Sono-Electroanalysis: Application to the Detection ofLead in Petrol

Alastair N. Blythe, Richard P. Akkermans, and Richard G. Compton*

Physical and Theoretical Chemistry Laboratory, Oxford University, South Parks Road, Oxford OX1 3QZ, UK

Received: April 5, 1999

Final version: June 2, 1999

Abstract

The quantitative detection of lead in petrol is shown to be possible by anodic stripping voltammetry in aqueous media under conditions ofinsonation-induced emulsi®cation. An immersion horn probe is introduced into a thermostatted conventional three-electrode cell opposite amercury plated platinum disk working electrode. Under ultrasonic emulsi®cation of the sample, lead is preconcentrated as an amalgam onthe Hg/Pt electrode surface via reduction at ÿ1.0 V (vs. SCE). The large mass transport associated with power ultrasound makes this stephighly ef®cient. Subsequently the lead is quanti®ed by applying an anodic linear sweep of the potential from ÿ1.0 V to ÿ0.15 V (vs. SCE)so as to oxidize the Pb(0) to Pb(II). The area under the stripping peak gives a measure of the lead formed during the initial step. By use ofstandard microaddition of lead to the solution the system can be calibrated to give the total amount of lead present in the petrol sample.Experiments using samples of 4 star leaded petrol gave a total lead content of 380+ 40 mgL71. This value was in quantitative agreementwith that obtained by an independent laboratory using atomic absorption spectroscopy (AAS). In addition to the high mass transport andemulsi®cation insonation offers the crucial bene®ts of ®rst surface activation and cleaning, helping to prevent electrode fouling by theorganic components of petrol and second the complete extraction of lead from the water-insoluble target phase.

Keywords: Ultrasound, Emulsion, Lead, Petrol, Sono-anodic stripping voltammetry, Sono-electroanalysis

1. Introduction

Owing to the continuing focus on total lead concentrations inpetrol [1, 2] in recent years, there is increasing demand for simpleand rapid methods of determination. The most widely usedmethods are based on atomic absorption spectroscopy (AAS) [3]and involve stabilization of the petrol prior to analysis. Severalauthors ®rst attempted analysis during the 1960s using iso-octaneor methyl ethyl ketone as the sample diluent [4±7] but dif®cultieswere encountered as different alkyl lead compounds gave dif-ferent responses. Subsequently, the use of petrol pretreatmentwith iodine or bromine at elevated temperatures prior to ¯ameAAS was employed to counter this [8±10]. Other reportedmethods for lead analysis in petroleum include inductively cou-pled plasma mass spectrometry (ICP-MS) [11] and differentialpulse anodic stripping voltammetry after decomposition of theorganolead compounds with concentrated nitric acid then ashingat 300 �C [12]. A method involving stripping potentiometry wasreported by Jagner et al. [13] with a sample pretreatment to dilutethe petrol sample in a matrix containing ethanol, nitric acid,mercury ions and detergent. Polarography after treatment withiodine monochloride was investigated [14] whilst the extractionof lead and other heavy metals from waste oils using ultrasoundand acid digestion has also been reported [15].

The scope of electroanalysis has been broadened in recentyears by the introduction of ultrasound [16±21]. The speci®cadvantages of using power ultrasound for electroanalysis are:

i) the extremely enhanced mass transport of redox species tothe electrode surface [22],

ii) the surface cleaning and activation properties of microjetsinduced byacoustic cavitation [23, 24],

iii) the possible extraction of electroactive species from bindingsites in organic matrices [25, 26] and

iv) the emulsi®cation of otherwise immiscible organic and aque-ous layers without the need for additional emulsifying agents[27, 25].

In the ®nal case, ultrasonic formation of emulsions fromliquid-liquid mixtures has been chie¯y attributed to cavitationprocesses [23, 24] which preferentially occur at the phaseboundary [29]. These mechanical forces act to divide dropletsagain and again, forming microdroplets and effectively`homogenizing' the heterogeneous system even in the absenceof surface-active agents. Thus, regardless of the relative den-sities of the two liquids, droplets of both are in constant contactwith the electrode surface during voltammetric scans.

The use of anodic stripping voltammetry (ASV) in themeasurement of trace metals in passivating media has beenwidely attempted (30±32]. The attraction is the sensitivity of thetechnique resulting from the effective in situ preconcentration ofthe target metal into a mercury ®lm electrode or, recently,microelectrode [33] combined with the relative simplicity andlow cost of the necessary equipment. However despite thesemerits problems have arisen. For example the organic matrixmay interfere with the electrochemical process [32]. Alter-natively sample pretreatments of varying complexity [34±39]which are frequently time consuming may be required if reliableresults consistent with independent methods are to be obtained.In addition issues arise as to whether the electrochemicalmethods probe the total metal content or merely re¯ect somefraction of the free ions present.

