DOCSLIB.ORG

Hydrodynamically Modulated Voltammetry in Microreactors

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Hydrodynamically Modulated Voltammetry in Microreactors Hydrodynamically Modulated Voltammetry in Microreactors Luwen Meng Department of Chemical Engineering and Biotechnology University of Cambridge This dissertation is submitted for the degree of Doctor of Philosophy Lucy Cavendish College September 2018 Abstract Hydrodynamically modulated voltammetry in microreactors Luwen Meng This thesis describes modulated methods using both voltammetric and microfluidic perturbations to study mechanisms of electrolysis reactions. The initial chapters provide an overview of applications and research development in the fields of micro-engineering and electrochemistry, including microfabrication methodology, electrochemical detection techniques and analysis methods. Some typical electrochemical reactions have been studied for different kinds of industrial applications. Also hydrodynamic modulation methods have been investigated. The result chapters begin in Chapter 3 with detailed investigation of various electrochemical reactions by using cyclic voltammetry (CV) and large amplitude Fourier transformed alternating current voltammetry (FTACV) under microfluidic conditions. Single electron transfer reactions with different kinetics were studied first by using potassium ferrocyanide and ferrocenecarboxylic acid (FCA). Dual electron transfer reactions with different pathways were investigated by using 2,5-dihydroxybenzoic acid for one step oxidation and N,N,N’,N’- tetramethyl-para-phenylene-diamine (TMPD) for two consecutive one-electron step oxidation. An irreversible reaction was explored by using borohydride solution. Examples of 4- homogeneous reaction mechanisms were studied by using the combinations of Fe(CN)6 /L- cysteine or TMPD/ascorbic acid. The current response of all the electrolysis reactions except single electron transfer reactions was first reported under microfluidic conditions with FTACV, which has shown sensitive with the change of volume flow rates and the substrate concentrations when homogeneous reactions are involved. The linear relationships between peak current and volume flow rates or substrate concentrations can be obtained in every harmonic component. In chapter 4, the modulated technique was applied to microfluidic hydrodynamic systems. A range of electrolysis mechanisms including single electron transfer reactions, dual electron transfer reactions, irreversible reaction and homogeneous reactions were studied under hydrodynamic modulated conditions. The system showed rapid response with the change of volume flow rates during one measurement. The linear relationships between peak current and flow rates, as well as substrate concentrations, can be obtained simultaneously in one scan, which reveals a promising approach to get more information in a short-time measurement. Chapter 5 demonstrated a new protocol by forcing an oscillation of the electrochemical active solution flowing. Analysis of transition time and its effect on limiting current are presented to begin exploration of this new tool for supporting researchers on understanding redox mechanisms. A short simulated study was carried out to help better understand the mechanism under different hydrodynamic conditions. For my family, Past, present and future Declaration I hereby declare that except where specific reference is made to the work of others, the contents of this dissertation are original and have not been submitted in whole or in part for consideration for any other degree or qualification in this, or any other University. This dissertation is the result of my own work and includes nothing which is the outcome of work done in collaboration, except where specifically indicated in the text. This dissertation contains less than 65,000 words including appendices, bibliography, footnotes, tables and equations and has less than 150 figures. Luwen Meng September 2018 Acknowledgements First and foremost, I would like to express my gratitude to my supervisor Dr Adrian Fisher, who offered me the opportunity to undertake my PhD study in the group and paid his unbelievable patience to me. I am grateful to my advisor, Dr Kamran Yunus, for his help and guidance. My appreciation sends to Dr Peng Song as well. Thank you for your advice in both research and life in Cambridge even if you are not around. A big thank you to all of my colleagues in CREST group: Dr Chencheng Dai, Dr Hongkai Ma, Dr Arely Gonzalez, Dr. Viet Nguyen, Dr. Xiangming Gao, Aazraa Pankan, Yian Wang for all the discussion and help in the last four years and especially Feng Zheng, who wrote me a MATLAB script to minimize my pain of analysing AC data. Special thanks to Dr Ashoke Raman for the discussion about the simulation work. Also, I’d like to express my appreciation to Ziyan Zhao, Yao Du and Andi Tao for accompanying for these years in the department. Special thanks extend to all other supporting staff in our department, Chris, Peter and other people in technician team, electronic team and domestic department. I am grateful for all friends based in Cambridge and UK. Thank you so much for being with me and making my life so colourful these four years, especially Molly, Hanae and Susan who are always my dinner and wine buddies to help me get through the tough time. Special thanks to Rui Li and Anastasiya Sybirna for being my case practice partners and all the support they gave me. Also, I can’t finish this PhD with all my friends back to China, Fangyuan Chen and Chengyan Zhan who always gave me mental support since we have known each other and Zhao Zeng who can always answer my video call whenever and wherever it is. Special thanks to Wayne (JJ) Lin, who inspired me through these 15 years with his music. Thank you for being such a great model to gain me courage and belief, which always