Indian Journal of Chemistry Vol. 44A, March 2005, pp. 497-503

Solvent extraction, separation and recovery of lanthanum(III) and cerium(IV) from monazite sand by N-phenylbenzo-18-crown-6 hydroxamic acid

Y K Agrawal"*, S B Yorab & G Shah"

"Analytical Laboratory, Nirma University of Science and Technology, Ahmedabad 382 418, India Email: drykagrawal@yahoo.com

bGMDC Science and Research Centre, Khanji Bhavan, 132 ft. Ring Road, Vastrapur, Ahmedabad 380 052, India

Received 23 September 2004; revised 28 December 2004

A new N-phenylbenzo-18-crown-6-hydroxamic acid is reported for the separation of lanthanum(lll) and cerium(IV) from monazite. Lanthanum(lll) and cerium(IV) have been ex tracted and separated in presence of thorium, uranium and other associated cations at pH 8.8 and 9.5, respectively, in dichloromethane. Lanthanum(III) gives a colourless complex with the reagent which is extracted into dichloromethane and has molar absorptivity of 9.0 x I 03 L mo1" 1 cnf 1 at 385 nm. Cerium(IV) forms a red colour complex with the reagent extracted in dichloromethane with Amax 450 nm and molar absorp­ti vity 6.5 x 103 L mol" 1 em·'. For trace determination, the extracts are inserted directly into the plasma for ICP-AES meas­urements of La(lll) and Ce(!V) in the range of 4-80 ng mL·' and 8- 120 ng mL·' , respecti vely with the detection limit of 0.5 ng mL·'. The complexes of lanthanum(lll ) and cerium(IV) are very stable, (log f:hk 20.20 and 20.50, respecti vely) as com­pared to those of thori um ( 1.60), uranium ( 1.80), and other related cations. The proposed method of preconcentration and analysis of La(lll) and Ce(IV) is not adversely affected by highly ionic med ia, rendering the method suitable for their trace determination in sea water.

IPC Code: Int. Cl.7 GOlN; C07C 59/00; C22B 59/00

The analysis of cerium(IV), lanthanum(III) elements is important in several industries, e.g., nuclear power production and waste water analysis. Cerium occurs as a major element in the monazite sand and is associated with lanthanides, thorium and traces of uranium. It is used as a catalyst in automobile and nuclear chemistry. Monazite sand is mainly associated with Ce, La, Th, Y, Fe etc. Usually, it is treated with alkali followed by acid digestion and then extracted with tetrabutylphos­phate for separation of lanthanides. The process is time consuming and also requires further purification. A large number of reagents have been described for the separation of cerium(IV) and lanthanum(III) , However, several closely related ions interfere in the separation and determination of cerium and lantha­num1 ·5. Macrocyclic compounds have also been re­ported for the complexation of lanthanides . However,

. . h b d "b dG-IO no systematic separatton as een escn e . Moreover, these reagents are less sensitive and non­specific and a large number of diverse ions including thorium and uranium interfere with the determination of lanthanum(lli) and cerium(IV). It also requires prior ion exchange separation. Atomic absorption spectro­photometry and ICP-AES cannot be used directly for trace determination of cerium and lanthanum as these

have high detection limits11.12 of 10 !lg mr1 with prior

ion exchange separation 13'14. Also, the photomultiplier receives high background radiation that can cause fa­tigue of the photo responsive surface of the cathode even though the continuous radiation is not amplified. The complexities of cerium and lanthanum emission lines with a large number of strong ionic lines cause some difficulties over line selection, causing overlaps and reduce the detection limits considerably'5·16. The very low levels at which the cerium and lanthanum are sought and potential interference from major elements, have led to the use of separation method to remove bulk of the major elements and at the same time con­centrate cerium and lanthanum into a small volume of solution. Keeping this in view, a new crown hydrox­amic acid, N-phenylbenzo-18-crown-6 hydroxamic acid (PBCHA) has been tested for the extraction and separation of cerium(IV) and lanthanum(Ill) from monazite sand in presence of several diverse ions. The extraction removes bulk of the major elements and at the same time concentrates the cerium(IV) and lantha­num(III) into a small volume of solution with high purity. The extracts are directly inserted into the plasma for ICP-AES determination, which increases the sensitivity and detection limits several folds.

498 INDIAN J CHEM, SEC A, MARCH 2005

Materials and Methods

Al l the chemicals used were of AnalaR grades of BDH orE Merck.

