Thermodynamics of the Pt-Cl system

Article abstract of DOI:10.1016/j.tca.2008.01.005

Using a static method three individual compounds in system of Pt-Cl: PtCl₄, PtCl₃, and PtCl₂ are shown to exist. PtCl was shown not to exist. The enthalpies of formation of platinum chlorides were measured by calorimetry by reduction of the compounds with gaseous hydrogen. The recommended values for the enthalpies of formation at 298.15 K are -137.7 ± 0.3, 194.2 ± 1.0, and 245.6 ± 1.9 kJ/mol for PtCl₂(s), PtCl₃(s) and PtCl₄(s), respectively.

Full text of DOI:10.1016/j.tca.2008.01.005

Available online at www.sciencedirect.com
Thermochimica Acta 469 (2008) 59–64
Thermodynamics of the Pt–Cl system
Tamara P. Chusova∗, Zinaida I. Semenova
A.V. Nikolaev Institute of Inorganic Chemistry, Siberian Branch of the Russian Academy of Sciences,
Academician Lavrentiev Avenue 3, Novosibirsk 630090, Russia
Received 26 October 2007; received in revised form 18 December 2007; accepted 7 January 2008
Available online 16 January 2008
Abstract
Using a static method three individual compounds in system of Pt–Cl: PtCl4, PtCl3, and PtCl2 are shown to exist. PtCl was shown not to exist.
The enthalpies of formation of platinum chlorides were measured by calorimetry by reduction of the compounds with gaseous hydrogen.
The recommended values for the enthalpies of formation at 298.15 K are −137.7 0.3, 194.2 1.0, and 245.6 1.9 kJ/mol for PtCl2(s), PtCl3(s)
and PtCl4(s), respectively.
© 2008 Elsevier B.V. All rights reserved.
Keywords: Platinum chloride; Calorimetry; Vapour pressure; Heat of formation
1. Introduction
ride. Because of this, we did not take into account the results
on PtCl in the calculation of heats of formation of platinum
chlorides using Wohler’s data (Table 1).
The existence of three individual condensed platinum chlo-
rides PtCl2, PtCl3 and PtCl4 was established in the investigation
of the P–T–x diagram of platinum–chlorine system [1–3]. The
compound which is assumed by some authors [4–6] to have the
composition PtCl is in fact a mixture of compounds described
by the general formula Pt6Cl12·nH2O containing H2O or [OH−]
along with platinum and chloride. Thermodynamics of plat-
inum chlorides have been investigated by different methods in
a number of works (see Table 1). Only the heat of formation
of platinum tetrachloride has been determined by calorimetry
[7]. Other authors [1–6,9–11] determined thermodynamic char-
acteristics of platinum chlorides on the basis of the investigation
of thermal dissociation of these compounds.
Wohler and Streicher [4] determined the dissociation temper-
ature of platinum chlorides in chlorine flow at a pressure of 1 atm
and used the approximate Nernst’s equation [8] to calculate heat
of dissociation for PtCl, PtCl2, PtCl3 and PtCl4. No convinc-
ing evidence for the existence of platinum monochloride was
reported therein [4]. Moreover, the analysis of that work gives us
grounds to state that chlorine used in the experiments contained
moisture and oxygen, which was the reason of the formation of
Pt6Cl12·HnO, interpreted by the author as platinum monochlo-
The step corresponding to the formation of PtCl3 did not
occur during the stepwise decomposition of platinum tetrachlo-
ride as studied in [5] by means of the isoteniscope procedure.
The equilibrium of dissociation 2PtCl3(s) = 2PtCl2(s) + Cl2(g)
was investigated with specially prepared PtCl3 samples. In some
experiments, after the step corresponding to PtCl2 dissociation,
the authors observed one more step which they attributed to plat-
inum monochloride. They failed to isolate and characterize that
product. The standard enthalpies of formation of PtCl3 and PtCl2
calculated by us on the basis of experimental data reported in
[5] are shown in Table 1.
The results of the investigation of PtCl2 dissociation pro-
cess by means of the effusion–torsion procedure were published
in [6]. It was stressed that the decomposition of platinum
dichloride resulted in the formation of an intermediate com-
poundalongwithmetalplatinumandchlorine. Thisintermediate
was assumed to be PtCl. Treating the experimental data
on the basis of the second law of thermodynamics the
authors determined the heat of platinum dichloride dissociation:
ꢀH◦T = 127.356 0.791 kJ/mol. This value cannot be consid-
ered as reliable, because effusion procedures can give distorted
results for platinum chlorides because dissociation equilibrium
is a kinetically hindered process.