push me forward to pursue my dreams. Last but not least, I would like to express my sincerest gratitude to my parents and all my family. Thank you for bringing me to this world and offering all the support through my whole life. I hope I didn’t let you down. And I love you to the moon and back. Contents Contents .................................................................................................................................... xi List of Figures .......................................................................................................................... xv List of Tables ........................................................................................................................ xxiii Abbreviations ............................................................................................................................. I Common Symbols ................................................................................................................... III Chapter 1 Introduction of Electrochemical Fundamentals ................................................... 1 1.1 Introduction .................................................................................................................. 1 1.2 Introduction of electrochemical applications ............................................................... 1 1.2.1 Synthesis ............................................................................................................... 2 1.2.2 Sensing ................................................................................................................. 4 1.3 Introduction of micro-engineering ............................................................................... 7 1.3.1 Microreactors ........................................................................................................ 8 1.4 Introduction to electrochemistry ................................................................................ 15 1.4.1 Electron transfer on the electrode ....................................................................... 15 1.4.2 Reversibility ....................................................................................................... 18 1.4.3 Mass transport .................................................................................................... 19 1.4.4 Structure of the electrode/solution interface ....................................................... 21 1.4.5 Voltammetry techniques ..................................................................................... 25 1.4.6 Electrolysis reaction mechanisms ....................................................................... 44 1.5 Research aims and thesis structure ............................................................................ 50 Chapter 2 Microfabrication Technology ............................................................................ 51 2.1 Introduction ................................................................................................................ 51 2.2 Microfabrication methodology .................................................................................. 51 2.2.1 Technologies for microsystems .......................................................................... 51 2.2.2 Microfabrication materials ................................................................................. 58 2.3 Fabrication of electrochemical micro-devices ........................................................... 58 2.3.1 Photolithographic process .................................................................................. 58 2.3.2 Microelectrode fabrication ................................................................................. 64 2.3.3 Microchannel fabrication ...................................................................................
Load more

Recommended publications
  • 07 Chapter2.Pdf
    22 METHODOLOGY 2.1 INTRODUCTION TO ELECTROCHEMICAL TECHNIQUES Electrochemical techniques of analysis involve the measurement of voltage or current. Such methods are concerned with the interplay between solution/electrode interfaces. The methods involve the changes of current, potential and charge as a function of chemical reactions. One or more of the four parameters i.e. potential, current, charge and time can be measured in these techniques and by plotting the graphs of these different parameters in various ways, one can get the desired information. Sensitivity, short analysis time, wide range of temperature, simplicity, use of many solvents are some of the advantages of these methods over the others which makes them useful in kinetic and thermodynamic studies1-3. In general, three electrodes viz., working electrode, the reference electrode, and the counter or auxiliary electrode are used for the measurement in electrochemical techniques. Depending on the combinations of parameters and types of electrodes there are various electrochemical techniques. These include potentiometry, polarography, voltammetry, cyclic voltammetry, chronopotentiometry, linear sweep techniques, amperometry, pulsed techniques etc. These techniques are mainly classified into static and dynamic methods. Static methods are those in which no current passes through the electrode-solution interface and the concentration of analyte species remains constant as in potentiometry. In dynamic methods, a current flows across the electrode-solution interface and the concentration of species changes such as in voltammetry and coulometry4. 2.2 VOLTAMMETRY The field of voltammetry was developed from polarography, which was invented by the Czechoslovakian Chemist Jaroslav Heyrovsky in the early 1920s5. Voltammetry is an electrochemical technique of analysis which includes the measurement of current as a function of applied potential under the conditions that promote polarization of working electrode6.