In the following we will show that the above bene®ts ofinsonation in electroanalysis and in particular the use of ultra-sound to promote emulsi®cation of the aqueous and organicphases allows reliable measurements without the need for ela-borate sample pretreatment or added emulsi®cation agents.Sonication also permits quantitative measurements of total leadin petrol via anodic stripping voltammetry without problems ofelectrode fouling. Moreover the total lead content as determinedby sono-ASV will be shown to agree quantitatively with thatestablished via independent atomic absorption spectroscopy(AAS) measurements.

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2. Experimental

The well-characterized sono-electrochemical cell [40]employed in this work is shown in Figure 1. The cell volume wasca. 20 cm3. The ultrasonic immersion horn was supplied bySonics & Materials (Model VCX400) and used with a 3 mmtitanium stepped microtip extension at a frequency of 20 kHz.The ultrasonic intensity was calibrated according to the methodsof Mason et al. [41] and Margulis et al. [42] over a range of 0±544 W cmÿ2. For the sono-ASV experiments the intensity was setto 52+ 1 W cmÿ2. Circulation of water from a constant tem-perature bath through a submersed stainless steel coiling coil wasused to achieve thermostatting of the sonoelectrochemical cell.An essentially constant temperature for voltammetric analysiswas maintained by limiting the length of any single sonicationperiod to a maximum time of 4 minutes.

A computer-controlled PGSTAT20 Autolab potentiostat (Eco-Chemie, Utrecht, Netherlands) was employed to control thepotential applied at the working electrode during all experiments.The need for bipotentiometric [43] control of the titanium hornitself was eliminated by insulating the transducer from the probewith a thin te¯on disk and connecting the two with a screw threadmachined from Delrin instead of titanium. A saturated calomelelectrode (SCE) and a platinum wire acted as reference andcounter electrode respectively. The working electrode was eithera 4 mm or a 12 mm diameter platinum (Aldrich, > 99 %) disk. Inorder to eliminate the introduction of lead from the electrode intothe experiment, the 12 mm disk was platinum welded to a pla-tinum wire and then onto a brass rod without the need for the useof any solder in its construction. The platinum electrode waspolished using diamond lapping compounds (Kemet, Kent, UK)of decreasing size down to 1 mm before use.

The platinum working electrode was coated with mercurydroplets by cathodically plating onto the platinum from an aqu-eous solution of 0.15 M mercury nitrate and 0.12 M potassiumcyanide (both Aldrich) using a 1.5 V cell for 30 s. The water usedin the sono-ASV plating solution was UHQ grade of not less than18 MO cm. The sono-ASV experiments were conducted with acell solution of 0.0968 M nitric acid (Aldrich) purged of oxygenby bubbling argon (Pureshield) through the solution for 5 min-

utes prior to electroanalysis and maintained with argon ¯ow overthe solution for the entire duration of the experiment. Iodine(BDH), ferrocene, copper(II) sulfate, sodium sulfate and octane(all Aldrich) used in calibration experiments were all of thehighest commercially available grade.

The working electrode was placed opposite the horn in a faceon geometry with a horn-to-electrode distance of 7.0+ 0.5 mm.An electrochemical pretreatment of 0 V (vs. SCE) for ten secondswas employed to strip out any residual lead. Electroreduction ofthe sample at ÿ1.0 V (vs. SCE) was conducted for 4 minutesunder 52 W cmÿ2 ultrasound. Each accumulation step was fol-lowed by a single `silent' anodic stripping scan at 0.1 Vsÿ1 fromÿ1.0 V to ÿ0.15 V (vs. SCE).

For the comparison of total lead content, independent experi-ments were conducted using atomic absorption spectroscopy(AAS) according to the Institute of Petroleum method no. 362=93[44] at Rooney Laboratories Ltd. (Basingstoke, UK).

