EFFECTS OF ORGANIC COMPOUNDS ON ANODIC STRIPPING OF METALS USING A MERCURY-FILM MICRO-PLATINUM

by

MARANAEK SIAGIAN

A Thesis presented for the degree of

Master of Science

at THE UNIVERSITY OF NEW SOUTH WALES

Supervisors :

1. A/Prof. P.W. Alexander. 2. Prof. D.B. Hibbert.

April, 1991 UNIVERSITY OF N.S.VV. 2 7 AUG 1993 LIBRARIES I hereby declare that this thesis is my own work and that, to the best of my knowledge and belief, it contains no material previously published or written by another person nor material which to a substantial extent has been accepted for the award of any other degree or diploma of a University or other Institute of higher learning, except where due acknowledgement is made in the text.

2-^ - o/\ - |

1 ACKNOWLEDGEMENTS.

In the name of Allah, most Gracious and most Merciful

I wish to express my sincere thanks to my supervisors, Associate Professor P.W. Alexander and Professor D.B. Hibbert, for their valued advice, guidance, suggestions, encouragement and help throughout this work.

Thanks to all members of the Department of Analytical Chemistry and School of Chemistry. I realize that all of you contributed help for this work. And also to my colleagues, who have encouraged me, shared experiences and helped me during this work, I extend my appreciation.

My appreciation goes to the Australian International Development Assistance Bureau (AIDAB) for financial support and guidance given and to the PPPTMGB "LEMIGAS" Jakarta, Indonesia, for the nomination and my leave of absence.

Finally, I wish to thank to my wife Nur and sons, Enru and Azri for their patience and understanding. May "Allah " guide us to the straight way, amen. ABSTRACT

This study evaluates the uses of a mercury-film micro-platinum electrode as a for anodic stripping voltammetry (ASV) of trace metals. The study of the response of this electrode which is of 5 mm length and 100 pm in diameter includes the effects of hydrocarbons, phenolics, pyridines, sulphur compounds, crude oils and

surfactants on the ASV scans.

The micro-platinum electrode plated with a thin mercury film by in situ deposition of mercury (5 x 10'5 M) in 0.1 M acetate buffer solution at pH 4.5 was chosen as the working electrode in preference to the hanging mercury drop electrode and the thin-film glassy carbon because of its stationary configuration, simple preparation and low cost. ASV scans of cadmium, lead, copper and mercury solutions are shown to give excellent resolution of the stripping peaks recorded at -0.80, -0.65, -0.20 V and +0.09 V (vs SCE), respectively. Zinc was detectable using the Pt-electrode only at high concentration in the ppm range.

The main objective of this study was the possibility of indicating the presence of various classes of organic compounds by their effects on the ASV scans of the above metal ion solutions. Addition of these organic compounds to standard mixtures of cadmium, lead, copper and mercury was studied. Hydrocarbon compounds showed no significant effect on the ASV scans, but non-hydrocarbon compounds had a variety of effects on the peaks observed. There was a very marked effect of crude oils on the ASV of the metal ion mixture when samples of 0.1 g of crude oil was dispersed in 100 ml of the aqueous solution containing 100 ppb of each the metal ions. It is shown, that the non­ hydrocarbon components in the crude oils are the main cause of the adsorption effects.

The non-hydrocarbon compounds studied had various effects on the ASV scans. For example, in the presence of a neutral surfactant (Triton X-100 at a concentration of 2.0 ppm) the Cd and Pb peaks disappeared but the other peaks were less severely affected. Similarly for the other organic compounds studied, different effects on the ASV peaks were observed, suggesting that this scanning method may be of value for indicating the presence of certain classes of organic compounds. Thus the possibility of the ASV scan as a rapid indicating method may be considered for detecting compounds such as crude oils, surfactants, detergents and dispersants which may be present in the environment. TABLE OF CONTENTS

CHAPTER 1. INTRODUCTION AND LITERATURE REVIEW 1

1.1. GENERAL INTRODUCTION

1.2. ELECTROCHEMICAL METHODS 2

1.2.1. Conductometric Analysis 2

1.2.2. Potentiometrie Analysis 3

1.2.3. Coulometric Analysis 4

1.2.4. Electrogravimetric Analysis 5

1.3. ANODIC STRIPPING VOLTAMMETRY (ASV) 5

1.3.1. Principle of AS V 6

1.3.2. Stripping Techniques in ASV 12

1.3.2.1. Linear Sweep and Staircase Voltammetry 12

1.3.2.2. Pulse Voltammetry 14

1.3.3. Comparison of ASV and CSV 16

1.3.4. Instrumentation 17

1.3.5. Electrodes 21

1.3.6. Reference and Auxiliary Electrodes 20

1.3.7. Working Electrodes 20

1.3.7.1. Mercury Electrodes 20

1.3.7.2. Solid Electrodes 24

1.3.8. Supporting Electrolyte 24

1.4. APPLICATION OF ASV ON METAL ANALYSIS 26

1.4.1. ASV Determination of Cadmium 28

1.4.2. ASV Determination of Lead 29

1.4.3. ASV Determination of Copper 30 1.5. ORGANIC INTERFERENCES ON ASV 30

1.5.1. Effect of Organic Compounds on AS V 32

1.5.2. Effect of Surface-active Substances on ASV 33

1.5.3. Effect of Metal complex-forming compounds on ASV 34

1.6. AIMS OF THE THESIS 35

1.7. REFERENCES 37

CHAPTER 2. EXPERIMENTAL 4 4

2.1. INSTRUMENTATION 44

2.1.1. Polarographic Analyzer 44

2.1.2. Electrodes 46

2.2. REAGENTS 46

2.2.1. Water 46

2.2.2. Metal Standard Solutions 46

2.2.3. Mercury Solutions 47

2.2.4. Sodium Acetate and Acetic Acid Solutions 47

2.3. PROCEDURES 47

2.3.1. Preparation of a micro-platinum electrode 47

2.3.2. ASV Scanning of Cd, Pb and Cu using a micro-platinum electrode 49

2.3.3. ASV Scanning of Zn, Cd, Pb and Cu 49

2.3.4. ASV Scanning of Cd, Pb and Cu using metal electrodes 49

2.3.5. ASV Scanning of Cd, Pb and Cu at Various Deposition Potentials 50

2.3.6. ASV Scanning of Cd, Pb and Cu at Various pH 50

2.3.7. ASV of Cd, Pb and Cu in Various Cone ofHg(II) 50

2.3.8. ASV of Cd, Pb and Cu in Aqueous Solutions with dissolved 51

Organic Compounds 2.3.9. ASV of Cd, Pb and Cu in Aqueous Solutions with Dissolved 51

Crude Petroleum [5,6]

2.3.10. Peak measurements 51

2.4. ASSESSMENT OF PRECISION 52

2.5. REFERENCES 53

CHAPTER 3. RESULTS AND DISCUSSION. 5 4

3.1. ASV OF Cd, Pb AND Cu USING A MERCURY-FILM MICRO-PLATINUM ELECTRODE 55 3.1.1. Effect of Deposition Potential 59 3.1.2. Effect of pH 60 3.1.3. Effect of Concentration of Mercury 60 3.1.4. Repeatability of ASV of Cd, Pb and Cu 62 3.1.5. Calibration Curve of Cd, Pb and Cu 63

3.2. EFFECT OF ORGANIC COMPOUNDS 66 3.2.1. Effect of hydrocarbon compounds 66 3.2.2. Effect of non-hydrocarbon compounds 68

3.2.3. Effects of organic sulphur compounds 71

3.3. EFFECT OF CRUDE PETROLEUM OILS 75

3.4. EFFECT OF SURFACTANTS 78

3.5. REFERENCES 83

CHAPTER 4. CONCLUSION 85

vii CHAPTER ONE INTRODUCTION AND LITERATURE REVIEW

1.1 GENERAL INTRODUCTION

Since the effect of heavy metals on biological processes tends to be toxic, many reports have been published on methods to determine very low levels of the heavy metals in an aquatic environment. However there exists a need for a direct and rapid method of determining trace metals in the marine environment. Anodic Stripping Voltammetry

(ASV) has been shown to possess high sensitivity and excellent resolution for certain metals in sea water [1], but its application is sometimes difficult or even impossible because of interference effects. The most frequent problems are an overlap of the analyte peaks or the presence of organic compounds, such as humic acid [2], fulvic acid, gelatine, alkyl phosphate, surfactants and polysaccharides [3], as interferences.

While ASV is a useful technique'for determination of heavy metals , its limitations and possible interferences appear to be inadequately appreciated. The effects of adsorption on ASV results, have been considered by some workers. Although adsorption of organic substances onto mercury electrodes is well-known from polarographic studies [4],

Brezonik et al. found effects of adsorption of a variety of model organic compounds, representative of types of organic substances occurring in natural waters, on hanging mercury drop electrodes in the anodic stripping voltammetric analysis of trace metals [3].

In this present study, the effect of organic compounds on anodic stripping voltammetry of trace metals using a mercury-film micro-platinum (0 = 100 pm) electrode is described. 2

It is proposed in this introductory chapter to briefly discuss electroanalytical methods with special emphasis on anodic stripping and the effect of organic compounds on metal analysis by anodic stripping voltammetry.

1.2 ELECTROANALYTICAL METHODS

Electrochemistry in an electrolyte always requires two electrodes and can be

carried out by means of either non-faradaic or faradaic methods. Non-faradaic methods of

electroanalysis, with a zero net electrical current, are represented by conductometric and normal potentiometric analysis. Faradaic methods of electroanalysis, with a-non zero net electrical current, are represented by voltammetric, electrogravimetric and coulometric analysis [5].

1.2.1' Conductometric Analysis

This method is primarily based on measurement of the electrical conductance df a solution from which, by previous calibration, the analyte concentration can be derived. \ Two types of methods are based upon the measurement of electrical conductance of solution, namely direct conductance methods and conductometric titrations. The principal

advantage of these methods is their simplicity and relatively good sensitivity [5,6].

Direct conductometric measurements suffer from a lack of selectivity, since any

charged species contributes to the total conductance of a solution. On the other hand, the high sensitivity of the procedure makes it an important analytical tool for certain applications. Perhaps the most common application of direct conductometry has been for estimating the purity of distilled or deionized water. The conductivity of pure water is only about 5 x 10'8 ohm-1 cm"1; traces of an ionic impurity will increase the conductance by an order of magnitude or more. Conductance measurements are also widely used to measure the salinity of sea water in oceanographic work. 3

1.2.2 Potentiometric Analysis

Normal potentiometric analysis is the determination of the analyte through measurement of the potential difference between an indicator electrode and a . Of more recent origin are methods in which ion concentrations are obtained directly from the potential of an ion selective electrode against an internal reference electrode. Such electrodes are relatively free from interference and provide a rapid and convenient means for quantitative estimations of numerous important anions and cations.

The response of ion-selective electrodes plays a major role in the way they can be applied to measurement processes. However, correction must be taken into account when dealing with solutions that contain solvents other then water. A correction factor for potential readings of glass electrodes has been determined for dimethyl sulfoxide [7]. The measurement of electrode selectivity is important for proper application of ion-selective electrodes.

Pungor and coworkers have reported two studies that deal with the behaviour of solid-state membrane electrodes at extremely low concentrations of the principal ion. In their first study, the response of the silver/sulphide electrode at low silver and sulphide concentration was investigated [8]. They explained that potential deviations at low concentrations of these ions are caused by excess silver at the membrane. In some cases, this excess silver appeared to be a contamination from previous measurements that was difficult to remove. In their second study [9], the response of silver chloride electrodes was measured at low chloride and silver ion concentrations. In this case, deviation from

Nernstian response was attributed to various processes occurring at the membrane surface such as adsorption-desorption and photoreduction. In related work, the use of the solid-state fluoride electrode under conditions close to its limit of detection [10,11] has been evaluated. Results indicate that experimental parameters and the method of operation can dramatically influence the overall electrode accuracy, precision, and response rate. 4

Finally, the linear response of cadmium [12] and copper (II) [13] solid-state membrane electrodes has been extended by use of metal ion buffers.

Many reports have presented solid-state membrane compositions which display ionic response to copper(II), copper(I), cadmium, nickel, lead, manganese, tin, mercury, arsenic silver etc. Table 1.1. shows some analytical applications of solid-state membrane electrodes.

Table 1.1 Analytical Application of Solid -State Membrane Electrode

Electrode A p 1 i c a t i o n Reference(s)

Copper (II) Determinarion copper in : peat soil 14 technical zinc electrolyte 15 Determination of: zinc in fertilizer 16 copper (II) , zinc 17 Lead Determination of lead in : cystein sample 18 wastewater from a battery 19 plant white spirit 20 Cadmium Determination of cadmium in water 21, 22

1.2.3 Coulometric Analysis

Coulometric analysis is the. determination of the analyte through measurement of the amounts of electricity (number of coulombs) required for complete chemical reaction of the analyte. There are three methods based on the number of coulombs. These methods are: constant-potential , constant-current coulometry or coulometric titrations, and [5,23]. 5

Controlled potential coulometric methods have been applied to the determination of some 55 elements in inorganic compounds and in the nuclear energy field for a relatively interference-free determination of uranium and plutonium [24]. A coulometric titration employs a titrant that is electrolyticaly generated by a constant current. The method has been developed for all types of volumetric titrations [25].