    [Show full text]
  • Hydrodynamic Voltammetry As a Rapid and Simple Method for Evaluating Soil Enzyme Activities
    Sensors 2015, 15, 5331-5343; doi:10.3390/s150305331 OPEN ACCESS sensors ISSN 1424-8220 www.mdpi.com/journal/sensors Article Hydrodynamic Voltammetry as a Rapid and Simple Method for Evaluating Soil Enzyme Activities Kazuto Sazawa 1,* and Hideki Kuramitz 2 1 Center for Far Eastern Studies, University of Toyama, Gofuku 3190, 930-8555 Toyama, Japan 2 Department of Environmental Biology and Chemistry, Graduate School of Science and Engineering for Research, University of Toyama, Gofuku 3190, 930-8555 Toyama, Japan; E-Mail: [email protected] * Author to whom correspondence should be addressed; E-Mail: [email protected]; Tel./Fax: +81-76-445-66-69. Academic Editor: Ki-Hyun Kim Received: 26 December 2014 / Accepted: 28 February 2015 / Published: 4 March 2015 Abstract: Soil enzymes play essential roles in catalyzing reactions necessary for nutrient cycling in the biosphere. They are also sensitive indicators of ecosystem stress, therefore their evaluation is very important in assessing soil health and quality. The standard soil enzyme assay method based on spectroscopic detection is a complicated operation that requires the removal of soil particles. The purpose of this study was to develop a new soil enzyme assay based on hydrodynamic electrochemical detection using a rotating disk electrode in a microliter droplet. The activities of enzymes were determined by measuring the electrochemical oxidation of p-aminophenol (PAP), following the enzymatic conversion of substrate-conjugated PAP. The calibration curves of β-galactosidase (β-gal), β-glucosidase (β-glu) and acid phosphatase (AcP) showed good linear correlation after being spiked in soils using chronoamperometry.
    [Show full text]
  • Effective and Novel Application of Hydrodynamic Voltammetry to the Study of Superoxide Radical Scavenging by Natural Phenolic Antioxidants
    antioxidants Article Effective and Novel Application of Hydrodynamic Voltammetry to the Study of Superoxide Radical Scavenging by Natural Phenolic Antioxidants Stuart Belli 1,*, Miriam Rossi 1,*, Nora Molasky 1, Lauren Middleton 1, Charles Caldwell 1, Casey Bartow-McKenney 1, Michelle Duong 1, Jana Chiu 1, Elizabeth Gibbs 1, Allison Caldwell 1, Christopher Gahn 2 and Francesco Caruso 1 1 Department of Chemistry, Vassar College, Poughkeepsie, NY 12604, USA; [email protected] (N.M.); [email protected] (L.M.); [email protected] (C.C.); [email protected] (C.B.-M.); [email protected] (M.D.); [email protected] (J.C.); [email protected] (E.G.); [email protected] (A.C.); [email protected] (F.C.) 2 Computing & Information Services, Vassar College, Poughkeepsie, NY 12604, USA; [email protected] * Correspondence: [email protected] (S.B.); [email protected] (M.R.) Received: 18 November 2018; Accepted: 25 December 2018; Published: 4 January 2019 Abstract: The reactions of antioxidants with superoxide radical were studied by cyclic voltammetry (CV)—and hydrodynamic voltammetry at a rotating ring-disk electrode (RRDE). In both methods, the superoxide is generated in solution from dissolved oxygen and then measured after being allowed to react with the antioxidant being studied. Both methods detected and measured the radical scavenging but the RRDE was able to give detailed insight into the antioxidant behavior. Three flavonoids, chrysin, quercetin and eriodictyol, were studied, their scavenging activity of superoxide was assessed and the molecular structure of each flavonoid was related to its scavenging capability. From our improved and novel RRDE method, we determine the ability of these 3 antioxidants to react with superoxide radical in a more quantitative manner than the classical CV.
