Reversible oxidation processes in dichloromethane for triethyl-, diethyl-, trimethyl-, and dimethyllead compounds at mercury electrodes

197

J Electroanal Chem, 218 (1987) 197-211 Elsevler Sequoia S A, Lausanne - Pnnted m The Netherlands

REVERSIBLE-OXIDATION PROCESSES IN DICHLOROMETHANE TRIETHYL, DIETHYL, TRIMJ3THYJx, AND DIMETHYLLEAD COMPOUNDS AT MERCURY ELECTRODES

FOR

A M BOND and N M McLACHLAN Dtvrsron of Chemrcal and Physrcal Snences,

(Raved

Deakrn Unrversrty, Waum Ponds, Kctona

3217 (Australra)

16th May 1986, m revised form 6th August 1986)

ABSTRACT Chemically reversible electrochenncal oxldahon processes have been observed at mercury electrodes m &chloromethane m the presence of Et3PbOAc (OAc- = acetate), Et$bCl, Et2PbCl,, Me3PbOAc, Me3PbCl and Me,PbCl* The tnalkyl denvahves R,PbX (R = Et, Me, X- = OAc-, Cl-) gve nse to a reversible oxidation process mvolvmg transfer of one electron, exchange of alkyl groups and X- and formation of a mercury-lead complex, whch m its simplest form can be represented as 2+ The oxldatlon process 1s therefore described by the equation [R@-Hg-PbR,] 2R,PbX+2Hg*[R3Pb-Hg-PbR3]*++HgX2+2

e-

On the longer time scale of controlled potennal electrolysis, HgXz can react Htlth R,PbX presumably [R,Pb-Hg-R,Pb12’ to produce RHgX, morgamc lead(I1) and electroactlve R2PbX, dlalkyl complexes R2PbX2 etiblt analogous oxtdatlon processes of the kmd 2 R2PbX2 +2 Hg =I [R,XPb-Hg-PbXR2]*++HgX2

and The

+2 e-

ullth short hme domam expenments However HgX, reacts ~th [R2XPb-Hg-PbXR2]2+ and R2PbX2 on longer hme domam expenments such as controlled potential electrolysis or dc polarography to generate morgamc lead(I1) and RHgX The existence of reversible polarographlc processes lmphes that It may be possible to develop extremely senslkve analytical methods for the determmatlon of these toxic and environmentally slgmficant compounds based on procedures mcorporatmg (1) extraction mto non-polar solvents, (u) lugh performance hqmd chromatography and (m) electrochermcal ondative detection wthout the need to remove oxygen as 1s required when usmg previously described reduction processes

INTRODUCTION

The analy&al determmatlon of alkyllead compounds has been of particular concern because of thezr mdespread use as mdustnal chermcals and theu hq$ toxlclty [l-3] For example, the use of Et,Pb and Me,Pb as petroleum ad&Wes at lngh concentrations IS well known [3] Spillage of these mater& and release of 0022-0728/87/$03

50

0 1987 Elsevler Sequoia S A

198

evaporative ermsslons has resulted m substantial quantltles of lead bemg released mto the environment Development of analytical methods for the specific determmatlon of tetraalkyllead and the decompoatlon products, tnalkyllead, dlalkyllead and morgamc lead(H), has attracted attention m the literature as mdlcated by perusal of refs l-10 Usually, a method based on solvent extractlon mto an orgamc solvent, followed by hqmd chromatography coupled mth detection by atonuc absorption spectrometry, voltammetry etc , or gas chromatography uflth mass spectrometnc detection 1s utlhzed In the particular case of electrochenucal detectlon, reduction processes are avalable for the determmatlon of tnalkyl and dmlkyl lead and related complexes [4,9-131. The techmque of ano&c stnppmg voltammetry which rehes on the formation of lead (amalgam) at a mercury electrode after reduction has also been employed for trace determmatlons m static cells [4,9] More recently, we have described some novel oxldatlon processes for Me,Pb, Et,Pb, and Ph,Pb whch involve alkyl exchange to form alkyl mercury species at the dropping mercury electrode [14,15] For example, m the case of Et,Pb the first step m the oxldatlon process at mercury electrodes would appear to involve an ethyl exchange process coupled mth electron transfer Et,Pb + Hg -+ Et,Pb’+

EtHg+ + e-

(1)