∗
Krustinson [10] investigated dissociation of platinum dichlo-
ride and tetrachloride by means of isoteniscope. These
Corresponding author. Tel.: +7 383 330 92 59.
E-mail address: chu@che.nsk.su (T.P. Chusova).
0040-6031/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.tca.2008.01.005

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T.P. Chusova, Z.I. Semenova / Thermochimica Acta 469 (2008) 59–64
experiments were carried out with better purity, so the decom-
position of platinum dichloride proceeded to metal platinum
without the intermediate step noticed in [4–6]. The mean value
of the heat of PtCl2 formation calculated by Krustinson using
the approximate Nernst’s equation and shown in Table 1 is equal
to −123 kJ/mol. Data treatment on the basis of the second law of
thermodynamics gives the value −247 121 kJ/mol. The data
of that author on the pressure of PtCl4 dissociation were not
considered by us because the decomposition of platinum tetra-
chloride proceeded to platinum dichloride leaving aside the step
of PtCl3 formation, that is, following the non-equilibrium route.
Even more unsatisfactory are the data reported in [10],
where measurements were carried out under essentially non-
equilibrium conditions (under continuous heating of the
sample).
Meanwhile, it was stressed as early as in [4,9] that the equi-
librium of dissociation of platinum chlorides is established very
slowly (within several days). So, the data of [10] cannot be
considered reliable.
Shafer and Wiese [11] used the equilibrium quenching
method to determine the pressure of PtCl2 dissociation at
723 K. Then, using the approximate Nernst’s equation he found
ꢀfH◦298.15 (PtCl2) = −134 kJ/mol.
The most reliable data on dissociation pressure of platinum
chlorides were obtained using the static method with a mem-
brane null manometer [1–3]. These experiments were carried out
in the modes of isothermal and non-isothermal thermostating.
The equilibrium pressure of dissociation of platinum chlorides
was settled within a very long time (24–200 h and more), so it
was possible to approach the equilibrium only from the side of
lower pressures. The heats of formation of platinum chlorides,
calculated from tensimetric data, required additional verifica-
tion with an independent procedure because these values could
be distorted due to the difficulty of equilibrium establishment
and unaccounted features of the behaviour of compounds. We
chose calorimetric procedure as such an independent method.
Since platinum dichloride and trichloride are almost insoluble
in solvents, the methods of solution calorimetry turned out to be
unsuitable. We used reduction as the calorimetric reaction:
PtClx(s) + 0.5 × H2(g) = Pt0(s) + xHCl(g) + ꢀrH
2. Experimental
2.1. Synthesis and identification of initial compounds
2.1.1. PtCl4
PtCl4 was synthesized by thermal decomposition of chloro-
platinic acid in chlorine flow (pCl = 1 atm) [12] obtained by the
2
interaction of HCl and KMnO4. The optimal synthesis tempera-
ture at which the final product had the composition nCl/nPt = 3.94
and contained a minimal amount of water (nH O/nPt = 0.01)
2
was 493 K. Gaseous chlorine was dried preliminarily by pass-
ing above H2SO4 and CaCl2. The duration of the reaction was
8–10 h, with the initial weighed portion of 300 mg. To deter-
mine platinum and chlorine content of platinum tetrachloride,
theweighedportionwasreducedinhydrogenflowat473–573 K,

T.P. Chusova, Z.I. Semenova / Thermochimica Acta 469 (2008) 59–64
61
HCl was absorbed with water; chloride was determined by
means of potentiometry. Platinum was determined gravimet-
rically after calcination in a crucible in the air at 873 K. The
synthesized compound was identified by elemental and X-ray
phase analyses, as well as by IR spectroscopy. Found (%): Pt,
57.60; Cl, 42.20. PtCl4—calculated (%): Pt, 57.86; Cl, 42.14.
Interplanar spacings for the samples of platinum tetrachloride
are satisfactorily described by the data obtained previously
[13,14]; the IR spectra coincide with the results reported pre-
viously [13,15]. A compound from one and the same lot was
examined in calorimetric experiments.
found(%):Pt, 73.26;Cl, 26.67. PtCl2—calculated(%):Pt, 73.34;
Cl, 26.66. Interplanar spacings for the samples obtained by syn-
thesis 1 are in satisfactory agreement with those calculated from
the structural data [18]. The interplanar spacings for samples
obtained by synthesis 2 coincide with those reported in [17] and
do not coincide with those for the samples obtained by synthesis
1. We established previously [19] that the compound obtained
in synthesis 1 gets transformed into the compound obtained by
synthesis 2 (judging form the set of interplanar spacings) during
thermal annealing at 777 K, whereas no reverse transition can
be observed. According to Shafer’s classification, the products
obtained in syntheses 1 and 2 were called ␤ and ␣ modifica-
tions of PtCl2, respectively. For ␣-PtCl2, the samples from three
lots were used in calorimetric experiments, and for ␤-PtCl2 the
substances from two lots were used. All the manipulations of
packing into ampoules were carried out in a dry box (argon,
drying agent: P2O5).
