the determination of the chromium speciation in sea water using catalytic cathodic stripping...

Analytlca Chumca Acta, 262 (1992) 103-115 Elsevler Science Publishers B V , Amsterdam

103

The determination of the chromium speciation in sea water using catalytic cathodic stripping voltammetry

Marc Boussemart I, Constant M G van den Berg * and Mohammed Ghaddaf 2

Oceanography Laboratory, Earth Sciences Department, Vnrverslty of Lwerpooh Lwerpool L69 3BX (UK)

(Received 5th November, revised manuscnpt received 30th January 1992)

Abstract

A voltammetnc procedure to determine chronuum m aqueous solution using cathodic stnppmg voltammetly (CSV) preceded by adsorptive collection of complex species with diethylenetnannnepentaacetlc acid (DTPA) on a hangmg mercury drop electrode LS optlmlzed for sea water The optmuzed conditions Include a pH of 5 2 and a DTPA concentration of 2 5 mM Comparison of the chemical speclafion of DTPA m sea water and chstdled water with the CSV senslttvlty mdicated that the adsorptwe species IS a complex of chrommm(II1) wtth H,DTPA The sensltmty of the CSV procedure for chrommm(VI) m sea water IS a decade lower than m dlstdled water due to major cation cornpetItion (of c&urn and magnesium) for DTPA m the sea water The hmlt of detection for chrommm(VI) m sea water IS 0 1 nM at a deposition tune of 2 mm Chrommm(VI) produces a stable peak using the optmnaed CSV procedures, whereas the peak due to chrommm(III) IS unstable due to probable conversion of the chrommm(II1) complex to an electrochenucally mert complex over a penod of = 30 mm The chfferent behavlours of chronuumCVI1 and -(III) were utdlzed to devise a procedure to determme reactnre chrommm(III) and chrotmum(VI) m sea water samples Experiments with an added chelator indicated that the reactive chrommm(III) concentration does not mclude complexes with organic matenal The total &solved chrommm concentration was determined after UV- uradlation of the sample at Its ongmal (neutral) pH Storage expenments indicated that neither the redox spectation of chronumn nor Its total dissolved concentration m sea water could be stored The CSV method was applied successfully to certified sea water samples and to the ship-board determination of chrommm(V1) and <III) in the water column of the NW Mechterranean

Keywords Catalyhc methods, Stnppmg voltammetry, Chrommm, Sea water, Waters

Chromium occurs naturally m two oxldatlon states, chrommm(VI) and chrommm(II0, m natural waters (see eg [1,2]) At the natural pH of sea water the predommant species of chrommm(VI) 1s CrOf- whereas of chrommm(II1) It 1s Cr(OH), and Cr(OH)i [3] Previous mvestlgatlons have shown that the predominant oxldatlon state of chrornmm 1s chrommm(V1) m oxygenated sea water whereas it 1s chrornmm(II1) m anoxlc condl-

tlons [l] The oxldatlon state of chromium 1s readily changed m the natural system as chrommm(II1) IS oxldlzed for mstance by MnO, [41 and H202 [51

The usual determmatlon of chromium m natural waters 1s by means of atormc absorption spectrometry with preconcentratlon by coprecrplta- tlon on lron(II1) hydroxide [2] Only chrommm(II1) adsorbs on the lron(II1) hydroxide whereas chrommm(VI) 1s preconcentrated sepa- rately after reduction with lron(II) However, other preconcentratton techntques include ad-

sorption of complexed chrommm(II1) on macro- porous resin 161 and hqmd-liquid extraction [‘7j

’ Present address RhBne-Poulenc Agriculture Ltd , Fyfield Rd , Ongar, Essex CM5 OHW, UK

* Present address Marme Science Centre, Aden, Yemen

0003-2670/92/$0.5 00 0 1992 - Elsevler Science Pubhshers B V All rights reserved

104 M Boussemart et al /Anal Chtm Acta 262 (1992) 103415

Chrommm can be determined directly m water (without previous extraction) using anodlc stnp- ping voltammetry [81 or polarography [9] A more Fen&we procedure consists of cathodic stripping voltammetry (CSV) preceded by adsorptive collection of the complex of chrommm(II1) with dlethylenetrrammepentaacetlc aad (DTPA) [lo] or with trlethylenetetraammehexaacetlc acid 0THA) ill1 on a hanging mercury drop electrode (HMDE)

The CSV method is here adapted for the determination of chrommm(VI) and total dissolved chrommm m sea water It will be shown that total dissolved chromium can be determined m sea water after W-irradiation of the sample, whereas chrommm(IV) can be determined speclflcally m the untreated sample after a suitable reaction period to eliminate interference by chromlum(III) The chrommm(II1) concentration can then be calculated by difference The reactive (morgamc) chrommm(II1) concentration can also be determined directly by measuring the peak height produced mnnedlately upon the addition of DTPA to the untreated sample

Comparative expernnents were carried out usmg TTI-IA as adsorptive chelating agent for chrommm(III), the results usmg this compound were generally similar to DTPA but no advantage was obtained with this hgand so only the results with DTPA are reported here

EXPERIMENTAL.