1.2.4 Electro gravimetric Analysis

Electrogravimetry is one of the oldest electroanalytical methods and generally consists of the selective cathodic deposition of the analyte metal on an electrode (usually platinum) followed by weighing. Although preferably high, the current efficiency does not need to be 100%, provided that the electrodeposition is complete, i.e., exhaustive electrolysis of metal of interest. This contrasts with coulometry, which in addition to exhaustive electrolysis requires 100% current efficiency [5,23].

1.3 ANODIC STRIPPING VOLTAMMETRY (ASV)

Voltammetry consists of a group of electroanalytical methods for the determination of the kind of analyte and its concentration through measurement of the current versus voltage curve. It was developed from the discovery of by Heyrovsky in 1922. The measurement of current is made as a function of applied potential obtained under conditions that stimulate polarization of the working electrode. The working electrodes in voltammetry are mainly characterized by their small surface area [5].

Anodic Stripping Voltammetry was first achieved by Zbiden [27]. He was attempting to determine copper electrogravimetrically by plating it onto a platinum electrode, but found that the amount of copper plated was too small to weigh accurately. 6

He devised the technique of reversing the current and stripping the copper from the electrode and made a quantitative determination by measuring the current consumed during the process.

Since the 1960's, a number of publications have appeared dealing with the theory of ASV for the hanging mercury drop electrode and the mercury film electrode [28-33]. But the technique of ASV has grown rapidly only in the past few years as a result of a greatly increased demand by environmental chemists and other scientists for sub-ppb metal analysis.

The technique of stripping voltammetry (SV) has gained considerable analytical importance owing to its sensitivity and selectivity. It also became known under several names such as linear-potential sweep stripping [144], stripping analysis [144], anodic amalgam voltammetry and inverse voltammetry [145] and re- dissolution voltammetry [146]. It is ordinarily used as an anodic stripping technique [28].

1.3.1 The Principles of ASV [34-37]

An ASV measurement involves a two-step technique. The first, or deposition step, involves the electrolytic deposition of a small portion of the metal ions in solution onto the mercury electrode, to preconcentrate the metals. The second, or stripping step, involves the dissolution of the deposit. Fig. 1.1 shows a typical potential wave form applied to the working electrode.

For the deposition step, a suitable electrode is maintained at a potential of the element(s) to be determined. The metals to be deposited arrive at the electrode surface at 7

Stripping

stirrer off

3tirrer

>- Time, t

Fig. 1.1 Principle of Anodic Stripping Voltammetry 8 rates determined by their respective concentrations, the diffusion properties of the electrolyte solution, and the area of the electrode used. The deposition time td which is the total time of the deposition step, is consequentially carefully measured. The deposition results in a preconcentration of the analyte(s) into a small volume (surface).

The metal ions reach the electrode surface by diffusion and convection-forced rotation of the working electrode or stirring the solution. They are there reduced and concentrated as amalgams in the mercury. The duration of the deposition is selected according to the concentration of the metal ions in question, from less then 1 minute at

0.1 ppm to about 10 minutes at 1 ppb [38].

At the mercury working electrode used for measurement of amalgam-forming metals the electrode reaction is :

Mn+ + ne' —► M (Hg) (1.1)

The deposition potential, E^, is applied to the working electrode to cause the metal of interest to deposit onto or into its surface. Deposition potentials for zinc, cadmium, lead and copper, are usually at -1.4, -1.1, -0.9 and - 0.6 V. However, in practice, it is best to check experimentally the effect of the deposition potential on the peak current of interest and to use the value that yields the largest peak current and minimum side reactions.

The deposition time, td, must be controlled carefully. Obviously, the longer the deposition step, the larger the amount of analyte available at the electrode during the measurement step.

In the deposition step the bulk concentration of the metal ion can be calculated according to Lingane [39] ; 9

^b,t — C^ ^exp ( ) (1.2) Vo where: Cb0 and Cbt are the bulk concentrations of the beginning of the deposition

step and a given time, t

A = Surface area of electrode

V = The solution volume

D = Diffusion coefficient of the ion

5 = The thickness of the diffusion layer

To strip this analyte from the electrode, its potential is swept back in the anodic direction. The faradaic current produced by the oxidation of each analytical species is measured. The reaction is : M (Hg) —► Mn+ + n e- (1.3)

The resultant current-potential voltammogram as shown in Fig. 1.2, provides the analytical information of interest. The peak potential, E , of each metal is a characteristic of that metal and is related to the standard potential of its redox couple. Thus, it can be used for qualitative identification. The peak current (height), i , is proportional to the concentration which is determined by a standard addition experiment or calibration curve.

Application of Faraday's Law enables the concentration of the metal in the amalgam, Ca, to be calculated :

r *Ltd (1.4) -nFVHg where: iL = Limiting current for deposition

F = Faraday VRg = Volume the mercury electrode Bi

Fig. 1.2 Voltammograms of Zn, Cd, Pb, Cu, Sb, and Bi, paniculate metter Deposition times for Zn, Cd and Pb : 2 min and Deposition times for Cu, Sb and Bi : 5 min From : G. Golan, Talanta, 29 (1982), 651. The amount of metal (in moles) accumulated on the surface (M) is given by :

iLtd M = (1.5)

According to the Nemst diffusion layer model, which is the simplest treatment of the convective system, the convection maintains the concentration of the metal ion uniform, at the bulk value, up to a certain distance from the electrode 5 (thickness of diffusion layer).

As a result of this concentration gradient, ions are moving by diffusion toward the electrode surface. The rate at which this transfer occurs is proportional to the concentration difference. Accordingly, the deposition current is proportional to the slope of the concentration profile at the electrode surface and can be described by:

nFADCb (1.6) 5

The equation above indicates that any means of reducing the value of 5 will increase the deposition current. Empirically, 8 is related to the convection rate by :

K 5 (1.7) Ua where U is the convection rate and.K and a are constants dependent on the flow regime and electrode geometry. For laminar flow conditions (smooth and steady mass transport) a usually ranges from 0.3 to 0.5. Therefore, when the convection rate increase, the diffusion layer thickness decreases. As a result, the concentration gradient becomes steeper, thereby increasing the deposition current.

The treatment of the stripping step assumes uniform concentration distribution of the metal in the volume of mercury. In contrast, during the deposition step the metal concentration in the mercury-drop or film approaches a parabolic distribution, with a higher concentration toward the solution direction [40,41]. The expressions for Ca(x,t), 1 2 obtained by solving the diffusion law for the flux of the metal in the mercury, indicate such a parabolic distribution (x and t being the location and time coordinates). The concentration becomes more and more uniform with longer deposition periods or thinner mercury films.

1.3.2 Stripping Techniques in ASV

Quantitative classic polarography with dropping mercury electrode is limited to solutions with concentrations greater than about 10'5 M. This limitation results from the non faradaic current associated with the charging of each mercury drop as it forms. Thus, when the ratio of the faradaic current (from the reduction of the analyte) to the non- faradaic current approaches unity, large uncertainties in determining diffusion currents are inevitable. To permit the quantitative determination of an analyte at lower concentrations, the classic method has been modified by modern techniques which increase the ratio between faradaic and non-faradaic currents by suppressing the latter. Some of the more important techniques which can be applied in voltammetry are described briefly in the paragraphs that follow.

1.3.2.1 Linear Sweep and Staircase Voltammetry

The theory of (LSV) for linear semi-infinite diffusion and the reversible electrode system was developed by Sevcik [42] and Randles [43].

Their calculations were revised [41,44] and extended also to partially reversible and irreversible processes [46] as well as to processes in which chemical reactions participate

[47-49]. The theory is valid for electrodes with a large volume; i.e. the effect of finite electrode volume has not been taken into account. The potential at any time, t, during the scan is given by :

E(t) = Ed + vt (1.8) where v is the potential scan rate. With the assumption that the rate of electron transfer in each direction is very fast, the Nemst equation holds :

Cb(0,t) = Ca (0,t) exp [ (Ed-E° + vt)] (1.9) where R is the gas constant and T is the absolute temperature. This equation serves as the first boundary condition. During the potential scan the metal concentration in the middle of the drop remains at its initial value; as a result, a concentration gradient and thus a flux of the metal towards the surface forms inside the drop. The second boundary condition assumes that the fluxes of the metal and of its ion, at the electrode surface, are equal.

Roe and Toni [50] treated the case of very thin mercury films. They assumed that there was no concentration gradient in the films, i.e., diffusion was negligible, a condition that could be met by thin films (< 10 pm thick) and a slow scan rate (<1 V min'l). They also assumed that the solution was stirred.

With the application of this technique, especially with stationary electrodes, LSV becomes fairly simple, under conditions of sufficient solubility of oxidation and reduction, because of the constant and undisturbed electrode surface.

The use of staircase voltage waveforms to reduce the charging current interference was suggested by Barker [51]. This technique achieves a result similar to that of current sampling in dc polarography, except that the additional complication of the dependence on drop time does not enter the argument.

A comparison of linear-scan currents with all staircase current points at the same potential under conditions of both restricted (Hg-film electrode) diffusion was carried out by using the finite element method in the Hermit version [52]. For strict equivalence, changes in the value of a, the fraction of the step length at which the current is sampled, with dE and the position on the stripping peak need to be taken into account; however, such deviations are not considered significant in comparison with the average experimental error [53].

1.3.2.2 Pulse Voltammetry

Another approach to minimize the residual current is by using differential pulse anodic stripping voltammetry (DPASV) [54,55]. Perhaps the most widely used stripping mode is DPASV. In the differential pulse stripping mode, pulses of equal amplitude are superimposed on an anodic potential scan as a shown in Fig. 1.3. The pulses have an amplitude of 25 or 50 mV, a duration of about ms, and a repetition rate of 0.5 - 5 s.

Usually, the basic potential scan rate is slow, 2-10 mV s'1 [56,57], so that the ramp potential does not change significantly during the pulse life. The decoupling effect occurs when the pulse is applied at rate at which the reduced and oxidized forms of the electroacdve species are present. This would be correspond to the potential region of the

ASV stripping peak for that species.

The total current in the system is increased if the potential is suddenly stepped.

This is because of the pulse termination. The charging current induced by the application of the pulse decays more rapidly than the faradaic current. This is governed in pan by the rate at which the reoxidized metal can diffuse away from the pulse application and again for the same period of time at the end of the pulse life. The difference in these two measurements is amplified and read out. The choice of the measurement periods permits a high level of discrimination against the charging current since the current measured near the end of the pulse is predominantly due to the faradaic reaction. Potential Fig.

1.3

2. 3. Stripping 1.

Current Pulse Pulse

potential

repetition amplitude measurement

waveform 1

5 period

for

DPASV

[57] Time 1.3.3 Comparison of ASV and CSV

Anodic Stripping Voltammetry is usually used to determine metals that form amalgams with mercury. For metal ion determination the species is first reduced at negative potentials in an amalgam which dissolves and preconcentrates on an electrode. The accumulated species is then stripped back into solution by applying a scan potential in the positive direction.

Cathodic stripping voltammetry involves the accumulation of a species at relatively positive potentials to undergo oxidation with the formation of an insoluble species as a film around the electrode surface. The adsorbed film may then be reduced using a cathodic scan which strips the material from the electrode surface [58]. The reactions involved in the deposition step can be represented :

M —► Mn+ + ne (1.10)

(l.H) where M is the electrode material, An~ is the analyte, and MA the insoluble salt forming the film on the electrode. The reactions in the stripping step is :

MA + n e —► M + An' (1.12)

Cathodic stripping voltammetry is the "mirror image" of anodic stripping voltammetry in term of the potential-time profile applied.

The sensitivity of cathodic stripping voltammetry depends on the amount that can be plated in a certain period and the dissociation rate of the insoluble mercury compound during the stripping step [59]. A large amount of mercury compound on the surface area is achieved using electrodes with large surface area such as mercury drop or pool. In contrast, the anodic stripping measurements of trace metals, mercury films are used to obtain higher amalgam concentration. Florence [60] recommended the use of a mercury pool electrode for CSV, because the presence of mercury in the cell (in situ deposition or using the HMDE) may result in lower peak currents and erratic results due to the formation of insoluble mercury compounds in the solution.