This class of reaction has led to the development of a method for the simultaneous determmatlon of Et,Pb and Me,Pb m motor and aviation gasohne No sample preparation 1s required and the method 1s based on direct mjectlon of samples onto a reverse phase high performance hquld chromatography column followed by specific detection and determmatlon at a static mercury drop electrode m a flow-through electrochenucal cell [16]. These oxldatlon processes at mercury electrodes are highly specific and avold the comphcatlons associated with removal of oxygen when reduction processes are bemg used This can be a non-tnvlal problem when using HPLC wth reductive electrochenucal detectlon [lo] so that the use of oxldatlon processes 1s generally to be preferred In the present mvestlgatlon, exammatlon of the oxldatlve electrochenustry at mercury electrodes of a range of tn- and dlalkyl lead complexes has been undertaken m dlchloromethane to ascertam d analytically useful responses may also be available for these highly toxic and environmentally unportant decomposltlon products of Et,Pb and Me,Pb EXPERIMENTAL

The techmques of dlfferentlal pulse polarography, dnect current polarography, controlled potential electrolyms and cychc voltammetry were performed urlth prevlously described EG & G Pnnceton Apphed Research Corporation (PARC) mstrumentatlon [14,15] The normal pulse polarographlc expenments employmg short pulse widths were undertaken wth instrumentation conslstmg of a Motorola 6800

199

mtcroprocessor function generator/data acqulsltlon system controlhng the potentlostat of a PARC Model 174 A polarographc analyser Further details concernmg this mstrumentatlon are avtiable m the literature [17] All electrochermcal expenments were performed m dlchloromethane with 0 1 M Bu,NClO, as the supportmg electrolyte Polarographlc data were obtained at (20 f 1)“C using Ag/AgCl (satd LKl, CH,Cl,) as the reference electrode and a platinum wire -as the auxlhary electrode. Electrolysis expenments were performed using the above reference electrode, a mercury pool workmg electrode and a platinum gauze auxlhary electrode m dlchloromethane (0 1 M Bu,NClO,) separated from the solution by a glass fnt The dlchloromethane (AR grade) and the Bu,NClO, (electrochermcal grade) were used wth drying where necessary or as supphed by the manufacturers (G Fredenck Srmth Chermcal Company and Malhnckrodt Australia respectively) Alkylmetals and other compounds were all used as supphed by the manufacturers (Sigma Chermcal Company, Alfa products and Strem Chermcals Inc ) RESULTS

AND DISCUSSION

Tnethyllead

acetate (Et, PbOAc)

In view of the low solublhty of tnethyllead chlonde m dlchloromethane, the majonty of the electrochermcal stu&es on tnalkyllead complexes were undertaken on the more soluble tnethyllead acetate Results from this compound are then extrapolated to tnethyllead chlonde which 1s the form of tnalkyl denvatlves expected to be found m marme and other chlonde contamng enwonments Figure 1 includes a dc polarogram for Et ,PbOAc at posltlve potentials m CH,Cl, (0 1 it4 Bu,NClO,) Two oxldatlon processes labelled 1”’ and II are evident pnor to the onset of the mercury electrode oxldatlon hrmt Reduction processes are also present and are closely related to those described m other media [4,11-131, but are not consldered m any detad m this study Table 1 provides a summary of E,,, values and other polarographlc parameters for Et ,PbOAc Process II (more positive process) m Fig lb 1s a complex process occumng over a very narrow potential range ( E3,4 - E,,4 = 10 mV) and etiblts a non-hnear hrmtmg current to concentration relatlonstip Data are consistent urlth thrs bang a tensammetnc type of response mvolvmg adsorptlon processes While process II 1s not analytlcally useful, Its presence 1s particularly valuable for ldentifymg products associated urlth process I”” Apart from this latter apphcatlon, the majonty of the remamng &scusslon 1s confined to the analytically useful process I” Process II may be attnbuted to formatlon of mercunc acetate as venfled by comparison urlth a polarogram of an authentic sample of mercunc acetate m dlchloromethane (see Fig la) That is, mercunc acetate also produces a sharp tensammetnc type response at essentially the same potentml as for process II The absence of a polarographlc reduction response of the kmd for mercunc acetate, Hg(acet), + 2 e- + Hg + 2 acetate-

(2)

55

70 80 80 90

Dc polarography

+052s

+053s +034 +036 co37

Compound

Et ,PbOAc Et,PbCI Me,PbOAc Me,PbCl EqbCl, Me,PbCl,

b -

G/4)

electrodes

+056 +042 +042 +042

+054

r

+046 iO28 +029 +029

+044

Ed /t

’

100 140 130 130

100

AEP /mV

for approximately

Cychc voltammetry

data at mercury

+051 +035 +035 +035

+049

E’B” /;I/:

1O-4 M solutions

h

‘d

+049 +043 +049 +035 +036 +035

Dulse

Dlfferenbal

compounds

pol~ogt@y E,/V ’

of alkyllead

_

’ Major omdation process described Other processes are referred to m the text ‘Droptime-05s ’ Scan rate = 500 mV s-lcommenced0 5 s after drop growth commenced I?? - oxidation peak potentml, EG4 = reduction separation, E~$ - calculated E1,2 value scan, *Ep = peak-to-peak d Pulse amphtude = 50 mV Ep = dlfferentml pulse peak potential ’ Controlled potentml held at potent& m hnutmg current repon of oxldatlon process Tnphcate detemunatlons f V vs Ag/AgCl E1,2 for oxldatmn of 2 X 10d4 M ferrocene IS + 0 48 V vs Ag/AgCl g A process IS also observed at 0 76 V and IS altnbutable to presence of acetate (see text) h Et,PbCl data obtamed from a saturated solution

/mV

( -%,4

and cychc voltammctnc at (20fl)°C ’

1

Polarographx Bu,NCIO,)

TABLE

peak potenllal

15*01 15*01 05_+01 08kOl

11+01

L

(0 1 M

on reverse

Controlled ootentml electrolysis ’ Apparent n value

m dlchloromethane

201

POTENTIAL /V

Fig 1 Companson of dc polarograms at positive potentials m dxhloromethane (0 1 M Bu,NCIO,) at 20°C and with a drop time of 0 5 s for (a) 1O-4 M mercunc acetate, (b) 2 4X1O-4 M tnethyllead acetate, (c) 0 7 X 10e4 M ðyllead dlchlonde, (d) 0 8 X low4 M tnmetbyllead acetate, (e) 1 5 X 10e4 M tnmethyllead chlonde and (f) approximately 0 6 X 10e4 M dlmethyllead dlchlonde

at about + 0 05 V m a polarogram of tnethylkad acetate confirms that mercunc acetate IS formed as a product of the first oxidation process and not through pnor dusoclatlon of the lead compound accompamed by an exchange process with the mercury electrode. In contrast to process II, process Iox has a well defined, dlffuslon controlled dc lunrtmg current which 1s a hnear function of concentration over the concentration range 2 X lo-’ M to lop3 h4. The dc titmg current per umt concentration 1s essentially the same as that observed for the oxldatlon process associated Hrlth tetraethyllead at mercury electrodes [15]. Smce the latter has been shown to be a one-electron process, the process associated with Et,PbOAc 1s also hkely to be a

202

POTENTIAL to60

I

000

-060

/ V -120

IOWA W-

MegPbOAc

\

Me3PbCl

Fig 2 Cychc voltammograms m dlchloromethane (0 1 A4 Bu,NClO,) at 20°C obtamed at a droppmg mercury electrode (scan rate = 500 mV s- ‘) for (a) 2 1 X 10e4 M tnethyllead acetate, (b) 1 5 X 10m4 M dlethyllead dlchlonde, (c) 2 2X10e4 M tnmethyllead acetate, and (d) 2 7X10e4 M tnmethyllead chlonde

one-electron process (dlffuslon coeffuxents for Et,Pb and Et,PbOAc should be sinular). Figure 2 contams a cychc voltammogram for trrethyllead acetate m dlchloromethane recorded at a droppmg mercury electrode ~th a scan rate of 500 mV s-l commenced 0.5 s after the commencement of the drop growth The potential on the lmtlal (oxldatlve) scan of the cychc voltammogram was svirltched soon after the first oxldatlon process to ensure that there were no comphcatlons due to the oxldatlon of elemental mercury The reverse (reductive) scan exhlblts three maJor reduction process(es) or regons which are labelled on Fig 2 as I&, III&, and IV’& respectively

203

The cychc voltammogram suggests, rather surpnsmgly, that process I 1s chemlcally reversible and consists of the oxldatlon and reduction couple Iox and Ired Additionally, no smular process was observed at platmum or glassy carbon electrodes, lmplymg that mercury 1s mtlmately associated with the oxldatlon step The peak to peak separation for the forward and reverse scans 1s (100 f 10) mV at a scan rate of 500 mV s-l At a scan rate of 100 mV s-l, the separation has decreased to 85 mV By way of comparison,, the peak-to-peak separation for oxldatlon of a 10F4 A4 solution for the known reversible standard compound, ferrocene, was (100 f 10) mV at a scan rate of 500 mV s-l and (80 f 10) mV at 100 mV s-l These data m&cate that the apparent devlatlon from complete electrochermcal reverslbdlty observed for tnethyllead acetate 1s due to uncompensated ohrmc resistance m the cell Process III m Fig 2 designated by III” and HIred, 1s attnbutable to the chenucally reversible mercury/mercunc acetate couple which m Its overall form can be wntten as m eqn. (3), although a mercury(I) mtermedlate 1s possibly associated with this process Hg(OAc), + 2 e- ,‘ Hg + 2 OAc-

