Determination of selenium in sea water by adsorptive cathodic stripping voltammetry

Analytrca Chlmrca Acta, 231 (1990) 221-229 Elsevler Science Publishers B V., Amsterdam

221 - Prmted

m The Netherlands

Determination of selenium in sea water by adsorptive cathodic stripping voltammetry C M.G. VAN DEN BERG Oceanography Laboratones,

Department

* and S H KHAN

a

of Earth Snences, Unrversrty of Liverpool, P 0 Box 147, Liverpool L69 3BX (Great Brrtarn)

ABSTRACT A procedure 1s presented for determtmng Se(W) and total dtssolved Se m sea water using cathodic stnppmg voltammetry m the presence of added copper. Expenments usmg cychc voltammetty mdicate that the preconcentratton step consists m adsorption of a Cu(I),Se complex species on the hangmg mercury drop electrode The optmuzed analytrcal conditions mclude a copper concentration of 40 PM and a solution pH of 16. Differential pulse modulatton is used. Interference caused by organic surface-active substances present m natural waters 1s ehmmated by UV photolysts of the sample. Cadmmm interferes wtth the determmatton of Se only when present at a concentration 100 times higher than normal. UV photolysts at pH = 8 is used to convert Se(V1) to Se(W), whtch is the electroactrve species. The response IS hnear for Se concentrations between 0 and 200 nM The hmrt of detection is 0 01 nM Se when a deposttion time of 15 mm IS used.

Selenium is an interesting element in the marine environment as it can occur in at least two and perhaps as many as four oxidation states. In sea water selenium occurs predominantly as Se(V1) [l], but the presence of Se(W) suggests that the ratio of the two species can be used as an indicator of the redox potential of natural waters. Measurement of selenium in environmental samples is important as at low concentrations it is an essential trace element for animals, whereas at higher concentrations toxic effects have been shown to occur [2]. The concentration of selenium in natural waters has been measured using gas chromatography after formation and extraction of a piazselenol [3], by neutron activation after complexation and adsorption on activated carbon [4] and by atormc absorption spectrometry of the hydride in combmation with cryogenic trapping [5]. Although all of these techniques are sufficiently sensitive to de’ Present address: National Institute of Oceanography, K/6, P.E C.H.S , Karacht 29, Pakistan

0003-2670/90/$03

50

0 1990 Elsevler Science

Publishers

37-

B.V

termine Se in sea water, they cannot be readily automated for monitoring purposes as can be done by voltammetric techniques. For this reason, a voltammetric procedure for the determination of Se in sea water has been developed. Although Se(V1) cannot be reduced at a mercury electrode, Se(W) is electroactive and is reduced in acidic solution to Se2- at -0.01 V [6,7]. The selemde thus formed is thought to react with Hg(I1) to form a poorly soluble precipitate of HgSe, which causes a secondary reduction wave at -0.54 V [6]. The formation of the mercury(I1) selenide on mercury electrodes has been used to precede the determination of Se usmg cathodic stripping voltammetry (CSV) in aqueous solution [8], mcluding extracts from biological materials [9] and contaminated waters [lo]. Another procedure involves the formation of an insoluble layer of CuSe or Cu,Se as a preconcentration step for the determmation of Se by CSV [11,12]. However, this technique suffers from non-linearity of response (the plot of current vs. Se concentration is slightly S-shaped) [ll], which makes it much more dif-

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ficult to use internal standard additions to calibrate the sensitivity. Polarography can be used to detect the piazselenol formed with 4-chloro-o-phenylenediamine [13]. The sensitivity of this technique is improved by adsorption of the piazselenol on the mercury electrode with quantification using CSV [14]. In preliminary experiments, the applicability to the determination of Se in sea water of CSV with adsorptive collection of the piazselenol and of CSV preceded by formation of HgSe and of Cu,Se was tested. It was found that the piazselenol procedure was not sufficiently sensitive and suffered from the inconvenience of a long reaction time (several hours) to produce the piazselenol. The HgSe procedure was also found to be insufficiently sensitive. A procedure is reported here for the determination of low levels of Se dissolved in natural waters, including sea water, using CSV in the presence of added copper. This procedure is adapted from previous work [11,12], but does not suffer from non-linearity of response and is more sensitive.

