Complex extraction of vanadium for atomic absorption spectroscopy

Analytica Chimica Ada

201

Elscvicr Publishing Company. Amsterdam Printed in The Netherlands

COMPLEX EXTRACTION SPECTROSCOPY DETERMINATXON IN LAKE WATERS

OF

OF

VANADIUM

MICROGltAM

QUANTITIES

FOR

ATOMIC OF

AUSORPTION

VANADIUM

Several methods for the determination of vanadium are available, including spectrographic and polarograpllic techniques. Early atomic spectrophotometric, absorption methodsl-3 were not sensitive. Application of the nitrous oxide-acetylene flame” and of complexing agents” provided better sensitivities; 0.8 mg/ml and 1.0 mg/ml of vanadium could be determined by the adclitions of aluminum ion and diethylene glycol to aqueous solution,pd by extraction of vanadium with cupferron CRURIP-WIBSNEH AND PUIWY’) in methyl isobutyl ketone (MISIC), respectively. evaluated eight chelating agents for extraction of vanadium into MIDI< and found cupferron to be most satisfactory. The biochemical ancl physiological roles of vanadium in biological systems arc not fully understood. In the aquatic environment it is well known that certain ascidians and tunicates concentrate vanadium 7. A concentration factor of 2So,ooo tilncs has been found in marine organisms compared to sea water”. In recent years tllcrc lias been an increasing interest in the study of vanadium as a biologically active trace element. Under normal aerobic conditions, vanadate is the thermodynami.cally most probable species in natural waters although vanadyl cation has been detected at a level of ca. 1% of the total vanadium 0. Vanadium(II1) exists in extremely low concentration in natural waters except for unusual waters with very high acidity. Vanadium forms a number of complexes extractable into organic solvents, and this allows concentration of vanadium from a very dilute solution. The present paper describes the investigation of 13 chelating agents for atomic absorption spectroscopy of vanadium and the development of a method for the determination of trace amounts of vanadium in lake waters. EXPERIMENTAL

A@aratzcs

A Jarrell-Ash atomic absorption spectrophotometer (moclel 820~52s) equipped with a “Triflame” burner (S-cm path) for nitrous oxide was used. The gas supplies were 30 psi for nitrous oxide and 15 psi for acetylene. Slits were IOO ,u and x50 ,u, respectively, for entrance and exit. The spectral source was a Westinghouse hollowcathode lamp operated at 12.5 mA. The 3184 A vanadiuln line was used. The output Asal.

Cl&n. Acla, 50 (1970)

zor-207

202

Y. K. CHAU,

K. LUM-SHUE-GHAN

was coupled to a Coleman-Hitachi recorder (model 165) operated at IO x expansion. A Nuclear-Chicago y-counting system (model 4454) with a thallium-activated NaI well was used to measure the 4aV activity.

All chemicals used were analytical grade. Solvents were distilled. Water was clouble-distilled from an all-glass apparatus. Vanadium(V) standard solution (IOO jAg/ml) was prepared by dissolving o.zzgG g of ammonium vanadate in water and diluting to I 1. A working solution (IO &ml) was prepared fresh daily by dilution. Vanadium(IV) solution (IOO /Ag/ml) was made by warming 0.0177 g of vanadium pentoxide with ca. IO ml of concentrated hyclrochloric acid until the solution turned blue. The solution was evaporated almost to dryness and diluted to IOO ml in a volumetric flask. Vanadium(III) solution (IOO /kg/ml) was prepared by warming an alicluot of vanadium(V) solut’ion with a piece of mossy zinc until the color changed to green. This solution is oxidizable by air and should be made fresh when required. A o.~~/~((w/v) solution of 5,7-dichloro-oxine in qt-butyl acetate was used. lJsV solution (as dioxovanadium chloride), carrier free, was supplied by Radiochemical Centre, Amersham, England. A solution of 0.1 ,X/ml containing I ,ug V/ml as carrier was used for tracer experiments. Acetate buffer contained 50 ml of 0.2 M sodium acetate and 950 ml of 0.2 M acetic acid. Vanadium-free lake water was preparecl by passing the filtered lake water which was acljusted to PH 5 through a Chelex-100 column 10. No vanadium was detected in I 1 of this stripped water when it was usccl as blank by the present methocl.

