THE ENTHALPIES OF FORMATION
1711
Table 1. Chemical analysis data on praseodymium halides,
quartz reactor of a special shape with a section for col-
lecting sublimed substances. The reactor with crystal
hydrate was heated to ~480 K at a reduced pressure
(5 × 10–1 Pa). The temperature was then increased to
1300 K, and the system was held at this temperature for
4 h. The stage of dehydration was followed by the stage
of sublimation, and both were performed in one reactor.
The sublimed substance was prevented from contacting
with the atmosphere.
at %
Found
Calculated
Halide
Pr
PrBr3 37.00 0.09 62.93 0.04 37.02
PrI3 26.98 0.11 72.92 0.05 27.01
Hal
Pr
Hal
62.98
72.99
Anhydrous praseodymium triiodide was synthe-
sized in the reaction between praseodymium metal
(99.8%) and iodine of os. ch. (special purity) grade in a
two-section quartz reactor. The reactor was preliminar-
ily degassed in a vacuum at 1200 K to remove impuri-
ties. Praseodymium metal filings were loaded into a
quartz crucible in the left reactor section; the filings
were prepared beforehand from an ingot in a dry box
under argon. Powdered crystalline iodine taken in an
excess twofold by weight was loaded into the first reac-
tor section to provide constant boiling conditions. The
reactor was then evacuated to 5 × 10–1 Pa. The temper-
ature of the section with iodine was maintained con-
stant (~490 K) during the whole synthesis time. The
temperature of the section with metal was gradually
(during 6 days) increased to 950 K with isothermal
intervals. These temperature conditions prevented
praseodymium and then praseodymium triiodide inter-
actions with the container and ensured reaction com-
pletion,
os. ch. grade (the total content of impurities less than
0.002 wt %) and distilled water with a specific conduc-
tivity of 6 × 10–6 Ω–1 cm–1.
The enthalpies of solution were measured in a
hermetic isothermic-shell swinging calorimeter at
298.15 K. A detailed description of the unit and proce-
dure for measurements can be found in [5, 6]. The cal-
orimetric vessel (V = 112 cm3) made of titanium was
filled with 80.000
0.003 g H2O and hermetically
closed. Temperature rise was measured with a platinum
resistance thermometer (R298.15 = 401.32, 1 Ω = 0.94 K)
using a bridge scheme. An F-116/2 microvoltmicoam-
meter connected to a KSP-4 automated recording
potentiometer was used as a null instrument. The poten-
tiometer recorded calorimeter temperature variations
during measurements. The thermometric sensitivity of
the unit was 3 × 10–5 K/mm recorder scale. The temper-
ature of water in the shell was maintained constant
automatically with an accuracy of 5 × 10–3 K.
(2/3)Pr + I2 = (2/3)PrI3.
The energy equivalent of the calorimetric system
was determined electrically with the use of a potentio-
metric scheme to within several thousandths of a per-
cent. The voltage on the heater and reference coil
(R = 1.00005 Ω) was measured by an R-363-2 high-
resistance potentiometer of the accuracy class 0.002%.
The duration of current passage was measured by an
F-5080 frequency meter-chronometer with an accuracy
of 0.002 s. The energy equivalent of the calorimeter
(W = 322.62 0.23 J/Ω) was found in a series of
10 measurements. Here and throughout, the confidence
intervals were calculated with a 95% probability.
The reactor was cooled starting with the section
with iodine (the furnace for the section with iodine
remained switched on during cooling). Next, the fur-
nace for the section with iodine was switched off. After
complete cooling, the reactor was transferred into a dry
box and accurately broken in such a way that the quartz
crucible with the product remained intact. The product
was in the sintered but not molten state and could be
easily removed from quartz crucible walls. The salt was
removed from the crucible, ground in a mortar, and
transferred into a Pyrex ampule, which was evacuated
for 6 h at a 5 × 10–1 Pa pressure and 500 K to remove
adsorbed iodine and then sealed for storage.
The praseodymium halide samples were identified
by quantitative chemical analyses for praseodymium
(complexonometric titration with Trilon B) and halo-
gen (gravimetry). The chemical analysis data are listed
in Table 1.
According to X-ray diffraction, the samples were
single-phase and corresponded to PrI3 (a PuBr3-type
structure) and PrBr3 (a UCl3-type structure). Praseody-
mium halides are strongly hygroscopic, and all opera-
tions with them were performed in a dry box in an inert
atmosphere to prevent the substances from contact with
air moisture.
Prior to calorimetric measurements, the substances
were placed into glass ampules and sealed in a dry box
in an inert atmosphere. The samples were weighed on a
Satorius balance with an accuracy of 0.00002 g.
RESULTS AND DISCUSSION
The results obtained in measuring the enthalpies of
solution of praseodymium bromide and iodide in water
are listed in Tables 2 and 3. We use the following nota-
tion: ∆R is the temperature rise corrected for heat
exchange, g is the sample weight, Q is the thermal
effect of measurements, and ∆H is the enthalpy of the
reaction. The enthalpies of solution were calculated
with corrections ∆Hr for reduction to a temperature of
Hydrochloric acid (1.07 N) used as a reacting liquid 298.15 K. There is no literature data on the temperature
was prepared from concentrated hydrochloric acid of coefficients for the enthalpies of solution of praseody-
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY Vol. 80 No. 11 2006
Goryushkina