Equrpment Voltammograms were recorded with an Auto-

lab polarograph (Ecochemle, Netherlands) con- nected to a Metrohm model 663 HMDE (drop surface area = 0 38 mm*), controlled by an IBM- compatible personal computer with a 80286 ml- croprocessor Some comparative expenments were carried out with a PAR 303 HMDE (drop surface area 2 94 mm2) The voltammetrlc cell was glass, and solutions m the cell were stirred usmg a rotating PTFE rod (Metrohm electrode) or a PTFE coated magnetic stirrer (PAR electrode) Potentials are given Hrlth respect to a Ag/AgCl, saturated AgCl m 3 M KC1 (SSCE)

reference electrode Samples were stored m high-density polyethylene bottles, which were cleaned usmg standard procedures (soakmg m 50% hydrochloric acid (BDH, AnalaR), followed by soakmg m 2 M nitric acid, and subsequently stored partially filled with dlstllled water of pH 2) All sample handling was carried out m a lammar-flow hood supphed with filtered air

Reagents Reagents were from BDH (AnalaR quality)

unless indicated differently Water was purified by reverse osmosis (Mllh-Ro, M&pore) and ion- exchange (Mllh-Q) Ammonia was purified by isothermal dlstlllatlon and hydrochloric acid by sub-bollmg dlstlllatlon

An aqueous solution of DTPA (Sigma) was prepared contammg 0 25 M DTPA An acetate pH buffer was prepared contammg 2 M sodium acetate (BDH, Anstar) and = 1 M ammonia Addition of 0 02 M of the acetate solution to sea water gave a pH of 5 2 A second acetate pH buffer was prepared contammg = 12 M ammoma gave a pH of 6 2 m dlsttlled water A stock solution of chrommm(V1) was prepared from potassmm chromate m water and contamed 1 mM chrommm(VI) This solution was found to be stable for at least 3 months A stock solution of chromrum(II1) was prepared from an atomic absorption standard solution for chrommm (BDH) containing chromium chloride and diluted with 1 M hydrochlonc acid

Contaminating chrommm was removed from an aqueous solution of 5 M sodium nitrate by co-precipitation with u-on(III) hydroxide, lron(II> chloride (0 1 mM) was thereto added to the solution and the pH was adJusted to neutral wrth ammonia, the lron(II) iron was omdlzed by the dissolved oxygen to lron(III) and allowed to pre- cipitate The suspension of lron(III) hydroxide was subsequently filtered to produce the purified nitrate solution Addition of 0 5 M of the thus purified nitrate to sea water contributed < 0 03 nM chrommm to the sample

Sea water used for preliminary experiments originated from the North Sea (salinity = 34 psu) or the Indian Ocean (sahnlty = 35 psu) (salmltles are expressed m practical salinity units (psu))

M Boussemari et aL /Anal Chm Acta 262 (1992) 103-115 105

Estuanne samples were collected from the Mersey estuary m collaboration with NRA Northwest, and from the Mediterranean durmg the “Cybele” cruise with the research vessel Marion Dufresne, April 1990 as part of the EROS project supported by the European Commumty The Mersey samples were filtered m the laboratory through 0 45 PM Oxold cellulose acetate filters The Mediterranean samples were collected usmg 15-1 Go-Flo bottles The samples were filtered through 0 4 pm Nuclepore membrane filters and analyzed on-board ship Comparative analyses were car- ned out m samples which were stored at the natural pH, frozen at -20°C

Procedure to determrne total drssolved chromium in sea water

Sea water was W-u-radiated for a penod of 4 h using either a l-kW high-pressure or a 100-W medium pressure mercury vapour lamp An ahquot of 10 ml of the sea water was plpetted mto the voltammetnc cell, then 1 ml of 5 M KNO, (final concentration of 0 5 M), 100 ~1 of 2 M acetate buffer (final concentration of 0 02 M glvmg a pH of 5 2) and 100 ~1 of 0 25 M DTPA (final concentration of 2 5 n&l DTPA) were added (the reagents can also be pre-mnced to mmlmlze the plpettmg during the reagent addl- tlons) The solution was deaerated by purging for 5 mm with oxygen-free, water saturated nitrogen Then a new mercury drop was made, starting the adsorption peirod of 60 s whilst the solution was being stirred Then the stirring was stopped and a qmescence period of 10 s was allowed Subse- quently the potential scan was carried out m a negative potential direction using the square-wave modulation, the step height was 2 5 mV wth a frequency of 100 Hz, the scan rate was 250 mV s-l Comparative measurements were carried out using the differential-pulse modulation with a pulse rate of 10 s-l, a scan rate of 20 mV s-l and a pulse height of 25 mV The peak corresponding wth the reduction of the adsorbed DTPA complex of chrommm(II1) appeared at - 122 V The sensltlvlty was calibrated by addition of chromlum(V1) standard solution sufficient to double the peak height

RESULTS AND DISCUSSION

Catalytrc CSV of chromrum m sea water The CSV scan of chrommm(VI) m sea water

(using the standardized conditions) produced a peak at - 122 V, at the bottom of the hydrogen wave The reduction current 1s due to the reduction of chrommm(II1) to chrommm(I1) and 1s enhanced by a catalytic effect m the presence of nitrate ions owmg to the chemical reoxldatlon of chrommm(I1) to chrommm(III) which 1s subsequently re-reduced at the electrode surface [lo] The scan was preceded by adsorptive collection of chromlum(II1) complexes with DTPA The chromnun(II1) 1s produced freshly during the adsorption step by reduction of the dissolved chrommm(V1) at the electrode surface durmg deposition at - 10 V (chrommm(VI) 1s reduced to chrommrn(II1) at potentials < -0 05 V, the reduction potential of chrommm(VI)), and subsequently forms a complex with the DTPA which adsorbs on the mercury drop

