Analytical Chemistry CH 382 PDF (2025-2024)

‫الكيمياء غير العضوية‬ ‫لطالب الفرقة الثالثة ميكروبيولوجي‪-‬كيمياء‬ ‫اعداد‬ ‫اعضاء هيئة التدريس‬ ‫قسم الكيمياء – كلية العلوم بنين بأسيوط‬ ‫للعام الجامعي ‪ 2025-2024‬م‬ ‫)‪In Organic Chemistry (C-382‬‬ ‫‪For Third year students‬‬ ‫اعداد‬...

‫الكيمياء غير العضوية‬ ‫لطالب الفرقة الثالثة ميكروبيولوجي‪-‬كيمياء‬ ‫اعداد‬ ‫اعضاء هيئة التدريس‬ ‫قسم الكيمياء – كلية العلوم بنين بأسيوط‬ ‫للعام الجامعي ‪ 2025-2024‬م‬ ‫)‪In Organic Chemistry (C-382‬‬ ‫‪For Third year students‬‬ ‫اعداد‬ ‫اعضاء هيئة التدريس‬ ‫قسم الكيمياء – كلية العلوم بنين باسيوط‬  Introduction to Chemical Analysis Chemical analysis includes any aspect of the chemical characterization of a sample material. Analytical Chemistry? “Science of Chemical Measurements” Areas of Chemical Analysis and Questions They Answer Quantitation: How much of substance X is in the sample? Detection: Does the sample contain substance X? Identification: What is the identity of the substance in the sample? Separation: How can the species of interest be separated from the sample matrix for better quantitation and identification? What is Analytical Science? Analytical Chemistry provides the methods and tools needed for insight into our material world…for answering four basic questions about a material sample? What? Where? How much? What arrangement, structure or form? Fresenius’ J. Anal. Chem. 343 (1992):812-813 1 Applied Analytical Chemistry Preparation and treatments of samples to analysis: Sampling: is one of the most important operations in a chemical analysis. Chemical analyses use only a small fraction of the available sample. The fractions of the samples that collected for analyses must be representative of the bulk materials. Knowing how much sample to collect and how to further subdivide the collected sample to obtain a laboratory sample is vital in the analytical process. All three steps of sampling, standardization, and calibration require a knowledge of statistics. 2 Analytical samples and methods Sample size Type of analysis > 0.1g Macro 0.01~0.1g Semimicro 0.0001~0.01g Micro –4 < 10 g Ultramicro Analytical level Type of constituent 1%~100% Major 0.01%(100ppm)~1% Minor 1ppb~100ppm Trace Census : all the material is examined  impracticable 2> Casual sampling on an ad hoc basis  unscientific 3> Statistical sampling 2) Sampling procedure 7  Bulk sample Increment homogeneous or heterogeneous Gross sample Sub sample Analysis sample Census vs Random sampling Census A complete enumeration, usually of a population, but also businesses and commercial establishments, farms, governments, and so forth. A complete study of the population as compared to a sample. 8 Random sampling A commonly used sampling technique in which sample units are selected so that all combinations of n units under consideration have an equal chance of being selected as the sample. (Statistical sampling meaning) A sampling method in which every possible sample has the same chance of being selected. 3) Sampling statistics : Total error = sampling error + analytical error sT = ( sS2 + sA2 )1/2 In designing a sampling plan the following points should be considered. 1> the number of samples to be taken 2> the size of the sample 3> should individual samples be analysed or should a sample composed of two or more increments (composite) be prepared. How much should be analyzed ? 2 mR  K S where m = mass of each sample analyzed, R= desired relative SD. How many portions should be analyzed ? 1/2 e = (tsS) / (n) 2 2 2  n = t sS / e where n = the number of samples needed t = Student’s t for the 95% confidence level and n1 9  degree of freedom. Statistics of sampling segregated materials 2 sS = [A / mn] + [ B / n] where sS is the standard deviation of n samples, each of mass m. The constant A and B are properties of the bulk material and must be measured in preliminary experiments. Sampling uncertainties Both systematic and random errors in analytical data can be traced to instrument, method, and personal causes. Most systematic errors can be eliminated by exercising care, by calibration, and by the proper use of standards, blanks, and reference materials. For random and independent uncertainties, the overall standard deviation s0 for an analytical measurement is related to the standard deviation of the sampling process ss and the standard deviation of the method sm by the relationship 2 2 2 s0 = ss + sm When sm< ss/3, there is no point in trying to improve the measurement precision. Size of the gross sample 10 Basically, gross sample weight is determined by (1) The uncertainty that can be tolerated between the composition of the gross sample and that of the whole, (2) The degree of heterogeneity of the whole, (3) The level of particle size at which heterogeneity begins. To obtain a truly representative gross sample, a certain number N of particles must be taken. The number of particles required in a 12 gross sample ranges from a few particles to 10 particles. Sampling homogeneous solutions of liquids and gases Well mixed solutions of liquids and gases require only a very small sample because they are homogeneous down to the molecular level. Homogeneous ? Which portion? Flowing stream? Gas sampling: Sampling bag (Tedlar® bag) with a Teflon fitting Trap in a liquid Adsorbed onto