Thermal decomposition of alkali metal, copper(I) and silver(I) thiocyanates

Article abstract of DOI:10.1016/s0040-6031(98)00391-8

Thermal decomposition of alkali metal thiocyanates of the general formula MSCN (M=Na, K, Rb, Cs), CuSCN and AgSCN has been studied. Thermal analysis curves and diffraction patterns of the solid intermediate, and final, products of their pyrolysis are pres

Full text of DOI:10.1016/s0040-6031(98)00391-8

Thermochimica Acta 319 (1998) 75±85
Thermal decomposition of alkali metal, copper(I) and silver(I) thiocyanates
*
B. Ptaszy n ski , E. Skiba, J. Krystek
Institute of General and Ecological Chemistry, Technical University, 90-924 è o d z , Poland
Received 15 November 1996; received in revised form 22 May 1998; accepted 29 May 1998
Abstract
Thermal decomposition of alkali metal thiocyanates of the general formula MSCN (MˆNa, K, Rb, Cs), CuSCN and
AgSCN has been studied. Thermal analysis curves and diffraction patterns of the solid intermediate, and ®nal, products of
their pyrolysis are presented. Gaseous products of the decomposition, SO and CO , were quanti®ed. Thermal, X-ray and
2
2
chemical analyses have been used to establish the nature of the reactions occurring at each stage of decomposition. # 1998
Elsevier Science B.V.
Keywords: AgSCN; Alkali metal thiocyanates; CuSCN; DTA; DTG; Pyrolysis; TG; X-ray analysis
1
. Introduction
decomposition of copper thiocyanate has been used
�
to analyse thiocyanates [3], in the presence of Cl ,
�
�
2�
2�
Intermediate products of thermal decomposition of
analytically important complex salts of bismuth(III)
Br , I , S and S2O3 , i.e. the anions which hinder
precipitation in redoximetric methods of determina-
�
with the general formulae M [Bi(SCN) ] (MˆLi, Na,
tion of SCN . Thiocyanates which are present in the
3
6
0
K, Rb), M[Bi(SCN) ] (MˆRb, Cs) and M M -
solution along with chlorides, bromides or iodides can
also be determined by precipitation of these anions
using silver nitrate and thermal decomposition at
6008C. The sulphur dioxide released as a result of
AgSCN decomposition is determined by alkalimetry
[4].
Copper(I) and silver can be simultaneously deter-
mined with satisfactory accuracy and precision by
precipitating CuSCN and AgSCN from a solution
containing both the metals, and decomposing these
compounds at an appropriate temperature [5].
Until now, thermal decomposition of thiocyanates
of alkali metals, copper and silver has not been a
subject of a closer investigation, and we intend to
study the reactions involved in their thermal decom-
position in this paper.
4
2
0
[
Bi(SCN) ] (MˆK, Rb, Cs; M ˆLi, Na), including
6
alkali metals thiocyanates, have been studied pre-
viously [1]. Some of these compounds have found
application in the so-called thermo-alkalimetric meth-
ods for quantitative determination of cesium [2] and
sodium [1], which involve heating of the compounds
to an appropriate temperature, determined from the
thermal analysis curves, absorption of the sulphur
dioxide liberated as a result of decomposition in
tetrachloromercurate solution and titration of the acid
with standard sodium hydroxide solution. Thermal
*
Corresponding author.
0040-6031/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved
P I I S 0 0 4 0 - 6 0 3 1 ( 9 8 ) 0 0 3 9 1 - 8