Addition of chronuum(II1) to the solution produced the same reduction peak at - 122 V mdlcatmg that the adsorptive complex 1s also produced when chrommm(II1) 1s present m solution However, the reduction current of the dissolved complex of chrommm(III) was not stable and gradually dunmrshed m height, disappearing altogether m approxunately 30 mm (discussed below) The CSV sensltlvlty for chrommm(II1) unmedlately upon the DTPA addition was less (ca 30%) than that for the same concentration of chrommm(V1) probably as a result of slower dlf- fusion of the dissolved chromnnn(III)-DTPA complex than of the uncomplexed chrommm(V1) to the mercury surface

Comparative experiments m distilled water and sea water showed that the sensltlvlty for chromuun(V1) differed considerably bemg 20-fold lower m the sea water The analytical conditions for the determmatlon of chronuum m sea water using CSV were therefore optmnzed

Optlmuatwn of the anulyttcal cond1tlon.s DTPA, pH, deposztwn potentuzl, and competrng cations

Several parameters affecting the CSV sensltlvlty were vaned whilst the reduction current of the

106

chrommm(III)-DTPA complex was monitored after 60 s adsorptlon at - 1 V from sea water of pH 5 2 contauung 2 5 mM DTPA, 0 5 M NO; and 10 nM chrommm(V1) unless indicated other- wise The results are summarized m Fig 1

Vanatlon of the DTPA concentration showed that the peak height for chromuun mcreased untrl 2 5 mM DTPA whereafter a small decrease was apparent (Fig 1A) The optunal DTPA concentration of 2 5 mM for analytical purposes 1s slightly smaller than that (5 mM DTPA) required m fresh water [lo], this fmdmg 1s unexpected m view of the competition by the major ions m the sea water for DTPA The decrease of the peak height for chromium at higher concentrations of DTPA m sea water may be due to the formation of higher order complexes such as chrommm(III)- DTPA,

The adsorption of the chrommm(III)-DTPA complexes was found to be strongly dependent on the deposition potentral (Fig 1D) reaching mm- mum adsorption at - 1 V The adsorption was very low at potentials more positwe than - 0 5 V where the charge on the mercury drop electrode

M Boussemart et al /Anal Chrm Acta 262 (1992) 103-115

IS positive m electrolytes contammg chlonde ions such as sea water [12], suggesting that the adsorb- mg chrommm(III)-DTPA complex has a posltme charge

Vanatlon of the pH showed that a pH between 5 0 and 5 3 1s optimal for the determmatlon of chrommm m sea water The optimal pH range 1s lower than for fresh water where a pH of 6 4 1s optimal [lo] which produces poor sensltlvlty m sea water pH values near 5 m sea water and near 6 m fresh water can both be conveniently buffered usmg acetate pH buffer due to calcuun and magnesium competition for the acetate Ions m sea water, so the same pH buffer can be used for fresh water and sea water analysis of chrommm although the actual pH values differ

Comparative measurements showed that the CSV sensltlvlty m sea water of pH 5 2 was consld- erably lower than m fresh water of pH 6 4 Vana- tlon of the concentrations of calcmm and magne- slum m dlstllled water showed that the CSV peak height for chromium decreased with increasing concentration of these major cations, at the same time shlftmg the peak potential to more positive

Collection potential WI

Fig 1 The effect of varymg the DTPA concentration (A), the pH (B), the magnesmm Ion concentrabon (0 and the deposItIon potential (D) on the CSV peak height for chrommm m sea water (A, B and D) and cbstdled water (C) Standard conchtlons are used unless Indicated differently The chrommm concentration was 2 nM as chrommm(VI)

M Bouweman et al /Anal Chmt Acta 262 (1992) 103-115

potentials The results for magnesium are shown m Fig 1C This fmdmg suggests that major cation competition for DTPA 1s responsible for the lower sensltlvlty m sea water as well as for the lower stab&y of the adsorbed chronuum(III)-DTPA complexes

Vanatlon of the mtrate concentration (not dn- played) showed that the CSV peak height for chrommm m sea water mcreased lmearly with the nitrate concentration until 0 5 M mtrate and non- lmearly thereafter until approxnnately 12 M m- trate whereafter the peak height started to dnnmlsh probably due to dllutlon of the DTPA concentration m the solutlon (a correction was made for the dllutlon of the chrommm concentration) The optunal nitrate concentration (12 M) is somewhat higher than that observed previously in fresh water (0 5 M) The hnear relationship of the sensitivity with the nitrate concentration at m- trate concentrations below ca 1 M 1s m lme with expectation as the reduction current depends on dlffuslon of the oxidant to the electrode surface durmg the scan

A nitrate concentration of 0 5 M was used by us for the optumzed analytical conditions as the solublhty of sodnun nitrate m the stock solution 1s lmuted to ca 5 M at room temperature, so the chrommm m the sample becomes slgmflcantly diluted when a concentration greater than 0 5 M 1s used unless the nitrate would be added as the salt