3. Results and Discussion

We consider ®rst experiments conducted using a mercuryplated platinum electrode and a solution containing 0.1 M nitricacid in the cell shown in Figure 1. The addition of small quan-tities of petrol to the latter produced a two phase system whichcould be emulsi®ed by 3.7 W of 20 kHz ultrasound delivered bythe horn transducer shown in Figure 1 typically using a horn-to-electrode separation of 7 mm. Under these conditions it wasfound that it was possible to obtain lead anodic stripping vol-tammetric signals by the following procedure. First the potentialat the electrode was held at 0 V to clean any residual target metalfrom the electrode surface. Second the electrode potential wasswitched to ÿ1.0 V for a period of typically tens or hundreds ofseconds under ultrasound. The system was only sonicated duringthe electrolysis and was `silent' at all other times. Third theelectrode was subjected to a linear potential sweep from ÿ1.0 Vto ÿ0.15 V. Figure 2 shows typical linear sweep voltammograms(LSVs) obtained using 120 gL of petrol detected on a 4 mmmercury plated platinum electrode. The anodic peak was attrib-uted to the oxidation of Pb(0) to Pb(II) ®rst as evidenced bycomparison to literature data [45] and second in the light of theobservation that addition of Pb(II) to the aqueous phase increasedthe size of the observed peak. Figure 2 also shows that the peakarea increased with the accumulation period over the range 7 s to240 s. Integration of the peaks showed that the total stripping

Fig. 1. The sonoelectrochemical cell used for the electroanalyticalstudies described.

Fig. 2. Anodic LSVs at 100 mV sÿ1 at a 4 mm Pt=Hg disk for 120 mL ofpetrol added to 12 mL of 0.1 M nitric acid. The accumulation times atÿ1.0 V (vs. SCE) under 3.7 W of ultrasound power were a) 7 s, b) 15 s, c)30 s, d) 60 s, e) 120 s, and f) 240 s.

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Electroanalysis 2000, 12, No. 1

charge accurately scaled in a linear fashion with the accumulationtime. The gradient of the linear plot of the charge stripped againstaccumulation times in Figure 2 was 3.31 mC sÿ1 and the linearleast squares regression analysis gave an R2 value of 0.9997.

Next the effects of the quantity of petrol added were investi-gated. Figure 3 shows that increasing the amount of the organicphase increases the magnitude of the lead stripping signal. Thedata shown were all obtained with an accumulation time of 240 s.In this case a 12 mm mercury-coated platinum electrode wasused.

PbR4�org� >��� PbR4�aq� �1�The data in Figures 2 and 3 clearly demonstrate the suitability

of sono-ASV for the quantitative determination of lead in petroldespite the insolubility of the latter in water. A likely mechanismfor the process is as follows,

PbR4�aq� � 4eÿ � 4H� ÿÿÿÿÿ?ÿ1:0 VPb�amalgam� � 4RH �2�

Pb�amalgam� � ÿ2eÿ ÿÿÿÿÿ?ca: ÿ0:3 VPb2��aq� �3�

The ®rst step involves the ultrasound assisted extraction of thetetraalkyl lead from the petrol phase. We do not exclude thepossibility of homogeneous chemical transformation of theaqueous PbR4 [46±48]; however the extracted lead clearlyundergoes reduction at the electrode surface whilst the latter isheld at a potential of ÿ1.0 V (step 2). Following accumulation thestripping step produces aqueous Pb2� (step 3) and if the analy-tical procedure is repeated the latter builds up in solution. It isnoteworthy ®rst that sonication in the absence of electrolysis ledto the release of no detectable lead even when employed for timeperiods suf®cient to release ca. half the available lead whenconducted in parallel with electrolysis. Second the assumptionthat the lead tetraalkyl transfers from the organic phase into theaqueous phase prior to electrolysis was tested by experiments inwhich the electroactive species iodine and ferrocene were dis-solved in octane, emulsi®ed via insonation and voltammetricallyinterrogated at a platinum electrode using a cell of the typereported. UV-visible spectroscopy showed that whilst appreciablepartitioning of iodine into water occurred, negligible ferrocenewas detected. Voltammetrically measurable currents were seenfor the former but not the latter, suggesting that step 1 is essentialin the mechanism proposed above.

Attention next focused on the development of a quantitativeprocedure based on the above observations. Accordingly

experiments were conducted in which three additions of 5 mL ofpetrol were made stepwise to 0.1 M nitric acid at the start of the®rst insonated accumulation. The measured stripping signalsusing a preplated 12 mm platinum electrode are shown as afunction of time in Figure 4. It can be seen that after ca. 2000seconds the response has increased to a steady plateau which isnot inconsistent with the complete extraction of organic leadinto the aqueous phase. Figures 4 and 5 show that furtheradditions lead to equivalent responses consistent with thisnotion.

