Journal of Crystal Growth
North-Holland
126 (1993) 493-498
10U'"><0'
CRVSTAL
GROWTH
Ca1cium oxalate monohydrate precipitation investigation
by thermometric method
O. Sohnel, A. Costa-Bauzá
1
and V. Velich
lnstitute of Chemical Technology, Department of lnorganic Processes, Nám. Legií 565, 532 lO Pardubice, Czechoslouakia
Received
27 March
1992; manuscript
received
in final form 9 October
1992
Calcium oxalate monohydrate
(COM) precipitation
from diluted solutions of 100 mol m -3 ionic strength at 25°C was studied by
an isoperibolic reaction twin calorimeter. The molar reaction enthalpy was determined as - 17.5 kJ mol-l. Results achieved with a
pure system were highly reproducible.
Citrate, pyrophosphate
and phytate retard COM precipitation
that is manifested mainly by
an induction period appearance
and a decrease of the initial precipitation
rateo Effect of the studied impurities on individual
precipitation
experiments carried out under identical conditions was to so me extent "random",
i.e. the reaction extent reached at
arbitrary time considerably
differed for individual experiments.
Impurity effectiveness
in retarding spontaneous
precipitation
< phytate.
increases in succession citrate < pyrophosphate
1. Introduction
Preeipitation kineties of ealcium oxalate monohydrate (hereafter COM) were studied using a
wide variety of experimental methods based on
evolution of erystal size distribution [1], deerease
of ealcium eoneentration in solution [2], development of the systems optieal turbidity [3], linear
displaeement of erystal faee [4], eonsumption of
reaetants by the system [5] and others. Most of
these methods follow a quantity diffieult to determine with reasonable aeeuraey. Furthermore the
quantity is often indireetly related to the erystal
growth and henee ealibration, usually nonlinear
and of limited preeision, must be introdueed.
Therefore, reliability of these results is somewhat
restrieted.
Thermometrie methods, espeeially those involving eompensation of eomplementary heat effeets assoeiated with dilution, stirring, heat losses,
ete., measure the heat evolved on reaetion and
this quantity is direetly, lineariy and unequivo-
1
Permanent address: Department
of Chemistry, University
the Balearic Islands, 07 071 Palma de Mallorca, Spain.
0022-0248/93/$06.00
© 1993 - Elsevier
Science Publishers
of
eally related to the mass of solid preeipitated
from the system. Moreover, measurement of temperature on whieh a ealorimetry is based is very
aeeurate and sensitive. Sinee an isoperibolie reaetion twin ealorimeter satisfies all requirements
given above, it represents a sensitive and reliable
teehnique for studying the kineties of preeipitation. This teehnique has already demonstrated its
value in studying homogeneous reaetion kineties
[6,7] and also a limited applieation to heterogeneous reaetions [8].
This eontribution reports on results aehieved
using an isoperibolie reaetion twin ealorimeter
and studying ealcium oxalate monohydrate preeipitation from diluted solutions. Speeial attention is given to the effeet of impurities, speeifieally phytie aeid, sodium pyrophosphate and eitrate, on the preeipitation kineties of COMo
2. Experimental procedure
The isoperibolie reaetion twin ealorimeter used
in the study of COM preeipitation from solutions
of CaClz and NaZCZ04 is shown in fig. 1. Reae-
B.V. All rights reserved
494
O. Sohnel et al. / COM precipitation
investigation
by thermometric
method
vessels were different, liquid in the reference
vessel had to be cooled or heated as appropriate.
After equilibrating temperature
of both vessels,
i.e. a strictIy horizontal line on a plotter was
recorded, precipitation was initiated by adding an
identical volume of oxalate solution to both ves-
:,.'
'
'"
. -==
..... ---- .
,.
.
~..
'"
'
..
,. . '..,
'
.•
,
--
~. ~-----
'1
."
~
".
,
z
. ~--;:1-
".
---=--"":.
J••
,
,
.~
'
- ----------_--:...---:..-----, , ------------. -'
.
-
-
Fig. 1. Cross section of isoperibolic reaction twin calorimeter.
(1) reaction or reference vessel; (2) insulating body of
calorimeter; (3) water jacket; (4) insulating cover; (5) syringe;
(6) stirrer.
tion and reference vessels (1) are situated in a
therma11y insulating block of polystyrene foam
which is placed inside a water jacket (3) thraugh
which water of constant temperature
is circulated. The water jacket is surraunded by a block
of foam polystyrene (2), representing the body of
the calorimeter. A vertica11y movable thermal insulating cover (4) contains mechanica11y coupled
syringes (5), thermistors and stirrers (6). Thermistors are connected to a differential
resistance
bridge with an amplifier and its voltage output is
recorded as a function of time by a compensating
recorder.