A !.Ox I o-3 M standard cerium(IV) solution was prepared by dissolving 0.4045 g cerium sulphate in a litre of doubly di stilled water (in the presence of 5-10 mL H2S04) . The cerium(IV) content was determined by EDTA titrati on and spectrophotometri cally 17

•18

•

Standard lanthanum so lution was prepared by dis­solving 0.276 g of lanthanum acetate hydrade in a li tre of doubly distilled water in the presence of HCl. Its final concentration (7.20xl0-3 M) was determined by spectrophotometry and EDTA titrati on17

.

The reagent, N-phenylbenzo- I 8-crown-6-hydrox­ami c ac id was synthesized by reacting ac id chloride of benzo-1 8-crown-6 at low temperature (0°C) in di­ethylether containing aquous suspension of sodium bicarbonate19

. Its 4.5x l0·3 M solution in acetone was prepared.

Absorption spectra were recorded on Hitachi 32 10 spectrophotometer with 10 mm quartz cell. The pH measurements were made with a Radi ometer pH me­ter PHM 94 using combined glass and calomel elec­trodes. Plasma Scan (Model 7 I 0) Sequential I nduc­tively Coupled Plas ma Atomic Emission Spectro­photometer (ICP-AES) with Plasma Scan multitask­ing computer and peri staltic pump was used.

The following operating conditi ons were set for ICP- AES: Rf 27.12 MHz; incident power 2000 watts; GMK nebuli zer; sample concentration I ng mr 1

; RF power 5 watts; observation height 14 mm; argon coo lant fl ow rate, 10 L min-1

; argon carri er fl ow rate 1 L min-1

; intergraph period 10 s; resolution 0.004 mm ; peri staltic pump fl ow rate I mL min-1 and wavelength 394.9 I nm and 4 I 8.66 nm, respectively for lanthanum and cerium.

Extraction procedure

An aliquot of lanthanum(III ) or cerium(IV) solu­tion containing (19.25-450 or 5.626 11g, respectively) was transferred into a 60-mL separatory funn el. Then I 0 mL of the reagent PBCHA so lution was added and p H was adjusted to 8.5 or 9.5 with 10 mL of buffer solu tion. The contents were shaken with I 0 mL of dichloro methane fo r 2 min and the organi c phase was all owed to separate. The organi c phase was dried over anyhydrous sod ium sulphate and transferred into a 25 mL volumetric fl ask to ensure complete recovery of lan thanum(III ) or cerium(IV). The ex tracti on was re-

peated by adding 5 mL of PBCH A along with 5 mL of dichloromethane. The sodium sulphate was washed with 2x2 mL dichloromethane and the combined ex­tracts and washings were diluted up to the mark with dichloromethane. The absorbance of the ex tract was measured against the reagent bl ank .

For ICP-AES determination, the lanthanum(II l) or cerium(IV) PBCHA dichloromethane extract was in­troduced into the plasma with a peristaltic pump after appropriate diluti on with dichloromethane.

The values of percentage ex traction (% £) and the distribution rati o (D) of lanthanum(Hl) and ce­rium(IV) were calculated at different p Hs and PBCHA concentrations.

Results and Discussion

The lanthanum(III)-PBCHA complex has max i­mum absorbance at 385 nm, and reage nt blank does not absorb at this wavelength. It obeys Beer's Law in the range of 0.77-18.00 ).lg mL-1 of lanthanum(m ) with Sandell sensiti vity 0.015 11g cm·2 and molar ab­sorptivity 9.0x l03 L tl)or 1 cm·1

• The regression analy­sis represents Cone. = l5 .33xabsorbance with corre­lation (r=0.999).

The red coloured cerium(I V)- PBCHA complex has maximum absorbance at 450 nm where the reagent blank does not absorb. The system obeys Beer's law in the range of 0.2-25 11g mL-1 of ccrium(IV) with sensitivity 0.02 ).lg cm·2, and molar absorpti vity 6.5x 103 I mor 1cm·1

• The regress ion analysis repre­sents cone. = 2 1.55xabsorbance with r = 0.999.

