Download Physical properties study for TBP-HNO3-diluent system

Chapter 4
Physical properties study
for TBP-HNO3-diluent
system
4. Physical properties study for TBP-HNO3-dodecane system
4.1. Introduction:
Extraction power of solvent depends upon the physical properties of the
system. TBP has been extensively used as a solvent in nuclear chemistry for fuel
reprocessing due to its excellent chemical resistance and physical properties which
results in better separation than other solvents (Chang et al. 2000). The extracting
power of TBP is mainly due to presence of phosphoryl group which form adducts or
solvates with the metal ions. The physical properties like viscosity, density, solubility
and interfacial tension play a very important role in solvent extraction studies. Drop
size, drop formation and extracting power of TBP depend on all these physical
properties.
TBP is generally used in conjunction with a hydrocarbon diluent which is inert
in nature. The diluent like n-dodecane, liquid paraffin, kerosene etc. improves the
physical properties of TBP by lowering the density and viscosity for better phase
separation (Schulz and Navratil 1984). Hence, it is important to study various physical
properties of TBP in presence of diluent. TBP is a polar compound having limited
solubility in water. The presence of nitric acid affects its solubility in water. It is
therefore informative to know the solubility of TBP in wide range of nitric acid
concentrations.
4.2. Literature survey:
Different investigators have studied different properties of TBP in the past.
Burger and Forsman (1951) have reported the solubilities of pure TBP and of TBP in
an inert diluent in water and in solutions of nitric acid of various concentrations. The
solubility of pure TBP in water was about 0.4 g/1 at 25°C. When the TBP was diluted
with an inert substance insoluble in water, the solubility of TBP was found to be
decreased. The presence of salts in the aqueous phase also decreased the solubility
markedly. Similarly, the solubility of TBP decreased slowly with increasing nitric
acid concentration because of competing effects such as the formation of TBP-HNO 3
complexes. The solubility of water in TBP-diluent mixtures varied from 64 g/l in pure
TBP to about 0.06 g/1 in the pure paraffin-type diluent. A comparison of several
analytical methods used for determining the solubility of TBP in aqueous solutions
was also done.
Mason et al. (1954) have measured viscosity and density of the nitric acidnitrogen dioxide-water system at 0, 25 and 40°C for compositions in the range of 0100% HNO 3 , 0-20% NO 2 and 0-5% H 2 O. The viscosity of HNO 3 solutions increases
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4. Physical properties study for TBP-HNO3-dodecane system
with an increase in NO 2 or H 2 O, and the density increases with increase in NO 2 and
decreases with an increase in H 2 O.
Higgins et al. (1959) have studied the effect of different electrolyte and
temperature ranging from 5 to 50°C on the solubility of TBP in water. The
Setschenow equation has been used to calculate the salting coefficient, k s . The slope
of k s vs. 1/T was found to be linear for most of the salts studied. This indicated that
the term λ/D o in the limiting Debye-McAulay equation is constant and independent of
temperature where λ is effect of organic solute on D o , the dielectric constant of water.
The salting coefficients at 25°C for all the electrolytes tested, excepting nitrates, have
been correlated with the Gurney unitary partial molal electrolyte entropy concept.
Tuck (1960) has carried out experiments to measure viscosity of extract
solutions of HNO 3 in TBP and TBP-water and TBP-anhydrous nitric acid mixtures
and discussed the interactions involved between like and unlike molecules. The
viscosity varies with the mole fraction of the species involved has been interpreted in
terms of strength of the interaction in the viscous-flow transition state. Since the
species involved have similar molar volumes, assumptions were made from the
viscosity to derive excess free energy of mixing at different mole fractions.
Bullock and Tuck (1963) have measured mutual solubilities of the two phase
water-TBP system. Nuclear magnetic resonance studies of solutions of water in TBP
showed that the structure of such solutions was more complex than suspected. The
results obtained were explained in terms of the formation of linear and chain polymers
of varying complexity. The model obtained has shown the dependence of the
solubility of water in TBP on both temperature and aqueous phase water acidity.
Hardy et al. (1966) have studied distribution of nitric acid between aqueous
phase and 100% TBP and measured the densities of both the phases at equilibrium.