Cathodic stripping voltammetry is used to determine a wide range of organic and inorganic compounds. Inorganic ions such as halide ions, selenide ions, vanadate ions, chromate ions can be determined by cathodic stripping voltammetry. These determinations are based on the reactions of the anion of interest with the electrogenerated Hg22+ ion to form a partially insoluble mercury (I) film that can be concentrated on the mercury electrode surface. Cathodic stripping procedures for the determination of organic compounds such as hydrogen sulphide [61 ], thiols [147], thio-urea [148], total sulphur in hydrocarbons [149], flavins [60] have been described.

1.3.4 Instrumentation

For the various electrochemical measurements usable in stripping determinations, there are a great number of commercial instruments available [37, 62-64], ranging from complete apparatus, e.g. d.c., a.c., and square-wave polarographs, to various electronic components, from which apparatus can be built, e.g. , galvanostats, current and voltage amplifiers and recorders. They include a polarographic analyzer with a three- electrode , voltage ramp generator, and current measuring circuitry; a cell with working, reference and auxiliary electrodes; and a recorder or other read device.

The basic electronic circuitry for ASV is shown schematically in Fig. 1.4. A d.c. power pack feeds a variable d.c. voltage supply and scan generator, which either holds the working electrode at a constant potential with respect to the reference electrode, or changes its potential in a continuous fashion.

The total voltage applied to the cell between the auxiliary and working electrodes is given by : Fig. 1.4 Block diagram of apparatus for controlled potential voltammetry

1. Voltage and scan generator 2. Potentiostat 3. Recorder 4. Reference electrode 5. 6. Working electrode 1 9

E = Va - Vw + i R (1.13)

where E is the total voltage, Va is the potential of the auxiliary electrode,Vw is the potential of the working electrode, i is the current through the cell, and R is the resistance of the solution between the auxiliary and working electrode.

The potential of the working electrode, Vw is the most important for ASV measurement. A reference electrode should be placed near the working electrode, in order to obtain its potential response. A potential measuring device continuously measures the potential difference between the working and reference electrode, and if the potential difference strays from the pre-set value an error signal is fed back to the power supply to tell it to change the total voltage across the cell (E) until the working electrode potential

(Vw) is restored to its pre-set value, irrespective of what happens in the cell. The electronic box that performs this funcdon is called a potentiostat.

The curve showing the relationship between the cell current (i) and the potential of the working electrode (Vw) is recorded on x-y or x-t recorder. In the case of a.c. or pulses of various waveforms are injected into the circuit [33, 65]

The three-electrode system in voltammetric analyses consists of an auxiliary electrode, a working electrode and a reference electrode. The function of auxiliary electrode is to complete the electrical circuit through the cell and the working electrode is the desired electrode reactions occur. The reference electrode, which is designed to have a constant potential and be independent of solution conditions, is used to monitor the potential of the working electrode. There is no current flow through the reference electrode circuit. The current for read out purposes is that flowing through the auxiliary and working electrodes. 20

1.3.6 Reference and Auxiliary Electrodes.

The most commonly used reference electrodes are the saturated calomel electrode (SCE) and the silver/silver chloride (Ag/AgCl). These electrodes are separated by a diaphragm from the test solution. As diaphragms, suitable sintered glass filters, possibly with a layer of agar are used. In some determinations, the ions of the reference electrode solution diffusing into the electrolytic space may interfere. Then a different reference electrode must be employed, or the reference electrode must be connected with the test solution by a salt bridge containing a solution of a suitable inert salt.

The auxiliary electrode is normally made from platinum wire or foil, but almost any chemically inert material with a reasonable surface area satisfies requirements for the auxiliary electrode.

1.3.7 Working Electrodes

The ideal working electrode should have favourable electrochemical behaviour to the analyte(s) of interest, a reproducible surface area, and low background current. Working electrodes used in ASV can be grouped into mercury electrodes and inert solid electrodes. And there are also two groups of electrodes used for solid electrode stripping analysis: carbon electrodes and noble metal electrodes.

1.3.7.1 Mercury Electrodes

Mercury electrodes are the most frequently used in ASV determinations due to their advantageous electrochemical properties especially the broad cathodic potential range. In neutral solutions, the potential range available is roughly from - 2.5 to + 0.2 V (vs SCE) [38,39]. There are two types of mercury electrode that have been widely used 21 for stripping analysis: stationary mercury electrodes and the mercury-film electrode (MFE).

The stationary drop electrodes which are most frequently used at present may be divided into two classes: (a) the hanging mercury drop electrode with a glass capillary (HMDE), (b) the stationary mercury drop electrode on a metallic or graphite support.

The Hanging Mercury Drop Electrode. The design of the HMDE electrode has been remarkably improved by Kemula. The Kemula-type HMDE allows easy and reproducible renewal of the mercury drop and use of relatively long deposition periods. The HMDE consists of a glass capillary tube connected to a micrometer head mercury reservoir. Connection of the mercury drop to the electrical circuit is made via a contact in the reservoir. The drop is usually dislodged at the end of the stripping step, and a new drop is dispensed for the next experiment. By turning the micrometer screw through a given number of degrees or by shifting the piston a certain length, a constant amount of mercury is pressed out of the reservoir. The reproducibility of the size depends on the accuracy of the device.

A low surface area to volume ratio of the HMDE is one of the disadvantages of the HMDE. The plating efficiency is reduced by the small area and the large volume causes a low concentration of the metal in the mercury and also broadening of the stripping peaks and so loss of selectivity. Another disadvantage of the HMDE is that only moderate solution stirring rates can be used to avoid dislodging the drop. Due to the relatively large volume of the HMDE, a rest period is required between the deposition and stripping steps to allow the amalgam concentration to become more homogeneous. The room temperature can affect the contraction or expansion of the mercury reservoir, causing the drop size to change. To minimize this effect, the reservoir should be as small as possible and the electrode temperature be thermostatically controlled. 22

Stationary Mercury Electrodes with an Inert Support. An inert contact is most often made of platinum [66,67], silver or gold [67-69].The contacts are usually made of platinum wire and a satisfactory design is that of Underkopler and Shain [70]. A platinum wire, 0.2-0.4 mm in diameter, is sealed in a glass tube, ground down level with the glass, and polished. Then the wire is etched with aqua regia to a depth of about 0.1 mm below the glass level (Fig. 1.5) and a mercury drops deposited on it. In this way, the mechanical stability of the drop is considerably improved, so that the electrode can even be rotated. The platinum contact can also be embedded in epoxide resin or in a suitable plastic. The preparation is very easy, and the electrode can even be used in media which are corrosive towards glass, e.g. hydrofluoric acid [71].

0.1mm

Fig. 1.5 Stationary mercury electrode with Pt contact design by Underkofler and Shain. (1. glass, 2. Pt wire and 3. mercury ) [70]

The Mercury Film Electrodes. In recent years mercury film electrodes (MFEs) have frequently been used in electroanalytical practice. By using such electrodes, metal ions present in the solution in trace amounts may be determined with satisfactory accuracy by their reduction on the surface of MFEs, by formation on a relatively concentrated amalgam, and by anodic oxidation from MFEs of the preconcentrated metal in a final step. In fact, a mercury film is produced in situ support forms a layer composed 23 of small droplets with a size dependent on the amount of mercury deposited and the deposition potential [72].

The ideal substrate for the MFE must meet relatively few criteria. It must have some reasonable electrical conductivity, be chemically inert to the mercury and the analysis solution, and be electrochemically inert at the potentials anodic of the most easily reduced elements of interest. Metals, although highly conductive, have several adherent oxide films (e.g. Pt, Ni), which inhibit uniform mercury deposition. Others have a low hydrogen overpotential and are slightly soluble in mercury ( e.g. Ag. Ni. Pt.). This results in a hydrogen overpotential for the film which decreases with time, making certain determinations highly irreproducible or impossible. If a substrate has a fmite solubility in mercury, intermetallic compounds between substrate and analyte (e.g. AuCu, AuGa, AuZn, Auln, AgZn, AgCd, etc.) may form, resulting in irreproducible stripping peaks [66]. Carbon has a reasonably high hydrogen overpotential and electrical conductivity, is insoluble in mercury, and is chemically inert. However, many graphite types are porous enough to allow the analyte solution or the mercury film to creep into the substrate. This results in a constantly changing electrode area and irreproducible behaviour. Wax impregnation of porous graphite or carbon prevents solution creep but can cause non- uniform mercury deposition if precautions are not taken to keep the polished surface free of wax films. Polishing is difficult even when wax impregnation is used.

The inert support is usually made of platinum or silver, but frequently also graphite [73] or glassy carbon [74]. Mercury film electrodes are commonly cylindrical on a wire with a length of 1-2 cm and a diameter of 0.5 - 0.8 mm. Mercury is deposited on a perfectly clean electrode by electrolysis from a mercuric salt solution. The most suitable film thickness is about 1-3 \im. An MFE can be made by in situ deposition, where a low concentration of mercuric ion is added to the test solution. Matson et al. [71] recommend adding 10'5 - 5 x 10'8MHg2+. 1.3.7.2 Solid Electrodes

There are a number of shape and material possible for solid electrodes used in stripping analysis. Cylindrical electrodes with small diameters (wires, 0 = 0.5-0.8 mm ) or larger diameters (rods), and disk electrodes have been chiefly used. There are two groups of electrodes being used for solid electrode stripping analysis: carbon electrodes and metallic electrodes. Carbon and Graphite Electrodes. Carbon has a relatively wide potential range. It is also chemically inert and relatively inexpensive. Glassy carbon has been used extensively for this purpose. Carbon paste, a mixture of graphite powder and viscous organic binder, has advantages that it can be renewed quickly and that its background current is very low. Other carbonaceous material used for solid electrode stripping measurements include pyrolytic graphite, obtained by thermal decomposition of hydrocarbon [75] and wax-impregnated graphite.

Metal Electrodes. Gold and platinum are the most common metallic electrodes used for stripping measurements. Solid metallic electrodes are not as chemically inert as carbon. All of them adsorb hydrogen on their surface. The deposition potential should be selected carefully to minimize the decreased sensitivity associated with hydrogen adsorption [76]. Mechanical, chemical and electrochemical pretreatment for metal electrodes are generally recommended, in order to obtain as reproducible a surface as possible. Mechanical pretreatment is using grinding and polishing with suitable media, chemical pretreatment by soaking in various acids and electrochemically, the electrode is frequently polarized at various voltages and in various electrolytes.

1.3.8 Supporting Electrolyte

Supporting electrolytes are required in solutions for voltammetric studies in order to conduct a current through the solution; the concentration of this must be much greater 25 than the analyte (usually 100-1000 times greater) to prevent the analyte from migrating to the working electrode by electrical transport [77,78].

The most common supporting electrolytes used in aqueous solutions are: potassium and lithium chloride, dilute sulphuric and hydrochloric acids, sodium hydroxide, and buffers such as acetate, citrate, borate and phosphate. For non-aqueous voltammetry tetraalkyl-ammonium salts (e.g. tetraethylammonium perchlorate) have been commonly used.

For voltammetric studies on some organic substances it may be necessary to carry out the investigation in an organic solvent, such as dimethylsulphoxide (DMSO), dimethyl-formamide (DMF), or acetonitrile. Flowever, there are many instances when a mixture of aqueous buffer solutions containing an organic solvent such as methanol are advantageous. Under these conditions organic reactions involving protons as well as electrons are still pH-dependent. It is possible to use this phenomenon to shift the peak potentials of species to potentials where interference may not occur [79]. Therefore, preliminary investigation's to study the effect of solvent concentration, buffer pH and ionic strength are usually performed.

Oxygen is capable of dissolving in aqueous solutions to produce millimolar concentrations. This can undergo reduction at a working electrode by the following mechanisms :

in an acid solution

02 + 2 e- + 2 H+ —► h2o2 (first reduction) (1.14)

H202 + 2e- + 2H+ —► 2 H20 (second reduction) (1.15)

in a neutral or alkaline solution

02 + 2 e~ + H20 —► ho2- + OH" (first reduction) (1.16)

H02- + 2 e- + H20 —► 3 OH (second reduction ) (1.17) 26

These reduction processes occur in the potential regions between about -0.05 and -1.3 V versus a saturated calomel electrode (SCE); therefore, voltammetric waves due to dissolved oxygen can overlap with many other electroactive analytes. In addition, the products of these processes may react with the species to be determined and cause erroneous results. For these reasons it is usual to remove dissolved oxygen, prior to voltammetric (polarographic) analysis, when scanning in the cathodic direction. This is normally achieved by passing nitrogen gas through the solution containing the analyte for about 10 min prior to the voltammetric measurement. Commercially available nitrogen contained in cylinders usually contains traces of oxygen so it is necessary to remove this before deaerating the sample solution; this is usually done by passing the gas through a vanadous chloride solution contained in a dreschel bottle [80]

1.4 APPLICATION OF ASV ANALYSIS OF METALS

The high sensitivity of anodic stripping analysis has led to a large number of analytical applications. The technique has been useful for the determination of various trace metals in environmental and clinical samples. Heavy metals may be the most harmful pollutants because, unlike many other pollutants, they are not biodegradable and are retained in the ecosystem indefinitely. Metals are in water, air and soil as a result of both natural and anthropogenic sources. Stripping analysis permits extremely sensitive determination of trace metals in the environment. Its environmental applications have been reviewed [81]

Over the last 20 years stripping analysis has been widely used in all branches of aquatic trace metal chemistry. In 1963, Ariel and Eisner [82] measured cadmium, zinc, and copper in Dead Sea brine. The major salt content in the Dead Sea water did not interfere with the trace metal determination. In 1965, Matson et al.. [73] reported that the sensitivity of ASV measurements of sea water could be increased by replacing the HMDE with a mercury-coated graphite electrode. 27

Among the heavy metals, Cd, Co, Cr, Cu, Pb, Ni and Zn can be advantageously determined by voltammetry using various approaches. Amalgam-forming heavy metals such as Cd, Cu, Pb and Zn are frequently determined by differential-pulse anodic stripping voltammetry (DPASV) at the hanging (HMDE) or stationary mercury drop electrode [83].

