Determination of Di-n-Butyl Phosphate in Organic Streams of FBTR Mixed Carbide Fuel Reprocessing Solution by Gas Chromatographic Technique

doi:10.4236/jasmi.2011.12005

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Journal of Analytical Sciences, Methods and Instrumentation, 2011, 1, 31-36

doi:10.4236/jasmi.2011.12005 Published Online December 2011 (http://www.SciRP.org/journal/jasmi)

Determination of Di-n-Butyl Phosphate in Organic

Streams of FBTR Mixed Carbide Fuel Reprocessing

Solution by Gas Chromatographic Technique

P. Velavendan, S. Ganesh, N. K. Pandey, U. Kamachi Mudali, R. Natarajan

Reprocessing Group, Indira Gandhi centre for Atomic Research, Kalpakkam, India.

Email: [email protected]

Received September 20th, 2011; revised October 25th, 2011; accepted November 16th, 2011.

ABSTRACT

The present work describes the amount of Di-n-butyl phosphate (DBP) produced when PUREX solvent (30% tri-n-butyl

phosphate (TBP) mixed with 70% hydrocarbon diluent) is exposed to intensive radiolytic and chemical attack during

the separation of uranium and plutonium from fission products of FBTR mixed carbide fuel reprocessing solution. DBP

is the major degradation product of Tri-n-butyl phosphate (TBP). Amount of DBP formed in the lean organic streams of

different fuel burn-up FBTR carbide fuel reprocessing solutions were analyzed by Gas Chromatographic technique. The

method is based on the preparation of diazo methane and conversion of non-volatile Di-n-butyl phosphate in to volatile

and stable derivatives by the action of diazomethane and then determined by Gas Chromatography (GC). A calibration

graph was made for DBP over a concentration in the range from 200 to 1800 ppm with correlation coefficient of

0.99587 and RSD 1.2%. The degraded 30% TBP-NPH solvent loaded with heavy metal ions like uranium was analyzed

after repeated use and results are compared with standard ion chromatographic technique. A column comparison study

to select of proper gas chromatographic column for the separation of DBP from other components in a single aliquot of

injection is also examined.

Keywords: Gas Chromatography, Flame Ionization Detector, Diazomethane, Di-n-Butyl Phosphate, PUREX Process,

Degradation of TBP, Lean Organic Streams, Carbide Fuel

1. Introduction

Tributylphosphate (TBP) is the most popular reagent in

liquid-liquid extraction. Di-n-butyl phosphate (DBP) is

one of the degradation products of Tri-n-butyl phosphate

(TBP) ((C4H9O)3P = O), used in the well known PUREX

(Plutonium Uranium Refining by Extraction) process [1]

for the separation of uranium and plutonium from fission

products in nuclear fuel reprocessing. During the solvent

extraction process the solvent TBP undergoes degrada-

tion by hydrolysis and radiolysis yielding mainly Di

-n-butyl phosphate (DBP) ((C4H9O)2P = O-(OH)), to a

lesser extent monobutylphosphate (MBP) ((C4H9O)P = O-

(OH)2), phosphoric acid and butanol [2]. The DBP com-

plexes with Zr, Pu and other metal ions [3] which leads

to loss of heavy metals to lean organic stream. Hence the

quality of PUREX solvent is generally measured in terms

of the concentration of DBP. Various analytical methods

were reported in the literature for the determination of DBP.

Bocek et al. [4] have used high-speed Isotachophoresis, a

form of capillary electrophoresis with conductivity detec-

tion, to analyze TBP for its degradation products in solu-

tions containing nitrates and nitrites. Muller et al. [5] have

determined trace amounts of DBP and TBP in nuclear

fuel reprocessing solutions by Liquid Chromatography [6]

while Wilkinson and Williams [7] determined DBP and

MBP by direct titration of irradiated TBP samples. The

latter method fails in presence of nitric acid. Krishna-

murthy and Sampathkumar [8] have used titrimetry to

determine DBP and MBP as degradation products in the

two component TBP-nitric acid system. Grant et al. [9]

have performed the separation and measurement of TBP,

DBP and MBP by Ion-pair Chromatography with refrac-

tive index detection. Infra Red Spectroscopy is used [10]

for determination of DBP concentration in the solvent

based on the P=O absorption at 1230 cm–1 with a poor

detection limit due to interferences from the TBP. Tech-

niques [11] of ionization at atmospheric pressure, i.e., elec-

trospray (ESI) or atmospheric pressure chemical ionization

(APCI) with Mass Spectrometry (MS) have been used

Determination of Di-n-Butyl Phosphate in Organic Streams of FBTR Mixed Carbide Fuel Reprocessing Solution by