Hala and Tuck (1970) have determined solubilities of lithium, sodium,
potassium, caesium and ammonium thiocynates and iodides and of rubidium
thiocyanate in TBP. They found that solubilities decreased with increasing cation size,
except for ammonium salts. The values of ammonium salts were high due to the
hydrogen-bonding between NH 4 + and the phosphoryl oxygen of the solvent.
Yamamoto (1987) has determined the solubility of nitric acid in the organic
phase by measuring the dielectric property of the system. The dielectric properties of
the organic phase i.e. 30% TBP-n-dodecane-HNO 3 -H 2 O system and HNO 3 -H 2 OUO 2 (NO 3 ) 2 system were measured with a concentric capacitance cell, for in-line
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4. Physical properties study for TBP-HNO3-dodecane system
HNO 3 monitoring in the organic phase. It was found that the variation in the dielectric
constant, caused by the variation in the extracted HNO 3 ,was markedly greater than
that caused by the same molar variations of H 2 O and UO 2 (NO 3 ) 2 .
The solubility of TBP in aqueous plutonium nitrate (PuN) solutions and in
highly radioactive liquid waste (HRLW) of PUREX nuclear fuel reprocessing has
been determined by Kuno et al. (1993). An empirical formula to derive the solubility
of TBP in PuN solutions in the range of 0~0.1M Pu and 1~8M HNO 3 was obtained.
The effect on nitric acid and temperature on the solubility of TBP in PuN solutions
was also investigated. The variation in solubility with nitric acid solutions suggested
that TBP dissolves by combining with H 2 O/HNO 3 . A linear plot of log S/S o vs. 1/T
was obtained which was found to be deviated at high temperature due to change in the
ionic form of Pu.
Swain et al. (1998) have measured viscosities and densities of different binary
liquid mixtures of TBP with benzene, toluene and o-xylene at 30, 35 and 40ºC. The
non-idealities reflected in mixture viscosities have been expressed in terms of excess
viscosities. A Redlich-Kister-type equation was fitted to the binary η-X-T data for
each system.
Tripathi and Ramanujam (2003) have examined the radiation-induced changes
in density and viscosity of 30% TBP-dodecane-nitric acid system. It was observed
that the increase in the density becomes significant with increasing nitric acid
concentration in the solvent, [HNO 3 ] TBP concentration, and absorbed radiation dose,
which concurrently leads to a much sharper increase in the viscosity of the solvent.
The extent of increase in the viscosity was found to be significantly enhanced by
gamma radiolysis and was a function of absorbed dose. Gas–liquid chromatography
(GLC) and infrared (IR) analysis of the treated solvent has revealed the radiationinduced polymerization and nitration of the hydrocarbon diluent which has resulted in
increased viscosity. They also found a considerable increase in the viscosity of the
solvent with the presence of small amount of radiolytic species remaining in the
solvent due to incomplete solvent purification.
Huang et al. (2008) have measured the kinetic viscosities for TBP-kerosenephosphoric acid-methyl isobutyl ketone system with content of P 2 O 5 in the range of
(0 to 17%) at temperature ranging from (10 to 50oC). They have proved that methyl
isobutyl ketone is better diluent for TBP then kerosene for purifying H 3 PO 4 in wet
process method.
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4. Physical properties study for TBP-HNO3-dodecane system
Wright and Hartmann (2010) have published a review paper by referring to
more than 100 publications on physical and chemical properties of TBP-diluent-nitric
acid systems. The data on the kinetics of degradation products of TBP reported by
different authors is also cited. The various studies carried out by different
investigators to measure physical properties like vapor pressure, solubility and density
of TBP and its degradation products at different temperatures in TBP-diluent-nitric
acid system have also been discussed in the review article.
Zheng et al. (2010) have determined viscosity of TBP-kerosene-phosphoric
acid system in the temperature range from (20 to 60oC) and mass fraction of H 3 PO 4 in
the range of (0 to 9%) and correlated them in well-developed regression equations.
According to the results, the mass fraction of TBP in the mixture of TBP+ kerosene of
82.8% is more appropriate for purifying H 3 PO 4.