EXPERIMENTAL

Voltammetric experiments were carried out with a PAR 174A polarograph with a PAR 303A hanging mercury drop electrode (HMDE). The polarograph had been altered to increase the pulse rate to 10 s-‘. Scans were recorded on a Houston Instruments flat-bed X-Y recorder. The reference electrode was Ag/AgCl, 1 M KCl. The surface area of the mercury drop was 2.8 mm2. The voltammetric cells were glass. Solutions were stirred using a PTFE-coated, star-shaped stirring bar, propelled by a PAR Model 305 electronic stirrer. Solutions and samples were prepared on a laminar flow bench supplied with filtered air to prevent contamination with Se from atmospheric or laboratory particles. UV irradiation of samples was carried out using a 1-kW high-pressure mercury vapour lamp (Hanovia) in silica tubes of 2.6 cm diameter. Distilled water was produced by a double fused-silica distillation apparatus. Standard solutions of Se(IV) were prepared by dilution of a

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BDH standard solution for atomic absorption spectrometry with 0.5 M HCl (BDH, Aristar). Stock solutions of various trace metals tested for interferences were also prepared by dilution of BDH standard solutions. A standard Se(V1) solution was prepared by dissolving sodium selenate in the 0.5 M HCl (BDH, Aristar). The stock copper solution was 10 mM copper sulphate (BDH, AnalaR). The HCl, (1 + 1) diluted with distilled water, was also used for pH adlustment. The analytical parameters were evaluated using aliquots of a 60-l sample originating from a depth of 1500 m in the Indian Ocean (salinity, 34.7) that had been filtered and stored at its natural pH. Additionally measurements were carried out on a sample originating from the North Sea that had been treated similarly. Procedure for determination of total dissolved Se

Samples were UV irradiated for 4 h to convert Se(V1) to Se(IV); the sample pH was > 7.8 during this process (sea-water samples can be used at their original pH). Then 10 ml of the sample were pipetted mto the voltammetric cell, 40 ~1 of 1 + 1 HCl were added, giving a pH of 1.6, plus 40 ~1 of 10 mM Cu to give a final concentration of 40 PM Cu. The solution was deaerated for 6 min by bubbling an mert gas (nitrogen or argon), saturated with water vapour, whilst stirring the solution. The potentiostat was switched on and set to the deposition potential ( - 0.4 V). A new mercury drop was made and deposition was carried out for a fixed time between 1 and 10 mm depending on the Se concentration to be determined. The stirrer was stopped and the scan was initiated 10 s later to allow the solution to become qutescent. The scanning conditions were: negative scan direction, differential pulse modulation, pulse height 25 mV, pulse rate 10 s-’ and scan rate 20 mV s-l. The scan was repeated after a standard addition of Se sufficiently large to at least double the peak height.

RESULTS

AND

DISCUSSION

Cyclic voltammetry

CSV of Se(IV) in sea water without added copper produced a single reduction peak at - 0.54

DETERMINATION

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(4

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1

3 l(b)

n

1

/I ,

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-04

-0 6 Potentlol(V)

-0 6

OL 0

50

100 scan

150

rate

(m V

ZOO

250

se’)

Rg. 1 (a) Cychc voltammetry of Se(W) (400 nM) m sea water m the presence of 40 PM Cu
V, to reduction of Hg(I1) in the deposited mercury(I1) selenide. This peak became suppressed and a second peak appeared at a more negative potential when copper was added (the actual peak potential depended on the copper concentration) as a result of the deposition of more stable CuSe or Cu,Se. The mechanism of the reduction process of the copper peak was investigated using cyclic voltammetry using the standard analytical conditions (40 PM Cu, pH 1.6). The first cyclic scan was preceded by deposition for 60 s at -0.3 V from a solution containing 400 nM Se(W). A reduction peak was apparent at - 0.73 V, immediately followed by a depression in the current indicating either the occurrence of an oxidation current or a decrease in the capacitance of the double layer (Fig. la). A second cyclic scan was carried out without prior deposition, causing a much smaller reduction peak and no visible depression of the current after the peak. In the reaction mechanism that seems likely to occur, Se(W) is reduced to Se( - II) during the deposition step: H,SeO,