the surface of a solid SPME SAMPLE PREPARATION (1) Is the sample a Solid or a Liquid? Liquids (2) Are you interested in all sample components or only one or a few? 11 If only a few then separation is necessary by extraction or chromatography. (3) Is the concentration of the analytes appropriate for the measurement technique? If not, dilute or concentrate with extraction, evaporation, lyophilization. (4) Is sample unstable ? If yes, derivatize, cool, freeze, store in dark (5) Is the liquid or solvent compatible with the analytical method? If not, do solvent exchange with extraction, distillation, lyophilization. (http://www.trincoll.edu/~dhenders/textfi~1/Chem%20208%20notes/sample_preparation. htm) 12  Sampling particulate solids Identify the population to be analyzed Randomly collect N particles to give a gross sample Reduce particle size of gross sample and homogenize Randomly collect N particles Store the lab sample Remove portions of the lab sample for analysis 13  Steps in sampling and measurement of salt in a potato chip. Step 1 introduces the sampling variance. Steps 2 to 4 introduce the analytical variance. 14 Linear regression Linear regression uses the method of least squares to determine the best linear equation to describe a set of x and y data points. The method of least squares minimizes the sum of the square of the residuals - the difference between a measured data point and the hypothetical point on a line. The residuals must be squared so that positive and negative values do not cancel. Spreadsheets will often have built-in regression functions to find the best line for a set of data. A common application of linear regression in analytical chemistry is to determine the best linear equation for calibration data to generate a calibration or working curve. The concentration of an analyte in a sample can then be determined by comparing a measurement of the unknown to the calibration curve. SAMPLE PREPARATION Most samples must be prepared before analysis. The preparation process varies depending on:- the sample matrix-the material to be analyzed - the analytical method. The most important processes include extraction and cleanup, digestion, leaching, dilution, and filtering. Depending on the sample matrix, other procedures such as grinding and chemical manipulations may be required. Sample Extraction The extraction processes are used more often for the organic analytes that are extracted to bring them into the appropriate solvent prior to analysis. The extraction method varies depending on whether the sample is liquid or solid. Extraction techniques for aqueous samples include liquid-liquid (separation funnel or continuous) and solid-phase. For solid samples, the methods include Soxhlett, supercritical fluid, and sonication. Some extraction processes are repeated multiple times (such as three) to improve the efficiency of extraction. Organic pollutants in potable or non-potable waters, soils, sediments, sludge, 15 solid wastes, and other matrices must be brought into an appropriate organic solvent for their injection into the gas or liquid chromatographic column. Sample Cleanup Samples may undergo a cleanup process to improve the analysis process and generate more reliable results. The sample extracts may be purified by one or more of the following techniques: 1.Partitioning between immiscible solvents 2.Adsorption chromatography 3.Gelpermeation chromatography 4. Destruction of interfering substances with acids, alkalis, and oxidizing agents 5. Distillation Digestion Samples analyzed for metals are usually digested. The digestion process uses strong acids and heat to increase the precision and accuracy of the measurement by providing a homogeneous solution for analysis by removing metals adsorbed to particles and breaking down metal complexes. Different digestion techniques are used depending on the analytical method and target accuracy levels. Dilution Sometimes it is necessary to dilute the sample prior to analysis. The reasons for this may be that the concentration of the analyte is outside the concentration range where the analytical technique is linear, or other substances in the sample may interfere with the analysis (matrix interference). A record of the dilution factor should be kept with the result. Dilution affects the result itself as well as the detection limit for the result. For non-detected results, the reported result based on the detection limit will be increased proportionately to the dilution, and this needs to be considered in interpreting the results. 