Kumar et al. (2011) have estimated PVT properties of TBP using group
contribution method and from the obtained data Wagner constants for TBP in the
range of 273.15K to critical temperature have been calculated.
Velavendan et al. (2012) provided a complete data on solubility of 5 %, 20 %,
30 % and 100 % TBP in various nitric acid concentrations (0–15.7 M). The effect of
heavy metal ion such as uranium concentration on the solubility of TBP in 4 M HNO 3
and the effect of various fission products on the solubility of 30 % TBP in 3–4 nitric
acids have also been presented. The solubility study done for aqueous solutions of
fuel reprocessing shows that presence of heavy metal ions, fission products,
concentration of nitric acid has significant effect on TBP solubility in the aqueous
phase.
Gill et al. have estimated the solubility of TBP and di-2-ethylhexyl phosphoric
acid (D2EHPA) in different media of water, oxalic acid and sulphuric acid solutions
by determining phosphorus contents employing molybdovanadophosphoric acid
method, after digesting and solubilising them in nitric and perchloric acid.
Although a number of studies have been carried out on physical properties of
TBP, but the reported results are incomplete, insufficient and also includes many
discrepancies. The data on density, viscosity and interfacial tension for nitric acid
containing different concentrations of TBP in presence of diluent is not yet available.
Thus, there is a need to do a complete study of various physical properties of TBP in
presence of diluent at wide range of nitric acid concentrations.
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4. Physical properties study for TBP-HNO3-dodecane system
Hence, the main objective of this research work is to study different physical
properties for TBP-diluent-nitric acid system which will be helpful in carrying out
different extraction studies. Thus, physical properties like density, viscosity,
interfacial tension and solubility have been measured for TBP-nitric acid-dodecane
system using pycnometer, viscometer, pendant drop method and High Performance
Liquid Chromatography (HPLC) respectively. All these properties have been
measured in a wide range of nitric acid concentration and in presence of n-dodecane
as diluent in the present paper. A complete study of physical properties for 30% TBP
in dodecane - nitric acid system which is useful in PUREX process has also been
done.
4.3. Experimental Section:
4.3.1. Materials:
TBP and n-dodecane was supplied by Prabhat chemicals, Mumbai, India.
Their purity was around 97% and 95% respectively. 70% commercial grade nitric
acid used was supplied by SD Fine chemicals Ltd., Mumbai, India. Water used was
freshly prepared de-ionized water from Millipore Milli – Q 50. Acetonitrile used as
solvent for High Pressure Liquid Chromatography were analytical grade purchased
from Hi Media Ltd., Mumbai. All the reagents used during the study were of
analytical reagent grade.
4.3.2. Instrumentation:
4.3.2.1. Density Measurement:
The density ρ was measured using pycnometer having interchangeable PTFE
stopper with a bulb volume of 10 cm3. The internal volumes of the bottle were
calibrated with deionized water initially. The thoroughly cleaned and dried empty
pycnometer was then weighed on an electronic balance with the precision of 0.1 mg
and then filled with the experimental liquid. The pycnometer was properly cleaned,
dried, and weighed for each sample. The density was then determined from the mass
of the sample and the volume of the bottle. The same procedure was repeated for
measuring densities of different solutions containing TBP. The readings obtained
from triplicates were averaged. The absolute uncertainties in the density
measurements were estimated to be within 0.005 g/cm3. The uncertainties observed in
the measurement have also been represented graphically using error bars.
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4.3.2.2. Viscosity Measurement:
The viscosity µ of TBP in different nitric acid concentrations was measured
using a commercial Ostwald capillary viscometer of U tube type. The instrument was
initially calibrated with distilled water and then thoroughly cleaned, dried and filled
with experimental liquid and placed vertically to measure the viscosity of different
solutions. The viscosity was measured by sucking the liquid into the upper bulb and
then allowing it to flow down through the capillary into the lower bulb. The time
taken by the liquid to pass between two calibrated marks was recorded with an
electronic digital stop watch with high precision (0.01s). The viscosity of the liquids
was calculated by the Eq. (1).
where µ, ρ and t and µ w , ρ w and t w are the viscosity, density andflow time of the
mixtures and water respectively. Viscosity of different samples containing varied TBP
concentrations was measured using the same procedure. The viscosity values reported
in this paper were the means of at least three replicates with uncertainty of 0.02
mPa*s. Errors bars have also been shown in the graphical representation of the data.