The specmon of DTPA The chemical speclatlon of DTPA (total con-

centration 2 5 mM) was calculated as a function of the pH for a solution snmlar to sea water (contammg 53 mM Mg*+) and fresh water m order to investigate which species is responsible for the formation of the adsorptive complex with chromnun(II1) and to explain the dtierent behavlours m sea and fresh water The following equlhbrmm constants for DPTA and its chelates were used 1131 pK,, = 182, pK, = 2 66, pK, = 4 3, pK, = 8 59, pK, = 10 55 and log K,,, = 9 3, log K,, = 5 84 (Y = unprotonated DTPA) It can be seen m Fig 2 that the concentration of the H,Y species reaches a maximum at pH 6 45 m fresh water and at pH 5 05 m sea water These

3 FRESH WATER

HZY

107

Ia

SEA WATER

9 H3Y HZY

00 20 40 60 80 100 120

PH

Fig 2 The protonatlon of 2 5 mM DTPA as a function of the

pH m fresh water (top) and sea water (bottom) Note the different vertical (concentration) scales

pH values agree very well with the optnnal sena- tlvltles obtained at pH 6 4 m fresh water [lo] and at pH 5 0 m sea water (Fig lB), suggestmg that a complex of H,Y with chrommm(II1) adsorbs on the mercury drop

Assuming that the H,Y speaes of DTPA (DTPAH,) is indeed responsible for the formation of the adsorptive complex with chrommm(II1) the calculated speclatlon explams that the optl- ma1 pH IS lower in sea water as a result of major cation competition for DTPA Further it can be seen that the H,Y concentration m sea water (= 1 PM) 1s much lower than m fresh water (= 2 5 n&I) at the optimal pH values due to the major cation competition m the sea water, clanfy- mg why the maxunal sensltlvlty m sea water 1s lower than m fresh water The stab&y constant for the formation of the chrommm(III)-DTPAH,

108 M Boussemarr et al /Anal Chum Acta 262 (1992) 103-115

__

02 03 04 05 08 Trkon X 100 lppml

Fig 3 The interference of Trlton X-100 m the determmatlon

of 6 nM chrommm(V1) m UV-xradlated sea water at two

different adsorptlon times (60 s and 120 s)

complex (log K value) 1s 3 67 [14] lllustratmg that the stability of this complex is not high m sea water where the dissolved concentration of DTPAH, 1s particularly low The actual degree of complexatlon of chrommm(II1) by the DTPA can not be calculated as the effective DTPA concentration at the mercury drop surface 1s m- creased by adsorption of the DTPA on the mercury

Interferences Interferences m adsorptive CSV include com-

petitive adsorption of surface active compounds or of other metal complexes on the mercury drop electrode The potential interference by surfactants was tested by addmg Trlton X-100, a non- lomc surfactant which 1s often used to model effects of surfactants occurring in natural waters [15,16], to sea water and monitoring the peak current using CSV Natural waters typically contam 02-2 kg ml-’ of compounds with a surface-active effect similar to that of Trlton X- 100 [15] It can be seen in Fig 3 that the sensitivity 1s strongly affected by addltlon of even low concentrations of surfactant, the peak being completely inhibited m the presence of 0 4 ppm Tn- ton X-100 at an adsorption time of 120 s Addl- tlon of humlc acid (Fluka) to the sea water also dmumshed the sensitivity, completely eliminating the peak for 10 nM chrommm(V1) at a level of 5

pg ml-’ The interfermg effect is less at an adsorption time of 60 s (Fig 3) lllustratmg that it may be possible to nnprove the sensltlvlty by lowering the adsorption time when the analysis is subject to interference by natural surface active compounds The interference by natural surface active compounds was found to be ehmmated by UV-irradiation of the sample

Other metals can interfere if they form adsorptive electroactlve complexes with a peak potential near to that for chromium No mterfermg effect was observed by the addition of cobalt (5 nM), mckel(50 nM), zmc (100 nM), tltamum (100 nM), manganese011 (100 nM) and rron(II1) (1

PM) A kmetlc effect was produced by higher iron

additions The sensltlwty for chromium was found to dnnmlsh gradually by the addition of 10 PM lron(III), showing a 90% decrease m 10 mm and dsappearmg altogether m 15 mm, presumably as a result of competition for DTPA The chromium peak did not reappear upon addition of chrommm(VI) mdlcatmg that perhaps a DTPA complex of lron(II1) adsorbed on the mercury drop and mterfered with the adsorption of the chromium complex 10 FM levels of iron do not normally occur in sea water from coastal or oceanic origin but can occur m mterstmal waters of sediments

The chromium concentration can be determined accurately also m the presence of low levels (sub hg ml-‘) of surface active compounds or high levels of non by using internal standard additions of chromium

It was found that high levels of hydrogen peroxide and nitrite interfered with the CSV determination of chromium by producing a broad peak positive of the chronuum peak and overlappmg with that of chromium respectively Hydrogen peroxide 1s sometnnes added (e g [lo]) to lm- prove the W-irradiation treatment, and our experiments indicated that this hydrogen peroxide was not destroyed d the radiation was carried out at a low temperature (ca 30°C) perhaps because the irradiation was carried out at neutral pH which increases the thermodynamic stabdlty of hydrogen peroxide somewhat Nitrite was found to be produced by UV-irradiation of samples

