chapter 4 part-b: density and vapor pressure...
TRANSCRIPT
87
Chapter 4 Part-B: Density and Vapor Pressure
Osmometry Studies of Aqueous Solutions of N-Butyl-Pyridinium Bromide at
298.15 K
87
4 (B).1.Introduction and Literature survey
In the first half of the past century several phenomenon were discovered which
are associated with the passage of electric current through a salt solution. The most
important discoveries in the field were made by Michael Faraday [227]. He made a
thorough study of electrolysis and classifieds substances into electrolytes whose
solutions conduct electricity and non-electrolytes whose solution does not conduct
electric current. Aqueous solutions of electrolytes have a number of properties that
distinguish them from solutions of non-electrolytes. They have high osmotic pressure,
higher boiling points and lower freezing points, they conduct electric current, etc.
these specific properties can be explained only if we suppose that the molecules of
electrolyte fully or partly separate into their constituent parts, ions. As reacting
particles, ions take part in dissociation, solvation, ionic sublimation, in
electrochemical, oxidation-reduction processes, etc. In other words, along with atoms,
molecules and radicals, ions are fundamental structural units of substances.
In our everyday life we deal with solutions whose concentrations vary within a
very wide range, from very dilute to saturated and supersaturated solutions.
Electrolytes solutions are characterized by various properties depending on the
concentration, significant qualitative changes are often observed when passing from
one region concentration to another. Dilute solutions having an infinitely small
concentration of the solute acquire the properties of an ideal solution. Here the
dissociation degree is unity and we deal with solutions in which ions play the role of
the particles. In addition to ions, solutions of high and medium concentrations can
also form molecules, associates, ion pairs, etc., which give new properties to the
solution. The situation is even more complicate with unstable supersaturated
solutions. The main difficulty in describing the properties of electrolyte solutions is
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that it is impossible to establish the boundaries of concentration and the ranges in
which the changes in properties obeys one law. As a rule, the properties of dilute
solutions are divided into two groups. One comprises the properties which, for a given
solvent, are independent of the nature of the solute. These are the saturated vapour
pressure of the solvent over the solution, elevation of the boiling point and lowering
of the freezing point of the solution (compared with the solvent), the osmotic
pressure, and others. The laws that govern their changes, associated with the variation
of the solute concentration, have become the foundation of the physical theory of
solution.
The classical trio of colligative properties, of which boiling-point elevation
and freezing-point depression are the first two members, is completed by the
phenomenon of osmotic pressure. In the course of investigating the properties of
aqueous solutions of electrolyte in this laboratory, it has become increasingly apparent
that reasonably precise thermodynamic data on these solutions are of fundamental
importance. At 00C, the determination of the freezing-point lowering is a convenient
and accurate method. Unfortunately, many compounds of interest both from a
theoretical and from a practical standpoint are quite insoluble in water at 00C.
Furthermore, the evidence for the rate of change of activity with temperature is
conflicting, making uncertain the extrapolation of properties measured at 00C, to room
and higher temperatures [228]. On the other hand the range and scope of applications
of vapour pressure osmometry over a large number of aqueous and non-aqueous
solutions of electrolytes including salts of drug molecules and non-electrolytes
molecules including enzymes and proteins have been investigated and documented by
researchers in the field of physical chemistry chemical physics.
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The thermoelectric differential vapour pressure method was first described by
Hill, but it was not widely used until the development of convenient means of
measuring small temperature differences became available. Brady, Huff, and McBain
used thermistors in an apparatus of this type and then proceeded to determine solution
properties of surface active solution. Several subsequent workers have built similar
devices using thermistors, but the major application has been in the determination of
molecular weight Since the observed temperature change in any instrument based on
this principle is determined by several mass transfer processes, the significance of the
results is uncertain until experimental evidence is provided to show that a definite
correlation exists between the observations and thermodynamic solution properties
[229].
