Separation of rhenium by extraction with crown ethers and flow-injection extraction—spectrophotometric determination with Brilliant Green

Analytrca Chrmrca Acta, 232 (1990) 287-292 Elsevter Sctence Pubhshers B.V , Amsterdam

287 - Pnnted

m The Netherlands

Separation of rhenium by extraction with crown ethers and flow-injection extraction-spectrophotometric determination with Brilliant Green a HIDEKO

KOSHIMA

and HIROSHI

ONISHI

*

Unroerslty of Tsukuba, Tsukuba-shr, Ibarakr-ken 305 (Japan) (Recetved

5th December

1989)

ABSTRACT The extractton behavior of perrhenate wtth crown ethers was studied and methods for the separation and determmation of rhenium were developed. The distribution ratio of perrhenate with dicyclohexano-18-crown-6 (DC18C6) increases wtth increases m the dielectric constant of orgamc solvents and in the potassium ion concentration of aqueous solutton. The molar ratios of crown ether to KRe04 m the extracted species are probably 1:l for DC18C6, dtbenzo-18-crown-6 and 18-crown-6 and 2:l for benzo-U-crown-5 and 15crown-5 Microgram amounts of rhemum were satisfactorily separated from large amounts of molybdenum(V1) by extraction wtth DC18C6 m 1,2-dichloroethane from 2 M potassium hydroxtde solution contarmng tartrate and by back-extraction wtth sodmm phosphate buffer solutton after the addition of a twofold volume of hexane to the organic phase. Rhemum was determmed by the flow-mlection extraction-photometnc method wrth Bnlhant Green Rhemum was sattsfactorily determmed m molybdemte and other ore samples

Crown ethers are excellent extractants for the chloro complexes of uon(III), gold(III), gallium (III), thallium(II1) and antimony(V) [1,2]. It was thought worthwhile to study the extraction behaviour of 0x0 anions, e.g., perrhenate (ReO;), with crown ethers. Caletka et al. [3] described the extraction of rhenium(VI1) with dicyclohexano18-crown-6 (DC18C6) in 1,2-dichloroethane from hydrofluoric acid and potassium fluoride solutions. Zhou and Yang [4] studied the extraction of rhenium with benzo-15-crown-5 (BlSC5) in nitrobenzene from sulphuric acid solutions containing potassium ion. The purpose of this work was to investigate systematically the extraction behaviour of rhenium wtth crown ethers and to develop useful methods for its separation and determination. For the dea Presented at the 38th Annual Meeting of the Japanese for Analytical Chemistry, Sendai, October 1989

0003-2670/90/$03

50 0 1990 - Elsevter

Society

Science Pubhshers

BV

termination of rhenium, a flow system based on discrete extraction photometry with Brilliant Green [5] was developed.

EXPERIMENTAL

Apparatus rhenium

for

flow-rnjectron

determmatlon

of

A diagram of the flow system is shown in Fig. 1. Water and benzene were pumped with two plunger pumps (Nihon Seimitsu Kagaku NP-FX3U and JASCO Tri Rotar-II). Phosphate buffer and Brilliant Green solutions were pumped with a peristaltic pump (Ismatec Mini-S-840) through Tygon tubes. Samples were injected from a six-way valve fitted with a lOO+l loop (0.5 mm i.d.). Flow lines were PTFE tubing (0.5, 0.8 and 0.25 mm i.d.). A cross-joint was used at the mixing point for mixing aqueous solutions and a T-Joint for