Gas Chromatographic Technique

for the direct quantification of MBP and DBP in a TBP

matrix, without any prior separation. Recently use of

ligand-sensitized Fluorescence Spectroscopy [12] is re-

ported for the determination of DBP in TBP/dodecane

solutions with Tb3+ as the fluorescent. This method is suit-

able for the determination of DBP over the concentration

of 0.1% - 10% DBP in TBP. Among the various methods

of analysis, application of chromatographic methods has

gained wider acceptance owing to their high sensitivity

and greater resolving power [13] Gas Chromatography

(GC) is the most widely used technique for the determi-

nation of DBP, mainly in the organic phase. Hardy et al.

[14] showed that DBP could be analyzed by gas chroma-

tography of the corresponding methyl esters formed by

reaction with diazomethane. Brignocchi et al. [15] devel-

oped the gas chromatographic method of Hardy [16,17]

into a quantitative procedure for the determination of

DBP and MBP in various organic streams [18,19]. Ex-

perimental details and results obtained by this method are

described in this paper

2. Experimental

2.1. Reagents

Tri-n-butyl phosphate (Merck) and Di-n-butyl phosphate

(Fluka). All other chemicals used were of AR or GR grade.

Diazomethane was prepared immediately before use

from a precursor in the laboratory fume hood and stored

in refrigerator. The precursor used in the present study is

N-methyl-N-nitroso-p-toluenesulfonamide (Diazald)

2.2. Instrumentation

A Shimadzu Gas chromatograph model 14 B equipped

with a Flame Ionization Detector (FID) was used to de-

tect the eluted components. Metrhom IC Net 2.3 software

is used for data acquisition. A stainless steel column of 4

meters long and 1/8’’ dia packed with 10% XE-60 was used

for the separation of components. Nitrogen gas with a flow

rate of 30ml/min was used as carrier gas. Hydrogen gas

and zero air with a flow rate of 30 and 300ml/min were

respectively used to generate flame. Column, Injector and

detector temperatures were 230˚C, 260˚C and 260˚C re-

spectively.

3. Gas Chromatographic Condition

Gas chromatograph: Shimadzu GC model 14 B

Analytical Column: Stainless steel packed column

Liquid phase: 10% XE-60

Nature of the column: Polar

Length: 4 meters

Diameter: 1/8”

Solid: Anakrom-ABS

Mesh range: 60/80

Maximum column

operating temperature: 250˚C

Column temperature : 230˚C

Injector temperature: 260˚C

Detector temperature: 260˚C

Fuel gas: Hydrogen

Supporting gas: Zero air

Carrier gas: Nitrogen

Carrier gas flow rate: 30 ml/min

Hydrogen gas flow rate: 30 ml/min

Zero Air flow rate: 300 ml/min

Detector: Flame Ionization Detector

Detector range: 2

Detector polarity: 1

Analytical mode: Isothermal

Injection volume: 5 μL

Run time: 15 min

Quantitation: Peak area

3.1. Preparation of Diazomethane

Diazomethane (CH2N2) is the most common methylat-

ing reagent for carboxylic acid and has found wide ap-

plication in the methylation of phenols, enols, and hetero-

atoms such as nitrogen and sulfur. Diazomethane is easy

to prepare and use. It is prepared immediately before use

from a precursor, after addition of base. The precursor used

in the present study is N-methyl-N-nitroso-p-toluenesul-

fonamide.

Preparation and handling of diazomethane requires

special precautions because it is a highly explosive gas at

room temperature. Diazomethane gas may explode vio-

lently even if it is diluted with nitrogen (OSHA, US De-

partment of Labor). Contact with sharp or rough surfaces,

or heat may cause diazomethane to explode. Diazome-

thane is a severe pulmonary irritant and causes coughing,

fever, fulminating pneumonia, and death on humans. High

velocity ventilation in the working area involving diazo-

methane is needed to guarantee minimum exposure to

diazomethane. Diazomethane was prepared by the use of

specially designed glassware kit (Aldrich Diazald Appa-

ratus) in order to avoid accidental explosion of CH2N2

with sharp or rough surfaces. A safety shield was placed

in the highly ventilated reaction hood to provide maxi-

mum isolation from exposure to diazomethane and pos-

sible explosion. The receiving flask was surrounded by a

NaCl-ice bath (33% NaCl by weight) giving a cool envi-

ronment at about –20˚C because diazomethane is a gas at

Determination of Di-n-Butyl Phosphate in Organic Streams of FBTR Mixed Carbide Fuel Reprocessing Solution by 33

Gas Chromatographic Technique

room temperature and liquifies at –23˚C (density 1.45),

and freezes at –145˚C. Ethanol (10 ml) was added into

KOH solution in the reaction flask, which was prepared

by dissolving potassium hydroxide (5 g) in water (8 ml).