Aliquots (I ml) of vanadium(V) solution containing IO ,ug of vanadium and 3 ml of acetate buffer were clilutecl to cu. 15 ml and the PH was adjusted to suitable values by dilute sulfuric acid. The solution was diluted to 20 ml ancl equilibrated for 15 min with IO ml of MIBK containing different chelating agents. The organic layer was separated and aspirated into the flame to compare the absorption intensity. Experiments on pxl dependence of extraction were carried out with IOO ml of distilled water, spiked with an appropriate amount of ‘@V and IO r(lgV as carrier. After addition of 5 ml of acetate buffer, the solution was adjusted to various PH values by dilute sulfuric acid or ammonia and equilibrated with 5 ml of 0.3% dichloro-oxine for 15 min in a mechanical shaker. The separated organic phase was made up to 5 ml with pure MIBK in a volumetric flask to compensate for the variation in solubility of the solvent in solutions of different acidity. A 3-ml aliquot was pipetted for scintillation counting of the y-activity against a suitable aliquot of standard. Similar experiments were carr.ied out with vanadium-free lake water. Iderference studies Vanadium-free lake water was used in order to provide a uniform ionic concentration for extraction. A Ioo-ml aliquot of lake water spiked with S,ugof vanadium(V) and IOOO ,ug of each of the interfering ions was extracted as detailed above but at A?$&. Cki+)a.Ada, 50 (1970) 201~207

DETERMINATION

OF VANADIUX

IN LAKE

203

WATERS

PH 2.8-3.2. The absorption signals of the organic estracts were compared with a control spiked with vanadium only. RESULTS

AND

DISCUSSION

Extraction of vanadium com$dexes Thirteen chelating agents different from those studied by CRunzi~-Wmsmm AND PURDYO were investigated for their extraction efficiency ancl flame behavior with the nitrous oxide-acetylene mixture. Their relative absorption intensities are listed in Table I; the optimal extraction pH for each reagent as described in the literature was used. For those that have not been reported previously, a pH value of 2-3 was used as the extractable species of vanadium are generally in the cationic formll.

CupfcrronG Di;~tninobcnzicliIlc~~ Acctylacctonc 1.1 l3cnzoylpl~cnyl1~yclroxylnminc~G 2-Rlcthyl-S-hyclrosycluinolincl~ I-(2-l~griclylazo)-211aphth01’

0

OxincG A’PDCl’ Dichloro-oxinc Diiodo-oxinc Dibrorno-osinc Tliio-oxine Dithizone I>ibcnzoylmeth~~nc Tlicnoyltrifluoroacctonc ‘IXnicthylglyoximc a All chelating agents extraction. 11No signal obtained.

2-2.5 I 2

5 ml aq. I y0 2 ml ;rq. 0.2 ';/o :j ml

12 11.s.” ‘2 5

2-3

7

4-5 r$o in ctlmnol 1 % 5 1111 aq. I y/o

5 TO ‘7 I.5 ’ 4 14 n.s.

0.3% 0.3%

0.3 ‘X, 0.5 % 0.5 “/” I “/”

l1.S.

3 3 n.8. wcrc tlissolvccl

3 *s--4 *5 2-3 1-2 3-3.5 3-3.5 3-3.5 3-3.5 3-3.5 3-3#5

0.1

0.5

1% in MIBK

‘%J

2-3 2-3

,.

or spccifictl

othcrwisc;

IO ml of RIII3TI; was usccl for

Of the ligands studied, four gave no signal (Table I) probably because no complex was formed or the complexes formed were not extractable in the solvent used. The APDC complex showed very sensitive absorption, but its rapid precipitation in MTBKD seriously limited its application. Attempts to stabilize the APDC complex by changing the solvent to methyl amyl ketone, butyl acetate and isopropyl ether failed; addition of acetic acid or pyridine to the extract also failed. The dihalo-substituted oxinates of vanadium gave the best extraction efficiency and absorption intensity. However, the low solubility of dibromo-oxine in organic solvents caused clogging of burner once it became hot. Diiodo-oxine was unstable; the reagent decomposed to liberate iodine even when shaken vigorously with solvent. Alaal.

Cl&t.

Acta,

50 (1970)

201-207

204

Y.

IC. GHAU,

K.