H KOSHIMA

288

AND

H ONISHI

3 15 Dal1

4

rig. I %ichematlc diagram of tTow-mjectlon system (1) pH‘ 6.U but% soiitlon at lJrS% (k/v) Enlliant Green at KU mi mm +; (1) benzene at @LIJ mi mm +, (cross-Jomt); (7J nuxmg coli (25 cm x ITS mm 16.); @j segmenter (‘i=Jomtj; (9 separator, (11) needle valve, (12) aqueous waste, (13) spectrophotometer (640 nm), I d ). (16) organic waste

segmenting

and aqueous phases. A PTFE (JASCO) containing a laminated PTFE membrane filter of 0.2~pm pore size (Millipore, FGLP) was used. A needle valve fitted to the outlet of the phase separator was regulated to obtain complete separation of organic and aqueous phases. Absorbances of organic phases were measured at 640 nm with a spectrophotometer equipped with an 8-~1 volume and lo-mm length flow cell (JASCO 870-UV) and were recorded with a data processor (Shimadzu Chromatopac C-RlA). phase

organic

separator

Reagents

A rhenium(VI1) standard solution was prepared by dissolving potassium perrhenate m water. Buffer solution of pH 6.0 (0.2 M NaH,PO,-0.03 M NaOH) and 0.15% (w/v) Brilliant Green solutlon m 50% (v/v) ethanol were filtered through a membrane filter (0.45~pm pore size) before use. The Brilliant Green solution could be used for at least 1 week. The crown ethers used were DC18C6 (mixture of syn-cu and anti-cis isomers), dlbenzo18-crown-6 (DB18C6), 1%crown-6 (18C6), B15C5 and 15-crown-5 (15C5). All the ethers except B15C5 (Tokyo Kasei Kogyo) were Aldrich products. Procedures Extractlon

volume

of rhemum

of 0.01-l

with crown ethers. A 2-ml

mM rhenium

aqueous

solution

u4u mi Illln-:‘, (2) water at ti.4u ti- mm-“; (3 (Sj sampie Injector (NV ~1)‘; (6)’ mung pomt extraction coli (z m Xu8 mm I 4), (r@j phase (14) data processor, (15) restnctor coil (0 25 mm

was shaken with an equal volume of 0.002-0.1 M crown ether in an organic solvent for 3 min and the rmxture was centrifuged. Rhenium in the aqueous phase was deterrmned by the flow-inJection method after adjusting the pH to about 6. Rhenium in the organic phase was determined by difference. The distribution ratio (D) of rhenium (organic to aqueous) was then calculated. Separatron and determmatron of rhenium. A lo30-ml volume of 2 M potassium hydroxide-O.01 M potassium sodium tartrate containing up to 20 pg of rhenium was transferred into a PTFE separating funnel and rhenium was extracted twice by shaking with 5 and 2 ml of 0.01 M DC18C6 in 1,2-dichloroethane for 3 min each time. The organic phases were combined in another PTFE separating funnel and the extract was washed by shaking with 5 ml of 2 M potassium hydroxide solution. The aqueous phase was washed by shaking with 1 ml of 1,2_dichloroethane and the organic phase was combined with the washed extract (total volume ca. 8 ml). Hexane (16 ml) was added to the extract and rhenium was back-extracted twice by shaking with 6 and 3 ml of the phosphate buffer solution for 3 min each time. The aqueous phases were combined and adjusted to 10.0 and 20.0 ml with the buffer solution. Rhenium was determined by the flow-injection method. A blank was run through the entire procedure. The cahbration graph was established by using O-l.5 mg 1-l Re working standards in the buffer solution.

SEPARATION

RESULTS

289

OF RHENIUM

AND

DISCUSSION 4_

Flow-qection determination of rhenium Among many photometric methods available [6], that based on the use of Brilliant Green [5] has been adopted because it gives a low blank and can be readily coupled with back-extraction of rhenium from the orgamc phase. The concentration of Brilliant Green in 50% (v/v) ethanol was varied from 0.03 to 0.4% (w/v) m the flow system (Fig. 1). The absorbance increased with increase in the reagent concentration up to 0.1% and was nearly constant from 0.15 to 0.4%; a 0.15% Brilliant Green solution was adopted. The pH of sample solutions was adjusted to about 6 before injection. The absorbance increased gradually with increase in the extraction coil length from 1 to 4 m; 2 m was chosen for the subsequent experiments. A linear calibration graph was obtained for O-l.5 mg 1-l Re in sample solution. The absorbance for 1.0 mg 1-l Re was about 0.54. Relative standard deviations were 2.9 and 2.3% (ten replicates each) for 0.1 and 1.0 mg 1-l Re, respectively. Twenty samples could be measured in 1 h. The effects of foreign substances on the determination are shown in Table 1. Extraction behavlour of rhemum wrth crown ethers DC18C6 was chosen to study the extraction of rhenium because it gave the highest distribution ratio among the five crown ethers examined in