M Boussemnrt et al /Anal Chum Acta 262 (1992) 103-115

which had been stored after ac&flcatlon with mtnc acid

Contammatwn from reagents The contnbutlon of chronuum from freshly

prepared reagents to the sea water was approxl- mately 0 3-O 5 nM It was observed that this contribution dlmuushed upon storage of the reagent solutions and was found to be due to chrommm(III) and chromuun(VI) present 111 the nitrate solution It was found that this chromuun could be readily removed by addltlon of 0 1 mM lron(I1) chloride to the nitrate stock solution and neutrallzatlon with ammoma, followed by oxldatlon of the lrodl1) iron to lron(III) hydroxide by the dissolved oxygen Hrlth concomitant reduction of chrommm(VI) to chrommm(III) and the subse- quent copreclpltatlon of all chrommm(II1) with the lron(III) hydroxide (this procedure 1s ldentl- cal to that used for the preconcentratlon of chrommm from sea water [l]) The preapltate was removed by fdtratlon (0 45 km Mdhpore filter) The contrlbutlon of chrommm from the thus pursed reagents was less than 0 03 nM

151 I __

0 li:.:..:-: 1 In,”

0 5o (“do0 150 Cr VI

5- B

5

4

109

Limit of detectwn and Hnear range of the deter- mmatwn of chromuun(V7) m sea water

CSV of chromnun added to UV-lrradlated sea water showed that the peak height increased lmearly with the chrommm concentration up to 20 nM (adsorption tune 60 s) and non-hnearly thereafter (Fig 4A) This linear range IS much greater than the range of chrommm concentrations occurring m sea water (normally well below 10 nM)

The peak height for 6 nM chromnun 111 sea water was found to increase lmearly with the deposition time up to 4 mm and non-linearly thereafter, the peak height after a deposition time of 15 mm betng 11 times greater than after

one of 1 mm The non-lmeanty IS probably caused by saturation of the drop surface with adsorbed complexes CSV scans for low levels of chrommm m sea water from which the chrommm had been removed by co-preapltatlon usmg lron(IIl)hy- droxlde (added as iron( are shown m Fig 4C The scans were carried out using the square-wave modulation at a frequency of 100 Hz

Variation of the square-wave frequency showed that the sensltlvlty increased with the frequency

” 5,

I

3O , 0' I

50 100 150 200 250 1 11 12 13 14

SW Frequency (Hz) Potential (VI

Fig 4 CSV of chrommm(VI) m sea water The lmear range at two deposlhon times, 60 s and 120 s (A), the effect of varymg the square-wave frequency (B), and scans for low levels of chrornmm(Vl) m punfied sea water (deposItIon time 60 s), the chrommm(VI) concentrations are mdxated on the drawmg

110

unttl levelhng off at frequencies between 50 and 100 Hz, whilst the sensltlvlty dlrmnlshed at higher frequencies probably due to poor electrochemical reverslblhty of the reduction step (Fig 4B)

The standard devlatlon of a determmatlon of 3 nM chrommm m sea water usmg the optlmlzed conditions was 3% (n = 6) The sensltlvlty was 5 8 nA nW’ nun-’ A hmlt of detection of 0 1 nM wth a deposmon time of 2 mm can be calculated from 3 x the standard deviation of this deten- nation This lumt can be reduced to ca 0 02 nM m UV-Irradiated sea water by mcreasmg the adsorption time to 15 mm The voltammetric scans for low levels of chrommm m punfled sea water are shown m Fig 4C It can be seen that the lnmt of detection 1s not determined by electronic noise but by the slope of the baseline as the chrommm peak 1s located at the bottom of the hydrogen wave The actual reduction current 1s large due to the catalytic effect of the mtrate

Chromrum (III) Addition of chrommm(II1) to the sea water

produced a peak which decreased with time, dls- appearing altogether after a period of approxl- mately 30 mm (Fig 5) leaving only the peak due to the chrommm(V1) which was orlgmally present m the sea water The same effect has been shown to occur m distilled water 1171

Rg 5 The decay of the peak height obtamed by CSV for 10 nM chrommm(II1) m the presence of 5 nM chrommm(VI) m sea water pH 8 Each scan was preceded by 60 s adsorptlon on the mercury drop

M Boussemart et al /Anal Chum Acta 262 (1992) 103-115

Measurement by atomic absorption spectrometry of the total dissolved chromuun(II1) concentration added to dlstllled water showed that the maJor part of the added chrommm was still m solution after the voltammetrlc peak had dlsap- peared, suggesting that the dunmlshmg peak height was not due to adsorptlon of the chrommm on the cell wall Apparently the electroactlve DTPA complex which is formed immediately upon the reactlon of the DTPA with chrommm- (III) 1s gradually converted to an electro-inactive complex One posslblhty IS that chrommm IS hy- drated m the first complex whereas It loses part or all of its hydration sphere m the second type Another posslblhty 1s that the second type of complex 1s polynuclear and does not adsorb on the mercury drop surface due to stereohmdrance

The reaction mechanism of the chrommm(II1) which 1s freshly produced from chrommm(V1) at the electrode surface could be qmte different from that which 1s orlgmally present m the solution as the freshly produced chrommm(II1) 1s known to be electrochemlcally very reactive [18] The peak due to chrommm(III) freshly produced from dissolved chrommm(V1) was found to be much more stable decreasmg only very slowly by ca 02% mm-’ m sea water of pH 5 2 This decrease was found to be due to the chemical reduction of chrommm(VI) by mercury waste m the voltammetrlc cell, this effect was stronger when a PAR303A electrode was used with a large mercury drop (drop area 2 9 mm’) than using the Metrohm mercury drop electrode with a surface area of 0 8 mm2 (thus producmg less mercury waste) Chrommm(V1) 1s known to be reduced by metalhc mercury [ 191