+ 6H++

6e-+

H,Se + 3H,O

(I)

This reaction occurs at a potential of -0.05 V [7]. Both the selenic and selenious acid are protonated at the experimental pH (1.6), as their acid dissociation constant (pK,) values are 3.9 and 11.6 (H,Se) and 2.3 and 7.8 (H,SeO,) [15].

Copper occurs predominantly as Cu(1) at deposition potentials between ca. 0.2 and -0.15 V in electrolytes containing Cl- as a result of complex stabilization [16]. The solubilities of Cu(II)Se and Cu(I),Se are very low, having solubility products (log values) of - 48.1 and - 60.8, respectively [15]. The calculated solubility of Cu,Se (1O-52 M in the presence of 40 PM Cu) is less than that of CuSe (1O-44 M), suggesting that the formation of Cu,Se is favoured thermodynamically. The thus stabilized Cu(1) is reduced at a much more negative potential than that of the chloro complex (which has a reduction potential of ca. -0.15 V), causing the reduction peak at -0.73 V: Cu,Se

+ 2H++

2e-+

2 Cu’(Hg)

+ H,Se

(2)

The reduced Cu is amalgamated, and the released selenide becomes protonated and diffuses away, causing a drop in the capacitance current at potentials more negative than the reduction peak at -0.74 V. The drop in the capacitance current was peak shaped at slow scan rates (20 mV s- ’ less), but the peak was considerably elongated at scan rates of 50 mV s-l (Fig. 1) and higher, owing to slow diffusion from the electrode surface. The height of the Se reduction peak (at -0.74 V) increased linearly with the scan rate when this was varied from 20 to 200 mV s-l and non-hnearly when the scan rate was increased to 500 mV s-l (Fig. lb). The linear increase with the scan

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rate is similar to the behaviour apparent for material adsorbed on the electrode surface, such as is observed for the reduction of metal ions in adsorbed complexes of copper with catechol [17] or quinolin-8-01 [18]. This linearity suggests that the Cu,Se is deposited as a monolayer on the electrode, as a multi-layered precipitate would exmbit non-linear behaviour as a result of stronger kinetic effects related to the then heterogeneous nature of the electrode process. The data suggest that it is possible that the deposited matenal is due to adsorptron of higher complexes of Cu,Se or CuSe rather than to precipitation of poorly soluble Cu ,Se. No stability constants are available for the formation of such complexes, but their formation would be analogous to that observed for copper with sulphide [19]. The peak potential shifted a small amount m the negative direction when the scan rate was increased, from -0.74 at 20 mV s-l to -0.75 V at 100 mV s-l and to -0.76 V at 200 mV s-i, but with a larger shift to -0.79 V at a scan rate of 500 mV s-‘. These comparatively small shifts are consistent with the homogeneous reduction of an adsorbed film of complex ions [17,18], as the heterogeneous reduction of a precipitate would be expected to behave less reversibly. Effects of varyrng pH and Cu concentratron

Copper was added to sea water of pH 1.6 contaimng 5 nM Se(W) whilst measuring the reduction current of the Cu,Se peak using CSV after deposition for 3 min at -0.3 V. A peak at -0.54 V due to the reduction of Hg(I1) in deposited HgSe was suppressed by 10% by the addttton of 1 PM copper and by 95% by the addition of 2 PM copper. A second peak due to the reduction of Cu(1) m adsorbed Cu,Se formed in the presence of 2 PM copper, and increased with increasing copper concentration up to ca. 40 PM (Fig. 2). The Cu,Se peak was narrower than that due to HgSe, consistent with the transition from a two-electron [reduction of Hg(I1) to Hg(O)] to a one-electron reduction step. Further, the peak was higher than that due to HgSe, illustratmg its analytical advantage. The peak potential shifted in a negative direction with increasing copper concentration (from

peak

height

VAN

DEN

BERG

AND

-42

-a

S H KHAN

(nA)

