Reactivity in the solid state between Ag2S and Ag2CrO4

Article abstract of DOI:10.1023/A:1011516206935

Reactivity, in the solid state between Ag₂S and Ag₂CrO₄, was investigated by DTA, XRD and IR methods. It was found that, according to a composition of an initial Ag₂S/Ag₂CrO₄ mixture, the products of a reaction of Ag₂S with Ag₂CrO₄ can be: solid solution with Ag₂CrO₄ structure (Ag₂Cr_1-xS_xO₄) and AgCrO₂; or solid solution Ag₂Cr_1-xS_xO₄, Ag₂SO₄, AgCrO₂ and metallic silver; or Ag₂S, β-Ag₈S₄O₄, Ag, AgCrO₂, Ag₂SO₄ and Ag₂Cr_1-xS_xO₄ solid solution.

Full text of DOI:10.1023/A:1011516206935

Journal of Thermal Analysis and Calorimetry, Vol. 64 (2001) 1087–1093
REACTIVITY IN THE SOLID STATE BETWEEN Ag2S
AND Ag2CrO4
E. Tomaszewicz and M. Kurzawa
Department of Chemistry and Environment Protection, Technical University of Szczecin,
Al. Piastów 42, 71-065 Szczecin, Poland
Abstract
Reactivity, in the solid state between Ag2S and Ag2CrO4, was investigated by DTA, XRD and IR
methods. It was found that, according to a composition of an initial Ag2S/Ag2CrO4 mixture, the
products of a reaction of Ag2S with Ag2CrO4 can be: solid solution with Ag2CrO4 structure
(Ag2Cr1–xSxO4) and AgCrO2; or solid solution Ag2Cr1–xSxO4, Ag2SO4, AgCrO2 and metallic silver;
or Ag2S, β-Ag8S4O4, Ag, AgCrO2, Ag2SO4 and Ag2Cr1–xSxO4 solid solution.
Keywords: DTA, IR, silver chromate(VI), silver sulfide, XRD
Introduction
Our earlier studies on the reactivity in the solid state between Ag2S and Ag2SO4
showed that both reagents enter into reaction to give two phases: β-Ag6S3O4 and
β-Ag8S4O4 [1–4]. These compounds had not been known in the literature before.
β-Ag6S3O4 is formed by heating in air atmosphere Ag2S and Ag2SO4 mixed at the mo-
lar ratio 2:1 according to the following reaction [1, 2, 4]:
2Ag2S(s)+Ag2SO4(s)=β-Ag6S3O4(s)
(1)
β-Ag8S4O4, the second phase occurring in the Ag2S-Ag2SO4 system, is formed as
a result of the reaction between Ag2S and Ag2SO4, at the molar ratio of the reagents –
3:1 [3, 4]:
3Ag2S(s)+Ag2SO4(s)=β-Ag8S4O4(s)
(2)
In the light of above information it deemed advisable to ascertain whether silver
chromate(VI), the phase crystallizes just the same as silver sulfate(VI) in ortho-
rhombic system (space group – Pnma, a=1.0063(11) nm; b=0.7029(4) nm;
c=0.5540(2) nm; Z=4 [5]), could form similar compounds.
1418–2874/2001/ $ 5.00
Akadémiai Kiadó, Budapest
© 2001 Akadémiai Kiadó, Budapest
Kluwer Academic Publishers, Dordrecht