The reaction and reference vessels of 200 mI
volume, each containing 100 mi of reference and
calcium containing
solution, respectively, were
kept in a constant temperature
water bath prior
to each experiment for at least 30 mino Then they
were quickly transferred
into the calorimeter
body, the cover tightIy closed, stirred and the
bridge output recording begun. If a recorded line
was inclined fram the base, ie. temperature
in
seis thraugh syringes. The temperature
development in the reaction vessel was recorded for
appraximately
10 min and then the experiment
was terminated.
Then a calibration determining the heat capacity of the reaction mixture, i.e. the heat required
for a defined increase in the output voltage, was
performed.
This calibration enabled conversion
to take place of the experimental voltage versus
time curves, so-caBed progress curves, into the
time development
of heat evolved at precipitation. The reaction vessel, corresponding thermistor and stirrer were carefu11y cleaned in order to
remove a11 adhered salid particles of COM, and
then dried. Construction of an isoperibolic reaction twin calorimeter and the way of performing
experiments were fu11y described in refs. [9,10].
Each experiment under certain reaction conditions was repeated several times. The result was a
family of pragress curves corresponding
to the
same experimental conditions.
100 mi of the reference solution placed in the
reference vessel always represented a solution of
NH4CI of ionic strength 100 mol m-3. 100 mi of
the reactant solution placed in the reaction vessel
contained the required amount of 100 mol m-3
CaClz stock solution and an appropriate amount
of NH4CI for adjusting the total solution ionic
strength to 100 mol m -3. If the influence of
admixtures was studied, the same amount of admixture, specifica11y phytic acid, sodium pyraphosphate
or sodium citrate, was added to the
solution in the reference and the reaction vessels.
Then an appropriate volume, 2 to 5 mi, of 100
mol m -3 stock solution of sodium oxalate for
achieving the stoichiometric conditions in the reaction vessel was added thraugh syringes to start
the reaction. AlI experiments were performed at
25°C.
The used chemicals of Analar grade were dissolved in distilIed water and the resulting solutions were filtered through a membrane
filter
O. Sohnel et al. / COM precipitation
investigation
by thermometric
495
method
types of reactions. A model of a bimolecular
autocatalytic reaction, A + B = 2B, with different
initial concentrations of reactants, i.e.
IV)
T
2 mV
1
1 - exp [ - kt (b + a)
1 + a exp [W - kt ( b
Os
100s
(t)
Fig. 2. Family of the progress, i.e. voltage versus time, curves
for COM precipitation
carried out at initial COM concentration of 2.439 mol m -3, 25°C and 1 = 100 mol m -3.
before use. The same stock solutions were used
for experiments carried out under identical conditions in order to eliminate any deviation caused
by a possible difference in the composition of
independentiy prepared stock solutions.
3. Results
Two series of experiments were performed:
the first series at different initial concentrations
of COM without the presence of admixtures and
the second series with solutions of the same initial concentration of COM, but containing different amounts of an admixture, specifically phytate,
pyrophosphate or citrate. In both cases, the ionic
strength of the reacting solution was adjusted to
100 mol m-3 with NH4Cl.
A typical family of progress curves obtained
from independent experiments with apure system, i.e. without an admixture present, performed at identical conditions, are shown in fig.
2. These experiments were well reproducible as
to the precipitation rate and the reaction extent
measured by the reaction enthalpy released at
any selected time. However, for experiments carried out at low supersaturations (c¡ < 1.9 mol
m -3), such reproducibilty was lost and individual
progre ss curves differed substantially. These
progress curves were fitted by a computer to give
kinetic expressions corresponding to different
]
+ a)
]/ b '
(1)
where ¡lU and ¡lUoo are the output voltages at
time t and at infinity, respectively, a and b are
the initial concentrations of the reactants and k
is the kinetic constant, describes best the actual
shape of the experimental curves.
The total reaction enthalpy evolved on reaction completion, ¡lHoo, from the employed volume
of reaction mixture was evaluated using eq. (1).
The arithmetic average of individual values from
independent experiments for each initial supersaturation studied,
are given in table 1.