    [Show full text]
  • Reaction Mechanism of Electrochemical Oxidation of Coo/Co(OH)2 William Prusinski Valparaiso University
    Valparaiso University ValpoScholar Chemistry Honors Papers Department of Chemistry Spring 2016 Solar Thermal Decoupled Electrolysis: Reaction Mechanism of Electrochemical Oxidation of CoO/Co(OH)2 William Prusinski Valparaiso University Follow this and additional works at: http://scholar.valpo.edu/chem_honors Part of the Physical Sciences and Mathematics Commons Recommended Citation Prusinski, William, "Solar Thermal Decoupled Electrolysis: Reaction Mechanism of Electrochemical Oxidation of CoO/Co(OH)2" (2016). Chemistry Honors Papers. 1. http://scholar.valpo.edu/chem_honors/1 This Departmental Honors Paper/Project is brought to you for free and open access by the Department of Chemistry at ValpoScholar. It has been accepted for inclusion in Chemistry Honors Papers by an authorized administrator of ValpoScholar. For more information, please contact a ValpoScholar staff member at [email protected]. William Prusinski Honors Candidacy in Chemistry: Final Report CHEM 498 Advised by Dr. Jonathan Schoer Solar Thermal Decoupled Electrolysis: Reaction Mechanism of Electrochemical Oxidation of CoO/Co(OH)2 College of Arts and Sciences Valparaiso University Spring 2016 Prusinski 1 Abstract A modified water electrolysis process has been developed to produce H2. The electrolysis cell oxidizes CoO to CoOOH and Co3O4 at the anode to decrease the amount of electric work needed to reduce water to H2. The reaction mechanism through which CoO becomes oxidized was investigated, and it was observed that the electron transfer occurred through both a species present in solution and a species adsorbed to the electrode surface. A preliminary mathematical model was established based only on the electron transfer to species in solution, and several kinetic parameters of the reaction were calculated.
    [Show full text]
  • Using Chronoamperometry to Rapidly Measure and Quantitatively Analyse Rate- Performance in Battery Electrodes
    Using chronoamperometry to rapidly measure and quantitatively analyse rate- performance in battery electrodes Ruiyuan Tian,1,2 Paul J. King,3 Joao Coelho,2,4 Sang-Hoon Park,2,4 Dominik V Horvath,1,2 Valeria Nicolosi,2,4 Colm O’Dwyer,2,5 Jonathan N Coleman1,2* 1School of Physics, Trinity College Dublin, Dublin 2, Ireland 2AMBER Research Center, Trinity College Dublin, Dublin 2, Ireland 3Efficient Energy Transfer Department, Bell Labs Research, Nokia, Blanchardstown Business & Technology Park, Snugborough Road, Fingal, Dublin 15, Ireland 4School of Chemistry, Trinity College Dublin, Dublin 2, Ireland 5 School of Chemistry, University College Cork, Tyndall National Institute, and Environmental Research Institute, Cork T12 YN60, Ireland *[email protected] (Jonathan N. Coleman); Tel: +353 (0) 1 8963859. ABSTRACT: For battery electrodes, measured capacity decays as charge/discharge current is increased. Such rate-performance is important from a practical perspective and is usually characterised via galvanostatic charge-discharge measurements. However, such measurements are very slow, limiting the number of rate experiments which are practical in a given project. This is a particular problem during mechanistic studies where many rate measurements are needed. Here, building on work by Heubner at al., we demonstrate chronoamperometry (CA) as a relatively fast method for measuring capacity-rate curves with hundreds of data points down to C-rates below 0.01C. While Heubner et al. reported equations to convert current transients to capacity vs. C-rate curves, we modify these equations to give capacity as a function of charge/discharge rate, R. We show that such expressions can be combined with a basic model to obtain simple equations which can fit data for both capacity vs.