Under the optimum conditions for the determina­tion of lanthanum(III) and cerium(IV) by ICP-AES, a linear calibration graph was obtained between 4 and 80 ng mL-1 for lanthanum([Il) and 8-1 20 ng mL-1 for cerium(IV). The determination of standard solution of lanthanum(III ) and cerium(IY) containing 10 ng mL-1

metal ion gave a relative standard dev iation 2. 1% and 1.9%, respecti vely with a detection limit of 0.5 ng mL-1•

Effect of pH on extraction

The optimum pH for max imum extrac tion was de­termined by carrying out the extraction with varyi ng concentrations of lanthanum(III ), cerium(IV) and PBCHA. The pH of the aqueous phase was varied using diffe rent buffer solutions. The ex tracti on of lanthanum(lll) or cerium(IV) increased with the in­crease in pH unti l it levelled off at pH 8.8 for Iantha-

AGRAWAL eta/.: EXTRACTION OF La(IJI) AND Ce(IV) BY PBCHA 499

num(IIl) and at pH 9.8 for cerium(IV). Thus, the op­timum pH for efficient extraction lies within the range of 8.2-8.8 and 9.0-9.8 for lanthanum(III) and ce­rium(IV), respectively. The low extractability at lower pH values may be attributed to the proton extraction to organic phase rather than the metal ion itself. How­ever, increase in pH facilities the cerium(IV) to hy­drolyse which form more stable complexes.

Effect of reagent concentration

The effect of PBCHA concentration on the extrac­tion of lathanum(III) and cerium(IV) were found by varying their concentrations. It was found that 10 mL of 4.5xl0·3 M solution in dichloromethane was quite adequate for complete extraction of lanthanum(III) and cerium(IV) . Lower concentrations of PBCHA reduce the percentage extraction, while an excess of reagent can be used without any adverse results.

Effect of solvent

The extraction was carried out with various sol­vents, viz., dichloromethane, chloroform, iso-amy I alcohol , iso-amyl acetate, toluene, carbon tetrachlo­ride and, octanol. It was found that dichloromethane is the most suitable solvent for the quantitative ex­traction of lanthanum(III) and cerium(IV).

Stoichiometry of the complex

The extraction of lanthanum(III) and cerium(IV) with N-phenylbenzo-18-crown-6-hydroxamic acid (PBCHA) may be represent as

La(OHh + 3 HA ~La3 A3 + 3H20; Ce(OH)4 + 2 HA ~(CeOA2) + 3H20

where HA is PBCHA

The stoichiometry of the complex was determined by slope ratio by plotting a graph of log of lanthanum and cerium distribution (log DM) against the pH and also by mole ratio method, which give lanthanum: PBCHA ratio as I :3 and cerium:PBCHA ratio as 1:2. The equilibrium between an aqueous solution con­taining lanthanum(III) or cerium(IV) and PBCHA can be represented as

mLa 3+ + pA <orgJ ~ [Lam Ap]org

laq) (1)

mCe(OH)(aqJ + pA<o•g> ~ [Cem 0 Ap]org + 3H20 (2)

where the subscript org refers to the dichloromethane phase and charges are omitted for simplicity . The ex­traction constant can be represented as follows:

Ke = [Lam 0 Ap]org I [La]111

[A] ~rg

or

Kc = [Cem 0 Ap]org I [CeO]"' [A] ~rg

(3)

. . . (4)

Assuming that only one species 1s formed in a given reagent concentration, then

Thus,

log A = m log [La]+p log[A] =log ft

log A = m log [CeO] + p log [A] + log E.t

. . . (5)

(6)

(7)

For La(III), m= 1.05 and p=3.0 1, while for Ce(IV), m = 1.01 and p = 1.77. The two phase stability con­stants can be obtained as

log ~zK'e =log Kex = n pKa + n logKe . .. (8)

where log ~2K'e and Kex are the respective overall formation and distribution constants respectively .

The calculated values of log ~2K'e and log Kex are 20.20 and 1.20x10-9 respectively for lantha­num(lll), and 20.50 and 2.0xl0-9, respectively force­rium(IV).

The extracted lanthanum(III) and cerium(IV) PBCHA complexes in dichloromethane were evapo­rated and the isolated solid complexes were analysed for elemental analysis. The most probable species formed are [La A3] and [CeO A2]. For [LaA3] Found (calc.) %; C: 56.09(56.16); H: 5.58(5.56) ; N: 9.43(9.41); La: 9.43(9.41) and for [CeOA2]; C: 52.73(52.21); H: 5.18(5.20); N: 2.69(2.67) and Ce: 13.39(13.38).

