)2 · nH2O, and calcium carbonate
CaCO3. The purity and sources of chemicals are given
in Table 1. The compounds were obtained by the fol-
lowing procedure. A small amount of distilled water
Table 3. Calcium and magnesium content in methanesul-
fonates according to complexometric titration. ω(M2+) is
weight content of magnesium (calcium) cation
ω(M2+), wt %
Table 2. Elemental analysis of magnesium and calcium
methanesulfonates, wt %
Compound
calculated
for molecular formula
experiment
Mg(CH3SO3)2 · 2H2O
Ca(CH3SO3)2
Calculation
ω(С)
ω(H)
ω(С)
ω(H)
Mg(CH3SO3)2
Mg(CH3SO3)2 · 2H2O 9.79 0.03
Ca(CH3SO3)2 17.19 0.07
11.35 0.02
11.34
9.71
Theory
Experiment
9.59
9.65
4.00
4.06
10.43
10.38
2.61
2.66
17.43
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 65 No. 5 2020

754
KOSOVA et al.
Table 4. Comparison of experimental and literature data [25] for the thermal degradation of anhydrous magnesium and
calcium methanesulfonates in air in temperature range 400–600°С. Td is onset degradation temperature on TGA curve. Δm
is weight loss, wt % calculated for anhydrous salt
Td, °C
Δm, wt %
literature data [23]
Compound
this work
literature data [4]
this work
Mg(CH3SO3)2
Ca(CH3SO3)2
440
450
476
474
24.74
43.98
21.85
41.87
was calculated automatically using software of the Table 5. The results of thermogravimetric analysis for water
content in magnesium and calcium methanesulfonates
Mg(CH3SO3)2 · 2H2O Ca(CH3SO3)2
chromatograph. Preliminary found calibration coeffi-
cients determined using standard samples, blank test
results, and sample weight were taken into account in
calculation. Experimental error was 0.5 wt %. Analysis
results are given in Table 2.
ω(H2O), wt %
Calculated from
molecular formula
Experiment
14.40
0
0
Titration. Calcium and magnesium content in
compounds was determined by complexometric titra-
tion with 0.005 M EDTA solution using Eriochrome
Black T as an indicator by the standard procedure
described in [29]. Analysis results are presented in
Table 3.
14.48
Compounds were placed in an aluminum crucible
(V = 56 mm3, D = 6 mm) with punctured cap or sealed
in the case of liquid sample. Sample weight was 3–10 mg,
weighting accuracy was 2 × 10–5 g. Solid samples were
carefully grinded and placed as a thin even layer on
crucible bottom. Liquid samples were prepared in
eppendorf vessels and next was transferred to crucible
using a syringe. Heterogeneous samples were prepared
immediately in crucible upon addition of solid com-
ponent and distilled water from a syringe.
Data treatment was carried out using Netzsch Pro-
teus Analysis software. First-order phase transforma-
tion temperatures were determined as Tonset (intersec-
tion point of base line extrapolated to peak region and
tangent drawn through inflection point on the left
shoulder of the peak on DSC curve) in accordance
with ASTM E 794. Phase transformation enthalpies
were determined from the corresponding peak areas
on DSC curves (ASTM E 793). Quantitative treat-
ment was used only for DSC curves obtained in heat-
ing mode.
Thermogravimetry. Thermogravimetric analysis
was performed on a Netzsch TG 209 F1 instrument.
Measurement were conducted in a dry air dynamic
atmosphere, flow rate of 40 mL/min. Heating rate was
10 K/min. Instrument calibration by temperature was
performed using melting points of standard com-
pounds of at least 99.999% purity. Standards used
were silver, aluminum, bismuth, indium, and tin. Sys-
tematic error for weight loss determined from the
decomposition of calcium oxalate monohydrate was
not larger 1%.Measurements were made in the tem-
perature range 30–600°C. Prior to measurements, the
compounds were carefully grinded and placed in
corundum crucibles. Sample weight was 10–20 mg.
The samples were weighed on an A&D GH-202 bal-
ances with accuracy of 2 × 10–5 g. Analysis results are
presented in Tables 4 and 5.
Differential scanning calorimetry. We used differen-
tial scanning calorimetry to study phase transitions of
individual crystalline phases and solid–liquid phase
equilibria. The measurements were performed on a
DSC 204 F1 Phoenix calorimeter under atmosphere
of nitrogen of high purity grade (flow rate through cell
was 40 mL/min, protecting gas flow rate was 70 mL/min).
An automated system of liquid nitrogen supply was
used for cooling. Instrument calibration was per-
formed for temperature and sensitivity in accordance
with ASTM E967 and E968. Standards used were
cyclohexane, mercury, gallium, benzoic acid, water,
indium, tin, bismuth, lead, cesium chloride, and zinc;
purity of the standards was at least 99.999%. The sys-
tematic error of the instrument after calibration was
0.1°С for temperature and 5% for heat. The measure-
ments were performed in the temperature range 150–
60°С.