(3) Process IV red 1s the reduction of tnethyllead acetate comphcated by formation of ethyl mercury species and by adsorption phenomena as described m detail m other studies [4,11-131 Et ,PbOAc + e- + Et,Pb’+ OAc-

(4a)

Et ,Pb’+ Hg + adsorbed products, etc

(4b)

The cychc voltammetnc data confirms the suggestion made on the basis of the appearance of process II m dc polarograms, that mercunc acetate 1s formed as a product of a one-electron oxldatlon process Equation 5 1s consistent with dc polarographtc and cychc voltammetnc data for process I 2 Et,PbOAc

+ 2 Hg + [Et,Pb-Hg-PbEtJ2+

+ Hg(OAc), + 2 P-

(5)

The formulation of a mercury-lead complex as the blmetalhc complex [Et,Pb-Hg-PbEt,12+ 1s made on the basis that It 1s the simplest species which can be formed to gve the observed stolchlometry and other data required for the oxldatlon process However, the formulation cannot be regarded as bemg proved A related tm compound Me,Sn-Hg-SnMe, has been reported m the literature [18] Oxldatlve controlled potential electrolysis at +0 60 V for different concentrations of tnethyllead acetate momtored by coulometry gave electron transfer numbers of 1 1 f 0 1 per molecule A grey precipitate formed early m the electrolysis was collected, dissolved m water (0.1 M HCl) and shown to gve a dc polarogram identical to that for morgamc lead(U) chlonde (or acetate) The course of the oxldatlve electrolysis was also momtored polarographlcally m CH,Cl, using the differential pulse techmque to improve adequate resolution of all polarographlc responses (Fig 3a) As the electrolysis occurs, all oxldatlve processes decrease m height whle a reduction process urlth a peak potential of -0 5 V vs

204

Ag/AgCl develops which 1s attnbutable to the formation of ethylmercury acetate as verified by comparison with a solution of an authentic sample A complicated process at - 1 25 V vs Ag/AgCl (negative differential pulse peak current), which 1s presumed to be associated with formation of adsorbed products of reduction (eqn 4b) remams after the electrolysis 1s completed TUB charactenstlc process 1s observed m polarograms of tnethyllead acetate, tnethyllead chlonde and dlethyllead dlchlonde The controlled potential electrolysis data indicate that with longer time domam expenments, reactions additional to those speclfled m eqn (5) must occur Mercury(I1) compounds are well known to react with alkylmetals [19] to produce monoalkylmercury compounds Reactions of mercunc chlonde and tnmethyllead chlonde with the lead dlmer, hexamethyldllead, have been studed m detail by Arnold and Wells [19,20] and a major product of these reactions was found to be morgamc lead Thus a reactton of the kmd [Et3Pb-Hg-PbEt312+

+ 2 Hg(OAc),

+ 2 EtHgOAc + Pb2+ + Et,Pb(OAc),

+ Et-Et

(6)

1s not unexpected, nor would be direct decomposltlon of [Et,Pb-Hg-PbEt,12+ to give Pb2+ plus other products Additionally, Hg(OAc), generated by electrolysis will react wth the starting material via eqn (7) Hg(OAc), + Et,PbOAc

-+ EtHgOAc + Et,Pb(OAc),

(7)

Furthermore, Et ,Pb(OAc), produced by th.s reactlon 1s electroactlve (see later) so that a complex bulk electrolysis result 1s predicted as well as observed Trrethyllead chlonde (Et, PbCI)

The hnuted solublhty of Et ,PbCl m dlchloromethane precluded the use of dc polarography m the study of tlus compound However, the more sensitive techmque of dlfferentlal pulse polarography provides a well defined oxldatlon response for a saturated solution of Et ,PbCl Data suggest that an analogous electrode process exists to that for the acetate denvatlve 2 Et,PbCl+

Hg s [Et,Pb-Hg-PbEt,]2++

HgCl, + 2 e-

(8)