Although others have made use of the thermoelectric differential vapor
pressure method to study solution properties, Prof. K. J. Patil and his colleagues [230-
235] have extended this to a broader class of electrolytes and have now shown that it
is a valid and useful technique for applications to most aqueous and non-aqueous
solutions. To date, a number of researchers have studied the thermo-physical
properties of aqueous solutions of ILs systems [236-251].We were confronted with a
problem of hydrophilicity of [bpy][Br]. In literature a research article by
Crosthwaiteet al. [186] provides an excellent account of the thermo-physical
properties like melting temperatures, freezing temperature, glass transition
temperatures, etc., of some pyridinium-based ILs. Very surprisingly [bpy][Br] salt
shows melting temperature 378K and freezing temperature 315K. Even in the mass
spectral analysis of [bpy][Br], it is not possible to predict the presence water in the
compound because the technique deals with high vacuum to remove solvent
molecules, and possibly the water attached to [bpy][Br] should be removed in this
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region. Therefore we decided to measure the exact molecular weight by performing
osmometry. The results obtained are supplemented by FT-IR, TGA and KF titrimetry.
The application of vapor pressure osmometry to the determination of
molecular weight of [bpy][Br] has been thus investigated. A discussion on the
results and the structural formula of [bpy][Br] is given in following pages. It was also
necessary to measure densities of aqueous solutions to convert concentration scales
appropriately i.e. to mole fraction, molarity and g.cm-3
etc.
4(B).2 Chemicals Used
In the present work, all the solutions were prepared on a molality basis using
doubly-distilled water and converted to molarity scale whenever required with the
help of density data. The salt NaCl and sucrose were of AR grade and dried under
vacuum at 393 K for 24 h before use. These were required for calibration purposes.
Synthesized N-butyl-pyridinium bromide was purified and dried in vaccum oven
before use.
4(B).2.1 Density Measurements
Densities of all the solutions were determined by a high precision ANTON
PAAR digital densitometer (Model: DMA 5000). This method is the most accurate
and convenient for the density measurements of liquids. The method involves
measurement of natural vibrational frequency of the oscillating tube containing liquid
under investigation. The natural vibrational frequency of the tube is related to the
density of the liquid inside it by
--- (1)
Where d is the density of the liquid, A and B are the instrumental constants and τ is
the oscillation period of the tube.
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The used digital densitometer with the oscillator excitation, amplitude control, and
time lapse measuring and data reception circuitry which was described in detail by
Leopold [252]. There is a software installed in the instrument by which with the help
of calibrating fluids, the constants A and B are obtained and can be used for
measuring the density of unknown liquid or solution at a given temperature.
A syringe holder is used to hold the syringe while it is being deployed for
filling the sample tube. While loading the vibrating tube with liquid sample, the
precaution is taken that the introduction of the liquid must be made slowly enough to
enable the sample liquid to properly wet the walls of the sample tube. The meniscus
of the liquid while filling the sample tube must be concave and not convex, to avoid
the trapping of micro air bubbles so that the period readings obtained are stable. The
sample tube is completely filled when the liquid meniscus passes the upper enlarged
portion of the sample tube. The opening of the upper part of the sample tube is to be
closed off with Teflon stopper and the illumination is to be turned off. Once the
illumination of the sample is turned off, the tube starts oscillating and the reading τ
(tau) will be displayed on the display unit after the interval of selected time period (k).
As a check of authenticity of the density measurements, the measurements for
aqueous NaCl and sucrose solutions were carried out and compared with the best
literature data [253, 254] reported. The plot of 103 x (d – d0) against molality for
aqueous NaCl and sucrose solutions at 298.15 K are shown in figure 4(B).2.1 and
4(B).2.2.
4(B).2.2 Accuracy
Accuracy of the instrument is depends on the temperature control since the A
and B constants are temperature dependent. The accuracy of density measurements in
the present work was estimated to be of the order of ±5 × 10-3
kg.m-3
.