Effect of diverse ions

The effect of various cations and anions in the separation and determination of lanthanum(III) and cerium(IV) was studied. Interference studies were carried out by measuring the absorbance of the ex­tracted organic phase and also by ICP-AES analysis of both extract as well as the aqueous phase. The tol­erance limit was set as the amount of foreign ion

causing a change of ±0.02 absorbance or 2% error in the recovery.

Lanthanum(III) or cerium(IV) was extracted in the presence of a large number of competitive ions at pH 8.5 and pH 9.5, respectively and none of them

500

For­eign ion

C 2+ u

INDIAN J CHEM, SEC A, MARCH 2005

Table !-Effect of diverse ions in estimation lanthanum(lll) and cerium(IV) 15 )lg mL- 1 [pH: 8.5 for La(III) and 9.5 for Ce(IV);

PBCHA: 10 ml (4.5x I o·3 M) ; Solvent: Dichloromethane; Amux: 385 nm for La(III) and 450 nm for Ce(IV)]

Added as

Pb(N01h

SnCI 2

SrCI2

BaCI2

Amount added,

mg

40

40

40

35

40

30

40

45

40

40

30

40

35

45

45

45

40

40

40

40

log K" . M

La( III) Ce(IY)

20.20

20.50

0.20 0.50

0.20 0.50

1.00 1.00

1.15 1.15

1.10 1.10

1.18 1.15

1.20 1.20

1.00 1.00

1.25 1.25

1.10 1.10

1.00 1.0

1.00 1.00

1.30 1.30

1.05 1.50

1.20 1.20

1.25 1.25

2.00 2.00

1.00 2.50

1.20 1.30

1.18 1.20

La(lll) Ce(IV)

1.0

1.0

1020.00

101 9.50

1019.02

101 8.50 1019.20

Metal ion found, )lg mL-1

La( III)

Spectra- ICP-photo- AES me try

14.99 15.003 ±0.02 ± 0.005

15.03 ±0.04

14.98 ±0.02

14.97 ±0.04

15.05 ±0.05

15.02 ±0.03

15.03 ±0.03

15.03 ±0.04

14.98 ±0.03

14.97 ±0.05

14.97 ±0.05

14.98 ±0.04

15.03 ±0.04

15.05 ±0.05

15.01 ±0.04

14.98 ±0.03

14.98 ±0.04

14.97 ±0.04

15 .01 ±0.03

14.98 ±0.03

15.Q2

±0.03

15.008 ± 0.003

15.005 ± 0.005

14.998 ± 0.004

14.995 ± 0.006

15.003 ± 0.008

15.007 ± 0.008

15.005 ± 0.006

14.999 ± 0.002

15 .003 ± 0.005

14.996 ± 0.005

14.995 ± 0.006

14.993 ± 0.008

15.003± 0.004

14.995± 0.006

14.997 ±0.005

14.997 ±0.005

15 .005 ±0.007

14.993 ±0.008

15.000 ±0.004

15.005 ±0.006

Ce(IY) Spectra- ICP-photo- AES me try

15.01 ± 0.03

i5.03 ±0.04

14.98 ±0.02

14.97 ± 0.04

15.05 ± 0.05

15.02 ± 0.03

15.03 ±0.03

15.03 ± 0.04

14.98± 0.03

14.97 ± 0.05

14.97 ±0.05

14.98 ± 0.04

15.03± 0.04

15.05 ± 0.05

15.0 1 ±0.04

14.98 ±0.03

14.98 ±0.04

14.97 ±0.04

15.01 ±0.03

14.98 ±0.03

15.Q2

±0.03

14.998 ±0.005

15.008 ±0.003

15.005 ±0.005

14.998 ±0.004

14.995 ±0.006

15.003 ±0.008

15.007 ±0.008

15.005 ± 0.006

14.999 ± 0.002

15 .003 ± 0.005

14.996 ± 0.005

14.995 ± 0.006

14.993 ± 0.008

15.003 ± 0.004

14.995 ± 0.006

14.997 ±0.005

14.997 ±0.005

15.005 ±0.007

14.993 ±0.008

15.000 ±0.004

15.005 ±0.006

(Contd)

AGRAWAL eta!. : EXTRACTION OF La( III ) AND Ce(IV) BY PBCHA 501

Table !- Effect of diverse ions in estimation lanthanum(III ) and cerium(IV) 15 !!g mL-1 [pH: 8.5 for La(III) and 9.5 for Ce(IY); PBCHA : 10 ml (4.5x 10·3 M) ; Solvent : Dichloromethane; Am•x: 385 nm for La( III) and 450 nm for Ce(IV)]-Contd