2.1.2. PtCl3
The method of synthesis of platinum trichloride by the
decomposition of PtCl4 in chlorine flow [12,16] is insufficiently
reliable because in many cases it results in the formation of a
mixture of PtCl3 and PtCl2. A possible reason is essentially non-
equilibrium conditions of PtCl4 decomposition. Because of this,
we carried out platinum tetrachloride decomposition in a closed
volume. About 8 g of platinum tetrachloride was placed into a
three-section ampoule with a volume about 15 ml, the ampoule
was pumped out and then sealed. The section with PtCl4 was
placed into a furnace; two other sections were kept at room tem-
perature. The furnace temperature was gradually increased to
483 K; after exposure for 2 days, the gas phase (H2O) was frozen
out and the corresponding section was sealed off. Then temper-
ature was gradually increased during 4 days up to 673–683 K;
the substance was kept at this temperature for 2 days. Then
the ampoule was put out of the furnace, quickly cooled in air
flow, then frozen with liquid nitrogen; the released chlorine was
sealed off. The temperature of decomposition of platinum tetra-
chloride was determined on the basis of tensimetric experiments
[1]. The product was identified by means of elemental, X-ray
phase analyses, and IR spectroscopy. Found (%): Pt, 64.67; Cl,
35.50. PtCl3—calculated (%): Pt, 64.72; Cl, 35.28. Interplanar
spacings in PtCl3 coincide with d␣ values calculated from the
structural data [17]. The IR spectrum of PtCl3 (υ, cm−1): 255
v.s, 302 s, 323 s, 362 m. No data on the IR spectrum of PtCl3
were found in literature. A compound from one and the same lot
was examined in calorimetric experiments.
2.2. Experimental procedure
In order to determine the enthalpies of formation of plat-
inum chlorides, we measured the heats of reduction of these
compounds with gaseous hydrogen. The following major
requirementsweretakenintoaccountwhenchoosingtheoptimal
reduction temperature:
1) First of all, it is necessary that the thermodynamic state of
the final products of reduction is rather well reproducible
in parallel experiments, that is, platinum should be formed
in the same structural from and disperse state. Experiments
showed that this requirement is met if the substance for use
in calorimetric experiments is ground (in a mortar). In this
case, platinum is obtained in the form of platinum black with
approximately the same grain size distribution for all the
three chlorides.
2) Reduction should proceed at a sufficient rate. Preliminary
investigation of the interaction of platinum chlorides by
means of differential thermal analysis and thermogravimetry
showed that even in the case of platinum dichloride reduction
occurs at a noticeable rate even at 343 K, while the maxi-
mal rate corresponds to the temperature of 403 K (with the
heating rate of 8◦ min−1).
2.1.3. PtCl2
Synthesis of ␤-PtCl2: according to a known procedure [12] by
the decomposition of H2PtCl6·6H2O in chlorine flow at 753 K
(fine olive-green powder was formed in the reaction (synthesis
1)).
3) It was necessary to take into account the possibility to carry
out calibration experiments with a reference substance. We
chose water as the latter; its heat of evaporation is known
with a high accuracy [20].
Synthesis of ␣-PtCl2: by means of sublimation of fine plat-
inum dichloride (synthesis 1, weighed portions of 0.5–0.7 g)
in a sealed pumped-out ampoule (15 cm3); transfer occurred
from the hot region (890 K) into the cool one (800 K). About
2/3 of the initial substance was deposited into the cool region
of the ampoule within 10 days. The product was a dark-violet
substance composed of dendrites 0.5–1.0 cm in size (synthesis
2).
The temperature point that met the listed requirements best
of all was 369 K.
Heats of reduction were determined in a flow calorimeter
with isothermal shell. The calorimeter was a glass silver-plated
Dewar vessel 0.7 l in volume, enclosed in a copper shell and
immersed into a thermostat filled with silicone oil. The liquid in
the calorimeter was mixed at a constant rate.
The synthesized products were identified by means of ele-
mental and X-ray phase analyses, as well as IR spectroscopy.