    [Show full text]
  • University of Cincinnati
    UNIVERSITY OF CINCINNATI Date:___________________ I, _________________________________________________________, hereby submit this work as part of the requirements for the degree of: in: It is entitled: This work and its defense approved by: Chair: _______________________________ _______________________________ _______________________________ _______________________________ _______________________________ AMPEROMETRIC CHARACTERIZATION OF A NANO INTERDIGITATED ARRAY (nIDA) ELECTRODE AS AN ELECTROCHEMICAL SENSOR A thesis submitted to The Division of Research and Advanced Studies of the University of Cincinnati In partial fulfillment of the requirements for the degree of MASTER OF SCIENCE In the Department of Electrical and Computer Engineering and Computer Science of the College of Engineering August 1, 2006 By Ashwin Kumar Samarao B.E. (Hons.) Electrical and Electronics Birla Institute of Technology and Science, India, 2004 Committee chairman Dr.Chong H. Ahn ABSTRACT The main goal of this research is to amperometrically characterize a ring type nano interdigitated array (nIDA) electrode as an electrochemical sensor and to verify the enhancements in the sensitivity of such a sensor when compared to its micro counterparts. Each electrode was fabricated in gold with 275 fingers, each of width 100 nm and spacing 200 nm, using electron beam lithography and nano lift-off processes on a SiO2/Si wafer. The reference and counter electrodes were fabricated using electroplating. P – Aminophenol (PAP) was used as the redox species to be detected by the nano- IDA electrochemical sensor. Using Chronoamperometry, concentrations of PAP as low as 10 pM were successfully detected using the fabricated sensor. The current output by the sensor for such low concentrations was in the pico-ampere range and was measured using a very sensitive pico-ammeter.
    [Show full text]
  • Modelling of the Rotating Disk Electrode in Ionic Liquids: Difference Between Water Based and Ionic Liquids Electrolytes
    Modelling of the rotating disk electrode in Ionic liquids: difference between water based and ionic liquids electrolytes A. Giaccherini1, A. Lavacchi2 1INSTM, Firenze, Italy, 2ICCOM - CNR, Firenze, Italy *Corresponding author: via Lastruccia 3-13 50019, Sesto Fiorentino (FI), [email protected] Abstract: The last few years experienced a The former allows to validate the model by rapid growth in the application of Ionic means of comparison with the experimental Liquids (IL’s) to electrodeposition. ILs voltammograms, the last allows to offer a variety of advantages over aqueous rationalize the peculiar mass transport electrolytes. In general ILs show large properties of the Ils. In particular, thanks to chemical and thermal stability, high ionic the comparison of the concentration profiles conductivity and an electrochemical window and fluxes at the steady and quasi-steady much larger than water. These properties states of the potential scan for both systems, together with their negligible vapor pressure we clarified the nature of the unexpected enabling their use at different temperatures peaks show by the experimental without any risk of generating harmful voltammograms. vapors and joined to the absence of hydrogen discharge interfering with Keywords: Levich equation, Ionic Liquids, electrodeposition processes, as they are Transport proprieties, RDE, essentially hydrophobic, make them the best electroanalytical, CFD. candidates to be used for the obtainment of homogeneous electrodeposited thin films. 1. Introduction This study focuses on the silver electrodeposition from a silver Research on electrodeposition has recently tetrafluoroborate solution in 1-butyl-3- focused on the quest for new electrolytes methyltetrafluoroborate BMImBF4. We alternative to water. This was mainly driven notice that practical deposition rate at even by the need to develop new and green concentrations were much lower in ionic electrodeposition processes.
