physical properties study for tbp-hno3-diluent...

Chapter 4

Physical properties study for TBP-HNO3-diluent

system

4. Physical properties study for TBP-HNO3-dodecane system

Studies in Extraction of tri-n-butyl phosphate form aqueous streams 80

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-HNO3

Mason et al. (1954) have measured viscosity and density of the nitric acid-

nitrogen dioxide-water system at 0, 25 and 40°C for compositions in the range of 0-

100% HNO

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.

3, 0-20% NO2 and 0-5% H2O. The viscosity of HNO3 solutions increases



with an increase in NO2 or H2O, and the density increases with increase in NO2 and

decreases with an increase in H2

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

O.

s. The slope

of ks vs. 1/T was found to be linear for most of the salts studied. This indicated that

the term λ/Do in the limiting Debye-McAulay equation is constant and independent of

temperature where λ is effect of organic solute on Do

Tuck (1960) has carried out experiments to measure viscosity of extract

solutions of HNO

, 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.

3

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.

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.

Hardy et al.

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

(1966) have studied distribution of nitric acid between aqueous

phase and 100% TBP and measured the densities of both the phases at equilibrium.

4+

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

and the phosphoryl oxygen of the solvent.

3-H2O system and HNO3-H2O-

UO2(NO3)2 system were measured with a concentric capacitance cell, for in-line



HNO3 monitoring in the organic phase. It was found that the variation in the dielectric

constant, caused by the variation in the extracted HNO3,was markedly greater than

that caused by the same molar variations of H2O and UO2(NO3)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 H2O/HNO3. A linear plot of log S/So

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.

vs. 1/T

was obtained which was found to be deviated at high temperature due to change in the

ionic form of Pu.

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, [HNO3]TBP

Huang et al. (2008) have measured the kinetic viscosities for TBP-kerosene-

phosphoric acid-methyl isobutyl ketone system with content of P

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 radiation-

induced 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.

2O5 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 H3PO4 in wet

process method.



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 H3PO4 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 H3PO

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.

4.

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 HNO3

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.

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.

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.



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.



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 µw, ρw and tw

4.3.2.3. Interfacial tension measurement:

are the viscosity, density and flow 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.

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, de 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).



where, ds is the drop diameter measured horizontally at a distance de

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.

away from the

apex of the drop (Arashiro and Demarquette 1999).

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

ds

de

de



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- HNO3

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 After Contact with 30%TBP

Properties Water 0.3M

HNO

3M

3 HNO

Water

3

0.3M

HNO

3M

3 HNO3

µ

(cP)

1.000 1.028 1.142 1.019 1.030 1.190

ρ

(g/cm3

0.996

)

1.026 1.098 1.017 1.028 1.145

S

(mg/l)

396.0 374.0 287.0 247.0 208.0 198.0

σ

(mN/m)

52.64 53.26 55.34 44.11 45.77 47.32

The effect of TBP on the physical properties viz., density, viscosity, interfacial

tension and solubility on TBP-dodecane- HNO3

4.4.1. Effect on density:

system has also been determined and

discussed below.

The effect of TBP on the density of TBP-dodecane-HNO3 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



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-HNO3, TBP-dodecane, TBP-UO2 (NO3)2,

30%TBP-dodecane-HNO3, 30%TBP-dodecane-HNO3-UO2(NO3)2 have been

reported in their review paper. It has been found that the density of aqueous phase

containing HNO3 and HNO3/UO2(NO3)2 solutions decrease with temperature and

increase with [HNO3] and ½UO22+. The results obtained are also in concordance with

that reported by Schulz and Navratil (1984) for 30%TBP-kerosene-HNO3-H2

O

system.

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-HNO3 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

0.95

1

1.05

1.1

1.15

0 100 200 300

Den

sity

(g/c

m3 )

Conc. of TBP (mg/l)

water3M nitric acid4M nitric acid



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-HNO3-H2

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.

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

0.9

1

1.1

1.2

1.3

0 50 100 150 200 250 300

Vis

cosi

ty (m

Pa.s)

Conc. of TBP (mg/l)water3M nitric acid4M nitric acid



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 HNO3 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 HNO3

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.

40

45

50

55

60

0 100 200 300

Inte

rfac

ial t

ensi

on (m

N/m

)

Conc. of TBP (ppm)Water3M nitric acid4M nitric acid



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 HNO3 and 3M HNO3 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 HNO3 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 HNO3 concentration.



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.

0

100

200

300

400

500

0 1 2 3 4 5

Con

c. o

f TB

P (m

g/l)

Conc. of HNO3 (mol/l)

physical properties study for tbp-hno3-diluent...

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