1

15 -

-R

-5R

-5ti

-54

-52

-5

-4R

-46

-44

-38

-38

1% [Cul Fig 2. Effect of varymg the Cu concentration on the peak height obtamed usmg CSV for Se m sea water of pH 1.6. Se concentration, 5 nM, deposltlon time, 3 mm; deposmon potenteal. -0 3 V.

- 0.62 V at 2 PM to - 0.70 V at 60 PM copper) as a result of complex stabilization. The peak height diminished at copper concentrations greater than 60 PM, perhaps owing to the formation of an insoluble Cu,Se phase on the electrode surface. A copper concentration of 40 PM was found to be optimum for analytical purposes. The hydrochloric acid concentration added to sea water contaming 40 PM copper and 5 nM selenium was varied in order to determine the optimum pH for the determination of selenium using CSV. The results (Fig. 3) showed that the height of the Cu,Se peak initially increased with increasing hydrochloric acid concentration up to 0.025 M (giving a sea-water pH of 1.6), whereas it diminished at higher concentrations. The reaction mechanism (Eqn. 1) explains that the reduction of Se(W) to Se( - II) is favoured by a low pH, whereas at pH < 1.6 the peak height diminishes as a result of protonation of the selenide competing with the formation of Cu ,Se complexes. Effect of varyng deposition time

the deposltron

potential

and

Variation of the deposition potential revealed that the peak height obtained for selenium using CSV with a depositron time of 3 min increases with diminishing deposition potentrals between -0.2 and -0.4 V (Fig. 4). The change in the peak

DETERMINATION

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0

belaht

OF SELENIUM

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(nA)

0 05

0 15

may be related to the charge on the electrode, which changes from positive to negative at potentials less than ca. - 0.5 V in chloride media [20]. It is possible that the absorption of Cu,Se complex species is enhanced at the more positive potentials if interaction of the selenide in the complex with the mercury electrode is required. The reduction current for selenium was measured using CSV as a function of the deposition time using the optimized analytical procedure. It was found that the slope of a plot of the peak current vs. deposition time increased shghtly with increasing deposition time at deposition times < 3 min, was constant at deposition times between 3 and 8 min, and decreased at deposition times > 8 min. The plot of the peak current as a function of the deposition time was therefore slightly S-shaped (Fig. 5). The less than linear mcrease at long deposition times is common to all techniques measurmg trace element concentrations using CSV with adsorptive collection, and is caused by saturation of the surface of the HMDE (e.g., [17,18]). The non-linear behaviour at short deposition times is unusual, and may be caused by an increase in the deposition of Cu,Se due to the concentration of Cu(1) being increased at the electrode surface during the lengthening deposition step as a result of adsorption of Cu(1) {adsorption of Cu(1) on the HMDE is known to occur from chloride electrolytes [21]}. A shift of the peak

02

Ftg. 3. Effect of varymg the HCl concentratton on the peak herght obtamed usmg CSV for Se (5 nM) m sea water. Cu potentral, -0 3 V; depost40 pM; deposrtron concentratton, tton ttme, 3 mm.

height was comparatively small at potentials between -0.3 and -0.4 V, so a deposition potential in that range was considered optimum. The absence of a CSV peak for selenium at deposition potentials more positive than -0.2 V may be caused by the progressively mcomplete reduction of Se(N) to Se(-II), for which the reduction potential lies near - 0.05 V [6,7]. A peak for this reaction is masked by the mercury oxidation wave in chloride-containing electrolytes such as sea water. The reduction in the peak height obtained at deposition potentials more negative than -0.4 V 4.

peak

he,ght

(nA)

I

i-0.

deposrtron of varymg the deposition potential on the C?X peak current for Se (1 nM) m sea water The standard analyttcal procedure was used Deposttron time, 3 mm.