Results for this compound are also included m Table 1 Dzethyllead

Figure dlchlonde extremely dependent

dlchlonde (Et2 PbCI,)

lc shows that a dc polarogram for a 7 x lop5 M solution of dlethyllead m dlchloromethane at posltlve potentmls 1s charactensed by a single, well defined, oxldatlon process The dc hnntmg current 1s hnearly on concentration up to 1 X 10m4 M where adsorptlon phenomena begm

205

to comphcate the response At low concentrations the hnutmg current 1s approxlmately equal to that of tnethyllead acetate per unit concentration, suggesting that this 1s basically a one-electron oxldatlon process on the time scale of dc polarography, as was the case with the tnethyl denvatlve. A reduction process 1s also observed for dlethyllead dlchlonde which has a snmlar hrmtmg current per umt concentration to the oxldatlon process, although It 1s comphcated by strong adsorption phenomena (mamma) Reduction of dlalkyllead compounds has been considered m other studies m different solvents as noted elsewhere and 1s not discussed m any detad m this paper A cychc voltammogram for dlethyllead dlchlonde m dlchloromethane recorded at a dropping mercury electrode (Fig 2b) demonstrates that the oxldatlon process 1s chemtcally reversible However, the cychc voltammogram 1s also comphcated by two sharp spikes attnbuted to specific adsorption phenomena which are equivalent to complications noted m dc polarograms at high concentration Three maJor processes or regons labelled m Fig 2b are apparent m cychc voltammograms. These processes may be explamed m an analogous manner to the descnptlon of the data for tnethyllead acetate Basically oxldatlon peak V“” and reduction peak Vmd constitute the chermcally reversible couple for oxldatlon process V The peak-to-peak separation for the forward and reverse scans 1s approximately 130 mV (scan rate = = 500 mV s-‘) but cannot be measured accurately because of the presence of the overlappmg sharp adsorption processes referred to above Regons VI”” and VIr4 are readily attnbuted to the mercury/mercunc chlonde redox reaction whch can be wntten m Its overall form as HgCI,+2

e-+Hg+2Cl-

(9)

Process(es) VII 1s the reduction of dlethyllead dlchlonde and associated ethyl mercury radical formation, etc (complicated process) These data are consistent with the occurrence of the one electron reversible reaction aven m eqn (10) 2 Et,PbCl,

+ Hg*

[Et,C1Pb-Hg-PbC1Et,]2++

HgCl, + 2 e-

(10)

Equations (9) and (10) are both wntten m their simplest overall forms, and may of course contam a number of additional intermediate steps Controlled potential electrolysis of dlethyllead dlchlonde at 0 50 V vs Ag/AgCl and monitored by coulometry 1s complete after the transfer of only 0 5 + 0 1 electrons per molecule Companson of differential pulse polarograms before and after electrolysis (Fig 3b) shows depletion of all processes other than the commonly observed response observed at - 125 V and the development of a response at - 0 5 V attnbutable to ethylmercunc chlonde A precipitate was observed dunng electrolysls which was redissolved m water (0 1 M HCl) and found to gve an ldentlcal polarographlc reduction response to standards of PbCl, added to the same solution Controlled potential electrolysis data suggest that mercunc chlonde formed by the mltlal electron transfer-exchange reaction 1s reacting with dlethyllead dlchlonde

206

1

Et.$‘bOAc

1OpA

I

+050

3a

\

l030

+o 10

-0

40

-080

-120

POTENTIAL /V 3 DIfferentA pulse polarograms (amphtude = 50 mV, drop time = 0 5 s) (1) before and (2) after exhaustwe electrolysis for (a) 1 1 X10e5 mol of tnethyllead acetate, (b) 8 3X1O-6 mol of dlethyllead dlchlonde, and (c) 7 2X 1O-6 mol of tnmethyllead chlonde m 20 ml of &chloromethane (0 1 M Bu,NClO,) at 20°C Fig

to produce morgamc lead and ethylmercunc representation of the reactlon Et 2PbCl 2 + HgCl 2 + Hg -+ PbCl 2 + 2 EtHgCl

chlonde

Equation (11) 1s a possible

01)

Since only approximately half an electron per molecule of dlethyllead dlchlortde IS transferred durmg exhaustive electrolysu, the above reactlon must be favoured over reactlons whxh may be possible between mercunc chlonde and the lead dlmer formed durmg the electron transfer reaction Polarographlc and voltammetnc parameters for the oxtdatlon process for Et ,PbCl, are summanzed m Table 1 Tnmethyllead

acetate (Me, PbOAc)