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B. Ptaszy n ski et al. / Thermochimica Acta 319 (1998) 75±85
2
. Experimental
tetrachloromercurate solution. The acid produced
was titrated with standard sodium hydroxide solution.
Carbon dioxide liberated on heating the thiocyanates
was adsorbed on ascarite and determined gravimetri-
cally.
2
.1. Preparation
NaSCN and KSCN (Merck) were used in the ana-
lyses. Rubidium and cesium thiocyanates were
obtained by dissolving their carbonates in fresh thio-
cyanic acid and crystallizing RbSCN and CsSCN.
2.3. Thermal analysis
3
In order to prepare AgSCN, 1 cm of nitric acid
� 3
The thermal analysis curves of the examined com-
pounds are shown in Fig. 1. The small loss of mass at
lower temperatures in the TG curves resulted from the
drying of the hygroscopic samples. Alkali metal thio-
cyanates melt before they decompose and the melting
points established on the basis of the TG curves are:
NaSCN ± 3208C, KSCN ± 1908C, RbSCN ± 1958C and
CsSCN±2108C.Silverandcopperthiocyanatesdecom-
posed without melting. Broad exothermic peaks in the
DTA curves over the ranges of 480±5908C (NaSCN),
540±6308C (KSCN), 500±6008C (RbSCN) and 500±
6208C (CsSCN) and mass losses in the TG curve
(14, 10, 7 and 5%, respectively) are due to the thermal
decomposition and oxidation of the decomposition
products. The endothermic peak at 8908C in the
DTA curve of sodium thiocyanate corresponds with
the polymorphous transformation of sodium sulphate,
a ®nal product of NaSCN decomposition.
3
(
nium thiocyanate solution, and the solution was then
1‡2) was added to 15 cm of 0.1 mol dm ammo-
�
3
heated to boiling. An aliquot of 0.1 mol dm of silver
nitrate was added dropwise with continuous stirring. A
slight excess of AgNO was necessary. The solution
3
was cooled and left to crystallize in the dark. The
precipitate was ®ltered off, washed with methanol and
dried in air.
In order to prepare CuSCN, water was added to
3
� 3
0 cm of 0.1 mol dm CuSO solution and 120 cm
4
3
4
of a reducing solution (NH Cl‡hydroxylamine) to
4
3
obtain 400 cm . The mixture was boiled until it lost
3
its colour. Thereafter, 40 cm of NH SCN solution
4
were added. CuSCN precipitated and was ®ltered off,
washed with methanol and dried in air at ca. 908C.
2
.2. Apparatus
Thermal analysis curves of AgSCN and CuSCN
indicate that these compounds decompose in several
steps. The endothermic peak at 9708C in the TG curve
of silver thiocyanate corresponds with the melting
point of silver.
Thermal analyses of the compounds were carried
out using an OD-102/15008C derivatograph. The sam-
ples were heated in air over temperatures in the 20±
10008C range at the rate of 58C min , using the mass
of 100 mg. TG sensitivity ± 100 mg, DTA sensitivity ±
�
1
1
reference material.
/15, DTG sensitivity ± 1/20. a-Al O was used as
2.4. Analysis of products of thermal decomposition
2
3
X-ray analysis of the compounds and their sinters
were carried out by means of a Siemens D5000
diffractometer, using CuK radiation monochroma-
tised by means of a secondary graphite monochroma-
tor. The curves were recorded over the 2<2ꢀ<908
range with a scan step of 0.048 and time of scan step
In order to examine the course of thermal decom-
position of the compounds under study, they were
�
1
heated in a furnace at a rate of 58C min up to
characteristic temperatures, determined from the ther-
mal analysis curves. Alkali-metal thiocyanates were
heated to 6008C, silver thiocyanate to 2808, 6008 and
9708C and copper thiocyanate to 4508, 6008 and
8508C. Figs. 2±7 show the diffraction patterns of
the examined thiocyanates and their sinters at different
temperatures.
a
1
s.
Sulphur dioxide evolved in the thermal decomposi-
tion of the thiocyanates was determined by the follow-
ing procedure. A weighed amount of the compound
was heated in a furnace which is a part of a C.I.
Electronics balance with programmed temperature
and computer recording of results. The released
SO2 was absorbed in washers ®lled with sodium
The gases liberated during the thermal decomposi-
tion of thiocyanates, sulphur dioxide and carbon
dioxide, were determined by the above-described
methods. The results are presented in Tables 1 and 2.