20

i-5

time

20

(minutes)

fig. 5 Etlect of increasing the deposrtton time on the CSW peak current for Se (2 nM) m sea water The standard analytrcal procedure was used.

226

potential in a negative direction with increasing deposition time (from - 0.71 V at 2 mm to - 0.80 V at 30 min deposition) as a result of complex stabilization is in agreement with this explanation. The Increasing sensitivity for selenium as function of the deposition time is obviously beneficial for the determination of low selenium concentrations. Interferences Possible interferences m adsorptive CSV include competitive adsorption of surface-active organic compounds present in natural waters, adsorption of interfering electroactive trace elements and organics and masking of the analytical peak as a result of complexation of the analyte by natural organic complexing ligands. The last effect is unlikely to happen to selenium, as Se(IV) occurs as an anion. Nevertheless, any orgamc complexmg ligands occurring in the water would be removed by the treatment used to remove the interference of surfactants and organics. The possible interference of surfactants is illustrated by the effect that Triton X-100 has on the CSV peak for Se. Additions of Triton X-100 were found to diminish considerably the peak current obtained for Se, the effect being stronger at a longer deposition time as the compound would start to saturate the surface of the HMDE (Fig. 6). Electroactive organic material present in untreated sea water from the North Sea and from the Tamar estuary was found to produce a peak between -0.45 and -0.6 V, more positive but close to that obtained for Cu,Se using CSV. This organic interference was removed by UV irradiation of the water for 3-8 h prior to the selenium determination. Similarly, poor sensitivity for selenmm in untreated sea water samples as a result of adsorption of organic surfactants was found to be much improved by UV irradiation of the water. The possible interference of various trace elements was investigated by addition of these elements to sea water and determining their effect on the CSV determination of selenium. The following did not produce any effects: Pb (10 nM), In (10 nM), Ni (200 nM), Co (50 nM), Zn (200 nM), V(V) (200 nM), Zr (100 nM), Ti (200 nM), Sb (100

C M G

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peak

heqht

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in,,)

I

”

01

02

Trlton

03

04

0:

X-100 (pprn)

Fig. 6. Effect of Interference by surfactants such as Tnton X-100 on the CSV peak height obtained for Se (5 nM) m UV-Irradiated sea water Data obtained after (B) l-mm deposltlon and (+ ) 3-mm deposition.

nM), Bi (10 nM), U(V1) (100 nM), Fe(II1) and Mn(IV) (lpM), Ge (50 nM), Sn(IV) (100 nM), Mo(V1) (500 nM), Ba (500 nM), W(V1) (100 nM), Al (200 nM), Cr(V1) (100 nM) and As(II1) (250 nM). A small peak was produced at -0.65 V by 100 nM Cd; this peak was due to the diffusion current of dissolved Cd2+ as this metal was neither plated nor adsorbed under the conditions used. Addition of 5 nM Te(IV) produced a CSV peak at -0.87 V; this peak was well removed from the selemum peak (about 150 mV more negative), but caused the latter to decrease in height by ca. 70%. As Te is chemically similar to Se, it is not surprising that a peak is formed for this element as well. However, the mechanism of formation of the Te peak is different from that for Se, as the former was found to be independent of the copper concentration. Perhaps it is caused by deposition of a mercury(I1) tellurite. The conditions used here to determine Se in sea water are not optimum for Te, as its peak occurs on the shoulder of the hydrogen wave. A procedure for determinmg Te m aqueous solution of pH 4.5 using CSV has been devised previously [12]. Neither Cd nor Te interfere with the determination of Se in unpolluted sea water, as their concentrations are normally much lower than those used here to illustrate their potential interfering effect.

DETERMINATION

OF SELENIUM

227

IN SEA WATER

The sensitivity (peak current/ selenium concentration) was the same in distilled water and in sea water, indicating that the major ions in sea water do not interfere with the determination of selenium using CSV.