Figure Id 1s a dc polarogram of a 8 x lop5 M solution of trunethyllead acetate m Ichloromethane. Two well defmed oxldatlon processes are observed as was the case with the ethyl analogue (labelled VIII” and II) Reduction processes presumably related to those described by other workers m different solvents [4,11-131 are also

207

I

0104 1

'. \ \ \ I

\

2 \ 3 / . ,/'

>-,

,

’

\ \ \.-__

1

+O?O

050

030 POTENTIAL

0 10 / V

4 Comparison of dlfferentml pulse polarograms for approximately 6X 10K5 M dlmethyllead dlchlonde (1) before and (2) after addltlon of sdver acetate with (3) a dlfferentml pulse polarogram of 6 x lo-’ M tnmethyllead acetate Fig

observed but again these are not discussed Polarographtc data are summarized m Table 1 Process II has a solar wave shape and occurs at a smular potential to that observed for tnethyllead acetate (Fig lb) and is agam asstgned to the formation of mercunc acetate durmg the first oxtdatlon process Process VIII”” for tnmethyllead acetate occurs at a smnlar potential to process I“’ for tnethyllead acetate However, the polarographtc wave is more drawn out and the hnntmg current per umt concentration is greater The dc limttmg current is hnearly dependent on concentration from 2 X lop5 M to 3 x low4 M (solubthty hrmt). Inspection of dtfferential pulse polarograms for tnmethyllead acetate (Fig. 4) suggests that process VIII” exhibits considerably greater complextty than was the case with Et ,PbOAc and m fact contams (at least) two unresolved processes The first of these unresolved steps is described by a sharp well defined dtfferentlal pulse peak This process at +0 48 V vs Ag/AgCl is assumed to be analogous to the chemically reversible tnethyllead acetate response The unresolved addmonal process(es) are observed as a broad differential pulse response characterized by a shoulder centered around +0 55 V Concentratton studies wtth the differential pulse waveform mdmate little vanatton of the relattve peak heights with concentratton Normal pulse polarograms with pulse wtdths less than 40 ms (m a dertvative format) show no mdtcation of the shoulder observed m process VIII”” durmg the longer time domam dc and dtfferential pulse polarographlc expenments, and the normal pulse polarograpmc hnutmg current per umt concentratton for tnmethyllead acetate is almost identical to that of tnethyllead acetate That is, under short ttme

208

domam expenments process VIII appears to be a reversible one electron oxldatlon step Cychc voltammetry at a DME m dlchloromethane (Fig 2c) also reveals that process VIII consists of at least two steps On the reverse scan, three major processes are observed which are analogous to those found m tnethyllead acetate cyclic voltammograms Process VIIIxd 1s the reduction process coupled to the reversible aspect of oxldatlon process VIII” Process IIIrd 1s the reduction of mercunc acetate formed durmg oxldatlon process VIII” and process Xred 1s the reduction of tnmethyllead acetate. Data mdlcate that at very short tune domams, such as associated wth normal pulse polarographlc expenments with pulse urldths of less than 40 ms, the electrode process 1s analogous to that for tnethyllead acetate 2 Me,PbOAc + 2 Hg G+[Me,Pb-Hg-PbMe,12+ + Hg(OAc), + 2 e02) The development of the broad response not found mth Et,PbOAc appears to mvolve reaction of the mltlally formed blmetalhc complex with mercunc acetate This 1s confirmed by observation of a small response at -0 65 V vs Ag/AgCl (process IXrd) m the reverse scan of the cychc voltammogram (Fig 2c) wluch 1s attnbutable to the reduction of methylmercury Oxldatlve controlled potential electrolysis for tnmethyllead acetate at +0 60 V vs Ag/AgCl momtored by coulometry resulted m the transfer of 1 5 f 0 1 electrons per molecule Differential pulse polarography of the resultant solution as compared to a polarogram for tnmethyllead acetate demonstrates the expected depletion of the responses due to this compound and the development of a response attnbutable to formation of methylmercury at -0 5 V vs Ag/AgCl (see Fig. 3a) A grey preclpltate was formed durmg electrolyas. These results are analogous to those observed for tnethyllead acetate and dlethyllead dlchlonde with the exceptlon of the coulometnc data which mdlcate that altematlve pathways for reactlon of Hg(OAc), must be avalable to produce addltlonal electroactive matenal The most likely electroactlve compound to be formed by interaction of mercunc acetate urlth tnmethyllead acetate or the mercury-lead blmetalhc complex, m addltlon to methyl mercury complex detected by cychc voltammetry, 1s dlmethyllead dlacetate A solution of this compound m dlchloromethane was prepared by reacting silver acetate wth dlmethyllead dlchlonde A preclpate of silver chlonde was formed immediately Furthermore, after addition of silver acetate the oxldatlon process at a mercury electrode for dlmethyllead dlchlonde was replaced by an oxldatlon process which occurs at agmflcantly more posltlve potentials Figure 4 shows a comparison of dlfferentlal pulse polarograms for a solution of dlmethyllead dlchlonde and a solution of dlmethyllead dlacetate prepared m the above manner The peak potential for the prepared solution of dlmethyllead &acetate 1s smular to that of the second response associated with process VIII for tnmethyllead acetate The reaction of mercunc acetate wth tnmethyllead acetate dunng controlled potential electrolysis expenments would not lead to an n value greater than 1 for the electron transfer step However, reaction with the bimetallic complex would aclueve ths