The amount of solid, ns, precipitated from the
actual volume of reaction mixture, determined as
the difference between the initial and equilibrium
total concentration of COM under experimental
conditions, i.e. 25°C and 1= 100 mol m - 3, is also
given in table 1. The latter concentration was
calculated from the thermodynamic solubility
product of COM, Ksp = 2.00 X 10-3 molz m-6
[11], valid for 1 = 100 mol m -3 and the thermodynamic association constant for ionic pairs of
CaCZ04,
K = 1.537 mol m-3 [12], as ceq = 0.123
mol m -3. The respective activity coefficient y:¡: =
0.3714 was calculated from the Davies extension
of the Debye-Hlickel theory [13].
The last column in table 1 gives the molar
reaction enthalpy, i.e. the change of enthalpy
accompanying formation of 1 mol of COM, from
¡lit,
Table
1
COM precipitation
solution at different
1.961
2.439
2.913
3.846
4.762
at 25°C, 1 = 100 mol m -3 from
initial concentration
- ¡jHoo
ns X 104
(mol)
(J)
- ¡jHoo/ns
(kJ mol- 1)
1.874
2.373
2.873
3.872
4.871
3.38
4.00
4.78
6.98
8.74
18.0
16.9
16.6
18.0
17.9
ns represents
the number of COM moles precipitated
reaction completion in the respective experiment.
pure
at
496
(v!!
O. Sohnel et al. / COM precipitation
investigation
by thermometric
(iii) the average slope of the progress curve
10 s after induction period termination, mlO, and
(iv) the difference between the maximum and
minimum value of reaction enthalpy observed at
a respective set of experiments after 100 s of
reaction duration:
iJ.IllOo
T
ZmV
1
8 = I HlOo(max)
Os
method
100s
HlOo(min) l.
(3)
diluted aqueous solutions of CaClz and NaZCZ04
at 25°C and 1 = 100 mol m-3.
The second series of experiments was designed
for studying the admixture influence on COM
precipitation. These experiments were performed
with reaction mixture of initial concentration c¡ =
2.439 mol m -3 COM at 25°C and varying amount
of impurity. A typical family of progress curves
determined in individual experiments carried out
under identical conditions are shown in fig. 3.
Each family of progress curves were characterized by (i) the average induction period, i.e. time
elapsed between the solution mixing and the onset of heat evolution, (ii) the arithmetic mean of
determined values of reaction enthalpy evolved
after 100 s from the induction period termination,
are also quoted in table 2. The corresponding
values for the reference state, i.e. characterization of progress curves produced by apure system
of the same supersaturation, is also given in table
2.
In all experiments crystals of characteristic
COM shape were formed.
3. Discussion
An isoperibolic reaction twin calorimeter
proved to be a suitable and sensitive technique
for studying spontaneous COM precipitation. Reproducibility of experimental progress curves determined for COM precipitation without impuri-
Table 2
= 2.439
(2)
These values are given in table 2. The standard
deviation corresponding to each calculated value
of iJ.HlOo, defined as
(ti
Fig. 3. Family of the progress (voltage versus time) curves for
COM precipitation
carried out at initial COM concentration
of 2.439 mol m-3, 25°C, 1 = 100 mol m -3 and 0.732 mol m-3
sodium citrate.
m
2.57
2.35
o0.66
1.86
2.08
1.50
2.77
2.59
0.050
2.73
0.066
3.74
0.133
0.065
0.067
.19
0.49
0.85
4.015
6.6
6.3
8.6
1.04
12
5I c 4.88
9.76
1.14
232.68
0.028
.69
0.56
81.95
1.28
1.61
0.44
4.01
0.083
0.035
0.046
.16
.13
0.79
o0.33
0.52
0.66
7.85
1254N
0.67
0.037
2.65
0.044
2.5
0.61
0.73
6.4
X 102(JS-I)
0.142
2.2
7.65
(J)
a9.4
t20.8
100
I2.72
L1 (s)
¡¡lOa
(J)
COM precipitation
at 2SOC, 1 = 100 mol m -3 and c¡
Impurity
-
mol m -3 in presence
of impurities
O. Sohnel et al. / COM precipitation
ties was satisfactory (see fig. 2) if initial supersaturation exceeded a certain minimum value. Fig. 2
presents a family of very similar progress curves
obtained in independent
experiments carried out
under identical conditions for apure system. The
rather small difference 8 given on the first line in
table 2 for systems without impurity confirms the
close similarity of independent
progress curves
determined under identical conditions.
The supersaturation
range within which a precipitating
system can be investigated
by this
method is limited. The lower limit is given by the
requirement
that a reasonable
reaction extent
must be reached within maximum 20 min beyond
which the calorimeter is not thermally stable. The
upper limit of supersaturation
is determined by
the promptness of the equipment response to a
temperature
change; if the heat evolution rate
exceeds the response, any reliable kinetic curve
cannot be obtained.