16 Filtering The sample may or may not be filtered, either in the field or in the laboratory. If the sample is not filtered, the resulting measurement is referred to as a total measurement, while if it is filtered it is considered a dissolved result. For filtered samples, the size of the openings in the filter (such as 1 micron) should be included with the result. Commonly, once a sample has been filtered it is preserved. This information should also be noted with the result. Some Definations: A sample is a small portion of a larger body of material that is obtained for laboratory analysis. A representative sample is a sample that possesses all the characteristics of a larger bulk system in exactly the same concentration levels as in the system. Acomposite sample when the substance to be determined is not homogeneously distributed throughout the entire system, a series of samples could be obtained from different parts of the bulk system and then combined into one sample. Aselective sample is a sample that is obtained from a particular part of the bulk system that is known, or assumed, to have a different composition. Arandom sample This would be just one sample taken from one location at random in the bulk homogeneous system. Some terms are used when a sample of a bulk system is divided, possibly a number of times, before actually being used in an analysis. For example, a water sample from a well may be collected in a large bottle (bulk sample or primary sample), from which a smaller sample is acquired by pouring into a vial to be taken into the laboratory (secondary sample, sub-sample, or laboratory sample), then poured into a beaker (another secondary sample or sub- 17 sample), before a portion is finally carefully measured into a flask (test sample) and diluted to make the sample solution. Analysis Steps: An analysis involves several steps and operations which depend on: the particular problem your expertise the apparatus or equipment available. The analyst should be involved in every step. 18 Different methods provide a range of precision, sensitivity, selectivity, and speed capabilities. 19 The sample size dictates what measurement techniques can be used. ©Gary Christian, Analytical Chemistry, 6th Ed. (Wiley) 20  Measuring the Amount of Analyte Spectrophotometer: Highly colored complex of arsenic was found to absorb light at a wavelength of 535 nm. 21  Calculating the Concentration ppm = (Absorbance -.005)/0.0282 Deer 1: (0.61 - 0.005)/0.0282 = 22 ppm Deer 2: (0.43 -0.005)/0.0282 = 15 ppm Arsenic in the kidney tissue of animals is toxic at levels above about 10 ppm. Grass Samples showed about 600 ppm arsenic. 22 (1) A type of chemical analysis by which the analyte or analytes in a sample are identified A type of chemical analysis by which the amount of each analyte or analytes in a sample is determined analyte(s)the species to be determinded in the sample matrix Matrix-analyte = concomitants Classification of Analytical Methods 1. Classical methods Qualitative – identification by color, indicators, boiling or melting points, odors Quantitative – mass or volume(e.g. gravimetric, volumetric) 2. Instrumental methods: Often, same instrumental method used for qualitative and quantitative analysis 23 Some Analytical methods and its measured signals Signal Example method Radiation emission Emission spectroscopy (X-ray, UV, visible), fluorescence, phosphorescence, Luminescence Radiation absorption Absorption spectroscopy photometry, NMR spectrophotometry, electron spin resonance Radiation Scattering Raman spectroscopy Radiation refraction Refractometry Radiation diferaction X-ray and Electron diffraction method Radiation rotation Polarimetry Electrical potential Potentiometry Electrical charge Coulometry Electrical current Voltammetry – amperometry , polarography Electrical resistance Conductometry Mass-to-charge ratio Rate of reaction Mass spectrometry Flow injection analysis Mass Gravimetry 24  Selecting an Analytical Method Defining the Problem A definition requires answers to the following questions: 1. What accuracy and precision are required? 2. How much sample is available? 3. What is the concentration range of the analyte? 4. What components of the sample will cause interference? 5. What are the physical and chemical properties of the sample matrix? 6. How many samples are to be analysed? Performance Characteristics of Instruments; Figures of Merit Precision: Indeterminate or random errors Absolute standard deviation (s) Relative standard deviation (RSD) Variance (s2) Coefficient of variation (a percentage) (CV) Standard deviation of means (sm) Absolute standard Deviation (s) 25  Absolute standard Deviation (s) Relative standard deviation (RSD) Coefficient of Variation (CV) Standard deviation of means (sm) Accuracy: determinate errors (operator, method, instrumental) 26 % Relative error (%Er) Sensitivity:“the ability to detect (qualitative analysis) or determine (quantitative analysis) small amounts of an analyte in a sample” x = analytical signal C = the analyte concentration Sensitivity : slope of calibration curve (Larger slope of calibration curve, more sensitive measurement) Limit of detection (LOD): “the lowest concentration of analyte in a sample that can be detected” Limit of Quantitation (LOQ):“the minimum injected amount that produces quantative measurements in the target with acceptable precision” 27  Selectivity:“ability of the method to measure one species of analyte in the presence of other elements or compounds” Example: Signal = mACA + mBCB + Signalblank mA, mB = calibration sensitivity of A, B CA, CB = concentration of A, B A+B sample Selectivity coefficient: kB,A = mB/mA k’s vary between 0 (no selectivity) and large number (very selective). Calibration methods: Basis of quantitative analysis is magnitude of measured property is proportional to concentration of analyte” Signal  [x] or Signal = m[x] + signal blank [x] = (signal-signal )/m blank 28  LOQ Instrument response (signal) LOD LOL Slope m Signal blank [x] 29 Historical background of some selected modern electrochemical techniques: Voltammetry was selected as the most popular type of modern electrochemical techniques. J. Heyrovsky (Czech chemist) was the first introduced polarography in 1922 and thus he received the Nobel Prize in chemistry (1959).  