    [Show full text]
  • Unit 1 Introduction to Electro- Analytical Methods
    Introduction to UNIT 1 INTRODUCTION TO ELECTRO- Electroanalytical ANALYTICAL METHODS Methods Structure 1.1 Introduction Objectives 1.2 Basic Concepts Electrical Units Basic Laws of Electrochemistry Electrode Potential Liquid-Junction Potentials Electrochemical Cells The Nernst Equation Cell Potential 1.3 Classification and an Overview of Electroanalytical Methods Potentiometry Voltammetry Polarography Amperometry Electrogravimetry and Coulometry Conductometry 1.4 Classification and Relationships of Electroanalytical Methods 1.5 Summary 1.6 Terminal Questions 1.7 Answers 1.1 INTRODUCTION This is the first unit of this course. This unit deals with the fundamentals of electrochemistry that are necessary for understanding the principles of electroanalytical methods discussed in this Unit 2 to 9. In this unit we have also classified of electroanalytical methods and briefly introduced of some important electroanalytical methods. More details of these elecroanalytical methods will be discussed in the consecutive units. Objectives After studying this unit, you will be able to: • name the different units of electrical quantities, • define the two basic laws of electrochemistry, • describe the single electrode potential and the potential of a galvanic cell, • derive the Nernst expression and give its applications, • calculate the electrode potentials and cell potentials using Nernst equation, • describe the basis for classification of the electroanalytical techniques, and • explain the basis principles and describe the essential conditions of the various electroanalytical techniques. 1.2 BASIC CONCEPTS Before going in detail of different electroanalytical techniques, let’s recapitulate some basic concepts which you have studied in your undergraduate classes. 7 Electroanalytical 1.2.1 Electrical Units Methods -I Ampere (A): Ampere is the unit of current.
    [Show full text]
  • Stationary Electrode Voltammetry and Chronoamperometry in an Alkali Metal Carbonate-Borate Melt
    AN ABSTRACT OF THE THESIS OF DARRELL GEORGE PETCOFF for the Doctor of Philosophy (Name of student) (Degree) in Analytical Chemistry presented onC (O,/97 (Major) (Date) Title: STATIONARY ELECTRODE VOLTAMMETRY AND CHRONOAMPEROMETRY IN AN ALKALI METAL CARBONATE - BORATE. MFT T Abstract approved: Redacted for Privacy- Drir. reund The electrochemistry of the lithium-potassium-sodium carbonate-borate melt was explored by voltammetry and chrono- amperometry. In support of this, a controlled-potential polarograph and associated hardware was constructed.Several different types of reference electrodes were tried before choosing a porcelain mem- brane electrode containing a silver wire immersed in a silver sulfate melt.The special porcelain compounded was used also to construct a planar gold disk electrode.The theory of stationary electrode polarography was summarized and denormalized to provide an over- all view. A new approach to the theory of the cyclic background current was also advanced. A computer program was written to facilitate data processing.In addition to providing peak potentials, currents, and n-values, the program also resolves overlapping peaks and furnishes plots of both processed and unprocessed data. Rapid-scan voltammetry was employed to explore the electro- chemical behavior of Zn, Co, Fe, Tl, Sb, As, Ni, Sn, Cd, Te, Bi, Cr, Pb, Cu, and U in the carbonate-borate melt. Most substances gave reasonably well-defined peaks with characteristic peak potentials and n-values.Metal deposition was commonly accompanied by adsorp- tion prepeaks indicative of strong adsorption, and there was also evi- dence of a preceding chemical reaction for several elements, sug- gesting decomplexation before reduction.
    [Show full text]
  • Thesis-1961-B586i.Pdf
    INVESTIGATION OF SOME POSSIBILITIES FOR AMPEROMETRIC TITRATION OF CERTAIN METAL IONS WITH OXINE By Donald George Biechler I I Bachelor of Science University of Wisconsin Madison, Wisconsin 1956 Submitted to the faculty of the Graduate School of the Oklahoma State University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE May, 1961 INVESTIGATION OF SOME POSSIBILITIES FOR AMPEROMEI'RIC TITRATION OF CERTAIN MEI'AL IONS WITH OXINE Thesis Approved: Thesis Adviser i i OKLAHOMA STATE UNIVERSITY llBRARY JAN 2 1962 PREFACE Oxine (8-hydroxyquinoline) is most generally used in analytical chemistry as a precipitant for metals and is known to form water- insoluble chelates with better than thirty metal ions (3). There exists in solutions of oxine a tautomeric equilibrium of the fol- lowing type: C C C C /~/'\ ,c/""/~ C C f ij 1 I II I C C C C C C ~/"'/C N ~/C "+/N r . _I I 0 H 0--------H Chelation of a metal ion involves replacement of the proton and for- mation of a coordinate bond with the nitrogen to form a stable 5 membered ring compound. Thus nickel, a bivalent cation, would form a compound with the following structure: 4810 90 iii iv The oxinates can be ignited and weighed as such or they may be further ignited to the metal oxides and then weighed. Alternately the oxinates may be dissolved in acid and quantitatively brominated (7)0 Considering the number of metal ions that are precipitated by oxine, it seemed that possibly more use could be made of the reagent in volumetric analysis.
