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GORYUSHKINA et al.
praseodymium metal and hydrochloric acid. The Note that the design of the reactor fully prevented sam-
enthalpy of this reaction was measured in several works
[2–7]. However, only in [7] was the metal thoroughly
analyzed and quantitative data on both metallic and
nonmetallic impurities in it were obtained. Quantitative
elemental analysis data are absent in the other works.
The enthalpy of solution of praseodymium trichlo-
ride in hydrochloric acid was measured in [3, 5]. These
data can, however, only be considered estimates,
because the enthalpies of the reaction are based on the
results of only one or two measurements. The enthalp-
ies of solution of praseodymium trichloride in water
were obtained in [8–12]. The PrCl3 sample used was
thoroughly analyzed and quantitatively characterized
only in [12]. Note that, in [13, 14], the enthalpy of solu-
tion of praseodymium trichloride in water is only given
for the infinitely dilute solution state, without mention-
ing how this value was obtained.
ple contact with air moisture not only during chlorina-
tion but also when the container with the anhydrous
product was removed from the reactor.
The product was identified by analyzing it for
praseodymium and chlorine. The contents of praseody-
mium (determined by complexometric titration with
Trilon B) and chlorine (determined gravimetrically in
the form of AgCl) were (wt %) 56.90 0.15 (calcu-
lated: 56.99) and 43.01
0.12 (calculated: 43.01),
respectively. According to the X-ray powder patterns,
the product was single-phase and corresponded to PrCl3
with a UCl3-type structure.
Praseodymium trichloride is very hygroscopic. To
prevent it from contact with air moisture, all operations
with this substance were performed in a dry box in an
inert atmosphere. Hydrochloric acid (1.07 n) used as a
reacting liquid was prepared from concentrated hydro-
It follows that rigorous calculations of the key
∆f H298.15 Pr3+(sln, ∞H2O) value from the available data chloric acid of os. ch. grade (the total content of impu-
°
rities was less than 0.002 wt %) and distilled water with
cannot be performed. Since reliable enthalpies of for-
mation of praseodymium compounds are necessary for
thermodynamic calculations, we deemed it worthwhile
to independently determine the enthalpy of formation
of the praseodymium ion. For this purpose, we used a
praseodymium metal sample characterized in detail and
anhydrous praseodymium trichloride synthesized and
analyzed by us. Measurements were performed on a
precision calorimetric instrument with a leak-proof
reaction vessel.
specific conductivity 6 × 10–6 S cm–1.
The enthalpies of reactions were measured in a
swinging isoperibol leak-proof calorimeter at 298.15 K.
A detailed description of the unit and the procedure for
measurements can be found in [16, 17]. A calorimetric
titanium vessel (V = 80 cm3) was filled with hydrochlo-
ric acid (1.07 n, 55.000 0.003 g) and sealed. Temper-
ature rise in experiments was measured by a copper
resistance thermometer (R298.15 = 223.90 Ω, 1 Ω =
1.05 K) using a bridge scheme. The null instrument was
an F-116/2 microvoltmicroammeter connected to a
KSP-4 self-balancing potentiometer, which recorded
calorimeter temperature changes during the whole
experiment. The thermometric sensitivity of the unit
was 3 × 10–5 K/mm recorder scale. Water temperature
in the shell was controlled automatically with an accu-
racy of 5 × 10–3 K.
EXPERIMENTAL
We used a praseodymium metal sample analyzed for
both metallic and nonmetallic impurities. The impurity
contents in the sample were (wt %): O, 0.042;
N, 0.0018; H, 0.00058; C, 0.089; Fe < 0.01; Ca < 0.01;
Cu < 0.01; Mo < 0.02; and La, Ce, Nd < 0.20.
The content of carbon was determined by burning
samples in oxygen and analyzing outgoing carbon-con-
taining gas on a coulometric analyzer. Analyses for
oxygen and nitrogen were performed by vacuum melt-
ing with analyzing gases released on a chromatograph.
The content of hydrogen was determined by heating
samples in a flow of an inert carrier gas followed by
chromatographing. Metallic impurities were deter-
mined spectroscopically.
The energy equivalent of the calorimeter was
determined electrically with an accuracy to several
hundredths of a percent. The voltage on the heater
and reference coil (R = 1.00005 Ω) was measured by
an R-363-2 high-resistance potentiometer of accuracy
class 0.002%. The time of current passage was deter-
mined using an F-5080 frequency meter-chronometer
with an accuracy of 0.002 s. The energy equivalent of
the calorimeter (W) was measured in six experiments
and was found to be 344.55 0.25 J/Ω. Here and
throughout, confidence intervals correspond to a 95%
probability.
Anhydrous praseodymium trichloride was prepared
by chlorinating praseodymium oxide Pr6O11 powder
(the major component content 99.9%) with carbon tet-
rachloride (os. ch., special purity grade) vapor. The syn-
thesis was conducted following the original procedure
[15], according to which the initial substance was
heated and held in CCl4 vapor at 950 K for 11 h, while
the reactor was rotated. Reactor rotations accelerate
chlorination and allow the reaction to be performed to
completion because the area of the reaction surface
Prior to calorimetric experiments, the substances to
be studied were placed into glass ampules and sealed in
a dry box in an inert atmosphere. The praseodymium
metal surface was preliminarily polished to glitter. The
samples were weighed on a Sartorius balance with an
increases as a result of reaction product granulation. accuracy of 0.00002 g.
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY Vol. 80 No. 5 2006
Goryushkina