Additions of chrommm(II1) to sea water showed that the CSV sensitivity was 3 3 nA nM_’ mm-’ immediately after the addition Measure- ments by CSV of chrommm(III) added to sea water indicated that it 1s possible to determine the concentration of reactwe chromzum(ZII) by measuring its peak height by CSV at pH = 5 immediately after the reagent addltlon to the sample The sensmvlty was calibrated by a standard addltlon of chromlum(II1) after a reactlon time of = 30 mm to allow the slgnal to stablhze

Prellmmary experiments were carried out to

M Boussemart et al /Anal Chm Acta 262 (1992) 103-115 111

mvestlgate the Influence of complexatlon of chrommm(lI1) by organic hgands other than DTPA on the CSV sensltlvlty Addition of 0 5 mM citrate (added as citric acid) to sea water containing 2 5 mM DTPA and acetate buffer (pH 5 2) was found to decrease the CSV sensitivity for 10 nM chrommm(V1) by 20% Slmllarly the CSV sensltlvlty for 10 nM chrommm(lI1) m purified sea water (contammg 0 8 nM chrommm(V1)) was dlmmlshed by 20% by the addition of citrate (and the peak gradually dlmuushed m height as usual) However, no CSV peak was produced by 10 nM chrommm(II1) when the citrate was added to the sea water and complexatlon of the chrommm(II1) by the added citrate was completed by heatmg the sample to = 60°C for 30 mm m a mlcrowave oven, prior to the addition of the DTPA, mdlcatmg that the chromnun(II1) was complexed by the citrate and was either not released to the DTPA or was released to form the electro-inactive complex This result suggests that chrommm(II1) complexed by organic material present m sea water samples would not be part of the “reactive chrommm(110” concentration

The determmatlon of reactive chromaun(VI) m sea water

The concentration of reactive chrommm(V1) can be determined m the presence of chrommm (III) without prior treatment of the sample by allowmg the sample to react for a short period (30 mm> with added DTPA as Illustrated m Fig 5 Experiments at different pH values indicated that the contrlbutlon of the dissolved chrommm- (III) to the reduction peak dlmlmshed more rapidly at higher pH values whereas the stability of the chrommm(V1) towards reduction by mercury waste improved Thus the disappearance of the contnbutlon from chromlum(II1) was completed m a period of approximately 15 mm at pH 6 8 whereas the decrease m the peak height due to chrommm(V1) was negligible at that pH

Probably all chrommm(VD orlgmally present m the sample 1s determined as reactive chrommm(V1) as its major species 1s anionic CrO,Z- which 1s unlikely to undergo any strong mteractlons with organic complexmg hgands

which could render it electro-inactive m sea water

The followmg procedure was therefore followed to determme the concentration of chrommm(V1) ongmally present m a sample ammoma was added to the sample m the voltammetnc cell m addition to the usual DTPA-acetate buffer nuxture to raise the pH to ca 6 8, and a reaction tnne of 15 mm was allowed whilst the sample was bemg deaerated Then the pH was lowered to 5 O-5 2 by addition of hydrochloric acid equivalent to the ammonia added previously The dissolved concentration of chrommm(V1) could then be determined usmg the optlmlzed CSV procedure Complete conversion of the chrommm(II1) to an electrochemlcally mert complex was Indicated by stab&y of the peak height

The determmutlon of total dusolved chromrum in sea water

The chrommm(II1) IS known to be converted to chrommm(VI) upon UV-madlatlon of the sample [lo,171 and also by hydrogen peroxide [5] We found that it was not necessary to add hydrogen peroxide to the sea water m order to obtam complete conversion to chrommm(VI) by UV- irradiation of samples from the North Sea or the Atlantic Ocean However, samples from estuarme origin (the Mersey estuary) benefitted from the addition of hydrogen peroxide (final concentration 0 03%) to the sample to fully destroy the natural surface active compounds

Tests indicated that not all chrommm(II1) was converted to chrommm(V0 if the sample was irradiated at low pH (pH 2) and losses occurred probably due to adsorption of chrommm(II1) on the slhca walls, but complete conversion was achieved if the irradiation was carried out at neutral pH values (pH 7-8 5, mcludmg the natural pH of the sea water)

The concentration of chrommm(II1) (as total chrommm(II0, mcludmg complexes wth organic material) can be calculated from the difference between the total chrommm and the reactive chrommm(V1) concentratrons

The accuracy of the CSV method was evaluated by determmatlon of the total dissolved chromium concentration m certlfled sea water

112

Unfortunately the certified sea water orlgmatmg from the North Atlantic (NASS-2 [19]> was stabl- hzed by aadlficatlon to pH 16 wrth mtric acid which precluded UV-treatment as this produced mterfermg quantltles of mtrrte It was found that the chrommm(II1) could be converted quantlta- tlvely to chrommmW1) by reacting the sea water with MnO, The conversion efficiency was cah- brated by addltlon of known amounts of chrommm(II1) and was found to he between 97 and 104% NASS-2 was found to contam a total chrommm concentration of 3 3 f 0 2 nM (n = 3) which compares well with a certified value of 3 37 & 0 02 nM A sea water sample ongmatmg from the North Sea and recently prepared for certlflcatlon by BCR (Bureau for the Certlflca- tlon of Reference Materials of the European Commumty) was analyzed by the same procedure and was found to contam 2 03 f 0 2 nM chrommm (the chrommm concentration has not been certified by BCR 111 this sample and this value is given for information only)