In the work, we used two main DSC temperature
programs:
the first program (I) was used to search and study
phase transitions of individual crystalline substances
and consisted in simple scanning at a rate of 10 K/min;
the second program (II) consisted of several stages and
was used in the study of water–magnesium methane-
sulfonate systems:
(1) heating at rate 5 K/ min up to 75°С;
(2) isotherm at 75°С, for 5 min;
(3) cooling at rate 10 K/min down to –150°С;
(4) heating at rate 10 K/ min up to –15°С;
(5) heating at rate 2 K/ min up to 75°С.
Control weighing was carried out after DSC exper-
iments with liquids to check up crucible air-tightness.
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THERMAL ANALYSIS STUDY OF PHASE TRANSFORMATIONS OF MAGNESIUM
755
obtained by the procedure described in Experimental.
Weight loss was detected only at temperatures above
450°С. The comparison of experimental and literature
data [25] on the thermal decomposition of calcium
methanesulfonate in air is given in Table 4, the pre-
sented values agree well with each other.
Δm, %
100
90
80
70
60
43.98%
Synthesized crystals of magnesium methanesul-
fonate were characterized by TGA without prelimi-
nary sample treatment. Dotted line in Fig. 2 shows the
results of experiment. It is seen that dehydration pro-
cess began even before recording in preceding isother-
mal segment at 30°C. Due to instability of this com-
pound in air, it is impossible to use it directly for anal-
ysis. Grinded crystals were remained to dry for several
days in air, then thermal analysis was performed; solid
line in Fig. 2 shows obtained TGA curve. Water con-
tent corresponds to dihydrate Mg(CH3SO3)2 · 2H2O
(Table 5). The character of decomposition of anhy-
drous magnesium methanesulfonate Mg(CH3SO3)2
agrees well with literature data (Table 4). Since mag-
nesium methanesulfonate is hygroscopic in air, all fur-
ther manipulations were performed with its dihydrate.
100
200
300
T, °C
400
500
Fig. 1. TGA of calcium methanesulfonate Ca(CH SO ) .
3
3 2
Δm, %
100
90
80
70
60
50
40
30
20
14.48%
40.70%
64.38%
38.84%
Melting of Mg(CH3SO3)2 · 12H2O. For the ther-
mal analysis of Mg(CH3SO3)2 · 12H2O, it was synthe-
sized immediately in DSC device by mixing of the cor-
responding amounts of distilled water and dihydrate
100
200
300
T, °C
400
500
Mg(CH3SO3)2
· 2H2O (theoretical content of
Fig. 2. TGA of hydrates of magnesium methanesulfonate
Mg(CH3SO3)2 is 50.2 wt %). The mixture was studied
by temperature program II described in Experimental.
DSC curve shows endothermal effect at 45.4°С corre-
sponding to the melting of twelve-water hydrate (Fig. 3,
Table 6). To establish melting type, we studied several
compositions of water–magnesium methanesulfonate
system. Special attention was paid to the region of
composition in proximity to the twelve-water hydrate.
For dilute solutions, we detected peaks of liquidus and
solidus (–4.5°С, the value agrees well with data [26]
(–5°C)). For composition containing 49.55 wt %, we
detected both solidus at –4.5°С and melting of twelve-
water hydrate at 45.4°С (Fig. 4). For compositions
with higher content of salt, we detected only melting
peaks of twelve-water hydrate. Based on the per-
formed analysis, we drew a conclusion on the incon-
gruent character of Mg(CH3SO3)2 · 12H2O melting.
DSC curves for the studied compositions are schemat-
ically drawn in phase diagram (Fig. 4) and shown in
Fig. 3.
Mg(CH SO ) . Solid line is Mg(CH SO ) · 2H O,
3
3 2
3
3 2
2
dashed line is crystals obtained by synthesis.
exo
–0.5
49.55%
–1.0
51.80%
52.09%
54.44%
–1.5
–2.0
–2.5
35
40
45
50
T, °C
55
60
Fig. 3. DSC curves for different compositions of H O–
2
Mg(CH SO ) system in the region of Mg(CH SO ) ·
3
3 2
3
3 2
12H O melting.