For synthesis 1, found (%): Pt, 73.42; Cl, 26.47. For synthesis 2,
A weighed portion of the substance in a sealed glass ampoule
was placed in the lower part of the glass reaction vessel shaped
as a tube with a tap in the lower part and immersed into the

62
T.P. Chusova, Z.I. Semenova / Thermochimica Acta 469 (2008) 59–64
Table 2
calorimetric liquid by 2/3 of the tube length. Hydrogen flow
entered the reaction vessel through a copper heat exchanger
located at the thermostat cap and then passed through the tap
into a glass spiral which served to increase the pass length of
gaseous products inside the calorimeter. The spiral was made of
a thin-wall glass tube 5 mm in diameter and about 2 m long; the
spiral was completely immersed into the calorimetric liquid. The
experiments were carried out with the optimal hydrogen flow
rate of 1.8 l/h, at a temperature of 293 K and total pressure of
1 atm.
Heat capacity (C◦p (J mol−1
K
−1) = a + bT + c/T2) of crystal platinum, plat-
inum chlorides, gaseous hydrogen and hydrogen chloride within temperature
range 298–900 K
Compound
a
b
c
Pt(␬p) [21]
PtCl2(s) [22]
PtCl3(s) [23]
PtCl4(s) [23]
H2(g) [21]
28.49
63.5
5.27 × 10−3
0.25 × 105
21.4 × 10−3
0.883 × 105
121.34
146.44
28.83
–
–
–
–
–
–
–
–
HCl(g) [21]
29.12
The ampoule with the substance was broken with a glass pin;
the design of the latter allowed us to prevent diffusion of gases
from the lower part of the reaction vessel into its upper part.
Temperature of the calorimetric liquid was measured with a
thermistor (r369 K = 11,650 ꢁ) and a direct current bridge with
an error of 0.0001◦, which corresponded to a change in ther-
mistor resistance by 0.03 ꢁ. The indications of the thermistor
were read from the chart strip of a plotter. The heat value of
the calorimeter was determined by means of calibration over
current after each experiment with the substance. The thermal
effect and mean temperature of the calibration experiment were
set approximately the same as those in the corresponding experi-
ment with the substance. Power conditioner was used as a power
supply. Voltage at the calibration heater and current strength in
the circuit was measured with a potentiometer. Pulse duration
was recorded with a printing chronograph. The working element
of the heater was a manganin wire 0.1 mm in diameter having
the resistance of 25 ꢁ, which was placed in a thin-wall glass
capillary filled with silicone oil. The electric circuit of calibra-
tion provided the accuracy of heat measurement with an error
not larger than 0.05%. Scattering in heat values did not exceed
0.3% of the mean value. Heat values averaged over the series
were used to calculate heats of reaction.
ples. As we have already mentioned above, platinum black was
formed as a result of reduction of platinum dichloride ground in a
mortar. In the cases when coarse crystals were loaded into calori-
metricampoules, platinumwasobtainedintheformoflight-gray
particles conserving the shape of initial crystals. To explain the
indicated difference, we assumed that the reduction of coarse
crystals results in a less stable state of platinum. Because of this,
we accepted the heat of ␣-PtCl2 reduction to be equal to the value
obtained in experiments with the ground substance. The values
of ꢀrH◦298.15 for reduction reactions (see Supplementary Table
S2) along with ꢀfH◦298.15 (HCl) = −92.3121 0.130 kJ/mol
[24] were used to calculate the enthalpy of formation of platinum
chlorides.
When evaluating the reliability of thus obtained heats of for-
mation, the following should be taken into account:
1) Reduction resulted in the formation of platinum black, that
is, platinum in thermodynamically unstable state.
2) Platinum (especially in the form of platinum black) readily
sorbsH2 andHCl. Thementionedcircumstancescanbetaken
into account by introducing the corresponding corrections.
The latter differ in sign; therefore, they partially compensate
each other. To our regret, we failed to estimate the heat of
the transition of platinum black into compact platinum and
the correction connected with possible sorption of HCl. It
may be assumed that both corrections are not large. We mea-
sured the heat of H2 sorption at 369 K experimentally; two
parallel experiments gave results −0.63 and −0.88 kJ/g-at.
of platinum.
The absence of essential systematic errors of the calorimetric
system was established by measuring the heat of water evapora-
tion. Themeanresultfortheheatofevaporationdeterminedin10
experiments at 369 K was 40.79 0.13 kJ/mol (here and below,
the error for the 95% confidence interval is shown), which is in
good agreement with the literature [20] value 40.8216 kJ/mol.
The cooling constant was 0.0072 min−1 as a mean. Devia-
tions from the mean value usually did not exceed 10%.