For- Added as Amount Io cr K" KM Metal ion found , !!g mL·' eign added, o M+

lOll mg La( III) Ce(IV) La(III) Ce(IV) La(III) Cc(IY) Spectra- ICP- Spectra- ICP-photo- AES photo- AES

• me try me try

ln3+ InC I3 40 1.20 1.45 1019.35 1019.05 15.03 15.004 15.03 15.004 ±0.04 ±0.005 ±0.04 ±0.005

Ce4+ Ce(S04h 35 1.70 101855 14.98 14.994 ±0.03 ±0.005

La3+ La(N03)3 40 1.60 1018.90 15.03 15.003 ±0.05 ±0.005

Th4+ Th(N03)4 50 1.60 1.60 101 8.60 101 8.90 15.03 15.005 14.98 14.993 ±0.04 ±0.006 ±0.05 ±0.007

u6+ UOiN03h.6H20 50 1.80 1.60 1018.40 1018.70 15.05 14.997 14.96 14.998 ±0.05 ±0.005 ±0.05 ±0.006

Gd3+ Gd(N03)J.6H20 50 1.65 1.80 1018.55 1018.90 14.97 14.993 15.03 14.996 ±0.04 ±0.007 ±0.04 ±0.008

Pr3+ Pr(N03)J.6H20 50 1.50 1.50 101 8.70 1019.00 14.96 15.002 15.01 14.998 ±0.04 ±0.004 ±0.03 ±0.005

Nd3+ Nd(N03)J.6H20 50 1.58 1.65 101 8.62 1018.85 14.99 15.008 15.04 14.995 ±0.03 ±0.005 ±0.03 ±0.007

y5+ NH4Y03 35 1.85 2.0 1018.35 101 8.50 14.99 14.995 14.99 14.995 ±0.02 ±0.005 ±0.02 ±0.005

Cl+ NH4Cl 45 0.70 0.50 1019.50 1019.00 15.01 15.001 15.01 15.001 ±0.02 ±0.003 ±0.02 ±0.003

No3· NH4N03 45 0.65 0.50 1019.35 1019.00 15.0 1 15.002 15.01 15.002 ±0.02 ±0.003 ±0.02 ±0.003

F NH4F 45 0.85 0.5 1019.35 1019.00 15.01 15.001 15.01 15.001 ±0.02 ±0.003 ±0.02 ±0.003

so/· (NH4h(S04) 45 0.75 0.5 1019.45 1019.00 15.01 15.001 15.01 15.001 ±0.02 ±0.003 ±0.02 ±0.003

Table 2-Determination of lanthanum(III) and cerium(IV) in standard geological samples and monazite sand

Sample Cert.values, mg Kg·' Metal ion found, mg Kg·'

La(III) Ce(IV) La( III) Ce(IV) Spectrophotometry ICP-AES Spectrophotometry ICP-AES

BCR-1 (52/19) 26.0 54.00 25 .8±0.2 26.002±0.005 53.95±0.08 54.001±0.003

AGY-1 (74/19) 35.0 35.05±0.01 34.985±0.015

GSR-1 (17/22) 190.0 190.8±0.05 189 .858±0.050

G2 96.0 95.9±0.01 95 .983±0.008

G2 (108/15) 150.0 150.05±0.08 149.995±0.008

PCC-1 (2/ 1) 0.20 0.21±0.05 0.200±0.008

WI 9.8 25.0 9.79±0.01 9.82±0.04 25.95±0.09 24.998±0.005

Monazite" 24.0 27.23 23 .98±0.02 23.999±0.009 27.20±0.08 27.238±0.008

S02 -Soil 46.0 115.0 45 .98±0.05 46.002±0.005 114.50±0.50 113.987±0.090

GSP-1 Granodiorite 180.0 423.0 180.05±0.05 179.980±.008 423.82±0.50 423.005±0.080 NBS 1632a coal 14.7 30.60 14.69±0.05 14.713±0.005 30.48±0.09 30.6 15±0.008

•values in %.