4.3.2.3. Interfacial tension measurement:
The interfacial tension was measured using the pendant drop method. This
method determines interfacial tension between fluid-fluid interfaces from the shape of
the drop. The profile of a drop of liquid suspended in another is determined by the
balance between gravity and surface forces (Lin et al. 1995).
The organic phase which was already equilibrated with the aqueous phase was
taken in the experimental cell and syringe containing the aqueous phase was dipped
into it. A drop was then gradually allowed to form at the edge of the needle. The
dimension of the drop was measured using the photographic technique. The profile of
the drop formed during the interfacial tension measurement is shown in Fig. 4.1. The
interfacial tension was then calculated using the Eq. (2).
where σ is the interfacial tension, Δρ is the density difference, d e is the equatorial
diameter of the drop, g is the acceleration due to gravity, H is a correction factor
which is related to the shape- dependent factor of the pendant drop, S, defined by Eq.
(3).
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4. Physical properties study for TBP-HNO3-dodecane system
where, d s is the drop diameter measured horizontally at a distance d e away from the
apex of the drop (Arashiro and Demarquette 1999).
During experiments, consecutive photographs of the drop for measurement of
interfacial tension for each sample were taken until the mechanical equilibrium of the
drop was reached. The mechanical equilibrium was determined when four
consecutive measurements of interfacial tension from the drop profile varied by less
than 2%. The values obtained after analysis of two to four drops were averaged and
reported as the equilibrium interfacial tension value. The deviation in the results
obtained is ≤ ±3%. These deviations observed have also been represented graphically
using error bars.
ds
de
de
Fig. 4.1. Profile of the drop formed in pendant drop method
4.3.2.4. Solubility measurement:
The solubility of TBP in the aqueous phase was determined by equilibrating
TBP in the organic phase with the aqueous phase for around 4 h using magnetic stirrer
at room temperature. The mixture was then transferred in to the separating funnel and
then allowed to stand overnight at 30°C ± 2°C for complete phase separation.
Samples were removed from the aqueous (lower) layer using a warmed and
thoroughly cleaned 10ml pipette. The amount of TBP in the aqueous phase was
determined on HPLC as described in detail in Bajoria et al. (2011). The solubility of
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4. Physical properties study for TBP-HNO3-dodecane system
TBP in different concentrations of nitric acid (ranging from 0.3M to 4M) was
measured using this method. All the samples were analyzed three times to check the
reproducibility of the results. The solubility values obtained were precise within ±2
%. The deviations observed during measurement have been described graphically
using error bars which signify the uncertainties in the result.
4.4. Results and discussion:
The physical properties for TBP- dodecane- HNO 3 system before and after
30% TBP contact have been studied and the results obtained have been summarized
in Table 4.1.
Table 4.1. Study of physical properties for 30%TBP-dodecane-nitric acid system
Before contact
Properties
µ
Water
After Contact with 30%TBP
0.3M
3M
HNO 3
HNO 3
1.000
1.028
1.142
0.996
1.026
396.0
52.64
Water
0.3M
3M
HNO 3
HNO 3
1.019
1.030
1.190
1.098
1.017
1.028
1.145
374.0
287.0
247.0
208.0
198.0
53.26
55.34
44.11
45.77
47.32
(cP)
ρ
(g/cm3)
S
(mg/l)
σ
(mN/m)
The effect of TBP on the physical properties viz., density, viscosity, interfacial
tension and solubility on TBP-dodecane- HNO 3 system has also been determined and
discussed below.