Trace metal levels of lead, cadmium, copper, indium, bismuth, and antimony in sea water were determined by Florence using a disk glassy carbon electrode in conjunction with linear scan stripping voltammetry [84]. Samples of marine organisms were also carried out by drying, ashing and washing with acid, filtering and diluting with supporting electrolyte solution. For these samples, Florence compared several ashing procedures and found no significant differences, although muffle ashing at 350° C was found to be the most convenient. In addition, the development stripping procedures for trace metal measurements in natural water, have been created by Nurnberg,-Valenta, Stoeppler, and their coworkers [85,86].

For applications involving quantitative analysis below the parts per billion concentration level, eg, thallium, lead or cadmium in sea and inland waters, the sensitive differential pulse stripping mode usually is employed. When this mode and a 15 min deposition are used, the detection limits for cadmium, lead and copper in sea water are 10, 10 and 30 ppt, respectively, with a precision of + 25 % at this level [87]. In addition to the differential pulse and subtractive stripping voltammetry have been applied successfully for trace metal quantitation in various water samples [88,89].

In the determination of heavy metals in sea water, anodic stripping voltammetry is perhaps the most widely use analytical technique. AS V is a fast and sensitive technique well suited for trace analysis of heavy metals at pg L'1 and sub pg L'1 levels. ASV is 28 especially useful for simultaneous multi-elemental analysis. The technique requires no elaborate pretreatment of sample, thereby minimising sample handling and contamination.

So far, most ASV analysis of heavy metals in sea water have been carried out with mercury film electrodes [84,90,91]. Compared to the conventional hanging mercury

drop electrode, the mercury film electrodes are more sensitive and more stable under

hydrodynamic conditions. Of various mercury film electrodes used, glassy carbon seems

to be the best support for the mercury film. The advantages of using glassy carbon

include its inertness, low porosity, a wide accessible potential range and simplicity of use

[84,95].

Ang et al. [93] reported that the levels of cadmium, copper, lead and zinc in

coastal sea water samples were determined by an automated anodic stripping voltammetry

with a flow injection system. The ASV analysis represented a first attempt to establish

baseline concentrations of cadmium, copper, lead and zinc in the coastal sea waters of

Singapore.

There are several heavy metals which are often found in the literature as an object

of observation, especially with mercury film electrodes. A review of the use of the metals

Cd, Pb and Cu, are described in the following (and resumed in Table 1.2).

1.4.1 ASV Determination of Cadmium

Cadmium (Cd), a member of Group IIB, is reduced reversibly to the metal at a

DME in non-complexing inert electrolytes, with half-wave potential around -0.6 to -0.8 V

and the wave-height is proportional to the Cd concentration over a wide concentration

range. On addition of complexing agents, the wave shifts to more negative potentials. In

slightly alkaline media, a well developed wave is also obtained, but in strongly alkaline

solutions the electrode reaction becomes irreversible. 29

Cadmium is quite soluble in mercury. The electrode reaction is rapid, and its determination on stationary electrodes is quite sensitive. However, electrodes with gold or platinum contacts are unsuitable, since the sensitivity is sharply decreased by formation of intermetallic compounds.

Indium, thallium and lead can interfere with the stripping measurements of cadmium, but this can be prevented in a number of ways such as by suitable choice of the base electrolyte. In hydrochloric acid solutions, cadmium can be determined in the presence of large amounts of In, since E]/2 Cd is more negative than Ej/2 In. A K2C03 solution is suitable for separation of Cd from T1 and Pb, but the determination is not very sensitive, because of the low solubility of Cd (II) in this medium. In KOH solutions, as well as alkaline tartrate, separation of Cu, Cd, and Zn is poor but simultaneous determination of Cu, Cd, Pb and Zn is possible in acidic tartrate solutions and in an acetate buffer at pH 5.4.

1.4.2 ASV Determination of Lead

Lead, Pb2+, can be reduced reversibly to the metal at the DME. In a solution of inert salts and acids, such as KC1, KNO^, HC1, HNO^, Ej^ is about -0.45 V. In alkaline solutions, the wave is shifted to about -0.7 (reduction of HPb00’ ion). On solid electrodes, the oxidation Pb(II) —► Pb(IV) proceeds with the formation of sparingly soluble Pb02 in neutral and weakly alkaline solution.

Mercury electrodes are generally used for the stripping determination of lead.

Lead can be stripped ffom solid inert electrodes after reduction to the metal. The reduction of Pb2+ to the metal can be employed for sufficiently precise determinations with platinum [94] or graphite [95]. However, the sensitivity is lower than that of the amalgam method. The determination can be carried out in ammoniacal or acetate buffer at 30 pH 5-6. The detection limit is about 5 x 10'9 M. T1 and Mn can interfere with lead [96].

With a carbon paste electrode, lead can be determined after oxidation to Pb02 even in the presence of a 100-fold amount of tin [97].

1.4.3 ASV Determination of Copper

Copper is reduced reversibly to the metal at a DME in non-complexing inert electrolytes, with a half-wave potentials on the addition of complexing agents and in some media (e.g. ammonia, pyridine, chloride, thiocyanate) two waves are obtained (Cu2+ —► Cu, Cu+ —► Cu). In solutions containing cyanide ions, Cu(II) is not 9 reduced to the metal since Cu(CN)4 ' decomposes to give the univalent copper complex and (CN)9 The reduction of the Cu(CN)4 complex proceeds at very negative potentials.

Cu can overlap Bi or Fe in the simultaneous ASV measurement. The Cu peak is shifted to more negative potentials in alkaline solutions. A suitable medium for separation from the Bi peak is 1 M KSCN (EpCu = -0.6 V, EpBi = -0.25) [143]

1.5 ORGANIC INTERFERENCES ON ASV

There are differing views on the effect of organic compounds on ASV signals.

For example, the addition of humic acid concentrate to synthetic sea water was observed

[116] to cause both enhancement and suppression of individual metal ion peaks but there where no significant changes in the peak positions. With a different base electrolyte at pH

6.8, addition of humic acid was found to alter both peak height and position [117]. The size, shape and position of Cu, Pb and Cd peaks has been shown [3,116,117] to be sensitive to the presence of wide a range of organic species (including surfactants), and as with the humic acids, the changes induced tend to be random in nature. Table 1.2 Stripping Determination of Cd, Pb and Cu

Metal Electrolyte Material Electrode Method Ref.

Cu Sea water Sea water HMDE DPASV 98

Cd,Zn Sea water Sea water RDE DPASV 99

Cd,Pb,Cu Am-Bromide SRM 1643b HMDE DPASV 100

Cd,Pb sea water sea water HMDE DPASV 101

Cu 0.2 M EDTA and 0.01 M Asc. Acid HMDE DPASV 102 Cd,Pb,Cu 38 % H2S04 Lead-Acid battery MME DPASV 103

Cd,Pb,Cu Agrc. by product HMDE DPASV 104

Cu Sea water Sea water HMDE LS/DPASP 105

Cd Sea water sea water 106

Cu Aqueous- natural water HMDE ASV 107

carbonate

Cd,Pb,Cu Extract soh sediment HMDE DPASV 108

Cd Sea water sea water MFE DPASV 55

Cd,Pb,Cu Hg HMDE d.c.V. 109

Cd,Pb,Cu hno3 hno3 MFE d.c.V. 110

Cd,Pb,Cu Sea water Sea water HMDE d.c.V. 111

Cd,Pb,Cu Water Water HMDE d.c.V. 112

Cd,Pb,Cu Sea water Sea water HMDE d.c.V. 113

Cd,Pb,Cu Water Water HMDE d.c.V. 114

Cd,Pb,Cu Sea water Sea water TMGE dcDPASV 115

HMDE = hanging mercury drop electrode

DPASV = differential pulse anodic stripping voltammetry

TMGE = tubular mercury graphite electrode. The lability of metal ions associated with organic species has not been clearly resolved. This situation is complicated by the fact that bonding modes tend to vary with the pH, relative concentrations and type of functional groups involved [118,119]. Extraction studies [120] indicate that a high proportion of the sorbed material is ion- exchangeable, and accordingly may be labile in ASV. The acid-soluble, lower molecular- weight, companion species, fulvic acid, is considered to form soluble metal complexes. The conditional formation constants reported vary with pH and the material used [121- 127], and under some conditions, sparingly soluble species have been isolated [128-

129]. The apparent stabilities of the complexes formed when metal ions are added to natural waters are often similar to those quoted for metal fulvate complexes [130].

1.5.1 Effects of Organic Compounds on ASV

Brezonik et al [3], described the effect on sorption by natural and model organic compounds, such as gelatine, alkaline phosphate (as representatives of soluble protein), triton XI00 (nonionic surfactant), and a variety of polysaccharides, including agar, alganic acid and starch. Effects of sorption included depressed ip values, shifts in Ep to more positive values, and broader peaks.

The sorption of surface active substances can affect both diagnostic parameters (ip and Ep) used in ASV. Ep may shift to more positive values if a sorbed molecule coats the electrode and renders the metal oxidation irreversible by creating a barrier to ion diffusion. Sorption affects peak current in two ways: by preventing metal deposition (a sorbed organic layer may hinder metal ion diffusion to the surface or retard chemical steps prior to electron transfer) and by changing the reversibility of the metal oxidation reaction. This phenomenon can be detected by measurement of w^, the peak width (in mV) at half the peak height. Theoretical reladonships for w^ as a function of sweep rate and film thickness have been developed by de Vries [131]. For linear sweep DC ASV with mercury film electrode the Randles-Sevcik theory applies [28] and is 102 mV for reversible reactions at 25 °C.

The effect of organic colloids, such as humic and fulvic acids on the ASV signals of Cd, Pb and Cu in acetate-buffer media has been investigated by Ugapo and Pickering [132]. The magnitude of the interference effect varied with the molarity of the medium and the presence of diverse anions (e.g., chloride, bromide) in the system.

1.5.2 Effects of Surface-active Substances on ASV

Comprehensive studies on the possible effects of surface-active substances on the response of anodic stripping voltammetric analysis were carried out by Brezonik et al. [3] and Sagberg and Lund [116]. A variety of model surface-active substances, represented in natural waters, were used. In both studies a hanging mercury drop was used as the working electrode. For certain surface-active substances, e.g. alginic acid, Triton X-100, or alkaline phosphatase, a 65-90 % decrease in the stripping peak current was observed. These studies showed that not all surface-active substances exhibit a depression effect, and that no general trend can be established. The effects produced by surface-active compounds depend on their chemical structure, their concentration, their environment, the metal determined, the deposition potential, and the type of electrode used. Similar peak depressions, coupled with smaller changes in the peak shape, were observed in a similar study utilizing the in situ plated mercury-film electrode [133]. In some cases the depression effect is reversed at high concentration of the surface-active substances. Various surface-active substances may result in significant enhancement of the stripping peaks. For example, certain long-chain alkyl amine and ammonium salts enhance the stripping peak of indium and tin in oxalic acid [134]. 34

1.5.3 Effects of Metal-complex forming compounds on ASV

Several workers have proposed that the extent of metal complexation by organic ligands can be determined by comparing ASV measurements (i.e. ip) on untreated samples with results from acidified or digested samples. Metal-organic complexes in natural waters are often assumed to be dissociated at low pH. For certain weak acid ligands, such as amino acids, this assumption is probably valid. Analysis of an untreated sample thus should measure the free ion and any reducible complexes, while an acidified or digested sample should yield the total soluble metal concentration. The difference between the two measurements has been interpreted as an estimate of the non-reducible (presumably organic ) complexes [135,136].

