1
82
O.A. Lukyanchenko, V.V. Samuskevich / Thermochimica Acta 327 (1999) 181±189
A peculiar feature of such compounds is their ten-
dency to the dispropotionation reaction which results
in the formation of less-proton phosphate and free
phosphoric acid [6]. When the disproportionation
proceeds on the stage of the water of crystallization
releasing, the resulting acid may react with the dehy-
dration products, complicating essentially the follow-
ing steps of thermal transformations [1].
For studying the reaction of disproportionation in
isothermal conditions, the weighted samples (100 mg)
were held at t358C to 958C inside of hermetic
vessels where the constant relative humidity was kept
by the aqueous solutions of sulfuric acid within
RH0.02% to 100%.
In isothermal experiments on dehydration kinetics
we use a high-vacuum setup having a quartz micro-
�
1
The available literature data on thermal transforma-
tions of Co(H PO ) Á2H O crystals [7±9] are few in
balance (spiral sensitivity 2 mg mm ) which permits
to investigate the process in vacuum (residual pressure
p10� hPa) or in water-vapor atmosphere (the values
2
4 2
2
5
number and discrepant. The reasons of the discre-
pancy are probably connected with the differences in
heating conditions as well as with the in¯uence of such
a factor as the composition of gaseous phase over the
sample which has not been taken into account in these
works.
of water-vapor pH O were ®xed with an accuracy of
2
Æ0.1 hPa). The sample of Co(H PO ) Á2H O was
2
4 2
2
spread as a thin layer (mass ca. 50 mg) in a ¯at-
bottomed platinum sample holder and thereafter
immersed in the previously thermostated part of the
reaction vessel. In the course of isothermal dehydra-
tion (t608C to 1108C) the mass loss Ám was peri-
odically registered.
In the present work the thermal transformations of
Co(H PO ) Á2H O were investigated in vacuum, in
2
4 2
2
water-vapor atmosphere and in air (when the values of
the relative humidity RH were ®xed) both in iso- and
non-isothermal conditions. The processes of anionic
condensation and disproportionation which attend, as
a rule, cobalt dihydrogenphosphate dehydration were
considered. The conditions when the removal of the
water of crystallization is not complicated by any side
process were revealed, the kinetic characteristics of
this reaction were obtained and its reversibility as well
as the possibility of proceeding of dehydration±rehy-
dration processes in cyclic regime were studied.
To study the rehydration process of the samples pre-
dehydrated in water-vapor atmosphere, the setup was
initially pumped and after cooling ®lled with water-
vapor again. During the isothermal holding (t208C
to 708C) at ®xed p
Á m was recorded.
20 hPa, the mass addition
H2O
0
The products which formed at different steps of iso-
and non-isothermal transformations were character-
ized by X-ray diffraction analysis (DRF diffract-
ometer � 2.0; Cu K radiation).
ꢀ
The anionic composition of heat-treated samples
was determined by quantitative ascending chromato-
graphy using FN 12 chromatographic paper; EDTA or
2
. Experimental
Na S were added to transfer the less-soluble products
2
The sample of Co(H PO ) Á2H O used in the work
of heat treatment in solution.
To determine the quantity of free phosphoric acid
2
4 2
2
(
crystal size D0.05 to 0.1 mm) was precipitated from
the solution of Co (PO ) Á8H O in 75% H PO (23 g
ꢀa
contained in the products, they were moved on
3
4 2
2
3
4
H3PO4
of Co (PO ) Á8H O per 100 ml of acid) by stirring at
the glass ®lter under the layer of acetone (25 ml) and
washed (with acetone), keeping out of contact with air.
The eluate was titrated with 0.01 N aqueous solution of
NaOH in the presence of bromocrezol green.
3
4 2
2
room temperature for 1 h. The precipitate was washed
from the remains of the mother liquor on the glass
®
lter with acetone and then with ether and dried at
relative humidity of air RH45% until the mass stop
to change. X-ray diffraction pattern of the sample
prepared coincides with the literature data for
Co(H PO ) Á2H O [10].
3. Results and discussion
2
4 2
2
When dehydration of Co(H PO ) Á2H O was stu-
3.1. Non-isothermal transformations
2
4 2
2
died in non-isothermal conditions, a derivatograph
OD-103 MOM (platinum crucible, sample weight
The derivatogram of Co(H PO ) Á2H O obtained
2
4 2
2
�
1
1
00 mg, heating rate 108C min ) was used.
when the sample was heated in air is shown in Fig. 1.