The molar enthalpy of reaction
+ CaClz + HzO
NazCz04
~ CaCZ04•
HzO(s)
+ 2 NaCl,
carried out in dilute solutions of ionic strength
100 mol m -3, is given in table 1 as i1¡t,/ns, and
is independent
of solution initial supersaturation
within the studied concentration range. The average molar reaction enthalpy was c~lculated from
values quoted in table 1 as i1Hr = -17.5 kJ
mol-I.
The reaction enthalpy can also be determined
from the standard formation enthalpies of products and reactants according to
i1Hor= Lv¡Ho¡-
Lv¡Ho¡,
(4)
p
where p and r denote products and reactants.
Inserting
appropriate
formation
enthalpies,
NazCzO/dil)
- 1299.2 [14], CaClidil)
- 873.2
[14], NaCl - 406.3 [14], Hz0(t')
- 286.2 kJ mol-I
[14] for 18°C and 1 atm and CaCZ04'
HzO(s) 1673.6 kJ mol-I [15] valid for 25°C and 1 atm into
Formation
(4) yields i1Hor = -27.6 kJ mol-I.
enthalpies denoted by (dil) apply to substances
dissolved in infinitely diluted solutions, (t') and
(s) correspond to liquid and solid state, respectively.
investigation
by thermometric
method
497
The experimental
average reaction enthalpy
( - 17.5 kJ mol- 1) is of a similar value as the
quantity calculated from eq. (4) ( - 27.6 kJ mol-I).
The calculated value is, however, to some extent
unreliable due to the difference in reference temperature of the formation enthalpies (l8°C) and
of the experiments (25°C) and al so due to uncertainty in the reported values of formation enthalpies. The formation enthalpies cannot even
be brought to a common temperature,
since the
required heat capacities are not available. Taking
into account these uncertainties,
the correspondence between experimental and calculated values of reaction enthalpies can be considered as
acceptable.
A mathematical model of the autocatalytic reaction fits the experimental
data well and also
gives realistic values of the initial concentration
of the reactants. Calculated
and actual initial
concentration
of reactants compare well. These
facts, howeverencouraging,
cannot be overestimated since the model in question applies strictly
to homogeneous
reactions [8], whereas in our
case a heterogeneous
reaction takes place. Therefore, this model was used only for estimating the
total reaction heat enthalpy, in which case it
provided reliable results. Further conclusions inferred from close correspondence
of mathematical model and experiment, mainly a dominating
role of secondary nucleation under employedexperimental conditions, could not be considered as
sufficiently safe.
The studied impurities - phytate, pyrophosphate and citrate - induce induction period at
COM precipitation
(see table 2). This effect is
generally more pronounced at higher concentration of impurity. The effectiveness of the impurity, estimated according to the concentration resulting in a similar duration of tind, increases in
the succession citrate < pyrophosphate < phytate.
The initial precipitation
rate, a measure of
which is mlO, is lowered by the studied impurities
compared to the rate in apure
system and decreases with increasing impurity concentration.
The effectiveness of impurity in influencing the
precipitation
rate follows the same succession as
given above.
An interesting conclusion can be inferred from
498
O. Sohnel et al. / COM precipitation
the observed spread of progress curves, expressed
as 8 and u in table 2, determined from independent experiments carried out under identical conditions when precipitation takes place in
the presence of impurities. This spread means
that in independent, but identical, experiments, a
different reaction extent is reached after the same
time has elapsed from the reaction start-up. The
same effect is illustrated in fig. 3 by dissimilar
shape and height of individual progress curves. It
contrasts with experiments in apure system where
the reaction extent is similar at any time. Such
behaviour indicates somewhat "random" influence of impurities on spontaneous precipitation.
An acceptable explanation of this observation
cannot be suggested at presento
4. Conclusions
An isoperibolic twin reaction calorimeter represents a convenient and reliable technique for
studying spontaneous crystallization of sparingly
soluble substances. The resulting progre ss curves
of precipitation, i.e. time dependence of heat
evolved due to precipitation, are highly reproducible. Precipitation of COM from diluted aqueous solution is retarded by the studied impurities,
compared to apure system. The effectiveness of
impurities in influencing precipitation increases
in the succession citrate < pyrophosphate <
phytate. The studied impurities affect sponta-
investigation
by thermometric
method
neous precipitation of COM carried out under
the same conditions in a "random" way, that is,
independently performed experiments are influenced to a different degree by the same amount
of an identical impurity.
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[14] International Critical Tables, Vol. V (McGraw-HiIl, New
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