2
Phase transition of Mg(CH3SO3)2 · 2H2O. On
DSC measurement under elevated external pres- heating and cooling at a rate of 10 K/min, we revealed
sure of 107 Pa was conducted on a Netzsch DSC 204
a reproducible anomaly for Mg(CH3SO3)2 · 2H2O with
HD instrument, heating rate of 10 K/min, tempera- maximum at –119°С, which is absent for anhydrous
ture range of 25–450°С.
salt and twelve-water hydrate (Fig. 5). No temperature
hysteresis was observed, the peak has typical lambda
form. Additional structural studies at decreased tem-
perature are required to determine the nature of the
observed process. No other phase transformation were
revealed on heating Mg(SO3CH3)2 · 2H2O in DSC cell
RESULTS AND DISCUSSION
Thermogravimetric analysis of salts. Figure 1 shows
the TGA curve of a calcium methanesulfonate sample
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 65 No. 5 2020

756
KOSOVA et al.
T, °C
exo
1.0
0.5
0
45.4
0
Mg(SO3CH3)2 · 12H2O
–0.5
–1.0
–1.5
Mg(SO3CH3)2
Mg(SO3CH3)2 · 2H2O
–4.5
H2O
41.31
51.80
Mg(SO3CH3)2
2.68
49.55
ω, wt %
0
–130
–125
–120
T, °С
–115
Fig. 4. Schematic representation of a fragment of phase
diagram of H O–Mg(CH SO ) system with superposed
Fig. 5. DSC curves for cooling (dashed line) and heating
(solid lines) of Mg(CH SO ) · 2H O, Mg(CH SO )
2
3
3 2
·
3 2
3
3 2
2
3
DSC curves for compositions containing 2.68, 41.31,
49.55, and 51.80 wt % of Mg(CH SO ) .
12H O, and Mg(CH SO ) .
2
3
3 2
3
3 2
that Mg(CH3SO3)2 ∙ 12H2O is unstable in air and
transforms in time into dihydrate Mg(CH3SO3)2 ∙
2H2O that has onset degradation temperature of
115°С. It was shown that Mg(CH3SO3)2 ∙ 2H2O,
Mg(CH3SO3)2, and Ca(CH3SO3)2 decompose with-
out melting not only under atmospheric pressure but
also under external pressure up to 107 Pa.
up to decomposition temperature (115°С) under
external pressure of 107 Pa.
Phase transition of Ca(CH3SO3)2. Calcium meth-
anesulfonate shows a reproducible anomaly in DSC
curves on heating and cooling (Fig. 6) that has tem-
perature hysteresis of ∼5°C due to supercooling of
high-temperature phase on cooling. Therefore, the
anomaly was related to first-order phase transition
whose temperature and enthalpy are presented in
Table 7. We revealed no other phase transformations
on heating Ca(CH3SO3)2 in DSC cell up to decompo-
sition temperature (450°С) under external pressure of
107 Pa.
It was found by differential scanning calorimetry
that Mg(CH3SO3)2 ∙ 2H2O and Ca(CH3SO3)2 undergo
phase transformations at –119 and –62.7°С, respec-
tively. DSC curves in the region of Ca(CH3SO3)2
phase transformation exhibit hysteresis on cooling and
heating; it was reliably found that there is the first-
order phase transition, its temperature and enthalpy
was determined.
CONCLUSIONS
Procedure for the synthesis of Mg(CH3SO3)2 ∙
12H2O immediately in DSC cell was developed, the
melting point of twelve-water hydrate Mg(CH3SO3)2 ∙
12H2O was found to be 45.4°С. Several compositions
of H2O–Mg(CH3SO3)2 system were studied by DSC,
it was found that Mg(CH3SO3)2 ∙ 12H2O undergoes
incongruent melting.
Thermal decomposition of Mg(CH3SO3)2 ∙ 12H2O,
Mg(CH3SO3)2 ∙ 2H2O, Mg(CH3SO3)2, and Ca(CH3SO3)2
in air was studied by thermogravimetry. It was shown
Table 6. Incongruent melting temperature for
Mg(CH3SO3)2 · 12H2O determined by DSC experiments for
mixtures of different composition of H2O–Mg(CH3SO3)2
system
ACKNOWLEDGMENTS
Mixture composition,
wt % Mg(CH3SO3)2
The authors thank the Center for Molecular Structure
Studies, Nesmeyanov Institute of Organoelement Com-
pounds, Russian Academy of Science, and Professor, Dr.
Sci. (Chem.) A.A. Goryunkov, head of Laboratory of Ther-
mochemistry, Moscow State University, for the possibility
to publish this material.
Tm, °C
Experiment no.
49.55
51.80
1
1
2
3
1
2
3
1
45.2
45.6
45.5
45.3
45.6
45.3
45.5
45.4
52.09
Table 7. Parameters of phase transformation of Ca(CH3SO3)2
Tpt, °C
∆Hpt, kJ/mol
54.44
Average value
45.4 0.1
–62.7 0.2
0.60 0.01
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THERMAL ANALYSIS STUDY OF PHASE TRANSFORMATIONS OF MAGNESIUM
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RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 65 No. 5 2020