TABLE

1

Effect of foreign rhemum Foreign substance MoPI)

WVI) Fe(II1) Cu(II) NO; ClO,-

substances

on flow-uqectlon

determmatlon

(mg 1-l)

Apparent Re found (mg 1-l)

100 1000 5000 100 100 100 10 0.1

0.043 0.14 0.46 0.004 0 018 0.004 0.15 0 17

Concentration

a Re taken=Omgl-’

a

of

2_

D !? O-

-2_

-41

I

I

I

05

10

15

log E

Fig. 2 Effect of organic solvents on extraction of rhemum with DC18C6 2 ml of 0.1 mM Re, (0) 0.1 M K,SO,, (0) 0 1 M Na,SO,; 2 ml of 001 M DC18C6 (1) Nltrobenzene; (2) methyl lsobutyl ketone, (3) 1,2-dlchloroethane, (4) dlchloromethane, (5) chlorobenzene; (6) chloroform, (7) toluene; (8) benzene; (9) carbon tetrachlonde

preliminary experiments. The influence of various organic solvents on the extraction of rhemum from potassium sulphate and sodium sulphate solutions with DC18C6 was investigated. Figure 2 shows that the potassium solution 1s suitable for extracting rhenium effectively and the distribution ratios increase steadily with increase in the dielectric constant (e) of the solvents. Nitrobenzene gave the highest distribution ratio. On the other hand, hexane, which had the lowest log E (0.276) among the solvents exammed, gave the lowest log D of less than - 3 (not shown in Fig. 2). In subsequent experiments, 1,2-dichloroethane was chosen as the extraction solvent from the standpoint of both extraction and back-extraction. As shown in Fig. 3, the log D values were dependent on the imtial concentrations of potassium and sodium in the aqueous solutions.

290

H KOSHIMA

The distribution ratio increased as the initial rhenium concentration decreased; a plot of log D for the extraction with 0.01 M vs. loiW1 aq,Inltlal DC18C6 from 0.1 M potassium sulphate solutions initially contaimng 5.4 x 10P5-4.3 X 10e3 M Re gave a straight line with the equation

D

log D = - 0.43 log[ Re]aq,,nltlar + 0.20

-F

The effect of acidity on the extraction of rhenium and molybdenum(V1) from solutions containing 0.1 M potassium sulphate was investtgated (Fig. 4). Rhenium was extracted nearly constantly over a wide pH range. On the other hand, molybdenum was very little extracted, especially from strongly basic solutions. Log D values for the extraction of rhemum and molybdenum from

AND

H ONISHI

O-1 -2-

-3_

I

I

I

I

2

1

01

I

I

I

00100010

I

002

aqueous

I 4

NaOH(M)

H2S04 (MI Inltml

II 12

I

02

solutaon

contamng

OlM

K2S04

Fig 4 Effect of acldlty on extraction of rhemum and molybdenum(W) 2 ml of 0.1 mM Re or 2 mM MO m aqueous solution 0 1 M m K,SO,, 2 ml of 0.01 M DC18C6 m 1,2-dlchloroethane (0) Re; (A) MO.

2 M potassium hydroxide solution (in the absence of potassium sulphate) were 2.19 and -2.38, respectively. The species extracted from potassium sulphate solution with five crown ethers were determmed from the dependence of the distribution ratio on crown ether concentration (Fig. 5). The abscissa is the initial crown ether concentration in the organic phase. The slopes are 1 .Ol, 0.99, 0.96, 2.04 and 1.92 for DC18C6, DB18C6, 18C6, B15C5 and 15C5, respectively. These values indicate that the molar ratios of crown ether to KReO, are 1:l for the first three ethers and 2:l for the last two. Zhou and Yang [4] also have shown that the molar ratio of B15C5 to rhenium is 2:l.

