Thermal decomposition of Bi(III), Cd(II), Pb(II) and Cu(II) thiocyanates

Article abstract of DOI:10.1023/A:1011501307497

Thermal decomposition of Bi(SCN)₃, Cd(SCN)₂, Pb(SCN)₂ and Cu(SCN)₂ has been studied. The thermal analysis curves and the diffraction patterns of the solid intermediate and final products of the pyrolysis are pre

Full text of DOI:10.1023/A:1011501307497

Journal of Thermal Analysis and Calorimetry, Vol. 65 (2001) 231–239
THERMAL DECOMPOSITION OF Bi(III), Cd(II), Pb(II)
AND Cu(II) THIOCYANATES
B. Ptaszyñski, E. Skiba and J. Krystek
Institute of General and Ecological Chemistry, Technical University, 90-924 £ódü, Poland
(Received December 14, 2000; in revised form April 2, 2001)
Abstract
Thermal decomposition of Bi(SCN)3, Cd(SCN)2, Pb(SCN)2 and Cu(SCN)2 has been studied. The
thermal analysis curves and the diffraction patterns of the solid intermediate and final products of the
pyrolysis are presented. The gaseous products of the decomposition (SO2 and CO2) were detected
and quantitatively determined. Thermal, X-ray and chemical analyses have been used to establish
the nature of the reactions occurring at each stage in the decomposition.
Keywords: decomposition of Bi(SCN)3, Cd(SCN)2, Pb(SCN)2 and Cu(SCN)2, DTA, DTG, pyroly-
sis, TG, X-ray
Introduction
Simple metal thiocyanates are intermediate products of thermal decomposition of an-
alytically significant thiocyanatobismuthates(III) [1] and thiocyanatochromates(III)
[2, 3]. They subsequently decompose and stoichiometric amounts of sulfur dioxide
are released. The sulfur dioxide can be determined by titration, spectrophotometry,
potentiometry or conductometry. On these grounds methods of determination of mil-
ligram and microgram amounts of thiocyanates by thermal decomposition of cop-
per(I) thiocyanates [4] and silver thiocyanates [5] were developed. Thermal methods
of determination of thiocyanates are noteworthy due to their high selectivity and pos-
sibility of simultaneous determination of thiocyanate and chlorate anions.
Reactions of thermal decomposition of thiocyanates may also be used to deter-
mine cations which form sparingly soluble thiocyanates. One of the examples is
thermo-alkalimetric method of determination of copper and silver, which involves
precipitation of CuSCN and AgSCN and thermal decomposition of these compounds
[6]. A thermal method was also developed to determine caesium by precipitation of
Cs[Bi(SCN)4], thermal decomposition of the compound and alkalimetric determina-
tion of sulfur dioxide released as a results of the decomposition [7].
Investigation of reactions of thermal decomposition of simple thiocyanates may
also help to explain the mechanism of thermal decomposition of complex thiocyan-
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atobismuthates and thiocyanatochromates with different metals in the outer coordina-
tion sphere.
Experimental
Preparation
Bi(SCN)3, Cd(SCN)2, Pb(SCN)2 and Cu(SCN)2 were prepared according to the meth-
ods described in the literature [8–11].
Apparatus
Thermal analysis curves of the compounds under study were made using an
OD-102/1500°C derivatograph. The 100 mg samples were heated over the tempera-
ture range of 20–1000°C at a rate of 5°C min–1. TG sensitivity 100 mg. α-Al2O3 was
used as reference material. X-ray analysis of the compounds and their sinters was car-
ried out by means of a Siemens D5000 diffractometer, using CuKα radiation mono-
chromatized by means of a secondary graphite monochromator. The curves were re-
corded over 2θ angle range 2–90° with scan step 0.04° and time of scan step 1 s.
Chromatograms of gases released as a result of decomposition of the
thiocyanates under study were made by heating the sample at a rate of 10°C min–1
within the temperature range of 20–600°C using air as the carrier gas.
Sulfur dioxide was absorbed in washers filled with sodium tetrachloromercurate
solution and determined by alkalimetry [12]. Carbon dioxide was adsorbed on
ascarite and determined by gravimetry.
Thermal analysis
The thermal analysis curves of the examined compounds are shown in Fig. 1. The
small mass loss observed on TG curves at lower temperatures result from the drying
of the hygroscopic samples.
TG and DTG curves for Bi(SCN)3 (Fig. 1a) indicate a three-stage decomposition
of the compound. Each consecutive stage corresponds with a higher exothermic peak
with maxima at 170, 270 and 430°C. The sample mass remains practically unchanged
within the temperature range of 500–850°C and decreases slowly over 850°C.
Cd(SCN)2 decomposes in four stages (Fig. 1b). The first three stages correspond
with exothermic peaks at 160°C (low, broad), 300°C (high, sharp) and 580°C (sharp).
The fourth stage corresponds with a small but sharp endothermic peak.