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2
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B. Ptaszy n ski et al. / Thermochimica Acta 319 (1998) 75±85
Table 1
high temperatures (>8508C), giving pure metallic
silver as the ®nal product of decomposition. This is
con®rmed by the DTA peak at Tˆ9508C, which
corresponds with the melting point of silver.
The postulated schemes of chemical reactions
occurring during thermal decomposition of AgSCN
are as follows:
Average results of determination of SO
2
released while thermal
decomposition of thiocyanates
Compound
SO
2
determined/%
2
SO calculated/%
NaSCN
KSCN
37.87
36.48
22.43
17.65
19.68
38.01
39.51
32.96
22.31
16.77
19.30
35.11
RbSCN
CsSCN
AgSCN
CuSCN
2
AgSCN ! AgSCN Á AgCN
The TG curve indicates that, within the 260±6008C
range, the following reactions occur simultaneously:
Table 2
AgSCN Á AgCN ‡ 3O2
Average results of determination of CO
decomposition of thiocyanates
2
released while thermal
!
Ag S ‡ 2CO2 ‡ SO2 ‡ N2
2
Ag S ‡ 2O2 ! Ag SO4
2
2
Compound
CO
2
determined/%
2
CO calculated/%
and above 6008C silver is formed, which was found by
diffractometric analysis [7].
NaSCN
KSCN
58.06
44.61
31.91
26.13
25.60
38.52
54.27
45.28
30.65
23.04
26.51
36.18
RbSCN
CsSCN
AgSCN
CuSCN
In the case of CuSCN, the 34.1% mass loss
observed in the TG curve at 4508C corresponds with
the following reaction:
6
CuSCN ‡ 11O ! 2Cu S ‡ 2CuO ‡ 4SO
2
2
2
‡
6CO2 ‡ 3N2
The theoretical loss in mass corresponding with this
3
. Discussion
X-ray analysis shows that the only solid product of
reaction is 34.6%.
pyrolysis of the alkali-metal thiocyanates is the sul-
phate of the alkali metal. Moreover, analysis of SO2
and CO in the gaseous products of decomposition
Similarly, the decomposition of copper(I) thiocya-
nate may involve formation of copper(I) oxide, which
is con®rmed by diffraction analysis of sinters obtained
by heating the initial compound within the 350±4008C
range. For this reason, the determined loss in mass
does not correspond with that calculated from the
proposed reaction.
2
indicates that alkali-metal thiocyanates heated to ca.
6
008C decompose according to the following scheme
of reaction:
MSCN ‡ 5O2 ! M2SO4 ‡ SO2 ‡ 2CO2
N2 ꢀM ˆ Na; K; Rb; Cs†
2
Above 4808C, the intermediate products of decom-
position are oxidized, which is re¯ected in the TG
curve by the increase of mass and by the exothermic
effect recorded in the DTA curve. The analysis of
difftaction patterns indicates the presence of the fol-
lowing products of oxidation: CuSO , Cu O(SO ) and
‡
The thermal analysis curves of AgSCN (10% mass
loss and a peak in the DTA curve at 2708C) show that
the sharp DTA peak at Tˆ2708C and the loss of mass
Ámˆ9.7% correspond with the conversion of the
initial compound AgSCN into a compound whose
proposed composition is AgSCNÁAgCN, which is in
agreement with literature date [6,7], but was not
con®rmed by X-ray analysis. Starting at 2808C, a
relatively slow mass loss follows to Tˆ6008C. The
shape of the TG curve indicates a complicated course
of reactions, whose stages we are unable to establish.
Diffractometric analysis shows explicitly that,
4
2
4
CuO, which is con®rmed by the calculated loss in
mass (12.6%), consistent with that established on the
basis of the TG curve (13%) (Fig. 7c).
The TG curve shows that within the 550 ± 8008C
range, further decomposition of the above-mentioned
compounds takes place, and the ®nal product of
CuSCN decomposition (>8008C) is CuO. According
to literature data this, in turn, is converted slowly into
beyond 2808C, Ag S begins to form and is gradually
Cu O, which corresponds with the slow loss in mass in
2
2
oxidized to Ag SO which, in turn, decomposes at
the TG curve.
2
4

B. Ptaszy n ski et al. / Thermochimica Acta 319 (1998) 75±85
85
[
2] A. Cyga n ski, T. Majewski, Talanta 30 (1983) 699.
3] T. Majewski, A. Cyga n ski, Anal. Chim. Acta 141 (1982)
The endothermic DTA peak at 8108C corresponds
with the reaction of thermal decomposition of CuSO4
to CuO.
[
329.
[
4] A. Cyga n ski, T. Majewski, Analyst 104 (1979) 167.
5] T. Majewski, A. Cyga n ski, Chem. Anal. (Warsaw) 32 (1987)
[
1
89.
References
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6] C. Duval, Inorganic Thermogravimetric Analysis, Elsevier,
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1] A. Cyga n ski, T. Majewski, B. Ptaszy n ski, Chem. Anal.
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[7] D. Czakis-Sulikowska, J. Kaøu zÇ na, J. Radwa n ska-Doczekals-
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(