Determrnatwn of the specuztwn of chromwm m the Mediterranean

The CSV method was applied to samples from the Mediterranean to test the procedures Meas- urements of reactwe chrommm(V1) and reactive chromuun(II1) were carried out on-board ship m samples from the Mediterranean during the “Cybele” cruise m the Gulf the Lyon, April 1990 [20] The results for two statlons @AD1 and EBB4) are shown m Fig 6 The station locatlons were 43”3’/5”5’ (RADl) and 40”3’/1”51’ (EBB4) The station depths were 140 m &AD11 and 1700 m (EBB4) The shipboard analyses were less re- producible than those carried out m the land- based laboratory due to engme vlbratlons Never- theless clear trends are apparent m the data

It can be seen m Fig 6 that the concentration of chrommmW1) was around 5 nM at both stations Some varlatlon m the chromlum(V1) concentration was apparent m samples from shallow depth perhaps due to uptake by orgamsms

The chrommm(V1) concentration could be determined readily without prior UV-treatment of the samples mdlcatmg that the concentration of mterfermg surface-active compounds was low

M Boussemart et al /Anal Chrm Acta 262 (1992) 103-115

Cr InM)

Statm IUD1

c.m.-

Cr (nM)

Fig 6 The concentrations of chrommm(VI) and reactive chrommm(III) at stations RADl and EBB4 m the NW Mediterranean These chrommm concentrations were determined on-board slup reactwe chrommm(II1) was estimated from the peak height obtamed lmmechately after the addltlon of the DTPA to the sample

(with an surface-active effect less than that for 0 5 pg ml-’ Trlton X-100)

The concentration of chrommm(II1) was much lower than that of chrommm(VI) and could be detected only m near-surface waters the chromlum(II1) concentration dununshed rapldly from ca 1 nM at the surface to undetectable at 30 m depth at RADl The chrommm(111) concentra- tlon was below the lunlt of detection in most samples from EBB4 except m the surface sample and a sample from 200 m depth It 1s temptmg to ascribe the presence of chrommm(II1) m the surface waters to photochemical effects but a more detailed study has to be carried out to confirm this observation

The detected levels of chrommm(V1) and -(III) m the Mediterranean are generally slmdar to those detected previously (usmg co-precipltatlon and atomic absorption spectrometry) m samples orlgmatmg from the Eastern Paafic 1211 The

M Boussemart et al /Anal Chum Acta 262 (1992) 103-115 113

deep water concentrations of chrommmW1) m the Pack were 4-5 nM, perhaps slightly lower than m the deep waters from the Mediterranean where the chrommmW0 concentrations were around 5 nM The surface concentrations of chrornmm(VI) m the Pa&c were lower at 3-4 nM [21] but metal concentrations m surface waters are affected by blogeochemlcal reactlons with temporal variations The absence of a clear re- ductron of the chrommm(VI) concentrations m the upper water column of the Mediterranean may therefore be due to comparatively recent mlxlng of the water column

The concentration of chrommm(II1) m the Pa- clflc was between 0 and 1 nM wth a maximum m the upper water aAnnn 1211, smllar to the Mediterranean data

The CSV method was sufficiently sensltlve to detect reactive chrommm(V1) and total dissolved chrommm m the sea water samples, but not usu- ally sufflclently sensltlve to detect the low levels of chrommm(II1) occurnng in sea water either directly (from the peak height unmedlately after the reagent addition) or by difference (total chrommm mmus reactive chrommm(VI)) due to the much higher concentrations of chrommmWI) Investlgattons are therefore currently underway to develop a procedure mvolvmg a preconcentratlon step to lower the detectlon hnut for chroml- urn(II1)

Sample storage Prelmunary experunents mdlcated that chro-

mmm(III) 1s removed rapidly Cm a matter of minutes to hours) from solution by adsorption onto the contamer walls at neutral pH, whereas chrommm(VI) IS known to be reduced to chrommm(II1) by organic material m aadlfled conditions [2] Several samples were stored frozen (m high dens@ polyethylene bottles) m an at- tempt to stabdlze the chrommm speclatlon The results of later laboratory determmatlons of the chrommm speclatlon and of total dissolved chromium can be compared m Table 1 with the chromium speclatlon determmed on-board ship munedlately after collection It can be seen that the chrommmW1) and the total dissolved chromium concentrations m the stored samples were much lower than the chrommm(V1) concentrations ongmally present m the samples As chrommm(VI) does not readdy partlclpate m adsorption reactions at the natural pH of sea water It IS hkely that some of the chrommm(VI) became reduced to chrommm(II1) durmg storage and that some of this was removed from solution by adsorption on the bottle walls or by preclpltatlon durmg freezmg or thawmg The chrommm(III) concentration was measured m two of the stored samples and was found to be much higher (at 15 and 2 1 nM) after storage than before (0 and 0 2 nM chrommm(III)), confummg that some of the

TABLE 1

Effect of sample storage on the concentrations of chrommm(II1) and (VI) m sea water (The samples were collected from the Mechterranean (StatIon RADl) and were stored by freezmg)

Sample depth (m)

ZQO 140 83 72 30

Analysts on-board

Reactrve chronuum(VI) (nM)

490 5 80 448 500 5 10

Reactwe chrommmUI1) (nM)