LUM-SHUE-GHAN

Dichloro-oxine was the only one that satisfied both extraction efficiency and absorption sensitivity and caused no trouble in the nitrous oxide-acetylene flame when a 0.5 ‘y! solution was sprayed. The dichloro-oxinate complex was chosen for further stu. dies. of vmadizmz as diclzlovo-ox&ate The effect of PH on the extraction of vanadium(V)

Extructio9z

by dichloro-oxine in MIRK indicated that maximum extraction of the complex occurred at pH z-4.5. The recovery of IO pg of V from 100 ml of solution was g6 + ~zO/~,. Increasing the concentration of the dichloro-oxine above 0.5’;/~ in MIRK clid not improve the recovery further. Increase of chelating agent concentration increases the extraction efficiency and hence tile absorption until a point is reached \vhen tile effect on aspiration rate due to cliange of viscosity becomes more significant; at tliis stage, there is a decrease in &sorption. This effect was observed for solutions of dichloro-osine greater tllan o.50io in MIBK; at tllis concentration, an emulsified layer developed. For 5-50 ,zg of vanadium in IOO ml solution, 5 ml of o.z-o.3°/o dichloro-oxinc was optimal. A shaking time of 15 rnin sufficed under these conditions; prolonged shaking did not improve the yield. The solvent effect was also studied 11y dissolving the dichloro-oxine in various organic solvents. After extraction, the organic layer was made up with the respective solvent to 5 ml to compensate for the difference in solubility of each solvent. Among MII3K, butyl acetate, ?z-butyl alcohol, methyl amyl ketone, amyl acetate, It-amyl alcohol, isopropyl etllcr, isoamyl alcohol and r,z-dichloroethane, MIRK and butyl acetate gave the best sensitivity. Butyl acetate was chosen because it gave a steadier base line tllan MIBK, especially when the burner became Ilot. Another advantage with butyl acetate was its low solubility in water (0.7 ml/xoo ml) ; the volume change after cstraction was thus less pronounced than wit11 MIRK (cu. z ml/loo ml). After extraction, the organic phase can be carefully separated into a volumetric flask ancl adjusted to a definite volume, or the extract can be aspirated directly into the flame. When a large volume of aqueous phase must be extracted with a small organic phase, the latter method is preferable. Provided that the volume and the p~ of the solution are well controlled for both samples and standard, the organic phase after extraction is quite reproducible and can be directly aspirateclafterccntrifugation. Quantitative separation of pleases can thus be nvoicled. Under the optimal conditions for extraction of vanadium(V), vanacliuni(IV) at similar concentration level (5 ,ug V/IOO ml) was also extracted completely. The extraction of vanadium(II1) was not quantitative under the present conditions; its extraction yielcl increased from C,SCvo to 90% with increase of shaking times from IO to 45 min, but times up to 5 11did not then improve the recovery. The vanaclium(III), -(IV) and-(V) c1’tc11 1 oro-oxinate compleses had the same color and ultraviolet spectra. The extracted complex absorbed strongly at 406 nm ancl coul.cl bc usecl as a spectrophotometric methocl for vanadium.

Stability

of vanadiwn

dichloro-oxinate

The stability of the vanadium dichloro-oxinate complex was investigated. The complex was brownish iu color in both MIBK and butyl acetate and was stable in A~~u.1. Cl&n..

Ach,

50 (1970)

zor-207

both solvents. No change in color and no deterioration in absorption intensity occurred in a period of up to two weeks. The cupferrate of vanadium has been the most recommended complex for atomic absorption of vanadium. This comples, however, was found to decompose at different rates depending on the pH of the extraction and solvents used. Vanadium(V) cupferrate extracted at PH I into MIRK decomposed rapidly in ca. 45 min, probably because of the instability of cupferron itself; at pH 3, the extracted comples was stable. for cu. 2 h.