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Table 4(B).1: Density data of aqueous NaCl and Sucrose solutions at 298.15K
NaCl Sucrose
Molality
(m)
1000*Δ ρ
(gm/cm3)
Molality
(m)
1000*Δ ρ
(gm/cm3)
0.00000 0.000 0.00000 0.000
0.00515 0.201 0.01173 1.518
0.00700 0.259 0.01856 2.409
0.01006 0.370 0.02367 3.076
0.01960 0.724 0.02954 3.826
0.02630 1.000 0.03573 4.643
0.03979 1.588 0.04134 5.361
0.05139 2.066 0.04663 6.027
0.06219 2.509 0.05024 6.478
0.08090 3.342 0.10058 12.907
0.10366 4.231 0.20227 25.395
0.11951 4.889 0.30320 37.644
0.15118 6.177 0.40137 48.260
0.20123 8.201 0.50130 59.028
0.25065 10.200 0.80320 89.780
0.30245 12.255 1.00007 106.499
0.35221 14.232 1.25043 127.502
0.40100 16.092
0.44960 18.091
0.50685 20.311
0.55237 22.137
0.59971 24.053
0.65466 26.151
0.70523 28.147
0.71871 28.912
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Figure 4(B).2.1: Comparison of density data of aqueous NaCl solutions with
literature at 298.15 K
Figure 4(B).2.2: Comparison of density data of aqueous Sucrose solutions
with literature at 298.15 K
0
5
10
15
20
25
30
35
40
45
0 0.2 0.4 0.6 0.8 1
10
3 ∙
(d-d
o)
/ g
∙cm
-3
m / mol∙kg-1
This Work
Literature
0
20
40
60
80
100
120
140
0 0.2 0.4 0.6 0.8 1 1.2 1.4
10
3 ∙ (
d-d
o)
/ g
∙cm
-3
m / mol∙kg-1
This Work
Literature
94
4(B).3 Osmotic Vapor Pressure Measurements
Vapor pressure is not measured directly due to difficulties in sensitivity, but is
measured indirectly by using thermistors to measure voltage changes caused by
changes in temperature. In the present work we have used the KNAUER K-7000
Vapor Pressure Osmometer (K-7000) and the complete experimental set-up is shown
in figure 4(B).3.1. In this model, two thermistors are placed in measuring chamber
with their glass enclosed sensitive bead elements pointed up. The thermistors are
covered with pieces of fine platinum screen to ensure a constant volume of the drop of
the analyte, which is present on the bead for each measurement. The chamber
contains the reservoir of solvent and wicks (the wick provides a large surface area for
the solvent and its transfer to the atmosphere inside the chamber) to provide a
saturated solvent atmosphere around the thermistors. If pure solvent on one thermistor
is replaced by a solution, condensation of solvent into solution from the saturated
solvent atmosphere will proceed. Solvent condensation releases heat so the thermistor
will be warmed. Condensation will continue until the thermistor temperature raises
enough to bring the solvent vapor pressure of solution up to that of pure solvent at the
surrounding chamber temperature.
4(B).3.1 Operational Procedures
The first step in preparing for a Vapor Pressure Osmometry run is to clean the
chamber assembly. The chamber was removed from the oven, revealing the baffles,
wicks and thermistors. The platinum gauze covering of the thermistors were carefully
removed and placed in a solvent, from which the new samples will be prepared, which
causes the wetting of the thermistor probes with the solvent. The uncovered
thermistors were fully rinsed with the solvent and new wicks placed in the assembly.
The chamber was reassembled with the gauze in place and 20 ml of solvent poured
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into centre cylinder over the thermistors. The injection syringe were then removed,
cleaned with solvent, loaded with appropriate solutions or solvent and returned to the
apparatus. The temperature control was set to the appropriate number (the cell at
250C and head at 27
0C) and the instrument was allowed to reach the operating
temperature. This was usually started before the cleaning and allowed to warm and
equilibrate for at least one hour.
4(B).3.2 Calibration of Vapor Pressure Osmometer
Using the material of known molecular weight, one can do the calibration of
the vapor pressure osmometer. Requirements for standards are vapor pressure no
more than 0.1 % of the solvent, high purity, complete solubility. Sucrose, mannitol
and NaCl are excellent standards for aqueous solutions while benzil, sucrose-
octaacetate are good for organic solvents. In the present work, we have used aqueous
NaCl and sucrose solutions of known osmolality for the calibration and hence
determined the instrumental constant, K, with the help of which the osmolality of
aqueous solutions of various samples were determined and further used for estimation
of practical osmotic coefficient values. The operating temperature for these
measurements was 298.15±0.001 K. The calibration constant K is represented by the
slope of the regression curve (measurement value as a function of osmolality of
aqueous NaCl solutions) passing through origin and is represented by the equation.