    [Show full text]
  • Standard Methods for the Examination of Water and Wastewater
    Standard Methods for the Examination of Water and Wastewater Part 1000 INTRODUCTION 1010 INTRODUCTION 1010 A. Scope and Application of Methods The procedures described in these standards are intended for the examination of waters of a wide range of quality, including water suitable for domestic or industrial supplies, surface water, ground water, cooling or circulating water, boiler water, boiler feed water, treated and untreated municipal or industrial wastewater, and saline water. The unity of the fields of water supply, receiving water quality, and wastewater treatment and disposal is recognized by presenting methods of analysis for each constituent in a single section for all types of waters. An effort has been made to present methods that apply generally. Where alternative methods are necessary for samples of different composition, the basis for selecting the most appropriate method is presented as clearly as possible. However, samples with extreme concentrations or otherwise unusual compositions or characteristics may present difficulties that preclude the direct use of these methods. Hence, some modification of a procedure may be necessary in specific instances. Whenever a procedure is modified, the analyst should state plainly the nature of modification in the report of results. Certain procedures are intended for use with sludges and sediments. Here again, the effort has been to present methods of the widest possible application, but when chemical sludges or slurries or other samples of highly unusual composition are encountered, the methods of this manual may require modification or may be inappropriate. Most of the methods included here have been endorsed by regulatory agencies. Procedural modification without formal approval may be unacceptable to a regulatory body.
    [Show full text]
  • Assessment of an Ultramicroelectrode Array (UMEA) Sensor for The
    Assessmentofanultramicroelectrodearray(UMEA)sensorfor thedeterminationoftraceconcentrationsofheavymetalsinwater ZurErlangungdesakademischenGradeseines DoktorsderNaturwissenschaften anderFakultätfürBauingenieur-,Geo-undUmweltwissenschaften der UniversitätKarlsruhe genehmigte DISSERTATION von XudongXie ausVRChina 2004 TagdermündlichenPrüfung:02.06.2004 Referentin:Prof.Dr.DorisStüben Korreferent:Prof.Dr.Rolf-DieterWilken Abstract Abstract Rapid development in silicon technology and microelectronics has nowadays enabled the mass fabrication of microelectrode arrays with well-defined and reproduciblegeometriesonmicronscalesbyusingthin-filmtechnology.Comparedto conventional macroelectrodes, microelectrodes possess several attractive features, suchashighmasstransport,reducedchargingcurrent,immunitytoohmicdrop,high signal-to-noiseratio,etc.,whichareveryadvantageousfordevelopingdecentralised analyticalequipmentsintracemetalanalysis. A novel iridium-based ultramicroelectrode array chip (Ir-UMEAs) was recently designed and fabricated by means of microlithographic techniques by one of the partners (Fraunhofer Institute of Silicon Technology, ISIT, Germany) of this joint research project. The Ir-UMEA chip is made up of 4048 individual ultramicroelectrodes (working electrodes) arranged in four separate arrays. Each arrayconsistsof46x22=1012discshapedmicroelectrodeswithadiameterof1.8 µmeach,givingatotalelectrodeareaof2575µm².Theelectrodesarerecessedby 0.2µmandarespacedwithaninterelectrodedistanceof25µm. Thisstudyismainlyaimedat(i)investigatingtheelectrochemicalbehaviorandthe
    [Show full text]