The bellavior of dichloro-oxine is in many ways analogous to osine, llencc several metals wc~uld be expected to form complexes and be co-estractcd. In the determination of IO iqg of V in 100 ml of spiked lake water samples, no interference in absorption signal was observed in the presence of 1000 pg of each of the following ions: Na,Ca,Cu, M~,Ni,Pl~,%n,Mn(tI),Al,Hi,Cr(III),R4o(~~I)andSb.Cr(T~I),l~e(EI1),Sn.(IV) and W(V1) suppressed absorption seriously; several masking agents were tried but only ascorbic acid was satisfactory in eliminating the interferences of iron and chromium. With 5 ml of 20/oascorbic acid, up to IOOO ,ug of Cr(VI), IOO ,ug Sn(IV) and 250,~g I’e(II1) could be tolerated. No masking agent was found effective for tungsten. In the determination of IO ,ug of V in IOO ml solution, up to 50 ,ug of W(Vl) could be toleratecl; lligller levels are unlikely to occur in natural waters or biological materials.

Various amounts of vanadium were added to IOC) ml of stripped lake water and extracted wit11 3 ml of dichloro-oxine in butyl acetate. The plots of absorption signals ZIS.concentration were linear up to at least x0 rug of V, hence the extraction yield in this concentration range was constant. The sensitivity of vanadium dichlorooxinate in butyl acetate was calculated to be 0.13 pugV/ml for a z’;/~absorption signal. When this extraction is applied to the analysis of vanadium in various materials, the sensitivity can be increased further by using a larger sample and also reducing the volume of the organic phase for extraction.

The natural occurrence of vanadium is cu. 0.5-5 lug/l in sea water and is cluitc variable in the lakes. A large sample has to be used to obtain reliable analysis. To test the validity of dichloro-oxine as a complexing agent to concentrate vanadium from a large volume, appropriate alicluots of 4”V and 5 ,q of V as carrier were spiked to different volumes of stripped lake water. The solutions were processed as described above except that larger amounts of organic solvent were used to compensate for its solubility. The concentration of the extractant was so acljusted that the final concentration of dichloro-oxine did not exceed 0.5~/& Shaking time was accordingly prolonged. After separation, the organic layer was carefully drained and the separatory funnel was washed with butyl acetate into a 5-ml volumetric flask. A 3-ml aliquot was counted and compared to a standard to calculate the recovery. For extraction of 5 ,ug of V from 500 and IOOO ml of lake water, radiometric recoveries of 93o/oand gz o/O were obtained. And.

Ciri,rn. flctu, 50 (1970)

zor-207

Y.

ZOG

K.

CHAU,

K.

LUivf-SHUE-CHAN

Spiking of different volumes of stripped lake water showed that excellent agreement could be achieved between different volumes, provided that the calibration graph for atomic absorption measurements was prepared in a similar way (Table II).

RlXOVERY

01’

VANAT>IlJhl(V)

PROM

1’ 3 ml of o.zOk, dichloro-oxinc 115 ml of o.I’%, tlichloro-oxinc 0 g ml 01 o.T% dichloro-oxinc

VAR’IOUS

VOLUMES

OX’

STRIPPISD

LAlClE

WATER

in butyl acctntc used; shaking time 15 min. in butyl ncctntc usccl; shaking time 30 min. in butyl ncctntc used; shdcing time 30 min.

Filter the sample through a membrane (0.5 ,u) immediately after collection. The sample may be acidified if it is intended. for other trace element analyses. Measure accurately I 1 of sample into a Iooo-ml separatory funnel, and add 5 ml of acetate buEEer and 5 ml 2% ascorbic acid. After adjustment of the PEI to 2.8-3.2, equilibrate the mixture with g ml of 0.1 o/0clichloro-oxine in butyl acetate for 30 min in a mechanical shaker. Leave the solution for 30 min to allow .the phases to separate. Drain off the aqueous phase slowly until ccc. 5-G ml is left with the organic phase, then drain the mixture into a centrifuge tube, and centrifuge to facilitate separation. Aspirate the organic extract from the centrifuge tube without separation. Since the stripped lake water contains no detectable vanadium and always shows zero absorption reading, the blank may be conveniently prepared by running the complete procedure with I liter of distilled water. Calibrate the method by spiking 2 ,ug of vanadium to I 1 of vanacliutn-free lake water and taking it through the entire process.