3. Discussion of results
The results of calorimetric experiments are shown in
Supplementary Table S2, where K is the constant of cooling, δ is
a correction for heat exchange, ꢀr is the corrected resistance of
thermistor in the calorimetric experiment, τ is the duration of the
main period, ꢀH◦369.15 is the heat effect of the reduction of the
corresponding platinum chloride. Enthalpies of reactions were
calculated for 298.15 K using thermodynamic values shown in
Table 2.
Attention should be paid to a substantial difference between
heats of reaction for the ground and non-ground platinum dichlo-
ride samples. It should be noted that in the latter case the main
period was about two times longer. In addition, a substantial
difference was also observed in the appearance of platinum sam-
The experiment was carried out as follows. A weighed por-
tion of ground ␣-PtCl2 was reduced with hydrogen under the
conditions simulating the calorimetric experiment. The result-
ing platinum black was placed into the calorimetric ampoule;
desorption of gases from the surface of platinum was carried out
by pumping out (10−5 Torr) at 323 K for three hours. After that,
the ampoule was sealed under vacuum and placed into the flow
calorimeter; the heat released after breaking the ampoule was
determined.
Heats of dissociation and heats of formation of platinum
chlorides, calculated on the basis of data of calorimetric and
tensimetric experiments [1–3], are shown in Tables 3 and 4.

T.P. Chusova, Z.I. Semenova / Thermochimica Acta 469 (2008) 59–64
63
Table 3
Heat of dissociation (ꢀrH◦298.15 kJ/mol)
Reaction
Tensimetric
Calorimetric
136.7 0.7 (second law)a
137.7 0.3 (third law)
138.6 4.6 (␣-PtCl2)
138.0 2.4 (␤-PtCl2)
PtCl2(s) = Pt(s) + Cl2(g)
2PtCl3(s) = 2PtCl2(s) + Cl2(g)
2PtCl4(s) = 2PtCl3(s) + Cl2(g)
123.1 1.7 (second law)
111.3 5.6
102.8 2.9
104.4 1.8 (second law)
101.70 1.76 (third law)
a
Calculation according to the second and third laws of thermodynamics.
Table 4
Heats of formation of platinum chlorides (−ꢀfH◦298.15 kJ/mol)
Compound
Tensimetric
Calorimetric
Recommended −ꢀfH◦298.15 kJ/mol
PtCl2 (s)
137.7 0.3
137.95 2.38 (␤–PtCl2)
138.62 4.60 (␣-PtCl2)
137.7 0.3
PtCl3 (s)
PtCl4 (s)
199.2 1.2
246.3 1.3
194.2 1,00
245.6 1.9
194.2 1.0
245.6 1.9
4. Conclusion
Using the value of the heat of formation of platinum trichlo-
ride [3] we calculated the standard heat of formation of PtCl4
(see Table 4). This value is in good agreement with the results
obtained by means of calorimetry, and in satisfactory agree-
ment with the heat of formation of PtCl4 reported in [7]
(−248.5 kJ/mol).
Close values of the heats of formation of platinum dichlo-
rides, calculated fromtensimetricmeasurementsanddetermined
by means of calorimetry, provide evidence of the relatively small
error connected with the failure of calorimetric experiments to
take account of the heats of gas sorption on platinum and the
heat of transition of platinum black into the stable form. This
error may be assumed to be approximately the same for all plat-
inum chlorides. Because of this, it should be expected that the
heats of dissociation of platinum tri- and tetrachloride calculated
from calorimetric data would not contain noticeable systematic
errors. The enthalpy of platinum dichloride dissociation under
standard conditions was calculated on the basis of the second and
the third laws of thermodynamics (see Table 3). One can see that
both types of treatment give results that are in agreement with
each other. Nevertheless, we prefer the results of calculation
according to the third law of thermodynamics because the good
accuracy of determination of the absolute entropy of PtCl2(s)
achieved earlier [22,25] provided smaller errors in these calcu-
lations. Heats of formation of platinum dichloride, calculated
from tensimetric and calorimetric data, are in good agreement
with each other. However, since the latter gives a larger error of
determination, we propose to consider the value calculated from
tensimetric data according to the third law of thermodynamics
as the recommended value.
We propose to consider the calorimetric value as the recom-
mended one for the heat of formation of PtCl4, because when
calculating the heat of formation on the basis of tensimetric data
we had to rely upon the estimated values of heat capacity for
PtCl4.
The calorimetric investigation carried out by us confirmed the
correctness of interpretation of the results of tensimetric exper-
iments [1–3], as well as the reliability of heats of formation
calculated on that basis for platinum chlorides (Table 4).
Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at doi:10.1016/j.tca.2008.01.005.
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