-2

”

-1 log [K or

Na] aq

Fig. 3. Effect of lmtlal potassmm and sodmm concentrations on extractlon of rhemum: 2 ml of 0 01 M DC18C6 m 1,2-dlchloroethane, 2 ml of 0 1 mM Re; (0) K,SO,; (0) K,SO, contammg 0 1 M Na,SO,; (A) Na$O,, (A) Na,SO, contammg 0.1 M K,SO,.

Separation and determination of rhenium The above extraction behaviour suggested that rhenium extracted with DC18C6 in 1,2-dichloroethane from aqueous solutions containing potassium ion might be back-extracted efficiently by adding hexane to the organic phase and shaking with sodium ion solution. A 2 M potassium hydroxide solution containing 0.01 M sodium potas-

SEPARATION

OF RHENIUM

to decompose molybdenite, interfered with the determination of rhenium. Therefore, the separation procedure was carried out after complete decomposition of 2 ml of nitric acid (sp. gr. 1.42) initially added to the foreign elements by heating twice with 2 and 1 ml of 90% formic acid [7] in the presence of 0.5 ml of 5 M sulphuric acid. As shown in Table 2, the recoveries of rhenium were satisfactory. Molybdenum and tungsten in the final solutions were determined by flame atomic absorption spectrometry and inductively coupled plasma atomic emission spectrometry, respectively; the separation for both elements was satisfactory.

cl 8

Appbcation

to molybdemte

and other samples

Although many photometric methods have been described for the determination of rhenium m molybdenite [8], a novel method based on the crown ether extraction-flow-injection method appeared to be worth studying.

TABLE

2

-1

-2 log [crown

ether&

Ftg. 5. Dependence of dtstnbutton ratto on tmtial crown ether concentration m extraction of rhemum. 2 ml df 0.1 mM Re in 0.1 M K,SO,, 2 ml of crown ether m 1,2-dlchloroethane; (0) DC18C6, (0) DB18C6; (A) 18C6; (A) B15C5, (0) 15C5.

sium tartrate to complex some other metals was used as an aqueous medium for extraction. To establish the conditions of the back-extraction, rhenium was extracted with 0.01 M DC18C6 in a mixed solvent of 1,2-dichloroethane and hexane from sodium phosphate buffer solution containing 1 mg l- ’ Re. Addition of hexane caused a decrease 111log D; a plot of log D vs. C, (volume percent of hexane) gave a straight line for which the equation log D = -O.O64C, + 0.78 held over the range O-408 hexane. In the procedure a twofold volume of hexane was added to the organic phase (extrapolated log D value, - 3.5). The effect of foreign elements on the separation and determination is shown in Table 2. The elements usually present in molybdenite were tested. As shown in Table 1, nitric acid, which was used

Effect of foreign elements on the separation and determmatton of rhenmm (Concomttant NO; was decomposed wtth formic aad before separation) Foreign element

None Mo(VI) + Fe(III) +cu(II) +se(1V) None None ’ Mo(V1) W(VI) Fe(III) Cu(I1) Se(Iv) Mo(V1) +Fe(III) +cu(II) +se(1v)

Amount

Re taken

Recovery

(mg)

kg)

(W) =

50 1 1 1

100 10 2 5 5 50 1 1 1

1.00

95 5

1.00

95.3

100 100 10.0 10.0 10 0 10 0 10 0

100 b 102 98.4 912 98 3 97.8 97 6

100

100

Foreign element found m final solution ( pg)

MO

21-94

MO W

43-55 07-36

MO

0.9-2.4

a Average of two determmatrons b Average of SIX determmattons; relative standard devratton, 3 4%. ’ Irutial volume of aqueous solution, 50 ml