Polarography is the class of voltammetry, in which the DME is used as working electrode. Here, will discuss many different forms of voltammetric analysis used for analytical purposes. 30  Some Selected Electrochemical Techniques Electrochemical Sensor Potentiometry Voltammetry Cyclic Voltammetry Square Wave Voltammetry Stripping Voltammetry Pulse Voltammetry Anodic Stripping Voltammetry Normal Pulse Voltammetry Cathodic Stripping Voltammetry Differential Pulse Voltammetry Adsorptive Stripping Voltammetry Potentiometric Stripping Voltammetry 31 What is the general idea of voltammetry? Voltammetry is based on voltage-current-time relationship arising in a cell of three electrodes: working electrode, reference electrode and auxiliary or counter electrode. This relationship could be explained when potential (E) is applied to the working electrode and the resulting current (i) flowing through the electrochemical cell will be recorded. The applied potential could be changed or the resulting current will be recorded over a period of time (t). This applied potential service as driving force for the reaction (oxidation or reduction) that causes the chemical species present in solution to be electrolyzed at the electrode surface. The special branch of voltammetry is a stripping analysis, which used to determine a very small concentrations of analyte (subnanograms) What is the general idea of stripping analysis? Zbinden suggested the general idea of stripping analysis in 1931. It involves the analyte is concentrated into or onto the surface of the working electrode. After the preconcentration step is completed, the preconcentrated analyte is stripped from the electrode surface by the application of a potential scan. The presence of preconcentration step makes stripping analysis much more sensitive than direct polarographic techniques. Modern variants of classical stripping started to appear in 1947-1960, fast linear sweep voltammetry was the first followed by the development of square wave and pulse polarography by Barker et al 32 I.1. Anodic Stripping Voltammetry (ASV): It is the most widely form of stripping analysis. ASV consist of two steps, preconcentration step, where the analyte is preconcentrated by electrodeposition into the small-volume mercury electrode as following: analyte is reduced at the mercury electrode forming amalgam at negative (cathodic) potential The second step consist of amalgamated analyte is reoxidized and stripped out of the electrode by applying positive (anodic) potential as follow: 33 I.2. Cathodic Stripping Voltammetry (CSV): It is used to determine many of organic and inorganic compounds that forms insoluble salts with electrode material. It involves positive deposition of the analyte when anodic potential is applied to the working electrode followed by stripping in a negative- going potential scan. The sensitivity of CSV depends on the amount of that can be plated in a given period, the density of the formed insoluble salt film and the dissociation rate of the insoluble mercury compound during the stripping step. This can be shown as following: 34  n deposition A  Hg HgA  ne  stripping I.3. Adsorptive Stripping Voltammetry (AdSV): It is quite similar to ASV and CSV methods. The main difference between them could be explained as using adsorption in preconcentration step. It is employed in the trace analysis of a wide variety of organic and inorganic analyte. Where, in case of metal ions, it could be easily determined by AdSV when it reacts with suitable ligand to form a complex which is adsorbed on the electrode surface. Also, metal ions will be determined by AdSV when it reacts with ligand adsorbed on the electrode surface I.4. Potentiometric Stripping Analysis (PSA): In this case, stirring play important role in stripping step to facilitate the transport of oxidant Oxidation will be occurs when constant anodic current passed through the working electrode. Then, the variation of the working electrode potential is recorded and the stripping curve is obtained. The transition time (tM) consumed during the oxidation process is quantitative measure of the concentration of the analyte: This means that when the oxidant concentration (Cox) decrease the obtained signal will be increase. 