0 02 0 07 0

Anaiysls m laboratory

Reactwe Total Total chrommm(VI) chronuum chrommm(II1) (nM) (nM) (“M)

280 2 10 360 150

350 260

150 420 270

114

chrommm(VI) had been converted to chrommm (III) These results illustrate the difficulty of pre- servmg the dissolved chrommm concentration as well as the redox speclatlon of chrommm m sea water

Conclusions The CSV sensltlvlty for chrommm m sea water

is approximately a decade poorer than m distilled water as a result of major cation competition for DTPA The optlmrzed conditions include a solutlon pH of 5 2 (approximately one pH unit lower than m freshwater) and a DTPA concentration of 2 5 mM The different sensitivity for chrommm, and the different optmuzed condltlons, as compared with the situation m fresh water, are due to competltlon by the major cations, magnesium and cahum, 111 sea water Calculation of the speclatlon of DTPA m sea water and fresh water showed that (H,DTPA) 1s the hgand of the adsorptive complex vvlth chromnun(II1)

Reactive chrommm(VI) and total dissolved chrommm can be determined m sea water using CSV with a hmlt of detection of ca 0 1 nM with an adsorptlon time of 2 mm The hmlt of detectlon is higher than m freshwater (ca 0 01 nM) The total dissolved chromium concentration 1s determined after UV-lrradlatlon of the sample at neutral pH The concentration of total chromlum(II1) (reactive morgamc and organically complexed) can then be evaluated by difference

Reactive chrommm(II1) can be determined by adsorption of the DTPA complexes onto the HMDE munedlately after the DTPA addition to the sea water The experiments with citrate mdr- cate that orgamc complexes of chrommm(III) do

,not contribute to this chrommm fraction Unfor- tunately the redox speclatlon of chromium m sea water cannot be stored, necessltatmg analysis as soon as possible upon samplmg The determmatlon of chrommm(II1) m oxygenated sea water 1s dd%cult as Its concentration 1s much lower than that of chromu.un(VI) Chrommm(II1) m oxygenated sea water IS therefore generally below

M Boussemart et al /Anal Chm Acta 262 (1992) 103-115

the limit of detection by the current CSV procedure

The authors are grateful for assistance by Lu- cla Campos with the chrommm analyses on-board ship, and for the cooperation of the other sclen- tests with sample collection during the cruise of the Marion Dufresne Thus research was fman- clally supported by NERC grant GR3/7247 whilst the field expenses were sponsored by the EROS programme of the European Commumty

REFERENCES

1 R E Cranston and J W Murray, Anal Chum Acta, 99 (1978) 275

2 F Ahern, J M Eckert, NC Payne and KL Wllhams,

3

4

5

6

7

8

9

10

11

12

13

14

1.5

16

17

18

Anal Chim Acta, 17.5 (1985) 147 C F Baes and R E Mesmer, The Hydrolysis of CatIons, Wdey, New York, 1976 E Nakayama, T Kuwamoto, S Tsurubo and T Fulinaga, Anal CIum Acta, 130 (1981) 401 M Pettme and F J Mllero, Llmnol Oceanogr ,35 (1990) 730 K. Is&u, Y Sohnn, H Karatam and E Nakayama, Anal Glum Acta, 224 (1989) 55 G J de Jong and U A Th Brmkman, Anal Ctum Acta, 98 (1978) 243 ST Crossman and T R Mueller, Anal Chim Acta, 75 (1975) 199 R Fuoco and P Papoff, Ann Chum (Rome), 65 (1975) 155 J Gohmowslu, P Valenta and H W Numberg, Fresemus’ Z Anal Chem ,322 (1985) 315 KM Samalk, MM Palrecha and R G Dhaneshwar, Electroanalysis, 1 (1989) 469 J Heyrovsky and J Kuta, Pnnclples of Polarography, Acadenuc Press, New York, 1966 R A Chalmers and M Masson (Editors), Atlas of Metal- hgand Eqmhbrla m Aqueous Solutions, Elhs Horwood, ChIchester, 1978, pp 108 and 441 IUPAC Chenucal data series, No 22 Stab&y constants of metal-Ion complexes, Part B Orgamc hgands, Pergamon, Oxford, 1979, p 997 B Cosov~ and V VoJvochc, Llmnol Oceanogr , 27 (1982) 361-369 CMG van den Berg, m JP Rdey (Ed), Chenucal Oceanography, Vol 9, Academic Press, London, 1989, pp 197-245 F Scholz, B Lange, M Drahelm and J Pelzer, Fresemus’ J Anal Chem , 338 (1990) 627 J Zarebsky, Chem Anal 22 (1977) 1037

M Boussenaart et al /Anal Chun Acta 262 (1992) 103-115 115

19 NASS-2 Seawater reference matenal, National Research cOunc11 Canada, Marme Analyttcal Chemistry Standards Program, Ottawa

20 J-C Bnm-Cottan, Cnuse and Sclentlfic Report of the MD/63 Cybele Crmse wtth the R/V Manon Dufresne, 12 Aprtl-10 May, 1990 Coptes of the report from the EROS

office at the Institute de Blogeochmue Marme, Mon- trouge, France

21 J W Murray, B Spell and B Paul, m C S Wang, E Boyle, KW Bruland, JD Burton and ED Goldberg 0% ), Trace Metals m Sea Water, Plenum Press, New York, 1983, pp 643-669

the determination of the chromium speciation in sea water using catalytic cathodic stripping...

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