502 INDIAN 1 CHEM, SEC A, MARCH 2005

affected the absorbance of lanthanum(III) or ce­rium(JY) complexes. This shows that Kex val ues are

greater than K~. for the competing metal cations

which are determined independently at pH 8.5 and 9.5 for lanthanum(III) and cerium(IY), respectively. The results obtained (Table l) show that selectivity factor,

Ku, (KLa =~zK'e/ K~.) or ~2 Kef K~- for lantha­

num(III) complex or Kcc for cerium has a high se­

lectiv ity with most of the cations and anions. ln order to examine the suitability of the present method, lan­thanum(III)/cerium(IV) was extracted in the presence of various commonly associated ions . Interference was determined by measuring the absorbance of the extracted organic phase by spectrophotometry and ICP-AES . Moderate amounts of commonly occurring

Sea Water Filter Mixing

Coil

Buffer---.--'

metal ions associated with lanthanum(lll ) and ce­rium(TV) were tolerated (Tab le 1).

Analysis of the standard samples and recovery from monazite sand

To check the validity and reliability of the method, lanthanum(III ) and cerium(IY) were separated and determined in monazi te sand, standard of geological samples (USGS) and sea water. Lanthanum(Jli) and cerium(IV) were separated from monaz ite sand after digesting 0.5 g sand with a mixture of cone. HCl and HC104 ( I: I) and diluting it to 250 mL with di s­ti lled water. An aliquot was extracted at pH 8.5 for lanthanum(IH) with l 0 mL of PBCHA in dichlo­romethane. The pH of the aqueous layer was then raised to 9.5 with buffer and Ce(IY) was extracted. Lanthanum(III) and cerium(IY) were stripped with

ICP­AES

Scheme 1

Table 3-Preconcentration and de terminati on of lanthanum(III ) and cerium(IV) in sea water and water samples

Sea water/ La(III), ~g mL - I Ce(l"/), ~g mL - I

Water samples NAA* Found

NAA* Found

Spectra- ICP-AES Spectra- ICP-AES photometcry photometry

Bombay High 3.35 3.36 ±0.03 3.358+0.09 1.70 1.72 1.71 8+0.009

Mahim 10.00 9.97 ±0.05 I 0.008+0.0 10 6.80 6.78±0.05 6.808+0.90

Appo1o bunder 11.00 11 .05 ±0.05 1 0.995+0.007 8.50 8.55±0.05 8.487+0.005

Cambay (Gujarat) 8.00 8.03 ±0.05 7.995+0.008 6.50 6.48 ±0.05 4.806+0.007

Marina (Chennai ) 6.00 5.98 ±0.04 6.003+0.005 4.80 4.76 ±0.05 4.8002+0.005

High purity HCI 0.8 1 0.81 ±0.02 0.799+0.003 !.50 1.52 ±0.05 1.500+0.03 (Merck)

Distilled water 1.75 1.78 ±0.04 1.749+0.005 2.20 2. 16±0.05 2.203+0.007

Quatz water* 0.55 0.50 ±0.05 0.553+0.003 0.95 0.92 ±0.04 0.948+0.005

La +Ce added

5+5 5.0 4.98±0.05 5.006±0.008 5.00 5.02±0.05 4.998±0.005

5+10 5.0 4.99±0.04 4.999±0.004 10.00 9.98±0.04 I 0.008±0.0 10

10+5 10.0 10.02±0.05 1 0.002±0.005 5.00 4.98±0.05 5.003±0.006

*NAA =Results of neutron activation analysis

AGRAWAL eta/.: EXTRACTION OF La(IIT) AND Ce(IV) BY PBCHA 503

0.5 M HN03 as nitrates with purity of 99.97% while the organic PBCHA layer was recycled for recovery. Lanthanum(III) and cerium(IY) concentrations ob­tained are in good agreement with the certified values (Table 2).

Preconcentration The lanthanum and cerium wer~ separated, precon­

centrated and determined in sea water (Scheme 1). The seawater was filtered and pumped to the mix­

ing chamber. The pH was adjusted to 8.8 for La(III) and to 9.5 for Ce(IV) and then valve V 1 was opened to pass the solution to the extraction chamber. Then V 2

was opened and the 10 mL PBCHA was added. After five min. the organic layer was transferred to the col­lection chamber through valve V 3 and the seawater was drain out from the extraction chamber through valve Y4• The organic extract was recycled via valve V 5 in to a reagent chamber and then into the extrac­tion chamber for the extraction. After 10-15 extrac­tions the organic layer is passed through the valve V 6

for measurement of the desired element by ICP-AES. Results show good agreement with those obtained by NAA analysis (Table 3)

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