35  I.5. Cyclic Voltammetry (CV): Cyclic voltammetry is a rapid voltage scan technique in which the direction of voltage scan is reversed. The scan rate in the forward and reverse direction is normally the same. CV can be used in single cycle or multicycle modes. I.6. Pulse Voltammetry (PV) Normal Pulse Voltammetry (NPV) Differential Pulse Voltammetry (DPV) 36  Normal Pulse Voltammetry (NPV): In this type peak current is measured at near the end of each pulse. At certain period, the resulting current equal zero, and the electrode potential is kept constant between the pulses and no chemical reaction occurs in the cell. Differential Pulse Voltammetry (DPV): Figure shown that current is measured at two points; before the application of the pulse ( point 1), and at the end of the pulse (point 2). The first current is subtracted from the second, and the current difference {Δi = i(t2) – i(t1)} is plotted versus the applied potential. The highest of the produced peak current is proportional to the concentration of analyte: Where α = exp [(nF/RT) (ΔE/2)] and ΔE is pulse amplitude 37 Square Wave Voltammetry (SWV): In SWV the current is doubled during each square wave cycle, one at the end of the forward pulse and the other at the end of the reverse pulse. The difference between the two measurements is plotted versus the base potential. As shown in this Fig., the forward pulse produce a cathodic current (A), the reverse pulse produce an anodic current (B) and (C) is the difference between them. The major advantage of SWV is its speed and more sensitive than DPV by about 4 and 3.3 times higher, for reversible and irreversible cases respectively, 38 39  Physical sensors Measuring physical quantities Biosensors such as temperature and Biosensor can be defined as a device pressure incorporating a biological sensing element connected to a transducer (such as enzyme and DNA). Different types of biosensors have many analytical applications Chemical Sensor Chemical reaction occurs I.8.andSensors and can be used for biosensors qualitative or quantitative determination of the analyte Electrochemical sensors are really a subclass of chemical sensors where the electrode is used as transduction element. The analyte that this sensor detects and measures may be organic, inorganic and biological components 40 I.9. Potentiometry: Potentiometry is a classical analytical technique in which the cell consists of two electrodes. Recently, this method was developed by suggestion of use ion selective electrode (ISE). This electrode characterized by its selectivity and sensitivity. Such electrodes show fast response, wide linear range, not affected by color or turbidity, not destructive and very inexpensive. 41 II. Instrumentation used for stripping analysis (as the most popular modern electrochemical methods): The apparatus used for stripping analysis consist of simple potentiostat circuits used for three-electrode cell; this could be shown in this Fig. The three electrode cell (about 10-mL volume used in general) is made up of three electrodes immersed in a solution containing the analyte and the nonreactive electrolyte called supporting electrolyte As the continuous development of stripping science, the shape of cell change and being depend on the type of used working electrode and the limit of concentration want be measured. 42  II.1. Types of working electrode: Mercury electrodes: Solid electrodes For stripping analysis, HMDE and MFE are used as working electrode due to the There are many different types of solid electrodes reproducible area and low background used as working electrodes such as gold, platinum, current over wide range of potential glassy carbon electrode, carbon paste electrode, carbon fiber electrode, and epoxy-bonded graphite electrode. The use of such electrodes requires precise electrode pretreatment and polishing to obtain reproducible results. Chemically Modified electrodes (CMEs): Modification will be used to improve the properties of the selected working electrode. The main idea of the modification depends on incorporating of a reagent on the electrode surface or into the matrix of the selected electrode This may be occurs by covering the electrode surface with the solution of the selected polymer and allowing the solvent to evaporate. Also, electroploymerization may be used to make the polymer film on the electrode surface. Incorporating also may be occurs by mix the preconcentrating agent with the electrode matrix (as done with CPE) or may be binding with functionalized polymeric film on the electrode surface 43  III. Analytical application of some selected modern electrochemical techniques Pharmaceutical compounds such as gastro-intestinal drugs, antibiotics Application for the trace and determination of metals in antibacterial drugs, antineoplastic different samples such as drugs, cardiovascular drugs, industrial samples, anesthetic food samples, drugs, flavonoids, vitamins, soft drinks and juices samples, antifungal biological and pharmaceutical drugs and antidepressant drugs. samples and These drugs were determined in environmental samples dosage forms (tablets, capsules, injections and suspension) and also in biological samples (real and spiked urine samples, blood and serum). 