Figure 1c presents thermal analysis curves for Pb(SCN)2. The analysis of TG
and DTG curves indicates that the decomposition of this compound proceeds also in
four stages corresponding with DTA peaks at 220 (exothermic), 320 (exothermic),
480 (exothermic) and 970°C (endothermic), higher than the respective peaks of cad-
mium thiocyanate. At the same time, the mass loss in each of the stages of decompo-
sition is lower than that determined on the basis of TG curve of Cd(SCN)2.
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PTASZYÑSKI et al: Bi(III), Cd(II), Pb(II) AND Cu(II) THIOCYANATES
233
Fig. 1 Thermal decomposition curves of: a – Bi(SCN)3; b – Cd(SCN)2; c – Pb(SCN)2;
d – Cu(SCN)2
The course of the thermal decomposition of copper(II) thiocyanate (Fig. 1d) is
different from that of the above discussed compounds. The first stage of the reaction,
which starts at 120°C corresponds with a slight mass loss on TG curve and a small
exothermic peak on DTA curve at 160°C. At the second stage, a high mass loss and a
broad DTA peak with a maximum at 400°C can be observed. An increase of mass oc-
curs within the temperature range of 400–480°C. The last stage of the reaction
preceeds over the temperature range of 650–800°C. It corresponds with a 28% mass
loss and endothermic DTA peak at 780°C.
Analysis of intermediate and final products of thermal decomposition
In order to examine the course of thermal decomposition of the compounds under
study, 100 mg samples were heated in an electric furnace in air atmosphere at
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Fig. 2 Diffractions patterns of a – Bi(SCN)3; b – its sinter at 600°C. • – Bi2O3;
– (BiO) SO
*
2
4
5°C min–1 up to of characteristic temperatures determined from the TG curves.
Diffractograms of the compounds and their sinters are presented in Figs 2–5.
Sulfur dioxide and carbon dioxide liberated during the thermal decomposition of
thiocyanates were determined by the methods described above. The results are pre-
sented in Tables 1 and 2.
Table 1 Average results of determination of SO2 released while thermal decomposition of
thiocyanates
Compound
Bi(SCN)3
Cd(SCN)2
Pb(SCN)2
Cu(SCN)2
SO2 determined/%
SO2 calculated/%
43.2
27.8
21.2
24.8
44.6
28.0
19.8
23.8
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PTASZYÑSKI et al: Bi(III), Cd(II), Pb(II) AND Cu(II) THIOCYANATES
235
Table 2 Average results of determination of CO2 released while thermal decomposition of
thiocyanates
Compound
Bi(SCN)3
Cd(SCN)2
Pb(SCN)2
Cu(SCN)2
CO2 determined/%
CO2 calculated/%
35.7
38.7
27.3
25.2
34.5
35.5
27.2
24.5
Fig. 3 Diffractions patterns of a – Cd(SCN)2; b – its sinter at 380°C; c – its sinter at
700°C; d – its sinter at 1000°C. o – CdS; ∆ – CdO; – CdSO ; • – Cd O SO
4
*
4
3
2
Discussion
The results of the thermal and X-ray analyses as well as the analysis of the gaseous
products indicate that the first stage of thermal decomposition of Bi(SCN)3 occurs in
the temperature range of 120–180°C, according to the following scheme of reaction
[13]:
6Bi(SCN)3+3O2 → Bi(CN)3·5Bi(SCN)3+3SO2
(1)
The mass loss determined from the TG curve and calculated according to Eq. (1)
are 4.5 and 4.2%, respectively.
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PTASZYÑSKI et al: Bi(III), Cd(II), Pb(II) AND Cu(II) THIOCYANATES
Fig. 4 Diffractions patterns of a – Pb(SCN)2; b – its sinter at 240°C; c – its sinter at
600°C. • – PbS; o – Pb2O(SO4); ∆ – PbSO4
The products of the second stage of decomposition (240–300°C) are bismuth
sulfide and a compound of bismuth, sulfur, carbon and nitrogen, with uncertain stoi-
chiometric composition:
Bi(CN)3·5Bi(SCN)3+nO2 → Bi2S3+4BiSxCyNz+gaseous products
(2)
Diffractograms of the sinter obtained by heating Bi(SCN)3 up to 600°C indicate
the presence of Bi2O3 and (BiO)2SO4, while SO2 and CO2 were found and determined
in the liberated gases. The results of analyses indicate that bismuth thiocyanate,
heated to 600°C decomposes according to the following reaction:
6Bi(SCN)3+41.5 O2 → 2(BiO)2SO4+Bi2O3+16SO2+18CO2+9N2
(3)
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237
The amount of SO2 calculated on the basis of Eq. (3) is 44.6, while that deter-
mined experimentally 43.2%. The amounts of CO2 are 34.4 and 35.7%, respectively.