Kcalib = Measurement value/Known Osmolality
The osmolality of the sample solution has been calculated with
Osmolality = Measurement Value/ Kcalib.
At the time of each measurement, at least 5-6 readings were taken for the fixed
time settings and the averaged value was used for further processing. Since KNAUER
K- 7000 vapor pressure osmometer is a commercial instrument and it works above the
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ambient temperature, one cannot do the measurements at 298.15 K or below this.
Thus it was necessary to have a proper cooling assembly so that the surrounding of
the instrument is kept colder than the working temperature. Fortunately we had air
conditioned lab of which temperature was maintained at 170C., so that measurements
could be made at 250C (298.15 K). The osmotic coefficient values (ϕi) for different
solutions were mole using the equations:
∑
Where i is the total number of species (= 2 for electrolyte) and mi is the molality of
the salt.
The accuracy in osmotic coefficient (ϕ) measurements was found to be better
than ± 1 x 10-3
. Comparison of the osmotic coefficient NaCl and sucrose in water
obtained in this work with the literature data [255] is shown in figure 4(B).3.2, which
shows the authenticity of the osmotic vapor pressure measurements done in this work.
Table 4(B).2: VPO data of aqueous solutions of NaCl and Sucrose at 298.15 K
NaCl Sucrose
m1/2
/mol.kg-1
Osmotic Coefficient
(ϕ)
m1/2
/mol.kg-1
Osmotic Coefficient
(ϕ)
0.00000 1.00000 0.00000 1.00000
0.16217 0.95704 0.05024 1.00199
0.19947 0.94886 0.10058 1.00776
0.22669 0.93986 0.20227 1.01568
0.28443 0.93758 0.30320 1.02759
0.32196 0.93188 0.40137 1.03048
0.38882 0.92695 0.50130 1.03812
0.44859 0.92290
0.50065 0.92141
0.54995 0.91992
0.59325 0.91908
0.63325 0.91862
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Figure 4(B).3.1: Knauer K-7000 vapor pressure osmometer (K-7000 vpo) and elements and design of measuring cell
98
Figure 4(B).3.2: Comparison of osmotic coefficient of aqueous solutions of Sucrose and NaCl at 298.15 K
1
1.01
1.02
1.03
1.04
1.05
1.06
0 0.2 0.4 0.6 0.8
Osm
oti
c C
oef
fici
ent
m/mol kg-1
Literature
Experimental
0.91
0.92
0.93
0.94
0.95
0.96
0.97
0.98
0.99
1
0 0.2 0.4 0.6 0.8 1
Osm
oti
c C
oef
fici
ent
m1/2
99
4(B).4.Results and Discussion
Molecular Weight determination of [Bpy][Br] by Vapour Pressure Osmometry.
We have used aqueous NaCl solutions of known osmolality for the calibration
and hence determined the instrumental constant (Kcalib).The Kcalib is represented by the
slope of the regression curve (measurement value as a function of osmolality of
aqueous NaCl solutions) passing through origin and is represented by the equation:
Kcalib = Measurement value/Known Osmolality (1)
The osmolality of the sample solution can be calculated with the following equation:
Osmolality = Measurement Value/Kcalib (2)
The osmotic pressure (π) is calculated by following equation:
π(atm) = Osmolality (mOsmol∙kg‐1)∙0.082056 (L∙atm∙K‐1∙mol‐1)∙298.15(K) (3)
The concentration c in (g∙cm‐3) is calculated with the help of density and weight
fraction data.