The precision of the method was tested by four analysts of 5oo-ml and IOOO-ml lake water samples spiked with 5 ,ug of vanaclium. The coefficients of variation were 1.30% and 2.05~/~, respectively, for 5oo- and rooo-ml samples. Other analyses (5) on 1-1 samples of a surface sample taken from Lake Ontario near Hamilton Hay showed an average vanadium content of 0.50 _C0.05 ,ug/l. The sensitivity of the method applied to lake water analysis was calculated from the calibration control. For a 5-,ug spike in I 1 of lake water (5 p.p.b.), the absorbance obtained was 0.164 (31.5”/~) ; the sensitivity was 0.27 pug (0.27 p.p.b.) for a signal of o.0088 (~‘3~ absorption). Storage of sanq%s Radiometric A~nl.

Cltim.

flctn,

investigation

50 (1970) 201-207

indicated

that

there was no appreciable

loss of

DETERMINATION

OF

VhNhnIUbI

IN

LAKE

207

WATERS

vanadium in filtered lake water samples when stored in pyres glass, polyethyl&ne and polypropylene containers at its natural PH of cn. S or at PH 2. Its stability in sea water was reported by CHAN AND RILEY~~. SUMMARY Thirteen chelating agents have been investigated for their suitability for estraction of vanadium for atomic absorption spectroscopy. Dichloro-oxine was found to extract vanadium(IV), vanadium(V) and 90% of vanadiurn(II1). The comples formed was very stable and gave lligh sensitivity for atomic absorption. A method combining extraction and atomic absorption has been developed for the determination of vanadium in lake water with a sensitivity of 0.3 1l.p.b.

Treize reactifs de chelation ont 6t6 examinds en vue de I’extraction du vanapermet d’esdium pour spectroscopic par absorption atomique. La dichloro-oxine trait-e vanadium(IV), vanadium(V) et 90% de vanadium(II1). Les complescs form& sent tr6s stables et permettent cl’arritvcr Q des grandes sensibilitds pour l’absarption combinant extraction et absorption atomique est prop&e atomique. Une n&hodc pour le dosage du vanadium clans l’eau cles lacs, avec une sensibilit6 de 0.3 p.p.1~.

Dreizehn chelatbildende Angenzien sind auf ihre Anwendbarkeit auf die Extraktion von Vanadin fiir die Atomabsorptionsspektroskopie untersucht wurden. Dichloroxin estrahiert Vanadin( IV), Vanadin( V) und I)Oo/o von Vanadin( III). Der gebildete Komplex ist sehr stabil und crgibt bei der Atomabsorption eine hohe Empfindlichkeit. Es wurde ein kombinicrtes Verfahren dcr IZxtraktion und Atomabsorption entwickelt fiir die Bestimmung von Vanadin in Seewasser mit eincr @mpfindlichkeit von 0.3 p.p.1’.

I

, Aforuic D. C. MANN~NC l’ahrfa, 15 (1968) 871. Atrnnic Absorpfimt NcuddIcr, 6

I,. C. ~DI~A~ADO AND

2 R. GoEcIcr~,

3 I>. MYERS, 4 J. X3.WILLIS,

Nutum, 5 S. .L. SACH~JGV, J. W.

0 J. CRUMP-WIESNER 7 1.5.D. GOI.DBER~,, 8 I. NODACK g M. KALIC,

AND

207

(1965)

ADsorpliou

Newslcller,

5 (196G)

I.

(1967) 8g.

715.

.I
12.

Nni,lrvc, rg8 (rgG3) 10x0. IO J. T’. RII.~~ AND D. TAYLOR, ,4mtl. CJhm

Acta, 41 (1968) 175. J. STAR+, Anal. CJrirn. Ada. 28 (1963) 132. 12 Ii. L. CHENC., Talu~tla, 8 (rgGr) 058. 13 I<;. hfOTOFIMA AND H. I‘TASIIITAN, Japmt fl M/J& (B~ZfitSEki ICUgU/iL1), I‘+ J. I?.MCICAVENEY AND I-I'. FI~EISZR, Amtl. CJwtr., 30 (1958) 526 15 z). 1:. RYAN, AnnZysl, 85 (IQGO) 509. IB S. WAKALIATSU, Jctpa7r Amtlysl (Btmscki ICngnk~u), 9 (1960) 284. 17 1-r.i%IALISSA AND S. GonrIscmc,%. ,4?tal.CJrcru., 169 (1959) 402. 18 I<. RI. CHAN AND J. 1’. RILEY, A7tal. Chim. AC/U, 34 (19G6) 337. II

Agtal. CJht.

9 (1960)

Actn,

1.51.

50 (1970) 201-207