44 III.1. Electroanalytical application for the determination of pharmaceutical compounds: This table shows some electroactive functional groups and their reactions. Group Reaction C C ROOC CN + H+ + 2e- ROOC H+ CN- C C C CH2 + 2H+ + 2e- CH2CH3 H H C C C C CHO + 2H+ + 2e- C C CHO H C X RCH2Br + H+ + e- RCH3+ Br- N O RNO + 2H+ + 2e- RNHOH NO2 RNO2 + 4H+ + 4e- RNHOH + H2O O O ROOR + 2H+ + 2e- 2ROH S S RSSR + 2H+ + 2e- 2RSH S O R2S O + 2H+ + 2e- R2S + H2O H H N N N N + 2H+ + 2e- N N 45 IV. Advantages of electrochemical methods: The previous survey shows that the number of publications dealing with the application of some selected modern electrochemical techniques (voltammetric techniques) to determine pharmaceuticals and metals in different samples. The importance of such applications increased steadily, and this due to the following advantages: The sensitivity is sufficiently high and can be increased more by modifications of classical voltammetric techniques (modified microelectrodes and ultramicroelectrodes) that enhance significantly sensitivity and selectivity of the method. Voltammetry coupled with different separation methods such as (HPLC, Flow Injection (FI) and Capillary Electrophoresis (CE)) enhancing the analytical properties for complex mixtures in different compounds. Turbid and colored solutions, which are a problem with other methods, can be easily analyzed. The separation of the excipients, in pharmaceutical analysis, is in many cases not necessary and this simplified the preparation of samples. Only small volumes of samples are necessary. Electroanalytical stripping procedures have been developed for the measuring down to sub-μg/L level. Modern voltammetry also continues to be a potent tool used by various kinds of chemists interested in studying oxidation and reduction process in various media, adsorption process on surfaces and electron transfer mechanism at chemically modified electrodes surfaces. Also, these techniques combine low maintenance costs with high sensitivity and selectivity that allows the determination of low levels of analytes with out prior treatments of the samples. These techniques have been developed for various cations, anions and organic molecules. Electroanalytical techniques (specially stripping analysis) are well known as excellent procedures for the determination of trace chemical species. 46  The developed stripping voltammetric methods are simple, time saving, selective and more sensitive for the simultaneous determination of trace substances. Electroanalytical methods especially square wave voltammetry is a very sensitive and rapid analytical method due to it is high scan rate in all cases where the reacting species is accumulated by adsorption on the electrode surface. The short analysis time in these methods makes it very attractive for routine determination of the analytes in different samples. 47  JS CH3403 Interdisciplinary Chemistry Module 1. 2013/2014. Analytical Chemistry: Electrochemical methods of analysis. Basic Electroanalytical Chemistry. Potentiometric,Voltammetric and Coulometric measurement techniques. Professor Mike Lyons School of Chemistry TCD Room 3.2 Main Chemistry Building [email protected] 48  Electro-analytical Chemistry. Electroanalytical techniques are concerned with the interplay between electricity & chemistry, namely the Electro-analytical chemists at work ! measurement of electrical quantities Beer sampling. such as current, potential or charge Sao Paulo Brazil 2004. and their relationship to chemical parameters such as concentration. The use of electrical measurements for analytical purposes has found large range of applications including environmental monitoring, industrial quality control & biomedical analysis. EU-LA Project MEDIS : Materials Engineering For the design of Intelligent Sensors. 49  Outline of Lectures Introduction to electroanalytical chemistry: basic ideas Potentiometric methods of analysis Amperometric methods of analysis Coulombic methods of analysis J. Wang, Analytical Electrochemistry, 3rd edition. Wiley, 2006 R.G. Compton, C.E. Banks, Understanding Voltammetry, 2nd edition, Imperial College Press,2011. C.M.A. Brett, A.M.Oliveira Brett, Electrochemistry: Principles, methods and applications, Oxford Science Publications, 2000. 50  Why Electroanalytical Chemistry ? Electroanalytical methods have certain advantages over other analytical methods. Electrochemical analysis allows for the determination of different oxidation states of an element in a solution, not just the total concentration of the element. Electroanalytical techniques are capable of producing exceptionally low detection limits and an abundance of characterization information including chemical kinetics information. The other important advantage of this method is its low cost. 51  Electroanalytical methods. Electrochemical reactions involve electron transfer (ET) processes at electrode solution interfaces. These ET reactions may be kinetically sluggish or kinetically facile depending on the details of the ET reaction and the nature of the electrode surface. Provided an analyte species exhibits electroactivity (can be oxidised or reduced) then it may be detected using the tools of electrochemistry. Thus, electrochemical methods may be split up into two major classes : Potentiometric and Amperometric. In potentiometry the ET reaction is kinetically facile and we measure the potential of a Galvanic cell under conditions of zero current flow. The cell potential responds to changes in the activity of the analyte species present in the solution in a well defined manner described by the Nernst equation. Indeed the cell potential varies in a linear manner with the logarithm of the analyte activity. In amperometry the kinetics of the ET reaction will have to be driven by an applied potential and so we measure the diffusion controlled current flowing across the electrode/solution interface. This current is directly proportional to the bulk concentration of the analyte present in the solution. 