209

An electrode mechamsm for the electrolysis of tnmethyllead electrodes may therefore include reactions such as 2 Me,PbOAc + 2 Hg s [Me,Pb-Hg-PbMe,12+ Me,PbOAc + Hg(OAc)? -+ Me,Pb(OAc), [Me,Pb-Hg-PbMe,]2+

acetate at mercury

+ Hg(OAc), + 2 e-

+ MeHgOAc

03a) 03b)

+ 3 Hg(OAc), + 4 MeHgOAc + Pb2’ + Me,Pb(OAc), 03c)

2 Me,Pb(OAc),

+ 2 Hg s [Me,OAcPb-Hg-PbOAcMe212+

+ Hg(OAc), + 2 e03d)

etc The oxldatlon processes associated wth dlmethyllead dlchlonde at mercury electrodes are presented m detal below and are analogous to the descnptlon for the process attnbuted to dlmethyllead dlacetate m eqn (13d) Trlmethyllead chlonde (Me,PbCl)

Figure le IS a dc polarogram for a 1 5 x 10e4 M solution of tnmethyllead chlonde m &chloromethane The half wave potential (+ 0 35 V) 1s sigmflcantly less posltlve than that observed for the oxtdatlon process observed for tnmethyllead acetate (+ 0 52 V) and no addltlonal dc polarographlc oxidation processes are found up to the mercury electrode oxldatlon hrmt The wave shapes and hn-utmg currents per umt concentration are sun&u to the ethyl analogue although current maxima are evldent m tnmethyllead chlonde polarograms at concentrations greater than lop4 M The dc hrmtmg current vanes hnearly wth concentration for concentratlons between 2 X 10v5 M and 2 X 10e4 M Cychc voltammograms for tnmethyllead chlonde (Fig 2d) are parallel to those observed for tnmethyllead acetate Vanatlon m the oxldatlon potential (Table 1) and the difference ansmg from the mercunc chlonde and acetate redox processes are as expected Controlled potential electrolysis at +0 50 V vs. Ag/AgCl for tnmethyllead chlonde solutions m dlchloromethane produced coulometnc and polarographlc data essentially Identical to that observed for tnmethyllead acetate The coulometry revealed an overall transfer of 1 5 f 0 1 electrons per tnmethyllead chlonde molecule and the resultmg differential pulse polarogram (Fig 3c) showed responses attnbutable to methylmercury chlonde at -0 45 V vs Ag/AgCl and surface phenomena at - 125 V vs Ag/AgCl as observed wth many compounds (see above) These results indicate that the electrode mechamsms for tnmethyllead chlonde are analogous to those observed for tnmethyllead acetate

210

Dlmethyllead drchlonde (Me, PbCI,)

Figure le shows a dc polarogram for an approximately 6 X 10d5 M solution of Qmethyllead dlchlonde m dlchloromethane at positive potentials The hrmtmg current 1s well defined and 1s proportional to concentration below 6 X low5 M whch 1s close to the solublhty hnut m dlchloromethane. Companson with polarograms of dlethyllead dlchlonde reveals slrmlar hrmtmg currents per umt concentration Cychc voltammetry at a dropping mercury electrode for dlmethyllead dlchlonde generated the now fanuhar three reduction processes on the reverse scan The most posltlve reduction process 1s part of the reversible couple as described for Et,PbCl, and the other processes are for the mercury/mercunc chlonde redox couple and reduction of dlmethyllead dlchlonde Electrolysis of dlmethyllead dlchlonde set at + 0 5 V vs Ag/AgCl generates coulometnc and polarographlc data sun&u to that previously reported for dlethyllead dlchlonde Differential pulse polarograms of the resultant solutions reveal a l~gh concentration of methylmercury and the previously described adsorption phenomena at negative potentials Coulometry mdlcates an overall electron transfer of 0 8 f 0 1 electrons per molecule This result is higher than that observed for dlethyllead dlchlonde and suggests that after lmtml electron transfer mercunc chlonde reacts wth both the lead complex and dlmethyllead dlchlonde itself Equations (14a, b, and c) provide a possible reactlon scheme 2 Me,PbCl,