In the stripping step, the kinetics of the electrodeposition [137] and chemical conditions at the electrode surface [138] may influence peak voltage and peak current if natural complexing agents or surface-active substances are present in the sample solution [139]. In a calculation of the percentage of labile metal in a natural sample from the relative height of the stripping peak, the direct proportionality between the quantity of the metal oxidized from the amalgam must be verified. If this is not the case, ASV performed with a suitable medium exchange may ensure that the ASV-labile measurement is controlled only by the deposition step.

Moreover, a slow rate of metal-ligand exchange in the bulk solution may cause the concentration of the electrochemically available metal to be only loosely related to the "natural" labile fraction, when this fraction is determined by standard addition. In fact, a metal spike may equilibrate very slowly with the natural pool of physicochemical species of the metal in the sample [140]. Thus, the use of a peak-height calibration graph based on additions of ionic metal may be suitable only when the reactions are well-known and free of any rate-limiting interferences. 35

Recent studies on Cu reduction in multiligand-chloride media (i.e. estuarine waters) have shown that the electrodeposition on the hanging mercury drop electrode

(HMDE) is controlled by a competitive reduction between Cu(II)-organic species and a Cu(I)-chloro complex intermediate and the involvement of latter will depend on the kinetics of dissociation of the complex and the electrode reaction [141]. In sea water at natural pH, the high chloride concentration increases the stability of the intermediate chloro complex. Nevertheless, at natural levels of Cu in untreated samples, the reaction remains irreversible and the shape of the oxidation peak is ill-defined.

Mercuric ion tends to form complexes with numerous ligands and these often exhibit relatively large formation constants [142]. Various surfactants are also complexing agents, and lowering of the peak may be the combined effect of both processes, complexation and adsorption. For example, fulvic and humic acid, which are strongly adsorbed on the mercury electrode, are known to complex heavy metals As both effects may be pH dependent, the difference in peak current between an untreated and acidified sample does not always define the complexed metal fraction [3]

1.6 AIMS OF THE THESIS

There are several types of mercury film electrodes that have been tested for anodic stripping voltammetry (ASV), but platinum mercury thin film electrodes might still be relevant to study, particularly in a micro-electrode. Only a few reports have been published on the application of a micro-platinum electrode on differential pulse anodic stripping voltammetry (DPASV) of trace metals.

The aim of this thesis is to study the characteristics of the micro-platinum electrode on ASV of metals and the effects of organic compounds. The main point of this work is to show that organic compounds such as crude petroleum oils, surfactants and 36 detergents affect the ASV voltammograms of cadmium, lead copper and mercury at a platinum electrode.

A micro-platinum electrode which had 100 fim in diameter and 5mm length was constructed and tested on the ASV of cadmium, lead copper and mercury in an acetate buffer at pH 4.5. The responses of the electrode on various concentrations of cadmium, lead,copper and mercury were also evaluated.

This electrode was chosen because it was easy to construct and of low cost.

Although the hanging mercury drop electrode can be used for simultaneous determination of zinc, cadmium, lead and copper, it is not suitable for use in a continuous-flow analysis, or on-line analysis of river, lake or sea water. It is difficult to control the mercury drop size, because the temperature may cause expansion or contraction of the mercury in the electrode reservoir. Carbon or graphite electrodes are used most frequently in practical stripping determinations, but the electrodes are expensive [150,151,45] and must be kept highly polished to achieve reproducible data.

ASV with the micro-platinum type of electrode was then attempted in an acetate buffer which was contaminated with hydrocarbon compounds such as n-heptane, n- hexane, cycloheptane, cyclohexane, benzene, toluene. Moreover, the effects of phenol, pyridine, crude petroleums, detergents and surfactants were evaluated based on ASV data using the micro-platinum electrode. 3 7

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97. H. Monien and K. Zinke, Z. Anal. Chem. 250 (1970) 178. 98. A. Nelson and R.F.C. Mantoura, J. Electroanal Chem. Interfacial Electrochem., 164(1984) 253. 99. Scarano, G.; E. Morelli; A. Seritti, and A. Zirino, Anal Chem. 62 (1990) 943 100. F.L.L.Muller, and D.R. Kester, Environ. Sci. Technol., 24,(1990) 234 101. G.E. Batley and Florence, J. Electroanal. Chem., 55 (1974)23. 102. A. Ciszewski, Talanta, 35 (1988) 329. 103. P.L. Buldini, P.Saxena, V. Saxena and A. Toponi, Analyst, 115 (1990), 104. P.M. Bersier and J. Bersier, Analyst, 114 (1989) 1531.

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CHAPTER TWO EXPERIMENTAL

The basic design for all the Anodic Stripping Voltammetry (ASV) experimental studies which were conducted, consisted of a three-electrode system in an electrochemical cell, connected to a polarographic analyzer and a strip chart recorder. A micro-platinum wire (0 = 100 |lA) was used as a working electrode on the ASV of metals. The electrode was assembled in a three-electrode cell, as shown in Fig. 2.1, together with a platinum- rod (0 = 1 mm) as an auxiliary electrode and standard calomel electrode (SCE) as a reference electrode.

Experimental details of the construction of the working electrode and all other instrumentation are given in the following sections.

2.1 INSTRUMENTATION

2.1.1 Polarographic Analyzer [1]

A Princenton Applied Research (PAR) model 174 Polarographic Analyzer was used for all ASV measurements in these experiments. The instrument controls were set as follows :

Potential Scan Rate : 5 mV per sec

Scan Direction : Positive

Potential Scan Range : 1.5 Volt

Initial Potential : -1.1 V

Modulation Amplitude : 50 mV c b a d

? H

cell top

cell bottom

Fig. 2.1 Voltammetric cell containing (a) Reference electrode, (b) Auxiliary electrode, (c) Working electrode and (d) Purge nitrogen gas. 46

Operation Mode Differential Pulse

Current Range Variable

Drop Time 0.5 sec

Output Offset Variable

Display Direction Negative

Low-Pass-Filter Off

2.1.2 Electrodes

A three electrode system was used for recording ASV scans. A Pt-rod (0 =1 mm), was used as an auxiliary electrode and a saturated calomel electrode was used as a reference electrode which against all potentials were measured. In these experiments, a micro- Platinum 100 |im in diameter, was used as a working electrode

2.2 REAGENTS

2.2.1 Water

Water for these experiments was deionized using a Milli-Q water purification system ( Millipore )[2]. The Milli-Q water used, had a resistivity of 18 MO cm"l.

2.2.2 Metal Solutions

10 ppm Cadmium (II), Lead (II) and Copper (II) standard solutions were prepared in 0.1 % HNO3 solution from AR grade CdCl2 21/2 H20, Pb(NC>3)2 and Cu(NC>3)2 supplied by May and Baker Ltd., England. Further dilutions were freshly made just prior to use. 2.2.3 Mercury Solutions

A stock solution of 0.1 M Hg (II) was prepared by dissolving AR grade Hg(NC>3)2 supplied by Merck, in deionized water. Further dilutions were made according to requirements.

2.2.4 Sodium Acetate and Acetic Acid Solution

A 0.2 M sodium acetate solution was prepared from a sodium acetate (anhydrous), AR grade reagent supplied by May & Baker LTD, England. A 5 M acetic acid stock solution was prepared from glacial acetic acid supplied by Ajax Chemicals LTD, Sydney. A 0.2 M acetic acid solution was then diluted from the 5 M acetic acid solution. A 0.1 M acetate buffer solution was then made from the 0.2 M sodium acetate and the 0.2 M acetic acid (1:1).

2.3 PROCEDURES

2.3.1 Preparation of a micro-Platinum electrode

A 2 cm micro platinum wire (100 pm in diameter) was connected with a 10 cm Cu-wire 1mm in diameter using silver epoxy. The copper-wire was covered with plastic tubing. A 5.5 mm length of the platinum-wire was uncovered (Fig.2.2.). This was used as the working electrode.

The electrode was cleaned by polishing with 1.0 pm alumina plus a water slurry on a polishing cloth followed by a thorough rinsing with deionized water. To clean the electrode electrochemically, a 50 ml buffer solution (25 ml. 0.2 M acetic acid and 25 ml 0.2 M sodium acetate ) was placed in a 150 ml cell and purged with nitrogen for 10 min. 48

1 1 0 mm

5 mm

Pt-wire 0 = 100 jim

Fig. 2.2 Construction of Micro-Platinum Electrode. A magnetic stirrer was used to stir the solution and the stream of nitrogen was continued

(during the plating and the stripping steps) to prevent oxygen from entering the solution.

Without depositing, it was scanned to the positive, from the initial potential -1.1 V (vs

SCE), to remove any trace element impurities deposited at the electrode [3,4].

2.3.2 ASV Scanning of Cd, Pb and Cu using a micro-Pt electrode [4]

The prepared micro-platinum electrode was placed in the cell, containing the buffer solution at pH 4.5 with Cd(II), Pb(II) and Cu(II) at the required concentrations and 5 x 10'^ M Hg(II). The solution was stirred continuously at open circuit. For ASV measurement of the metals, deposition potential of - 1.1 V (vs SCE) was applied and the deposition time was 2 min. A stirrer was used during deposition in the same position in all experiments. Stirring was discontinued at the conclusion of the deposition. After 15 sec at E^ to ensure quiescent conditions, the stripping voltammogram was recorded at a scan rate of 5 mV s"* with a modulation potential of 50 mV.

2.3.3 ASV Scanning of Zn, Cd, Pb and Cu

The ASV of Cd, Pb, Cu and Zn was also carried out to test the ASV of Zinc. A certain concentration of zinc was added into buffer solutions containing 40 ppb of cadmium, lead and copper and 5 x 10'5 M Hg(II). The ASV measurement procedure above was then followed. A deposition potential of -1.2 V (vs SCE) was applied.

2.3.4 ASV Scanning of Cd, Pb and Cu using metal electrodes

Other metal electrodes were also used as working electrodes as a comparison for the ASV of the metals using a micro-platinum electrode. The metal electrodes which were gold rod (0 = 1mm), platinum rod (0 - 1 mm) and copper wire (0 = 0.5 mm) were electrochemically plated with mercury in an acetate buffer solutions containing 10"5 M 50

Hg2+ for 10 min. Each metal electrode was placed in the cell containing an acetate buffer solution at pH 4.5 and 200 ppb Cd, Pb and Cu. For ASV measurement of the metals, the deposition was for 5 min and a deposition potential of -1.2 V (vs SCE) was employed.The ASV measurement procedure in 2.3.2 was then followed.

2.3.5 ASV of Cd, Pb and Cu at Various Deposition Potentials

The deposition potentials of -1.0, -1.1, and -1.2 V (vs SCE) were applied for the ASV of Cd, Pb and Cu. The ASV of the metals was conducted in an acetate buffer solution (pH 4.5) which contained with 100 Cd, Pb and Cu and 10'5 M Hg2+. The ASV measurements were repeated three time.

2.3.6 ASV of Cd, Pb and Cu at Various pH

ASV measurements of cadmium, lead and copper were also carried out in buffer solutions which contained 100 Cd, Pb and Cu and 5 x 10'5 M Hg^T The buffer solutions were obtained by mixing 0.2 M acetic acid and 0.2 M sodium acetate solutions proportionally. A deposition potential of -1.1 V (vs SCE) was applied.

2.3.7 ASV of Cd, Pb and Cu in Various Concentration of Hg(II)

For the evaluation of an effect of the concentration of mercury on the ASV of cadmium, lead and copper, the ASV of the metals was conducted in an acetate solution which contained with 100 Cd, Pb and Cu at pH 4.5 and the deposition potential at -1.1 V 2.3.8 ASV of Cd, Pb and Cu in Aqueous Solutions with dissolved Organic Compounds

The effect of organic compounds, such as pentane, hexane, cycloheptane, cyclohexane, benzene, toluene, sulphur compounds and surfactants were tested by adding a certain quantity of each compound into an acetate buffer which contained 100 ppb Cd, Pb, Cu and 5 x 10~5 M Hg2+. To obtain a homogeneous solution, each solution was placed in an ultrasonic bath at 40 °C for 10 min. The solutions were filtered with a 0.45 pm Millipore filter prior to the ASV measurements. The ASV measurement procedure in 2.3.2 was then followed

2.3.9 ASV of Cd, Pb and Cu in Aqueous Solutions with Dissolved Crude Petroleum [5,61

0.5 g of crude petroleum was added to 50 mL of a 0.1 M acetate buffer. The solution was then placed in an ultrasonic vibration bath at 40°C for 10 min. The undissolved crude was filtered with a 0.45 |im Millipore filter. The filtrate was used as a crude petroleum-dissolved aqueous solution. For testing the effect of crude petroleum on ASV of metals, 100 ppb Cd, Pb and Cu and also 5 x 10"^ M Hg2+ were added to the soludon. The above ASV measurement procedure was then followed.