4.4.1. Effect on density:
The effect of TBP on the density of TBP-dodecane-HNO 3 system has been
investigated. The study has been performed by varying the concentration of TBP in
the aqueous phase from 5 mg/l to 270 mg/l. It is observed that the density of solution
increases marginally with the concentration of TBP in the aqueous phase as shown in
Fig.4.2. This small variation in the density is because of very minute changes in TBP
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4. Physical properties study for TBP-HNO3-dodecane system
concentration in the aqueous phase. It is also found that density of solutions like
water, nitric acid increases after 30% TBP contact as shown in the Table 4.1. The
results obtained revealed that there is no substantial increase in densities after
contacting with 30% TBP. This is due to limited solubility of TBP in the aqueous
phase which is resulting in to small variation in the densities. The density of aqueous
phase is also found to be increased with nitric acid concentration as nitric acid is
heavier than water (Tripathi and Ramanujam 2003). The results obtained are in
agreement with that reported by Wright and Hartmann (2010). The densities of many
single and two phase systems viz. TBP-HNO 3 , TBP-dodecane, TBP-UO 2 (NO 3 ) 2 ,
30%TBP-dodecane-HNO 3 ,
30%TBP-dodecane-HNO 3 -UO 2 (NO 3 ) 2
have
been
reported in their review paper. It has been found that the density of aqueous phase
containing HNO 3 and HNO 3 /UO 2 (NO 3 ) 2 solutions decrease with temperature and
increase with [HNO 3 ] and ½UO 2 2+. The results obtained are also in concordance with
that reported by Schulz and Navratil (1984) for 30%TBP-kerosene-HNO 3 -H 2 O
system.
Density (g/cm3)
1.15
1.1
1.05
1
0.95
0
100
200
Conc. of TBP (mg/l)
300
water
3M nitric acid
4M nitric acid
Fig. 4.2. Effect of TBP on density of the aqueous solutions
4.4.2. Effect on viscosity:
The effect of TBP on the viscosity of TBP-dodecane-HNO 3 system has also
been studied. It is found that the viscosity increases slightly with the concentration of
TBP in the aqueous phase from 5 mg/l to 270 mg/l as shown in Fig.4.3. It has also
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4. Physical properties study for TBP-HNO3-dodecane system
been observed that viscosity increases to some extent after 30% TBP contact as
shown in Table 4.1. Viscosity is the ease with which the solution flows. As the
viscosity of TBP is greater than that of nitric acid, its presence in the aqueous phase
affects the viscosity of the aqueous solutions. The weak hydrogen bonding between
the P=O oxygen of TBP and hydrogen of water increases the resistance to flow and
thus, the viscosity increases (D. G. Tuck 1961). But due to limited concentration of
TBP the impact on viscosity is not very large. Hence, the viscosity of the aqueous
phase increases little after contacting with 30% TBP. It is even noticed that viscosity
increases with the concentration of nitric acid because nitric acid is more viscous than
water. Schulz and Navratil (1984) have also obtained similar values for viscosity for
30%TBP-kerosene-HNO 3 -H 2 O system. It has been described in their work that the
viscosity of 30% TBP-kerosene solution increases with nitric acid concentration in the
aqueous phase.
Viscosity (mPa.s)
1.3
1.2
1.1
1
0.9
0
50
100
150
200
250
300
Conc. of TBP (mg/l)
water
3M nitric acid
4M nitric acid
Fig. 4.3. Effect of TBP on viscosity of the aqueous solutions
4.4.3. Effect on Interfacial tension:
Interfacial tension between the organic and aqueous phases was measured in
presence of TBP. It was found that, the interfacial tension of dodecane contacted with
water or nitric acid increased marginally with an increase in the concentration of TBP
from 5 mg/l to 270 mg/l in the aqueous phase as shown in Fig.4.4. This is due to the
adsorption of TBP at the interface. The lone pair of electrons on oxygen atom of the
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4. Physical properties study for TBP-HNO3-dodecane system
phosphate bond is responsible for this adsorption. TBP acts as the surfactant and
reduces interfacial tension. But this decrease in the interfacial tension was not
substantial due to limited increase of TBP in the aqueous phase. It was also noticed
from Table 4.1. that the interfacial tension values decrease with 30% TBP contact.