52 53 54 55 56 57 58  Electron sink electrode Electron source electrode (Anode). (Cathode). P A ne- Q ne- B Oxidation or de-electronation. Reduction or electronation. P = reductant(electron donor) A = oxidant (electron acceptor) Q = Product B = Product In potentiometry an interfacial ET reaction is in equilibrium and the interfacial potential is governed by the Nernst equation. In voltammetry an analyte species is oxidised or reduced at an indicator electrode giving rise to a current flow which is directly proportional to the bulk analyte concentration. 59 Device Type Potentiometric Amperometric Method of Measure Measure potential at transport limited operation Zero current current Electrode Must be fast Electrode potential can kinetics drive reaction Response Concentration Concentration is depends a linear function exponentially on of current potential via Nernst equation Mass Unimportant Must be controlled transport Sensitivity Ca. 10-6 M but Ca. 10-9 M can be less (ca. 10-8M). 60  Voltammetry. Voltammetry is an electroanalytical method in which the controlled parameter, the potential of the indicator electrode varies in a definite manner with time, and in which the current flowing through the indicator electrode is the measured parameter. The voltammetry method relies on the fact that the current measured reflects rate determining diffusion of the analyte species from the bulk solution to the surface of the indicator electrode where it is readily oxidised or reduced. Under such conditions of diffusion control the measured current is linearly proportional to the bulk concentration of the analyte species. Voltammetric techniques are classified according to the type of voltage perturbation applied to the indicator electrode, i.e. the way that the voltage signal input varies with time. The form of the input V(t) function will determine the form of the resulting current response. The current/potential response curve is called a voltammogram. Linear potential sweep voltammetry A  ne  B E i Ef Ef Input Response E = F(t) I = G(E) input Ei response Ei t E 14 61 62 63 64  e- A0 A kD region of kET amperometric B0 B measurement 1.0 i  iD  nFAk D c  Material transport via diffusion region of rate limiting  = i / iD potentiometric 0.5 measurement k ET  k D ET kinetics iD rate limiting k ET  k D 0.0 net current zero -6 -4 -2 0 2 4 6 c interfacial ET balanced  = F(E-E0)/RT Amperometric Nernst equation E S Measurement pertains Potentiometric Measurement Log c 18 65 G. Denault, Ocean Sci., 5 (2009) 697-710. 66 67  Transport and kinetics Transport via Molecular diffusion in electrochemical systems. only Diffusion layer (stagnant) Simple diffusion layer Double layer model neglects region  convection effects and also simplifies c analysis of Fick Interfacial A A diffusion equations. ET kET kD B Electrode Bulk solution c0 (well stirred) Material f   D c   c0  Hydrodynamic flux  layer 68 Transport and kinetics at electrodes. Current density 1.0 i j ET & MT Diffusion f   Control nFA nF  = i / iD Mixed 0.5 Control Net flux D MT kD   Kinetic Control  dc  f   D   D     c  c0  k D c   c0  0.0  dx  0  -6 -4 -2 0 2 4 6  = F(E-E0)/RT ET  c c0  k  k ET k D c  1  ET f  1  1  1 D k D  k ET f  k ET c  k D c  f   k ET c0 Normalised k ET  k exp    0 potential 22 69  Mediated vs unmediated ET at electrodes. Redox groups bound to support surface as 2D monolayer or as 3D multilayer. e- S A S B P e- P Heterogeneous redox catalysis : mediated ET via surface bound Direct unmediated ET. redox groups.  ne   ne  A   B S   P BS A  P 23 70 A schematic experimental arrangement for controlled potential i (t ) measurement is outlined across. W denotes the C EC potentiostat R working or W cell indicator electrode, R represents the Waveform reference electrode, generator E controlled and C is the i (t ) counter or auxiliary electrode. The potentiostat controls the voltage between the working electrode and the counter electrode according to a pre-selected voltage time programme supplied by a waveform generator or computer. The potential difference between the working and reference electrodes is measured by a high impedance feedback loop based on operational amplifiers. Current flow is measured between the counter and the working electrodes. 24 71  Electrochemical Cell Large variety of digital waveforms can be generated via computer software. Potentiostat RE WE Electrochemical Cell stand Electrochemical water Splitting in electrolyte Filled cell (OER) 72 73 74  Working electrode Most common is a small Mercury and amalgam sphere, small disc or a short electrodes. wire, but it could also be  reproducible metal foil, a single crystal of homogeneous surface. metal or semiconductor or  large hydrogen evaporated thin film. overvoltage. Has to have useful working Wide range of solid potential range. materials – most common Can be large or small – usually are “inert” solid < 0.25 cm2 electrodes like gold, platinum, glassy carbon. Smooth with well defined  Reproducible geometry for even current pretreatment procedure. and potential distribution.  Well defined geometry.  Proper mounting. 