2.3.10 Peak Measurements

The measurement of peak height can be carried out using the procedure shown in Fig. 2.3. The only condition is that peaks be measured in the same way in all related measurements. The procedure given has a roughly identical precision with others procedures which are commonly used [7]. 52

Fig. 2.3 Method of measuring the peak height [7]

2.4 ASSESSMENT OF PRECISION

The repeatability of the ASV of cadmium lead and copper was carried out. The

ASV of solutions containing 100 ppb of the metals and 5 x 10"^ M Hg2+ was repeated. 8 of the best repeats were then evaluated, to determine the RSD of the ASV of the metals.

For evaluation of the ASV of metals for each experiment, triplicate ASV measurements were conducted. A calibration curve of the ASV of cadmium, lead and copper (a plot of the peak current of each metal against the concentration of the metal in the acetate buffer solution) was constructed.

The calibration was based on the standard addition of a certain quantity of the metals to obtain a metal concentration between 0 - 100 ppb. This was 100 pL of 10 ppm ppm metal standard solution for each addition into 50 mL of acetate buffer solution at pH 4.5. A correlation coefficient of the calibration plot was also evaluated.

2.5 REFERENCES

1. Instruction Manual of PAR Model 174 Polarographic Analyzer. 2. Milli-Q, Water Purification Manual, Millipore, Cat. No. JMAS 285. 3. F. Vydra, K. Stulik and E. Julakova, Electrochemical Stripping Analysis. Hoorwood, Chichester, 1976. p. 149,150 4. J. Wang, Stripping Analysis; Principles. Instrumentation and Applications. Ed. J. Wang, Deerfield Beach, Florida, VCH Publishers, 1985.p. 19-23. 5. T. Ugapo and W.F. Pickering, Talanta, 32 (1985), 131. 6. L. Huynh-Ngoc, N.E. Whitehead and B. Oregioni, Wat. Res. 22 (1988) 571. 7. F. Vydra, K. Stulik and E. Julakova. Electrochemical Stripping Analysis. Hoorwood, Chichester, 1976. p.186. 54

CHAPTER THREE RESULTS AND DISCUSSION

The application of stripping voltammetry has continued to expand, with many new electrochemical techniques being applied to analytical measurements. Many studies of Anodic Stripping Voltammetry (ASV) and various mercury film electrodes have been reported, several of which were focused on the characteristics of the electrodes. Stripping voltammograms with [1] and without [2] the use of semi differentiation at thin mercury film electrodes have been reported. However, many reports described interference problems on the mercury film electrodes, such as organic compounds as reviewed in Chapter 1. Additional information on the effect of organic compounds on mercury film electrodes is still needed, not only in terms of chemical characteristic responses on the specific electrode, but also electrochemical characteristics of the electrode itself.

Studies of an application of a mercury-film micro-platinum electrode and the effect of organic compounds were carried out using cadmium, lead, copper and mercury as a means of testing the electrode. Results were focused on the responses to different concentration of Cd2+, Pb2+, Cu2+ and Hg2+, the pH of the supporting electrolyte and the concentration of mercury in the test solution. Also presented the results of the effects of added organic compounds such as aliphatic, cyclic and aromatic hydrocarbons, and non-hydrocarbons (O-, N- and S-). Results were also obtained of the effects of crude oils and surfactants. 55

3.1 ASV OF Cd, Pb, Cu AND Hg USING MERCURY-PLATED

MICRO-PLATINUM ELECTRODE

To demonstrate the concept of anodic stripping voltammetry on a mercury-film micro-platinum electrode, mercury was deposited in situ onto the platinum electrode. Figs. 3.1 and 3.11 show that the stripping potentials for Cd, Pb, Cu and Hg in an acetate buffer solution containing 5 x 10’5 M Hg2+ at pH 4.5 for two minutes deposition time, were -0.80, -0.65, -0.20 and +0.09 V (vs SCE). The separation of Cd, Pb, Cu and Hg was sufficiendy large to gain an excellent resolution of adjacent peaks.

ASV measurements of zinc using the micro-platinum electrode were not as sensitive as cadmium, lead and copper as shown in Fig. 3.2. There was no zinc peak observed on simultaneous ASV measurements of 200 ppb Zn and 40 ppb Cd, Pb and Cu in the acetate buffer solution at pH 4.5. A broad peak was observed at ppm level of zinc in the buffer solution. The response of zinc did also not alter on the ASV of zinc individually. Thus, the mercury-film micro-Pt is not suitable for determination of zinc. Although the good reason for this not available yet, it might be possible that the zinc peak merges with that of hydrogen evolution in acidic solution [19]. The peak potential of zinc was about - 1.0 V (vs SCE) at pH 4.5.

Compared to copper-wire (0=O.5mm), gold-rod (0=1.0 mm) and platinum rod (0= 1.0 mm) electrodes, the micro-platinum electrode might be the most suitable for analytical requirements (Fig. 3.3). Cadmium and lead could not be separated properly by using the gold, copper and platinum-rod electrodes. Copper was also not separated by the gold electrode (Fig. 3.3 A), but it was clearly defined using the platinum electrodes (Fig. 3.3 C and D). 56

Fig. 3.1 Scan for Cd, Pb and Cu in 0.1 M Acetate buffer, 5 x 10*5 M Hg2+, at pH 4.5, 2 min deposition time at -1.1 V (vs SCE), 5 mV/sec Scan rate, 1. Control (blank), 2-4 : 20, 40 and 100 ppb Cd, Pb and Cu. 57

/ *v ! \

V (vs SCE)

Fig. 3.2 Voltammograms of Zn, Cd, Pb and Cu 1. Control sol: acetate buffer containing 5 x 10-5 M Hg^+; pH 4.5 2. 2000 PPB Zn in control sol. 3. 2000 ppb Zn; 40 ppb Cd, Pb and Cu in control sol. 4. 200 ppb Zn; 40 ppb Cd, Pb and Cu in control sol. 5. 40 ppb Cd, Pb and Cu in control sol. 58

-1.20 -0.9 -0.6 -0.3

Hg c

-1.20

Fig. 3.3 The voltamnx)grams of Cd, Pb and Cu using metal electrodes in Acetate buffer, 5 x 10"^ M Hg^+; pH 4.5; 2 min deposition time Deposition Potential at: -1.2 V (vs SCE).

A : 200 ppb Cd, Pb and Cu using a Gold-rod (0= 1 mm) B : 200 ppb Cd, Pb and Cu using a Copper-wire (0=0.5 mm) C : 40 ppb Cd, Pb and Cu using a micro-size platinum-wire D : 200 ppb Cd, Pb and Cu using a platinum-rod (0= 1 mm) The data shown here demonstrate the feasibility of depositing mercury on

electrode having micro dimensions, thus making them highly useful for analysis in

microenvironment.

3.1.1 Effects of Deposition Potential

An acetate buffer solution containing 100 ppb Cd2+, Pb2+,Cu2+ and 5 x 10'5 M

Hg2+> has been employed to study the dependence of the stripping peak- current on

deposition potential. Voltammograms have been obtained at scan rate of 5 mV s"l

following 2 min preconcentration and deposition potentials between -1.20 and - 1.00 V.

The magnitude of stripping peak-current remains essentially unchanged. There were no

significant effects for the deposition potentials between -1.20 V to -1.00 V for cadmium,

lead and copper, as shown in Table 3.1

Table 3.1 The Stripping Peak-Current of 100 ppb Cd2+, Pb2+ and Cu2+ at Various

Deposition Potentials ( average of triplicate measurements )

Deposition Potential Cd Pb Cu /V (vs SCE) /|iA /|iA /jiA

- 1.0 8.4 (0.29) 21.5 (0.71) 25.2 (1.77)

- 1.1 9.0 (0.35) 23.1 (0.69) 24.9 (1.66)

- 1.2 8.9 (0.49) 22.3 (0.83) 23.0 (1.81)

This potential is usually several tenths of a volt, (about 0.3 - 0.5 V), more

negative than the reversible potential calculated from the Nemst equation for the least

easily reduced ion to be determined. For analysis of cadmium, lead and copper, the

deposition potentials are usually applied at - 1.1, - 0.9 and - 0.6 V (versus Ag-AgCl

electrode). The optimum deposition potentials of Pt-ring ultramicro-electrode were -1.00 60

V for copper, lead and cadmium and -1.20 V for Zinc [3]. These data might support the deposition potential which applied in this work.

3.1.2 Effects of pH

Obviously, the pH of the solution and supporting electrolyte has a very important role in stripping analysis. The effects of pH of the supporting electrolyte on deposition and stripping process have been described by many authors.

In order to distinguish the inherent properties of a micro-size platinum-wire electrode from the other factors, the effect of pH on ip for cadmium, lead and copper in concentrations at the ppb level has been studied in an acetate buffer solution. The dependence of ip on pH is shown in Fig. 3.4, the optimum pH for the three metals in the buffer solution using the platinum electrode, was between 4.1 and 4.8. This result agrees with using a glassy carbon electrode [4] and also agrees with Karadaki using HMDE who obtained an optimum pH range of 3.6 - 4.2 [5].

3.1.3 Effects of Concentration of Mercury

In electrolysis with solid electrodes, a film is formed on the electrode surface and the situation is more complicated than in the case of amalgams [6]. Accordingly, in situ deposition provides a method for producing a very thin film mercury layer [7]. A thin mercury film deposited in situ on glassy carbon substrate has been shown [8] to be the most sensitive and convenient electrode for trace heavy metals by anodic stripping voltammetry.

Experimental conditions during film formation (related to e.g Hg(II) concentration and deposition time) determine the characteristics of the Mercury Film Electrode. An optimum film has a linear calibration curve and has maximum sensitivity. A film that is U

Fig. urrent i, (|iA)

20 3.4 10

- - Pb Effect a ♦ □ and

of

Cu

varying ♦ D

(

100 a

ppb pH ♦ a 6

v

1

of Cd,

4 the

Pb J 0□□□□

acetate

and ♦♦ pH

Cu a

buffer for ♦

q 2

solutions min. gfl Q □

□ ♦ B

deposition

Lead Cadmium Copper g □ on

the

DPASV rime

) 7

of

Cd,

62 too thin has decreased sensitivity ; a film that is too thick adversely affects the calibration curve [9].

Since the deposition and preconcentration of metal ions is performed simultaneously with Hg2* deposition, it is important to examine the relationship between

Hg2* concentration and stripping voltammetry. In these experiments, the deposition potential was set at - 1.1 Y for 2 min preconcentration, in acetic buffer solution containing

100 ppb cadmium, lead and copper. The dependence of the stripping peak-current on

Hg2+ concentration has been evaluated in lx 10~6, lx 10'5, 5 x 10‘5 and 1 x 10'4 M

Hg2+-

Table 3.2 shows the ASV measurements of Cd, Pb, Cu and Hg in acetate buffer solutions in various concentration of Hg (II). The effect of the concentration on ASV of the metals was related to the thickness of the mercury film on which the metals were deposited. An increase in I upon the mercury addition thus indicate the capacity of the mercury film for deposition of the metal becomes considerably higher. The depositing ability of the mercury film for the metals could be maximum in a certain concentration of the mercury. The optimum concentration of mercury in this experiment was 5 x 10"5 M

3.1.4 Repeatability of ASV of Cd, Pb and Cu

According to the previous experiments, pH and the concentration of mercury in the buffer solution, have potential effects on anodic stripping analysis of cadmium, lead and copper. The stripping peak-currents of the metals at the optimum conditions have been evaluated. Table 3.3 shows that the relative standard deviation (RSD) of the metals,

were less then 5 % except copper. Although the response of copper was most sensitive, the RSD was also largest. Table 3.2 The ASV Measurements of Cd, Pb,Cu and Hg in Acetate Buffer Solution

in Various Mercury Concentration (in situ deposition)

Peak Height, \iA Cone., of

Hg Repeat Cd Pb Cu Hg

10-6M 1. 0.6 0.6 0.8 —

2. 0.6 1.0 1.2 —

3. 0.6 1.0 0.6 - io-5m 1. 1.2 8.0 12.0 20.2

2. 2.8 14.8 29.6 33.0

3. 2.4 13.8 24.6 34.8

5xl0-5 M 1. 8.4 20.0 25.3 205.2

2. 9.0 22.0 26.3 202.0

3. 7.9 21.0 25.9 213.2 io-4m 1. 9.0 18.0 24.9 219.0

2." 8.2 22.0 27.0 204.0

3. 7.4 23.6 26.7 220.0

3.1.5 Calibration curve of Cd, Pb and Cu

The analytical uses of a micro size platinum electrode have been evaluated by measuring the stripping peak current of cadmium, lead and copper and referring these to a standard calibration graph of ip versus concentration. All parameters except concentration are kept constant so that the current is directly proportional to the concentration. A calibration graph is constructed by making standard additions directly to the acetate buffer solution in the voltammetric cell. The peak current was obtained by applying a deposition potential of -1.1 V (vs SCE), for 2 min deposition time at pH 4.5. 64

Table 3.3 Repeatability Measurement of Cd, Pb, Cu and Hg Stripping Peaks

Peak Height, pA Run No. Cd Pb Cu Hg

1. 8.00 22.00 23.00 204.00

2. 7.60 21.90 26.60 224.00

3. 7.80 22.10 25.40 207.20

4. 8.20 21.80 28.20 210.40

5. 8.10 22.40 26.00 198.40

6. 7.80 22.60 26.60 206.00

7. 8.00 20.70 23.40 200.00

8 7.40 23.00 24.60 216.40

n 8 8 8 8

Average 7.86 22.06 25.47 208.30

STD - 0.27 0.68 1.75 8.52

RSD ( %) 3.43' 3.08 6.87 4.09

A typical DPASV plot of an acetic buffer solution, containing 20 ppb, 40 and 100 ppb and made in 5 x 10'5 M Hg2+> is shown in Figure 3.1. The calibration plot (Fig. 3.5) for cadmium under these conditions show a correlation coefficient for unweighted linear regression of r = 0.998. The sensitivity of copper is slightly higher than that of lead but the linearity for copper was the worst ( r = 0.995).