The solution of Diazald (5.0 g Diazald in 45 ml ether)

was added in the reaction flask, and was heated to 65˚C.

The rate of addition of Diazald solution was approxi-

mately the rate of distillation. Some extra ether was ad-

ded in the reaction flask after the Diazald solution was used

up in order to trap the entire diazomethane in the receiv-

ing flask. The reaction was terminated when the distillate

become colorless. The strength of diazomethane was de-

termined Spectrophotometrically at 410 nm where its ex-

tinction coefficient, ε, is 7.2.

3.2. Procedure

Stock solution of DBP was prepared in n-dodecane and

from this stock solution various concentration of DBP was

diluted in n-dodecane and treated with diazomethane so-

lution which was prepared immediately before use from

a precursor. Solution was warmed at 60˚C using a water

bath. The ether and the excess diazomethane were evapo-

rated by the flow of nitrogen gas through the solution.

After the conversion of DBP into its methyl ester, solu-

tion was injected into Gas chromatograph using Hamilton

microliter syringe.









4922493 2

C H OPOOHCHNCH OPOOCHN 

DBP Diazomethane MDBP

Radioactive lean organic solutions of various burn-up

of FBTR mixed carbide fuel reprocessing solution was

washed with 2N sulphuric acid to remove any heavy me-

tal ions and then subsequently treated with diazomethane

before injection in to Gas chromatograph.

In another experiment degraded solvent of repeatedly

used 30% TBP-NPH phase loaded with 70 g/L concen-

tration of uranium was analyzed for its DBP content by

this procedure. 3 ml of organic phase was equilibrated

twice with 0.01 M nitric acid and subsequently washed

twice with 2N sulphuric acid to remove heavy metals.

Then 0.25 ml of organic phase was taken in a 5 ml stan-

dard flask and made up to the mark with n-dodecane.

From this 0.2 ml of aliquot was taken in a 15 ml centri-

fuge tube and followed the above procedure to determine

the concentration of DBP.

3.3. Studies on the Selection of Gas

Chromatographic Column for

DBP Separation

Studies were carried out to find best gas chromatographic

column for the separation of DBP from hydrocarbon

peaks (C10 to C14) and TBP in a single aliquot of inject-

tion. Columns of different stationary phases SE-30 and

XE-60 were taken for studies. Detailed column parame-

ters were described in Table 1

4. Results and Discussion

Diazomethane is an ideal derivatization reagent. The reac-

tion is fast, the yield is high, side reactions are minimal,

the by-product is nitrogen gas and reaction conditions are

very mild. Diazomethane is a yellow gas so the progress

of the reaction can be easily is followed. The reaction for

the conversion of DBP to methyl ester is outlined below:

Table 1. Comparison of Column parameters used for the

separation of DBP from other components.

Properties of Column XE-60 SE-30

Material S.S packed column S.S packed column

Liquid phase 10% XE-60 10% SE-30

Nature Polar Non-Polar

Length 4 meters 4 meters

Diameter 1/8” 1/8”

Solid Anakrom-ABS ChW/HP

Mesh range 60/80 60/80

Maximum

Temperature 250˚C 300˚C

Column temperature 230˚C 230˚C

Injector temperature 260˚C 260˚C

Detector temperature 260˚C 260˚C

Fuel gas Hydrogen Hydrogen

Supporting gas Zero air Zero air

Carrier gas Nitrogen Nitrogen

Carrier Gas flow rate 30 ml/min 30 ml/min

Hydrogen gas flow rate 30 ml/min 30 ml/min

Zero Air flow rate 300 ml/min 300 ml/min

Detector FID FID

Detector range 2 2

Detector polarity 1 1

Analytical mode Isothermal Isothermal

Injection volume 5 μL 5 μL

Run time 15 min 15 min

Quantitation Peak area Peak area

Determination of Di-n-Butyl Phosphate in Organic Streams of FBTR Mixed Carbide Fuel Reprocessing Solution by

Gas Chromatographic Technique

Two of the protons in the resulting methyl ester origin-

nate from the diazomethane. The other one is the donated

acidic proton from the DBP. A calibration graph (Figure

1) was made for DBP concentration in the range from

200 to 1800 ppm in n-dodecane with correlation coeffi-

cient of 0.99587 and RSD 1.2% and each standard was

esterified using diazomethane procedure as described above.