+ Hg + [Me,ClPb-Hg-PbC1Me,12+

+ HgCl, + 2 e-

(14a)

Me, PbCl 2 + HgCl 2 + Hg --) PbCl 2 + 2 MeHgCl

04b)

[ Me,ClPb-Hg-PbClMe,]‘+

W)

+ HgCl 2 + PbCl 2 + Pb2+ + 4 MeHgCl

In all bulk electrolysis expenments, it 1s clear that a number of side reactions occur and that the apparent charge transferred per molecule reflects the relative proportions of the dfferent side reactions CONCLUSIONS

Tnalkyl- and dmlkyllead compounds are observed to gve nse to a new class of reversible oxldatlve electrode process m dlchloromethane at mercury electrodes Reversible processes are analytically very sensitive when used with transient techmques such as differential pulse polarography Consequently, these processes may prove supenor to the well studied but comphcated reduction processes utlhzed m all previous work AdditIonally, removal of oxygen 1s not requned for use of these oxldatlve responses Many methods for the determmatlon of alkyllead compounds mvolve solvent extraction mto non-polar solvents as part of the overall analytlcal procedure Solvent extraction mto dlchloromethane followed by HPLC and oxldatlve electrochermcal detectlon may provide a very speclfrc and senatlve analytical method for determmatlon of alkyl lead compounds A research program aimed at

211

mvestlgatmg the analytical usefulness of these oxldatwe processes has commenced m these laboratones and results wdl be presented m due course REFERENCES 1 M Bramca and K Zolenka (Eds ), Lead Mar Environ , Proc Int Experts Discuss on Lead Occurrence, Fate and Polluuon m the Marme Environment, Rovq, Yugoslavia, 1977, Pergamon Press, Oxford, 1980 and references ated therein 2 Heavy Metals m the Environment, Proc 3rd Int Conf , Amsterdam, 1981, CEP Consultants Ltd , Edinburgh, Scotland, and references cited therem 3 P GrandJean and T Ndsen, Residue Rev, 72 (1979) 97 4 M P Colombim, R Fuoco and P Papoff, Sci Total Einvlron , 37 (1984) 61 and references cited therem 5 H R Potter, A W P Jarvle and R N Markall, Water Pollut Control, 76 (1977) 123 6 S A Estes, PC Uden and R M Barnes, Anal Chem , 53 (1981) 1336 7 M Brondl, M Dall’Agho, E Ghlara, C tignuw and G Tlravanti, Scl Total Envuon ,19 (1981) 21 8 Y K Chau, P T S Wong, GA Bengert and 0 Kramar, Anal Chem ,51 (1979) 186 9 AM Bond, J R Bradbury, G N Howell, HA Hudson, P J Hanna and S Strother, .J Electroanal Chem , 154 (1983) 217 10 W A MacCrehan, Anal Chem , 53 (1981) 74 and references ated therem 11 N B Fouzder and B Fleet m W Franklm Smyth (Ed ), Polarography of Molecules of Blologcal Slgmflcance, Acadenuc Press, London, 1979, Ch 9 and references cited therein 12 A J Bard and H Lund (Eds ), Encyclopedia of Electrochenustry of the Elements (Orgamc SectIon), Vol 13, Marcel Dekker, New York, 1979, p 60 13 R E Dessy, W Qtchmg and T Chlvers, J Am Chem Sot , 88 (1965) 453 14 A M Bond and N M McLachlan, J Electroanal Chem ,182 (1985) 367 15 AM Bond and N M McLachlan, J Electroanal Chem ,194 (1985) 37 16 AM Bond and N M McLachlan, Anal Chem , 58 (1986) 756 17 A M Bond, I D Hentage and M H Bnggs, Anal Chem ,56 (1984) 1222 and references cited therem 18 U Blaukat and W P Neumann, J Organomet Chem , 49 (1973) 323 19 D P Arnold and P R Wells, J Organomet Chem , 111 (1976) 285 20 D P Arnold and P R Wells, J Organomet Chem , 113 (1976) 311