With the hypothesis that the total lead content in the petrolcan be inferred by making microadditions of standard leadsolution to the aqueous phase, Figure 5 also shows the cali-bration of the cell sensitivity in this manner. Using the latterand taking the mean value of the three limiting values for eachpetrol addition gives a value of 380+ 40 mg Lÿ1 for the con-centration of lead in petrol detected by sono-ASV. For com-parison of the total lead content a petrol sample was sent toRooney Laboratories Ltd. (Basingstoke, UK) for independentanalysis using atomic absorption spectroscopy (AAS) accordingto the Institute of Petroleum method no. 362=93 [44].The leadconcentration was found by this method to be 400+ 20 mg Lÿ1

which is in excellent quantitative agreement with the sono-ASVresult.

The electrochemical procedure outlined above permits aquantitative determination of the total lead content in petrol in atotal time of ca. 2 hours without any sample pretreatment. Therapidity of the procedure relies crucially on the cell shown inFigure 1 which is the result of continued optimizationthroughout the course of the present study. In particular throughthe use of a large area (1.13 cm2) electrode together with theminimization of the total cell contents (9.0 mL) and theavoidance of `dead volumes' in the cell the time taken forexhaustive electrolysis of fully dissolved cell contents is esti-mated as 6.2+ 0.3 minutes on the basis of the two-electronreduction of 1 mM CuSO4 in 0.1 M Na2SO4. Experiments withlarger volumes or smaller electrodes produced correspondinglyinferior performances and suggested that for a fully dissolvedsubstrate (X) in a solution volume, V , at an electrode of area, A,the rate of concentration depletion is given by

Vd�X�dt� ÿDx�X�A

d�4�

Fig. 3. Anodic LSVs of lead at 100 mVsÿ1 at a 12 mm Pt=Hg disk.Sweeps were performed after 4 minutes of accumulation at ÿ1.0 V (vs.SCE) under 3.7 W ultrasound for a) no petrol added, b) 5mL of petroladded, c) 10 mL of petrol added, and d) 15mL of petrol added to 9 mL of0.1 M nitric acid.

Fig. 4. Variation of charge under the lead stripping peaks as a functionof time for consecutive additions of 5mL of petrol to 9 mL of 0.1 M nitricacid. A 12 mm Pt=Hg disk electrode was used. Each point corresponds toan accumulation period of 4 minutes under 52 W cmÿ2 ultrasound at ahorn-to-electrode distance of 7 mm.

18 A. N. Blythe et al.

Electroanalysis 2000, 12, No. 1

for transport limited conditions where d is the thickness of thediffusion layer. The half-life for the removal of X is therefore,

t1y2 � dV ln 2

DxA�5�

suggesting values of ca. 5.4mm for d consistent with valuesmeasured under similar conditions [40]. Quantitative examina-tion of Figure 4 shows that the half-life for the full sono-elec-trochemical extraction of lead from the petrol emulsion is of thesame order of magnitude as measured for the fully dissolvedcopper sulfate. This suggests that if the extraction from theorganic phase provides a kinetic barrier then the rate of masstransport must be augmented in the case of the electrolysis of theemulsion by virtue of electroactive material leaving the organicdroplets inside the diffusion layer so enhancing the rate oftransport. Similar effects have been reported for electroactivesolutes dissolved in water=organic solvent emulsions [27].

The technique described above shows currents as large as1 mA corresponding to the stripping of lead present at levels ofca. 400 mg Lÿ1. It is evident that useful measurable signals couldbe obtained with signi®cantly lower lead levels, possibly of asmuch as two orders of magnitude which would be consistent withdetection of the increasingly lower levels being required bylegislation. For many years tetraethyl lead was routinely added topetrol as an antiknock agent but its use has now been curbed dueto its incompatibility with catalytic converters and environmentalconcerns. Indeed only unleaded petrol is now available in, forexample, the Netherlands and Germany, and the use of leadedpetrol will be phased out across the EU from 2000 [2]. Experi-ments to investigate lower lead levels and detection limits arecurrently active.

4. Conclusions

The level of agreement between the AAS and ASV resultsreported in the preceding section is highly satisfactory suggestingthat ASV can be reliably used as an alternative to the presentlyaccepted procedures [3±11] with corresponding bene®ts of speed,cost and simplicity of approach; a particular advantage being theabsence of any necessary sample pretreatment. In addition the

possibility of portable electrochemical equipment for reliablelead-additive analysis appears viable.

5. Acknowledgements

We thank the EPSRC for a studentship for RPA and for®nancial support (grant nos. GR=L36413 and GR=L81 123). Weare also grateful to CeÂsar Agra-GutieÂrez for some preliminarymeasurements.

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