The interfacial tension is high for pure dodecane/water and pure dodecane/nitric acid
systems. However, after the 30% TBP contact some amount of TBP is transferred into
the aqueous phase and hence, the interfacial value decreases due to its adsorption at
the interface. It has also been observed that, interfacial tension increases with the
concentration of nitric acid in the aqueous phase. It is assumed that adsorption of TBP
at the interface is less in presence of nitric acid and therefore, the interfacial values are
high. The interfacial data available in the literature also confirms the same. Schulz
and Navratil (1984) mentioned the interfacial tensions for different systems
containing TBP in their work and found that a few percent of TBP drops the
interfacial tension of TBP- diluent- water system. Nave et al. (2004) measured
interfacial tension of TBP-diluent system contacted with water or HNO 3 solutions of
increasing concentrations and found that the adsorption is governed by the ability of
the P=O groups to coordinate with water molecules. The P=O groups bound to HNO 3
are less surface active than water. Thus, the interfacial tensions are higher when acid
is present. Hence, this confirms that interfacial tensions decreases with the TBP
concentration in the aqueous phase and increases with nitric acid concentration in the
same phase.
Interfacial tension (mN/m)
60
55
50
45
40
0
100
200
Conc. of TBP (ppm)
300
Water
3M nitric acid
4M nitric acid
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4. Physical properties study for TBP-HNO3-dodecane system
Fig. 4.4 Effect of TBP on IFT between dodecane-aqueous solutions
4.4.4. Effect on solubility:
Solubility of TBP in the aqueous phase was measured in presence of different
concentrations of the nitric acid ranging from 0.3M to 4M. It has been observed that
the solubility decreases with the concentration of nitric acid in the aqueous phase as
shown in Fig.4.5. This is due to the presence of nitric acid which is the salting agent.
The presence of the nitric acid in the aqueous phase restricts the solubility of TBP and
therefore, it is salted-in the organic phase. Hence, solubility of TBP is a function of
nitric concentration and is inversely proportional to its concentration in the aqueous
phase. The solubility of TBP in water, 0.3M HNO 3 and 3M HNO 3 was also measured
after 30% TBP contact. The result obtained reveals that, presence of dodecane which
is diluent also reduces the solubility of TBP in the aqueous phase. It is also found
from Table 4.1. that the solubility of TBP is lowest in 3M HNO 3 due to presence of
nitric acid in the aqueous phase in higher amount. Thus, the result obtained concludes
that the solubility of TBP in the aqueous phase decreases after 30% TBP contact.
Alcock et al. (1956) have also obtained similar results and measured solubility of TBP
in water and nitric acid in presence of different diluents. It has been mentioned that,
TBP is less soluble in the aqueous nitric acid solutions than water. Thus, its effect on
most of the physical properties of the aqueous phase will be negligible. The literature
survey also reveals the evidence for the solubility of TBP in water. The infra-red
spectra shows that when water is added to pure TBP the peak due to the P=O group at
1283cm-1 decrease while that at 1267cm-1 increase. This shift of 16cm-1 suggests
weak hydrogen bonding of the P=O oxygen with the hydrogen of water. It was also
observed that the solubility of TBP in water decreases with the presence of diluent
like kerosene. Higgins et al. (1959) determined the solubility of TBP and studied the
effect of electrolytes on it. The solubility of TBP in water was found to be decreased
with the presence of electrolytes like nitric acid and hence, termed them as the salting
agent for TBP. Kuno et al. (1993) observed that the solubility of TBP in the
concentration of nitric acid was dependent upon the concentration of nitric acid in the
aqueous phase. Wright and Hartmann (2010) also described in their review paper that
TBP solubility decreases with HNO 3 concentration.
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4. Physical properties study for TBP-HNO3-dodecane system
500
Conc. of TBP (mg/l)
400
300
200
100
0
0
1
2
3
4
5
Conc. of HNO3 (mol/l)
Fig. 4.5. Effect of nitric acid concentration on the solubility of TBP in the
aqueous phase
4.5. Conclusions:
The effect of TBP on physical properties of dodecane- nitric acid system has
been successfully studied. The results obtained reveal that density and viscosity
increases but interfacial tension and solubility decreases with the concentration of
TBP in dodecane-nitric acid system. Physical properties measured for 30%TBP-nitric
acid-dodecane system will be useful in PUREX process. The present study will be
useful in the field of nuclear reprocessing involving TBP as the solvent.
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