Conversion of Se(VI) to Se(IV) The CSV peak for selenium appears only in the presence of Se(W), as Se(V1) cannot be reduced electrochemically [6,7]. This was confirmed by additions of Se(W) to sea water, which did not increase the CSV peak for selenium. Se(V1) therefore has to be reduced to Se(W) prior to the determination of total dissolved selenium using CSV. Se(V1) can be reduced to Se(W) by boiling with 4-6 M hydrochloric acid [5,22] or by UV irradiation of the sea water at controlled pH [3]. The latter method was considered most convenient as sample losses could occur during boiling, and as UV irradiation has to be used anyway to eliminate the interference of natural organic surfactants and electroactive organic compounds. The effect of the solution pH on the conversion of Se(V1) to Se(W) was tested by irradiating sea water (originating from the North Sea), to which 4 nM Se(V1) had been added, at pH values between 3.7 and 8.3. The following recoveries were obtained, final pH values after UV irradiation being given in parentheses: 5% (pH 3.7), 4% (pH 6.6), 79% (pH 7.5), 87% (pH 7.7), 100% (pH 8.19). In order to ascertain the conversion efficiency at pH

values between 8.1 and 8.3, tins experiment was repeated with sea water to which 2 nM Se(V1) had been added, giving the following recoveries: 99.6% (pH 8.11), 101.4% (pH 8.19), 100% (pH 8.21) 101% (pH 8.24) 100% (pH 8.29) and 100% (pH 8.30). The average recovery was 100.3 f 0.6% at pH values betweenc8.1 and 8.3, covering the natural pH range of sea water. The increased conversion efficiency of Se(V1) to Se(W) with increasing pH is in general agreement with previous data [3], but the previous data only reached a maximum efficiency of between 74 and 86% in the presence of hydrogen peroxide, depending on the type of UV irradiation apparatus used. Other work has shown that the conversion is inhibited by hydrogen peroxide in addition to dissolved oxygen [lo]. It is therefore recommended to check the working order of the UV lamp regularly by determining the Se(VI)/Se(IV) conversion efficiency.

Dynamic range and kmit of detection The limit of detection was calculated

from repeated determinations of a low level of selenium in a sample from the Indian Ocean, which was found to contam a selenium concentration of 0.47 + 0.02 nM (mean f s.d., n = 11). The electronic noise of the measurements was ca. *6%. The 3a limit of detection using these data was 0.06 nM using a deposition time of 3 min. This limit of detection could be reduced by using a longer

(b) 2w 1

(4

I

JL 2nA

3

2

1 * -06

-07 Potential(V)

-08

0

50

100 150 se (nM)

200

250

Fig T Determmatlon of Se m sea water usmg C?F (a) CXVscans for Cr.5, I 5 and- 2 5 nM_Se m sea water. DepositIon time, 1XG s (6) CSV peak current for Se m sea water as a function of Se concentration Deposmon time, 1 mm.

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depositton time, as shown in Fig. 5; thus the detection limit would be ca. 0.01 nM with a deposition time of 15 mm. This limit of detection 1s well below that (3 nM) using adsorptive collection of the ptazselenol [14] and that previously quoted (ca. 0.5 nM) for the determination of selenium in the presence of copper [ll]. A CSV scan and two standard additions to the sample are shown in Fig. 7a. The decrease in the capacitance current at potentials immediately negative of the selenium peak (apparent in the cychc voltammetric experiments) was not visible at low selenium concentrations and when differential pulse modulatton was used. The linearity of response of the CSV method was ascertained by standard additions of Se(W) to sea water wlnlst measuring the peak current by CSV wtth an adsorption time of 60 s. The peak current was found to increase linearly with increasing selenium concentration between 0.2 nM (the Se concentration onginally present in the water) and 200 nM (the highest concentration tested (Fig. 7b). Stmilar tests carried out at longer deposition times of 180 and 300 s showed that the response was linear over a tested range of 0.2-20 nM selemum m both sea water and distilled water. It is to be expected that at enhanced selenium concentrations of > 200 nM (outside the range tested), the response will become non-linear as a result of saturation of the electrode surface with adsorbed Cu,Se complex species, causing the sensitivity to diminish. This non-linearity was not observed even though CSV was tested over a very wade range of selenium concentrations, perhaps because the linear range is very long owing to the small size of the adsorbing complex species compared with that of adsorbing complexes of Cu(I1) with catechol, for instance [17]. The described method was used to determine total dissolved selemum in samples from the North Sea and the Indian Ocean, giving 0.2 and 0.5 nM, respectively, m line with expectation for such waters [23]. Prelimmary measurements carried out on samples from the Tamar estuary revealed that the labile Se(W) concentratton could not be measured as a result of interference by dissolved surface-acttve orgamcs. The total dissolved selenium was