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TOMASZEWICZ, KURZAWA: REACTIVITY BETWEEN Ag2S AND Ag2CrO4
Experimental
The following reagents were used for experiments: pure Ag2S (POCh, Gliwice, Po-
land) and Ag2CrO4 (a precipitate obtained as a result of the reaction between Ag+ and
CrO24– , filtered, washed and dried at 110°C for 24 h). The mixtures of Ag2S with
Ag2CrO4 were prepared with a range of silver sulfide from 10.00 to 90.00 mol%.
These mixtures were heated in air, in the following cycles: 250 (8 h); 300 (8 h); 350
(8 h); 400 (8 h); 450 (8 h); 475 (8 h); 525 (2×8 h); 550°C (13 h). After each heating
cycle the samples were gradually cooled to ambient temperature, weighed, ground
and examined for their contents by XRD method and then heated again. After the fi-
nal heating cycle, the samples were examined by DTA and IR methods. The DTA ex-
aminations were made by using a TA Instruments SDT 2960 apparatus. These mea-
surements were carried out in air, at the heating rate of 10 K min–1. X-ray diffraction
phase analysis of the samples was performed by a diffractometer DRON-3 using
CoKα radiation (λ=0.179021 nm). The IR spectra were recorded on a Specord M-80
spectrometer (Carl Zeiss Jena).
Results and discussion
Table 1 shows the contents of the initial mixtures of Ag2S with Ag2CrO4 and results of
XRD analysis of samples obtained after the final heating cycle. Mass change investi-
gations show mass increases for the samples obtained by heating initial mixtures
comprising initially to 70.00 mol% of Ag2S and mass decreases of the samples, the
initial mixtures of which contained over 70 mol% of Ag2S.
Table 1 Contents of initial mixtures and results of XRD analysis of samples obtained after the fi-
nal heating cycle
No.
1
Ag2S content in initial mixtures/mol%
10.00; 20.00; 30.00; 40.00
50.00; 60.00; 66.67; 70.00; 75.00; 80.00
90.00
Phase identified
Ag2Cr1–xSxO4; AgCrO2
2
Ag2Cr1–xSxO4; Ag2SO4; AgCrO2; Ag
Ag2S; β-Ag8S4O4; Ag; AgCrO2(traces)
3
The data of Table 1 and the results of mass change studies point out that Ag2S
and Ag2CrO4 are not mutually inert in air. It was found that, at the concentration range
to 40.00 mol% of Ag2S, the products of reaction between Ag2S and Ag2CrO4 are solid
solution with Ag2CrO4 structure (Ag2Cr1–xSxO4) and AgCrO2. The result of solid solu-
tion formation is moving the position of diffraction lines, recorded on diffraction pat-
terns of the samples obtained by heating the Ag2S/Ag2CrO4 mixtures, in comparison
with the position of diffraction lines for pure Ag2CrO4 (Fig. 1).
At the concentration range from 40.00 to 90.00 mol% of Ag2S it was found that
three solid phases, i.e. Ag2Cr1–xSxO4 solid solution, AgCrO2 and metallic silver (Ta-
ble 1), occurred in the samples on treatment.
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Fig. 1 Dependence of interplanar distances on Ag2S concentration in initial
Ag2S/Ag2CrO4 mixtures for four diffraction lines: d=0.2879 nm – pure Ag2CrO4
(hkl 2 2 0) (a); d=0.2870 nm – pure Ag2CrO4 (hkl 2 1 1) (b);
d=0.2844 nm – pure Ag2CrO4 (hkl 2 1 1) (c); d=0.2769 nm – pure Ag2CrO4
(hkl 0 0 2) (d)
XRD measurements of the sample, obtained by heating the mixture comprising
initially 90.00 mol% of Ag2S, showed the presence of four solid phases, i.e. Ag2S,
metallic silver, β-Ag8S4O4 and traces of AgCrO2.
Figure 2 shows DTA curves for the samples obtained after the last heating cycle of
Ag2S/Ag2CrO4 mixtures comprising initially 10.00, 20.00, 30.00 and 40.00 mol% of
Ag2S as well as, for comparison purposes, DTA curves for pure Ag2CrO4 and an equi-
molar Ag2CrO4/Ag2SO4 mixture. DTA curves of the samples obtained after heating the
mixtures comprising initially 50.00 and 90.00 mol% of Ag2S are shown in Fig. 3.
The endothermic effect recorded on the DTA curve of the equimolar Ag2CrO4/
Ag2SO4 mixture at 426°C (Fig. 2b) is characterised by polymorphic transition of Ag2SO4
[6, 7]. The second endothermic effect, with its onset at 480°C, is due to polymorphic tran-
sition of Ag2CrO4 [8, 9]. The last endothermic effect starting at 665°C is associated with
the melting of the Ag2CrO4/Ag2SO4 mixture. On each DTA curve of the samples obtained
after heating Ag2S/Ag2CrO4 mixtures (Figs 2c–2f) two endothermic effects were re-
corded. The first effects starting at: 445°C (curve c), 405°C (curve d), 367°C (curve e)
and 326°C (curve f), are associated with polymorphic transition of Ag2Cr1–xSxO4 solid so-
lution. The second effects with their onset at: 662°C (curve c), 659°C (curve d), 658°C
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TOMASZEWICZ, KURZAWA: REACTIVITY BETWEEN Ag2S AND Ag2CrO4
Fig. 2 DTA curves for Ag2CrO4 (a), an equimolar Ag2CrO4/Ag2SO4 mixture (b) and
the samples obtained after the final heating cycle of Ag2S/Ag2SO4 mixtures
comprising initially 10.00 (c), 20.00 (d), 30.00 (e) and 40.00 mol% of Ag2S (f)
(curves e and f) correspond to melting Ag2Cr1–xSxO4 solid solution. The appearance of