A background current was detected positively for copper as shown in Fig. 3.2

Known amounts of analyte are added to the acetate buffer and the calibration graph

(Fig.3.5) shows the variation of the measured signal with the amount of analyte added. In Fig. Current, i (|iA)

3.5

0 containing Calibration ♦ □ B

Lead Cadmium Copper 20

5 plots

x

10"^

of Concentration,

40 DPASV M

Hg^+

65

of

;

Cd, 60 pH

Pb 4.5.

and

Cu 80

(ppb) in

acetate 1

00

buffer

solution, 1

20

66 this way the background current effect can be considered as an analytical problem. It is poor practice to substract the blank signal from those of the other standards before plotting the graph [10].

3.2 EFFECT OF ORGANIC COMPOUNDS

The effect of organic compounds such as humic acid, fulvic acid, surfactant etc. on Anodic Stripping Voltammetry (ASV) of metals with various stripping techniques have been summarized in Chapter 1 (literature review). However, there is little information about the effect of hydrocarbon compounds and crude petroleum oil on ASV of the metals. The organic compounds which were used in these experiments, were hydrocarbon and non hydrocarbon compounds, surfactants, detergents and also crude petroleum oils.

The hydrocarbon and non-hydrocarbon compounds chosen in these experiments, were related to the crude petroleum oil components. The types of surfactants and detergent used, due to the availability of these compounds in the laboratory at that time. The effects of these compounds on ASV of metals, are described in the following subsection.

3.2.1. Effect of Hydrocarbon Compounds

Hydrocarbons, as the name implies, are compounds whose molecules contain only carbon and hydrogen atoms [11]. The effect of the presence of hydrocarbon compounds such as n-pentane, n-hexane, cycloheptane, cyclohexane, benzene and toluene were examined on anodic stripping voltammetry of cadmium, lead and copper.

Figures. 3.6 A to F, show that the stripping current of the three metals were not affected significantly by the presence of 0.1, 0.2 and 0.3 % (vol) of n-pentane, n-hexane, 6 7 n-Heptane 30 Hexa.DC

♦ w

Come. of % (mi) I-Uolll Cmc^ % (mi) Cyclobeptane Cyclohexane

Coec. Cjctoitpum, % (rol) Cjcicbezmot Coac-, % ((*ol)

Benzene Toluene

C»Ot. of lillUII, * (rol) Crnrx. of To4o*>a,

Fig. 3.6 Effects of hydrocarbon compounds on ASV of Cd, Pb and Cu Control sol: acetate buffer 0.1 M containing 100 ppb Cd, Pb and Cu and 5 x 10-5 M Hg2+, pH 4.5 68 cyclopentane, cyclohexane, benzene and toluene in the acetate buffer at pH 4.5. The relatively stable response observed in this study might be a feature of the in-situ plated

MFE, at which a continuously renewed surface, was maintained.

There was also no a significant change on the stripping potentials of the metals by addition of these hydrocarbon compounds. These compounds do not affect the shape and potential of the cadmium, lead and copper.

3.2.2 Effect of non-hydrocarbon compounds

A further experiment determined the effects of non-hydrocarbon compounds, phenol and pyridine on the ASV of cadmium, lead and copper. An acetate buffer solution which contains 100 ppb metals and 5 x 10'5 M Hg2+> was analysed in the normal manner to obtain a control value and then 0.5, 1 and 2 % (vol) of phenol and also pyridine were added to the solutions. To obtain homogeneous solutions, the solutions were placed in a ultrasonic bath for 5 min. Anodic stripping measurements were then carried out under optimum condition for the solutions. Fig.3.7 A shows that there was a slightly decrease of the stripping peak current of lead after addition of pyridine into the acetate buffer solution at pH 4.5, but it did not occur with the cadmium peak. In contrast, the copper peak gradually decreased with successive addition of pyridine. Phenol has an obvious effect on ASV of cadmium, lead and copper as shown in Fig. 3.7.B. The addition of phenol depresses the peak-current for all three metals: a slight (15 %) decrease is observed for lead, the cadmium and copper peak-current decrease sharply (40 %) on the addition of 0.2 ppm (vol) phenol. These compounds might cause a disturbing on the plating process of mercury film and the surface area of the electrode could be reduced.

Electrochemical surface characteristics of platinum electrodes such as the surface coverage, hydrogen adsorption-desorption [12], have been studied. An alteration of the 69

Pyridine

M Cadmium

♦ I .P-ari

■ Copper

Cone, of Pyridine, % (vol)

Phenol

Cadmium

♦ Lead

H Copper

Cone, of Phenol, % (vol)

Fig. 3.7 Effects of Pyridine (A) and Phenol (B) on ASV of Cd, Pb and Cu Control sol: acetate buffer 0.1 M containing 100 ppb Cd, Pb and Cu and 5 x 10-5 M Hg2+. pH 4.5 surface state of a platinum electrode must be attributed to the accumulation of poisonous species on the electrode surface [13]. The present of phenol or pyridine in the test solution might be adsorbed onto the surface of the platinum electrode and as a result, the peak-current of cadmium, lead and copper decreased.

If the adsorbate and the surface of the adsorbent interact only by van der Waals forces, the adsorbed molecules are weakly bound to the surface and heats of adsorption are low. If the adsorbed molecules form chemical bonds with the surface, the phenomenon is called chemisorption. Chemisorption does not go beyond the formation of a monolayer on the surface. For this reason an isotherm of the Langmuir type which predicts a monolayer and nothing more is well suited for interpreting data. The Langmuir adsorption isotherm predicts a heat of adsorption which is independent of 0, the fraction of the surface covered at equilibrium [14]

If the rate of adsorption of phenol is ka x CphOH x A> and the rate of desorption of phenol is k^ x ( A° - A ), where Aq is total area and A is free area, at equilibrium

ka x cPhOH x A = k0 x (A0 - A ) (3.1) A_ Kd x A° (3.2) ka x CphOH + Kd

The peak current is proportional to the free area, A, therefore the equation can be simplified as :

I = k’ A (3.3)

i k’ Kd x A° (3.4) ka x CphOH + Kd

| = ka x CphQH + Kd (3.5) I k' Kd x A° 1 I k x cPhOH + Kd’ (3.6) where k and K^' are constants. It is noted that by Eq. 3.6

It is assumed that as C —► oo, I —► 0. However, from the data it is clear I attains a constant value 1^. Therefore -jK— is plotted against Cp^oH as shown in Fig. 3.8 A for phenol and 3.8 B for pyridine. Fig. 3.8 A shows clearly that the relationship of the amount of phenol added and the coverage of the surface of the electrode is linear. The effect of the addition of phenol can be identified a decrease of the peak current of the cadmium, lead and copper, but it only occurs on copper with the addition of pyridine. 3.2.3 Effect of organic sulphur compound

An effect of 2-aminoethanethiol hydrochloride (HSCH2NH2HC1), as an organic synthetic sulphur compound on ASV of cadmium, lead and copper, was tested. This compound is also as a complex-forming agent. Fig. 3.9 shows that in the presence of 2- aminoethanethiol hydrochloride in an acetate buffer solution, the cadmium, lead, copper and mercury peak currents were reduced. A slight (25%) reduction of the cadmium peak current was observed on addition of 3 ppm 2-aminoethanethiol hydrochloride, and decreased gradually for 15 ppm 2-aminoethanethiol hydrochloride. A significant depression (70% and 55%) of the lead and copper peak currents was also observed, on the addition of 3 ppm 2-aminoethanethiol hydrochloride, and then 80% and 65 % with

15 ppm 2-aminoethanethiol hydrochloride (Fig. 3.10)

An interesting response observed in this study was that another peak appeared between the lead and copper peaks. The copper peak potential shifted slightly to the positive. This alteration might be caused of the metal complexing agent ability [15] of 2- aminoethanethiol hydrochloride. 72

□ Cadmium

♦ T paA ■ Copper

Cone, of Phenol, % (vol)

□ I p,arl ♦ Copper

Cone, of Pyridine, % (vol)

Fig. 3.8 Plot of l/(Io - Ico) of stripping peak current of Cd, Pb, Cu versus the concentration of A.: Phenol, B : Pyndine in acetate buffer, containing 100 ppb Cd, Pb, Cu and 5 x 10"5 M Hg2+; pH 4.5. 73

I \l \v—*7

V vs SCE

Fig. 3.9 The Effects of 2-aminoethanethiol hydrochloride (HSCH2NH2HCI) on ASVofCd, Pb, Cu and Hg 1. Control sol.: acetate buffer 0.1 M containing 100 ppb Cd, Pb and Cu and 5 x 10-5 M Hg2+; pH 4.5 2. 3 ppm 2-aminoethanethiol hydrochloride 3. 15 ppm 2-aminoethanethiol hydrochloride 7 4

2-aminoethanethiol hydrochloride (AETHC1) 30 □ Cd ♦ Pb n Cu 20

10 ~-Q— - —-a-

0

Cone, of AETHCI, ppm

10 The Effects of 2-aminoethanethiol hydrochloride (HSCEbNTbHCl) on ASVof Cd, Pb, Cu and Hg Control sol: acetate buffer 0.1 M containing 100 ppb Cd, Pb and Cu and 5 x 10-5 M Hg2+; pH 4.5 75

3.3 EFFECT OF CRUDE PETROLEUM OILS

Petroleum is not a uniform material. It is a complex mixture of hydrocarbons plus organic compounds sulphur, oxygen, and nitrogen as well as compounds containing metallic constituents, particularly vanadium, nickel, iron and copper [11]. In these experiments, the effect of crude oil, which is well known as a pollutant, on anodic stripping voltammetry (ASV) of trace metals using a micro size platinum wire electrode was studied.

Fig.3.11 shows that there was a remarkable effect of crude petroleum on the ASV of cadmium, lead, copper and mercury. The presence of 0.5 g crude petroleum oil, which was shaken into 50 ml acetate buffer and filtered by using 0.45 Jim filter (Millipore), caused a huge reduction of the peak-current.of the metals. It was observed that the cadmium and lead peaks disappeared on addition of 1 g/100 mL crude oil (Kalimantan and Java crude oil). A 75 % reduction of the copper peak-current is observed at the same concentration of Kalimantan crude oil and a 80 % reduction on addition of Java crude oil. With a hanging mercury drop electrode, the addition of 18 ppm gelatin depressed the copper peak-current up to 70 % and 85 % in addition of 3 ppm humic acid, but the peak- current for cadmium and lead was reduced only slightly [16].

The effect of crude oil on ASV of mercury was also evaluated. A 90 % reduction of the mercury peak-current is observed when the micro-platinum electrode was used following the addition of 0.5 g crude oil (Kalimantan and Java crude oil) into 50 mL acetate buffer at pH 4.5. The high proportion of carbon and hydrogen indicated that hydrocarbon compounds are the major constituents of crude oil or petroleum. The effect of hydrocarbon compounds on ASV of cadmium, lead and copper has been discussed. It is a well-established fact that hydrocarbons can account for more than 75 % of the constituents of many crude petroleums. Crude oils contain appreciable amounts of organic non-hydrocarbon constituents, mainly sulphur-, nitrogen-, oxygen-containing 7 6

V vs SCE

^8* ~>m ^ The effect of Crude Oil Petroleum on ASV of Cd, Pb, Cu and Hg. 1. Control sol: acetate buffer 0.1 M containing 100 ppb Cd, Pb and Cu and 5 x 10-5 M Hg2+; pH 4.5 2. 0.5 mg Kalimantan Crude Oil saturated into 50 ml Control sol 3. 0.5 mg Java Crude Oil.saturated into 50 ml Control sol (Preparation Ch. 2 : 2.3.9.) 77 compounds and, in smaller amounts, organometallic compounds in solution and inorganic salts in colloidal suspension. The solubility of the non-hydrocarbon constituents in aqueous solution is much better than the hydrocarbon component. Phenol and pyridine, as representative of the non-hydrocarbon compounds, indicate a considerable effect on ASV of cadmium, lead and copper (3.2.2). Sulphur compounds are among the most important heteroatomic constituents of petroleum. The total sulphur in crude can vary from, perhaps, 0.04 % for a light oils paraffin oil to about 5.0 % for heavy crude oil. An ultimate analysis of the crude (Table

3.4) appears that the sulphur in both crude oils, Kalimantan petroleum and Java crude, is different, but the ASV of metal in the buffer solutions which were contaminated with the crude oils, were not significantly different. Sulphur may not contribute very much to the decrease of the peak current of the metal.