By using this calibration graph the concentration of DBP

present in the unknown samples were calculated. Table 2

gives results of gas chromatographic determination of DBP

and TBP in radioactive lean organic streams of various

fuel burn-up of FBTR carbide fuel reprocessing solution.

The table confirms that the TBP percentage remains more

or close to 30% and the concentration of DBP is fairly

high, its concentration depends on strength of the nitric

acid, concentration of plutonium and contact time during

the solvent extraction process. The higher DBP concen-

tration in the low burn-up fuel is probably due to delayed

stripping of heavy metal ions during the commissioning

of the reprocessing plant with low burn-up fuel. Figure 2

Figure 1. Typical calibration graph for the determination of

DBP by gas chromatographic technique after methylation

using diazomethane.

Table 2. Results of gas chromatographic determination of

DBP and TBP in lean organic streams of various fuel burn-

up of FBTR mixed carbide fuel reproce ssing solution.

S.No Fuel Burn-up

(MWd/T)

Nature of the

sample

% of TBP

determined

Conc. of DBP

(g/L)

1 Low burn-up Lean Organic29.971 2.654

2 25000-1 Lean Organic29.508 1.664

3 25000-2 Lean Organic29.383 2.387

01 23 4 56 7 8 910111213 min

500

1000

ch1

DBP

Figure 2. Typical Gas chromatogram of standard Di-n-

butyl phosphate (1. 659 g/L).

shows the typical gas chromatogram of standard DBP

(1.659 g/L) after methylation using diazomethane. Figure

3 refers the typical gas chromatogram of DBP and TBP

in radioactive lean organic solvent of FBTR carbide fuel

(25,000 MWD/T burn-up) reprocessing solution. In Fig-

ures 2 and 3 the DBP peak was well separated from hy-

drocarbon and TBP peaks respectively with the separa-

tion factor (α) greater than 1. Table 3 shows the results of

comparison of Gas Chromatographic and Ion chroma-

tographic methods from repeatedly used degraded 30%

TBP-NPH solvent loaded with uranium. Errors estimated

in the present gas chromatographic method is about 5%.

All the estimated values of different concentration range

of DBP were higher in GC than the estimated values of

DBP by IC. This positive error in GC estimation of DBP

may be due to pre-concentration of the solution during

the conversion of DBP into its methyl ester under hot

condition. The peak area obtained was compared with

standard calibration graph of DBP obtained by injecting

DBP standards every day after methylation using diazome-

thane. The relative standard deviation was 1.2% for the

repeated nine injections. Tab le 1 refers the comparison of

column parameters used for the separation of DBP from

other components. The column selection studies carried

out for the separation of DBP is reveal that the best col-

umn is XE-60 than SE-30. This is due to the polar nature

of column XE-60 separates the non polar hydrocarbon

(C-10 to C-14) peaks as a single peak from DBP and

TBP (Figure 2 and 3), hence there is no interference of

hydrocarbon peak for the separation, identification and

quantitative estimation of DBP in the degraded solvent

using XE-60. Whereas non polar nature of column SE-30,

hydrocarbons C10 to C14 (non polar) peaks were indi-

vidually well separated. It was found that use of column

SE-30 the retention time of peak C-14 (tetradecane, re-

tention time 4.5 min) and the retention time of DBP peak

were same. Therefore SE-30 column is not suitable for the

Determination of Di-n-Butyl Phosphate in Organic Streams of FBTR Mixed Carbide Fuel Reprocessing Solution by 35

Gas Chromatographic Technique

01 2 3 4 56 7 8 910min

200

400

600

ch1

DBP

TBP

Figur3.pical Gasra oB(1.664 g

able 3. Estimated values of DBP by present method and

Concentration of DBP(g/L) estimated by

e Ty chromatogmf DP /L) &

TBP (29.51%) in lean organic solution of FBTR mixed car-

bide (25,000 MWD/T Burn-up) reprocessing solution.

Ion chromatographic method.

Sample no Gas chromatography method Ion-chromatography

(described in this work)

method

1 14.71 14.42

2 7.82 7.45

3 2.63 2.41

aError the estimates by Gaatographic method is a.

ualitative and quantitative estimation of DBP in pres-

ntent in the radioactive lean organic

incere thanks to Shri S. C.

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5. Conclusions

Analysis of DBP co

streams of FBTR mixed carbide fuel reprocessing solu-

tions by gas chromatography method was demonstrated.

It is simple, only needs the conversion of DBP into its

methyl ester prior to injection. The major advantage of GC

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6. Acknowledgement

Authors wish to express their s

Chetal, Director, IGCAR for motivating them to excel.

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