At temperatures of over 900°C, bismuthyl sulfate decomposes [13], according to
the following reaction:
(BiO)2SO4 → Bi2O3+SO3
(4)
Thermal decomposition of Cd(SCN)2 starts at about 130°C. Chromatographic
analysis demonstrated that at 300°C all gaseous products are released, and X-ray
analysis showed that the only solid decomposition product is cadmium sulfide
(Fig. 3b). The amounts of SO2 (27.8%) and CO2 (38.7%) determined in the gaseous
products are in accordance with the amounts calculated according to Eq. (5)
(SO2 – 28.0%, CO2 – 38.5%).
Cd(SCN)2+3O2 → CdS+SO2+2CO2+N2
(5)
Further heating of Cd(SCN)2, connected with a slight increase of the sample
mass, leads to oxidation of CdS (exothermic DTA peak at 580°C) according to the
following equation:
CdS+2O2 → CdSO4
(6)
The X-ray pattern of the sinter obtained at 700°C indicates the presence of the
following compounds: CdSO4, CdS, Cd3O2(SO4) and CdO (Fig. 3c). At higher tem-
peratures (>830°C) sulfates and oxysulfates decompose, SO3 is liberated and the final
solid product of thermal decomposition of cadmium thiocyanate is CdO (Fig. 3d).
Thermal decomposition of Pb(SCN)2 is very similar to that of Cd(SCN)2. It
starts at 140°C. In the sinter obtained at 240°C, X-ray analysis demonstrated only the
presence of PbS (Fig. 4b). The analysis of gaseous products liberated rapidly at about
320°C (peak at 320°C on the chromatogram) indicates the following course of the
first stage of thermal decomposition reaction of Pb(SCN)2:
Pb(SCN)2+3O2 → PbS+SO2+2CO2+N2
(7)
The amounts of SO2 and CO2 determined (21.2 and 27.3% of the initial sample
mass) are close to those calculated from Eq. (7) (19.8 and 27.2% respectively).
The increase of the sample mass over the temperature range 460–600°C results
from oxidation of lead(II) sulfide to lead sulfate (exothermic DTA peak at 480°C).
PbS+2O2 → PbSO4
(8)
Diffractometric analysis of the sinter prepared by heating Pb(SCN)2 to 600°C in-
dicates that, apart from PbSO4, it also contains Pb2(SO4)O and slight amounts of PbS
(Fig. 4c). Over 900°C the TG curve exhibits further decrease of the sample mass. The
final solid product of thermal decomposition is lead(II) oxide.
Thermal decomposition of lead(II) thiocyanate over the temperature range of
20–1000°C may be presented as the following summary equation:
Pb(SCN)2+5O2 → PbO+SO2+SO3+2CO2+N2
(9)
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PTASZYÑSKI et al: Bi(III), Cd(II), Pb(II) AND Cu(II) THIOCYANATES
Fig. 5 Diffractions patterns of a – Cu(SCN)2; b – its sinter at 170°C; c – its sinter at
400°C; d – its sinter at 530°C; e – its sinter at 850°C. o – Cu(SCN)2;
× – CuSCN; Ÿ– Cu2OSO4; ∆ – CuSO4; o – Cu2O; * – CuO
The first stage of decomposition of Cu(SCN)2, which starts at about 120, corre-
sponds with a DTA peak at 170°C. Diffraction patterns (Fig. 5b) of the sinter demon-
strated the presence of CuSCN which is confirmed by Wendtland’s [14] and
Söderback’s [15] theory’s that at lower temperatures Cu(II) is reduced to Cu(I).
A highly hygroscopic Cu(SCN)2 reacts with H2O according to the following reaction:
6Cu(SCN)2+4H2O → 6CuSCN+5HSCN+HCN+H2SO4
(10)
While heating above 200°C a broad exothermic peak on DTA appears as a result
of two consecutive decomposition reactions. The first, connected with the mass loss
~35%, is an effect of decomposition of CuSCN according to the reaction:
6CuSCN+11O2 → 2Cu2S+2CuO+4SO2+6CO2+3N2
(11)
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PTASZYÑSKI et al: Bi(III), Cd(II), Pb(II) AND Cu(II) THIOCYANATES
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The second one, represented by approriate peaks on DTA, TG and DTG curves,
results in oxidation of Cu2S.
On the diffraction pattern of sinter obtained at 400°C (Fig. 5c) apart peaks char-
acteristic of the products of both above-mentioned decomposition reactions peaks
characteristic of Cu2O are present. Over the temperature range of 380–450°C, a 7% in-
crease of sample mass is observed on the TG curve, related to oxidation of CuS to
CuSO4. On the diffraction patterns of the sinter obtained at 530°C (Fig. 5d) peaks
characteristic of CuO, CuSO4 and Cu2OSO4 are present. Over the temperature range of
650–800°C, the sample mass decreases, first slowly and then at a gradually growing
rate, which is connected with the decomposition of copper sulfate and oxysulfate to
CuO. Diffractometric analysis of the sinter obtained at 850°C (Fig. 5e) indicates, that
the only final product of thermal decomposition of Cu(SCN)2 is copper(II) oxide CuO.
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