Table 3.4 Elemental analysis of Kalimantan petroleum and Java crude (in % wt. )

Crude C H S

Kalimantan 86.5 12.4 0.4 Java 87.1 12.5 0.0

The action of interfacial forces at the boundary between two phases, here an electrode in solution, leads to the formation of an interface with thickness usually comparable to molecular dimensions [18]. The ions of a number of metals form poorly soluble compounds with various reagents, especially complexing agents; these compounds may be adsorbed on the surface of electrodes.The stripping potential of the ASV of metal can be shifted to the positive or broadening the peak shape in the presence of complexing agents in the test solution [15]. In the presence of the crude oil in the buffer solution did not affect the peak potential and the peak shape. It might be suggested 78 that the crude oil components are not a complexing agents. The components are more likely as compounds which could be potentially absorbed on the surface of the electrode.

As a result, the sensitivity of the electrode of ASV of metals decreases.

3.4 EFFECTS OF SURFACTANTS ON ASV OF Cd, Pb, Cu AND Hg

Agar, as a model of gel and surface active substance, was evaluated. Fig. 3.12 indicates that the effects of varying concentration level of agar on ASV of cadmium, lead, copper and Hg. There was no obvious effect of the agar at the concentration up to 20 mg

L"1 on ASV of all the metals. A 20 % to 30 % reduction of cadmium, lead and copper

peak-currents on addition of 100 mg L'1 agar, but the peak potential of mercury was stable. Observation of the peak potentials of voltammograms of the metals indicated that they were relatively stable in the presence of agar in the buffer solution.

In the presence of hexadecyltrimethyl ammonium bromide (HTABr), a cationic surfactant, the peak current of cadmium and lead was sharply depressed. The cadmium

peak current was reduced by 75 % of the control value, on addition of 40 mg L'1 HTABr,

and the peak could be even disappear on addition of 200 mg L"1 HTABr (Fig. 3.13 B)

The lead peak current was also depressed 50 % by the presence of 40 mg L"1 HTABr and

80 % reduction on the addition 200 mg L'1 HTABr. The copper peak current was reduced

less by HTABr (15 % at 40 mg L"1 and 25 % at 200 mg L'1)

The effect of Triton X-100 (a nonionic surfactant) on anodic stripping voltammetry of metals has been often observed [15,17]. A 20 % reduction of the cadmium peak-current is observed and 35 % of the lead peak-current on addition of 0.4

mg L'1 Triton X-100 into the acetate buffer solution. In the presence of 2 mg L"1 Triton X-100 produced an overlap peak for cadmium and lead, while the copper peak became broad, as shown in Fig.3.13 C. 79

V vs SCE

Fig. 3.12 The effect of Agar on ASV of Cd, Pb, Cu and Hg. 1. Control : acetate buffer 0.1 M containing 100 ppb Cd, Pb and Cu and 5 x 10~5 m Hg^+; pH 4.5 2.20 ppm Agar in control 3. 100 ppm Agar in control 80

Fig.3.13 The effect of Surfactants on ASV of Cd, Pb, Cu and Hg. A. Control : acetate buffer 0.1 M containing 100 ppb Cd, Pb and Cu and 5 x 10-5 M Hg2+; pH 4.5 B. 0.2 ppm Hexadecyltrimethyl ammonium bromide (HTABr) in A C. 2.0 ppm Triton X 100 in A D. 5.0 ppm the Anionic Surfactant A 81

The Ep of the copper shifted slightly to the positive in the presence of 2 mg L'1

Triton X-100 in the acetate buffer solution. This evidence may indicate that Triton X-100 possess a metal complexing ability [15]. The addition of Triton X-100 could also reflect

the slowness of sorption and indicates that mercury surface coverage had not quite

reached its adsorption equilibrium limit [17].

A general purpose detergent which is known by its brand name "APPEAL", was

used as a model of anionic surfactant. Fig. 3.13 D illustrates the effect of the detergent on

ASV of cadmium, lead, copper and mercury. In the present of 5 mg L'1 the detergent,

caused no effect on ASV of cadmium, lead and copper, but the mercury peak-current

disappeared. There was also no a significant effect on the peak potential (Ep) of all the

metals.

The effect of varying the detergent concentration in the 0 - 1 ppm (vol) range was

evaluated. Fig. 3.14 shows that there is a proportional relationship between the addition

of the detergent and the reduction of the mercury peak-current. A plot of the percentage of

the reduction of the mercury peak current versus the concentration of the anionic

surfactant in the buffer solution containing 100 ppb Cd, Pb and Cu and 5 x 10-5 M

Hg(D) was shown to be linear with a correlation coefficient of 0.985. (lo - Ic) I o

Fig. 100 40 20 80 60

3.14 0 0.0

Cone, Plot pH buffer, and

4.5 Hg of

0.2

of the ppm (jiA)

percentage

the

(vol) versus

containing anonic

the 0.4

of

concentration

reduction

82

surfactant, 100

ppb 0.6

stripping

of Cd,

the

Pb,'Cu

anionic peak ppm 0.8

and current

surfactant

(vol) 5

x

of 10

Cd, ‘ ^

1.0 in

M

Pb, acetate

Hg^

Cu +

;

83

3.6 REFERENCES

1. J. Wang and Gaodeng Xuexiao Huaxue Xuebao, 6 (1985) 783. 2. D. L.Huizenga and D.R. Kester; J. Electroanal. Chem. , 164 (1984) 229. 3. Tay, E.B.; Soo-Beng Khoo , and Sow-Wai Loh, Analyst, 114 (1989)1039. 4. Stewart, E.E. and R.B. Smart, Anal Chem,1984,56, p. 1131-1135. 5. (Karadakhi), Talanta 34 (1987), 995. 6. T.M. Florence, J. Electroanal.Chem., 26(1970), 293. 7. T.M. Florence, J. Electroanal.Chem., 27(1970), 273. 8. H.W. Nurnberg, Sci. Total Environ., 12(1979), 35. 9. V.D. Nguyen, P. Valenta and W. Nurnberg, Sci. Total Environm., 12(1979)151 10. J.N. Miller, Analyst, 116 (1991), 3. 11. J.G. Speight, The Chemistry and Technology of Petroleum. Marcel Dekker, INC. 12. J. Clavilier, in Electrochemical Surface Science Molecular Phenomena at Electrode Surfaces. M.P. Soriaga (Ed.), A.C.S., Washington, D.C., 1988. 13. Inada Ryuhei, Katsuaki Shimazu and Hideaki Kita., J. Electroanal Chem.,1990,277', p. 315-326. 14. G.W. Castellan, Physical Chemistry. Second Edition, Addision-Wesley Publishing Company, Massachusets, 1971. 15. G.M.P. Morrison, T. M. Florence and J.L. Stauber., Electroanalysis, 2 (1990) 9. 16. J. Wang and D.B. Luo, Talanta 31 (1984), 703. 17. P.L.Brezonik, P.A. Brauner and W. Stumm, Water Research, 1976, 10, p. 605-612. 18. R. Kalvoda and M. Kopanica, Pure & Appl. Chem. , 1989, 61, p. 97-112. 19. S.I. Sinyakova, I.V. Markova, L.S. Chulkina, A.M. Demkin and V.I. Shirokova, Zavodsk. Lab. 35 (1969) 769. 84

20. M. Bernhard and E. Zattera in Proceeding 2nd Int. Congres Marine Pollutants in

the Marine Waste Disposal. San Remo 1973, E.A. Pearson and E. De Fraja

Frangipane (Eds.), pp. 195-300.

21. H.W. Nunberg, P. Valenta, L.Mart, B. Raspor, and L. Sipos., Z. Anal.

Chem., 282 (1976) 357. CHAPTER FOUR CONCLUSION

A mercury-film micro-platinum has been tested as a working electrode for differential pulse anodic stripping voltammetry (DPASV). The electrode has been shown to be applicable for the ASV determination of cadmium, lead, copper and mercury, but not suitable for the determination of zinc. The metals could be determined after purging with nitrogen, then depositing them onto the electrode by in situ deposition with mercury for 2 min. The deposition potential was applied at - 1.10 V (vs SCE), followed by an anodic scan at 5 mV/sec rate and 50 mV modulation amplitude. The determination of the metals was carried out in 0.1 M acetate buffer solution (at pH 4.5) containing 5 x 10'5 M Hg“+. The stripping potentials of cadmium, lead, copper and mercury were -0.80, -0.50, -0.20 and +0.09 V (vs. SCE). The optimum pH of the buffer solution on the ASV of cadmium, lead and copper was in the range of between 4.1 and 4.8. The calibration of cadmium, lead and copper in the range 0-100 ppb, was found to give curved plots with correlation coefficients of 0.998, 0995 and 0.995, respectively.

The effects of hydrocarbon compounds on the ASV of cadmium, lead, copper and mercury using a micro-size platinum wire electrode have been evaluated. In this study, the effects of hydrocarbon compounds were studied by addition of various amounts of n- pentane, n-hexane, cyclopentane, cyclohexane, benzene and toluene into 0.1 M acetate buffer solutions. In the presence of 1 to 4 % (by vol) the hydrocarbon compounds in the buffer solutions there was no a significant effect on the ASV of cadmium, lead, copper and mercury 86

Effects of phenol, pyridine and 2-aminoethanethiol hydrochloride, which represent oxygen, nitrogen and sulphur compounds, have also been attempted on the

ASV of cadmium, lead, copper and mercury. A considerable effect of phenol on the ASV of the metals could be observed in the presence of 1 to 4 % (by vol) phenol in an acetic buffer solution. In solution of 1 to 4 % (vol) pyridine, only the copper stripping peak was slightly affected. A sulphur compound, 2-aminoethanethiol hydrochloride, influenced the ASV of cadmium, lead and copper, not only the peak-current of the metals, but also the peak potential. This alteration is indicative of the metal complexing ability of 2- aminoethanethiol hydrochloride [4].

The presence of 0.1 mg of crude oil in 100 ml 0.1 M acetate buffer containing 100 ppb Cd, Pb and Cu and 10‘5 M Hg^+ affected the heights of the stripping peaks for cadmium, lead and copper, but they did not alter the peak potentials of the metals. Although the difference between the adsorption and complexation effects on ASV results cannot be easily differentiated, the evidence strongly suggested that the crude oil components have adsorption effects on the electrode surface rather than reactions due to formation reaction with the metals. Hydrocarbon compounds are the major components in crude oils and were shown to have no significant effect on the ASV of cadmium, lead and copper, but non-hydrocarbon compounds such as phenol or pyridine show considerable effects on the ASV of the metals.

The effects of cationic, neutral and anionic surfactants on the ASV of cadmium, lead, copper and mercury were different. In the presence of 100 ppm agar, the stripping peaks of cadmium, lead and copper were slightly (20 %) affected, but the peak of mercury was not significantly affected. The presence of a cationic surfactant (200 mg L*1 HTABr) caused the three metal peaks currents to decrease more then 70 %, but the mercury peak current was not altered. Triton X-100 (2.0 mg L"1) depressed the cadmium, lead and copper peak-currents gradually and the cadmium and lead peaks became overlapped, without significant effect on the mercury peak current. An interesting 87 response observed in this study was that the presence of an anionic detergent ( 0 to 1 ppm by vol) showed less than 30 % peak height reduction on cadmium, lead and copper peak currents, and a linearly proportional reduction on the mercury peak current. A plot of the decrease in of the mercury stripping peak versus the concentration of the anionic surfactant in buffer solution containing 100 ppb each of Cd, Pb and Cu and 10"5 M Hg2+ was shown to be linear with a correlation coeficient of 0.985.

This work demonstrates the advantages of using a mercury-film micro-platinum electrode for studies on the effects of organic compounds on the ASV of metals. Crude oils, surfactants, hydrocarbon compounds, phenolics and pyridines have a large effects on the ASV of cadmium, lead, copper and mercury. The possibility of using the ASV scan to indicate the presence of a certain classes of organic compounds in environmental samples is suggested for future studies.