The values of parameter (π/cRT) are estimated with help of following equation:
π/cRT (mol∙g‐1) = π (atm)/[c (g∙cm‐3)∙82.056 (cm3∙atm∙K‐1∙mol‐
1)∙298.15 (K)]
In Figure 4(B).5, parameter π/cRT is plotted as a function of concentration
(cin g∙cm‐3) for the studied compound. The intercept of the said plot yielded the value
of reciprocal of molecular weight while the slope value gave the measure of osmotic
second virial coefficient. The intercept of the plot reveals that the molecular weight of
studied compound is 283.53 g∙mol-1
. The theoretical molecular weight of the
compound is 216.12 g∙mol-1
. Therefore, we concluded that the salt contains four water
molecules as water of hydration. This conclusion is also supported by our KF
titrimetry and TGA analysis [256].In FT-IR analysis of [bpy][Br] (Figure 4(B).7) a
broad band found in the spectrum indicates the presence of water, as well KF
titrimetry (0.3%) and TGA profile (30%) (Please refer fig. chapter 3.1) also reveal
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water and in conformity with osmotic pressure measurements. These results indicate
that the salt is a hydrate containing 4 water molecules, which cannot be easily
removed simply by heating. It is the main reason for the difference in melting point
and freezing point of this compound. Probably, the IL is in a gel like state between
freezing point and melting point region, having a glass transition temperature in
between. The calculated molecular weight was used to correct the molalities and
collected in Table 4(B).3.
The density (d) data as a function of concentration of ionic salt molecule in
aqueous solutions at 298.15 K are reported in Table 4(B).3. The apparent molar
volume (V ) as a function of molality of the drug molecules were calculated by using
the following equation:
d
M
mdd
ddV
2
0
0 )(1000
(4)
Where m is molality of ionic salt molecules in aqueous solution (mol∙kg-1
), d and do
are the densities of solution and solvent respectively in kg∙m-3
and M2 is the molar mass
of the solute (kg∙mol-1
).
TheV data can also be expressed as [257]
cBcA VVVV 21
0 (5)
Where 0
V is apparent molar volume of the salt at infinite dilution, AV is Debye-
Hückel limiting law coefficient (1.868 for 1:1 electrolyte solutions at 298.15 K), BV is
deviation parameter and c is the concentration of the salt on molarity scale. The
variation of (V – 1.868c
1/2) as a function of concentration of ionic salt(c/mol∙dm
-3) in
aqueous solutions at 298.15 K are shown in Figure 4(B). 6. When (V – 1.868c
1/2)
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values extrapolated to infinite dilution, yield limiting apparent molar volumes ( 0
V ) of
ionic salt.
Table 4(B).3: Molality (m), Density (d), Apparent Molar Volume , for Aqueous
Solutions of [Bpy][Br] at 298.15 K
m
/
d
/
103 /
mm3 mol
-1
0.00000 0997.043 162.03*
0.01984 0998.117 162.14
0.02735 0998.523 162.07
0.04333 0999.388 161.93
0.05223 0999.870 161.85
0.06294 1000.450 161.76
0.07738 1001.231 161.63
0.09862 1002.381 161.45
0.12224 1003.659 161.24
0.16454 1005.949 160.88
0.23438 1009.729 160.27
0.31217 1013.394 159.61
0.39501 1018.424 158.90
*Extrapolated value at infinitely dilute solution of [Bpy][Br] at 298.15 K
102
Figure 4(B).3.3: The plot of parameter ( /cRT) against concentration (gm∙cc-1
) of
[Bpy][Br] in aqueous [Bpy][Br] solutions at 298.15 K
Figure 4(B).3.4: Variation of (V - 1.868c1/2
) as a function of concentration
(c/mol∙dm-3
) of [Bpy][Br] in aqueous solutions at 298.15 K.
0.000
0.001
0.002
0.003
0.004
0.005
0.006
0 0.02 0.04 0.06 0.08 0.1 0.12
π/c
RT
c / gm∙cc-1
150
155
160
165
170
0 0.1 0.2 0.3 0.4
10
3 ∙ (
V -
1.8
68
c1/2
) /
mm
3m
ol-1
c / moldm-3
103
Figure 4(B).5: FT-IR spectrum of n-butyl-pyridinium bromide
5007501000125015001750200025003000350040001/cm
0
7.5
15
22.5
30
37.5
45
52.5
%T
3454
.62
3215
.44
2935
.76
2870
.17
2717
.79
2416
.89
2052
.33
1882
.59
1631
.83
1579
.75
1485
.24
1381
.08
1321
.28
1215
.19
1172
.76
1066
.67
952.
87
887.
28
771.
55 734.
90
684.
75
solid
N+
CH3 Br-