H KOSHIh4A

292

A 0.1-g sample was decomposed with 2 and 1 ml of nitric acid (sp. gr. 1.42) m a covered 20-ml Pyrex beaker and the solution was evaporated with 0.5 ml of 5 M sulphuric acid. NBS SRM 330 copper ore mill head and CANMET PR-1 molybdenum ore were decomposed with 3 ml of nitric acid and 5 ml of 46% hydrofluoric acid. The nitrate remained was decomposed by heating twice with 3 and 2 ml of 90% formic acid. The residue was dissolved in 5 ml of water, 2 ml of 0.1 M potassium sodium tartrate and an adequate volume of 5 M potassium hydroxide solution, and then the solution was filtered through a filter-paper (medium texture). The filtrate was transferred into a PTFE separating funnel and was finally adjusted to 20 ml of solution 2 M in potassium hydroxide and 0.01 M in potassium sodium tartrate. Rhenium was then separated and determined according to the proposed procedure. Table 3 shows good recoveries of rhenium initially added to the samples. In Table 4 rhenium concentrations obtained in this work are compared with values obtained by neutron-activation analysis [9,10]. The agreement is generally good. The average blank values obtained by decomposition with nitric acid and with nitric and hydrofluoric acids were 0.19 pg Re (three determinations) and 0.31 pg Re (seven determinations), respectively. The standard deviation for the latter was 0.08 pg Re. The limits of determination are considered to be 4 and 6 mg kg-’ Re for decomposition with nitric acid and with nitric and hydrofluoric acids, respectively, based on a 0.1-g sample.

TABLE

3

Recovery of rhemum from molybdemte (A 0 l-g sample was taken) Sample

Molybdemte 66 Hn-02 66 Sk-18 Molybdenum CANMET

ore, PR-1

Copper ore null head, NBS SRM 330

and other samples

Re added

Recovery

(rg)

(W) a

20 0 100

970 b 101 b

10 0

97 7 c

10 0

96 9 =

a Average of two determmatlons. with HNO, HNO, ’ Decomposltlon

b Decomposltlon and HF.

mth

TABLE

AND

H ONISHI

4

Determmatlon

of rhemum

Sample

m molybdemte

and other samples

Re (mg kg-‘) ThlS

Literature value b

work a

Mo% (W) b

Molybdemte: 66 Dt-06, Druto (Shlmane)

186

188

99.4

66 Hn-02, Diuto (Hmotam, Shlmane)

131

157

93.4

66 Hn-04, Druto (Hmotam, Shlmane)

224

281

66 Hn-08, Data (Hmotam, Shlmane)

97 3

66 Sk-13, Selkyu (Chukan-211, Shmane)

177

66 Sk-18, Selkyu (Uwa-hl, Shmane) Molybdenum ore, CANMET PR-1 Copper ore null head, NBS SRM 330

98.1 ~6 ~4,

16’

106

100 95 9

173

100

102

100

’ 0.30 d

a Average of three determmatlons Decomposltlon mth HNO, with HN03 and HF b Refs 9 and 10 ’ Decomposltlon d Certified value.

Professor K. Terada of Kanazawa University is thanked for providing the molybdenite samples and for valuable discussions.

REFERENCES 1 H. Koshma and H ON&, Anal. Scl , 1 (1985) 389. 2 H Koshlma and H ON& Analyst (London), 111 (1986) 1261 3 R Caletka, R. Hausbeck and V. Knvan, Talanta, 33 (1986) 219 4 Z Zhou and J. Yang, Huaxue S~IJ~, 9 (1987) 50. 5 A.G. Fogg, C. Burgess and D.T Bums, Analyst (London), 95 (1970) 1012; D T. Bums and N Tungkananumk, Anal. Chlm Acta, 204 (1988) 359 6 H. Omshl, Photometnc Determmatlon of Traces of Metals, 4th edn, Part IIB Indlvldual Metals, Magnesmm to Zlrcomum, Wdey, New York, 1989, p 285. 7 EM. (Penner) Donaldson and W.R. Inman, Talanta, 13 (1966) 489. 8 H. ON& Photometnc Determmatlon of Traces of Metals, 4th edn, Part IIB: Indlvldual Metals, Magnesmm to Zlrcomum, Wdey, New York, 1989, p 293. 9 K Terada, S Osaka, S Istiara and T I(lba, Geochem J, 4 (1971) 123 10 S. Osalu, MS Thesis, Kanazawa Umversity, 1967