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determined after UV photolysis, giving a value of 2.4 nM at the low salinity end (S = 0.1) and 0.12 nM at the high salinity end (S = 34) in water from the Plymouth Sound. A detailed study of the estuarine geochemistry of selenium is currently being carried out using the method presented here.

The authors gratefully acknowledge encouragement given by Prof. J.P. Riley to this proJect. The research of S.H. Khan was supported by a fellowship of the Ministry of Science and Technology of Pakistan.

REFERENCES

1 C.1 Measures, B.C Grant, B J Mangum and J M Edmond, m C.S Wong, E. Boyle, K W Bruland, J D Burton and E.D Goldberg (Eds.), Trace Metals m Seawater, Plenum, New York, London, 1983, p. 1. 2 R J Shamberger, Blochermstry of Selenium, Plenum, New York, 1983. 3 C.1 Measures and J.D Burton. Anal Chm. Acta. 120 (1980) 177. 4 HA. van der Sloot, D Hoede, J Wykstra, J C Dumker and R.F Noltmg, Estuarme Coastal Shelf SIX., 21 (1985) 633. 5 S.C Apte and A.G. Howard, J Anal At. Spectrom, 1 (1986) 379 6 I M. Kolthoff and J J. Lmgane, Polarography, InterscIence, New York, 1952, pp. 564-561 I G D Chnstmn, EC. Knoblock and W.C Purdy, Anal Chem , 35 (1963) 1128 8 G Jarzabek and Z. Kubhk, Anal Chum. Acta, 143 (1982) 121 9 S B AdeloJu, A M. Bond and M H Bnggs, Anal. Chem ,56 (1984) 2397 10 G E Batley, Anal. Chum. Acta, 187 (1986) 109 11 U Baltensperger and J. Hertz, Anal Chlm. Acta, 172 (1985) 49 12 G Henze, P Monks, G Tdlg, F Umland and E Wesshng, Fresemus Z. Anal Chem., 295 (1969) 1 13 A G Howard, M R. Gray and A R Oronuehe, Anal. Chlm Acta, 118 (1980) 87 14 V Stara and M. Kopamca, Anal Chum Acta, 208 (1988) 231. 15 R M. Snnth and A E Martell, Cntlcal Stabthty Constants, Plenum, New York, 1976. 16 M von Stackelberg and H von Freyhold, Z. Electrochem , 46 (1940) 120 17 C M G van den Berg, Anal Chrn Acta, 164 (1984) 195 18 C M.G van den Berg, J. Electroanal Chem., 215 (1986) 111

DETERMINATION

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19 D Dyrssen, Mar. Chem , 24 (1988) 143 20 J. Heyrovsk and J KBta, Pnnclples of Polarography, Acadenuc, New York, 1966, p. 21 21 A. Nelson, Anal. Chm. Acta, 169 (1984) 273 22 J. Pettersson, L. Hansson, U ijmemark and A. Olin, Clm Chem., 33 (1988) 1908.

229 23 K.W. Bruland, m J.P Rdey and R. Chester (Eds ), Chemlcal Oceanography, Vol. 8, Acadenuc, London, 1983, p 157