solid solution (Ag2SO4 in Ag2CrO4) resulted in reducing temperature of polymorphic
transition (∆tmax=155°C) and melting point (∆tmax=8°C) of Ag2Cr1–xSxO4 in comparison
with the temperature of polymorphic transition and melting point of pure Ag2CrO4, re-
spectively.
On the DTA curve of the sample obtained after heating the mixture composed of
50.00 mol% of Ag2S and 50.00 mol% of Ag2CrO4 (Fig. 3) three effects were re-
corded. The endothermic effects with their onset at 367 and 653°C, are associated
with polymorphic transition and melting solid solution Ag2Cr1–xSxO4, respectively.
Additional studies are necessary for determining a reason for the appearance of the
exothermic effect starting at 324°C (Fig. 3).
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Fig. 3 DTA curves of the sample obtained after the last heating cycle of the mixtures
comprising initially 50.00 mol% of Ag2S (a) and 90.00 mol% of Ag2S (b)
The endothermic effects recorded on the DTA curve (Fig. 3, sample comprising
initially 90.00 mol% of Ag2S) starting at 172 and at 389°C, correspond to the poly-
morphic transition of α-Ag2S to β-Ag2S [10] and melting β-Ag8S4O4 [3, 4], respec-
tively. The endothermic effect recorded at 427°C is associated with the polymorphic
transition of Ag2SO4 [6,7]. This effect shows the presence of Ag2SO4 traces in the
products of reaction. The presence of Ag2SO4 was not proved by XRD method.
Ag2SO4 can appear, during DTA measurement, as a result of incongruent melting
β-Ag8S4O4. The exothermic effect starting at 553°C corresponds to the crystallization
of metallic silver [4]. Metallic silver is a product of the thermal decomposition of a
liquid obtained after melting β-Ag8S4O4 [3, 4]. In air decomposition of β-Ag8S4O4
starts at the melting point of this phase [3, 4]. The endothermic effect with its onset at
653°C is probably due to the melting traces of Ag2Cr1–xSxO4 solid solution. The pres-
ence of Ag2Cr1–xSxO4 solid solution was not confirmed by XRD method.
Figure 4 shows the infrared absorption spectra for an equimolar mixture of
Ag2CrO4 with Ag2SO4, initial Ag2S/Ag2CrO4 mixtures contained 20.00, 50.00 and
90.00 mol% of Ag2S as well as the samples obtained after the last heating cycle of the
Ag2S/Ag2CrO4 mixtures mentioned above. As it is seen the spectra of the samples, ob-
tained after heating Ag2S/Ag2SO4 mixtures, show a general similarity with the spec-
trum of Ag2CrO4/Ag2SO4 mixture. Only absorption bands, recorded in the spectrum
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TOMASZEWICZ, KURZAWA: REACTIVITY BETWEEN Ag2S AND Ag2CrO4
Fig. 4 IR absorption spectra for: an equimolar mixture of Ag2CrO4 with Ag2SO4 (a);
initial Ag2S/Ag2CrO4 mixtures contained 20.00 (b), 50.00 (d) and 90.00 mol%
of Ag2S (f); the samples obtained after the final heating cycle of mixtures com-
prising initially 20.00 (c), 50.00 (e) and 90.00 mol% of Ag2S (g)
of the sample obtained after heating the mixture comprising initially 90.00 mol% of
Ag2S, are very weak in comparison with the absorption bands for the remaining sam-
ples. The broad absorption bands, with the maximum at ~1100 cm–1, can be assigned
to the stretching mode of S–O bonds in isolated SO4 tetrahedra [10–13]. The bands
occurring at 616 cm–1 are related to the stretching modes of O–S–O bonds in SO4
tetrahedra [10–13]. In the light of literature information [14] the broad bands with
maximum at 884 cm–1 correspond to the stretching vibrations of Cr–O bonds in CrO4
tetrahedra. Then the absorption bands occurring at ~400 cm–1 can be due to the
stretching modes of O–Cr–O in CrO4 tetrahedra [14]. The simultaneous presence of
absorption bands, which are related to the stretching modes S–O and O–S–O bonds in
SO4 tetrahedra as well as to the stretching vibrations of Cr–O and O–Cr–O bonds in
CrO4 tetrahedra, is due to a big difference between ion radii of S6+ and Cr6+ (ion radius
of tetrahedrally coordinated S6+ – 12 pm; ion radius of tetrahedrally coordinated Cr6+
– 26 pm [15]).
Additionally, very weak bands at ~670 and at ~540 cm–1 were recorded in the
spectra of samples obtained after heating the mixtures comprising initially 20.00 and
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TOMASZEWICZ, KURZAWA: REACTIVITY BETWEEN Ag2S AND Ag2CrO4
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50.00 mol% of Ag2S. These bands can be assigned to the stretching mode of Cr–O
bonds in CrO6 octahedra, which form anion lattice of AgCrO2 [16].
Conclusions
The experimental results obtained have led to the following conclusions:
– redox reaction of Ag2S with Ag2CrO4 runs with an involvement of oxygen
from air,
– at the concentration range from 10.00 to 90.00 mol% of Ag2S in initial Ag2S/
Ag2CrO4 mixtures a presence of the solid solution with Ag2CrO4 structure (Ag2Cr1–xSxO4)
was found.
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