75 Working Electrodes Size  Analytical macro 1.6 - 3 mm diameter  Micro 10-100 m diameter From BAS www-site: http://www.bioanalytical.com/ 76  Counter (Auxiliary) Electrode Serve to supply the current Area must be greater than required by the W.E. without that of working limiting the measured response. Usually long Pt wire (straight Current should flow readily or coiled) or Pt mesh (large without the need for a large surface area) overpotential. Products of the C.E. reaction No special care required for should not interfere with the counter reaction being studied. It should have a large area compared to the W.E. and should ensure equipotentiality of the W.E. From BAS www-site: http://www.bioanalytical.com/ 77  Reference Electrode (RE) Aqueous The role of the R.E. is to SCE provide a fixed potential Ag/AgCl which does not vary during Hg/HgO the experiment. RHE A good R.E. should be able to maintain a constant potential Nonaqueous even if a few micro-amps are passed through its surface. Ag+/Ag Pseudoreferences  Pt, Ag wires Ferrocene/ferricinium couple (a) response of a good and Micropolarization test. (b) bad reference electrode. 78 Saturated calomel electrode SCE Cl-(aq)/Hg2Cl2/Hg(l) Hg22+ + 2e- = 2Hg(l) E0 = 0.24 V vs. SHE @ 250C Advantages Most polarographic From BAS web site: data referred http://www.bioanalytic to SCE al.com 32 79 Silver/silver chloride reference electrode Ag/AgCl Ag wire coated with AgCl(s), immersed in NaCl or KCl solution Ag+ + e- = Ag(s) E0 = 0.22 V vs. SHE @ 250C Advantages Disadvantages chemical processing solubility of KCl/NaCl industry has temperature standardized on this dependent electrode dE/dT = -0.73 mV/K convenient (must quote rugged/durable temperature) From BAS site http://www.bioanalytical.com/ 80  Silver/silver ion reference electrode Ag+/Ag Ag+ + e-= Ag(s) Requires use of internal potential standard Disadvantages Advantages Potential depends on Most widely used  solvent Easily prepared  electrolyte (LiCl, Works well in all TBAClO4, TBAPF6, aprotic solvents: TBABF4  THF, AN, DMSO, Care must be taken to DMF minimize junction potentials From BAS site: http://www.bioanalytical.com/ 34 81  Mercury-Mercuric oxide reference electrode Hg/HgO Metal/metal oxide reference electrode. HgO( s )  H 2O  2e   Hg  2OH  E  E 0  Hg , HgO, OH    0.059 log aOH  E 0  Hg , HgO, OH    0.0984 V  vs SHE  E  0.0984  0.059 log aOH   0.0984  0.059 pOH  0.924  0.059 pH Used in particular for electrochemical studies in aqueous alkaline solution. 1.0 M NaOH usually used in inner electrolyte compartment which is separated from Test electrolyte solution via porous polymeric frit. Hence reference electrode in like SHE system in that it is pH independent. 82 Comparing various Reference Electrode potential scales to SHE scale. E (vs SHE) = E (vs REF) + EREF (vs SHE) 83  Electrolyte Solution Consists of solvent and a high concentration of an ionized salt and electroactive species Functions to increase the conductivity of the solution, and to reduce the resistance between  W.E. and C.E. (to help maintain a uniform current and potential distribution)  and between W.E. and R.E. to minimize the potential error due to the uncompensated solution resistance iRu 84 85 86 87 88 89  Potential reading The potentiometric measurement. Device (DVM) In a potentiometric measurement two electrodes are used. These consist of the indicator or sensing electrode, and a reference electrode. A Electroanalytical measurements relating potential to analyte B concentration rely on the response of one electrode only (the indicator electrode). Reference Indicator The other electrode, the reference electrode electrode electrode is independent of the Solution containing solution composition and provides analyte species A stable constant potential. The open circuit cell potential is measured using a potential measuring device such as a potentiometer, a high impedance voltameter or an electrometer. 90 91  Fundamentals of potentiometric measurement : the Nernst Equation. The potential of the indicator electrode is related to the activities of one or more of the components of electron flow the test solution and it therefore determines the overall Ee equilibrium cell potential Ee. Under ideal reference H2 in indicator circumstances, electrode Pt electrode the response of SHE the indicator electrode to changes in analyte A(aq) Pt H 2 (g) species activity at the indicator electrode/ H+(aq) B(aq) solution interface should be rapid, reversible and test analyte governed by the Nernst equation. salt bridge redox couple The ET reaction involving the analyte species should be kinetically facile and the ratio of the analyte/product concentration should depend on the interfacial potential difference via the Nernst equation. 92  Ecell  Eind  Eref  E j The net cell potential at equilibrium is given by the expression across where Eind denotes the potential of the indicator electrode, Eref denotes O+ne- -> R the reference electrode potential and Ej is the liquid junction potential which is usually small. The potential of the indicator electrode is described by the Nernst equation. Hence the net cell potential is given 2.303RT  aR  Eind  Eind  0 log   by the expression across, nF  aO  where k denotes a constant and is given by The expression outlined. The constant k may be determined by 2.303RT  aR  measuring the potential of a standard Ecell  k  log   solution in which the activities of the nF  aO  oxidised species O and the reduced species R are known. Usually we are interested in k  Eind 0  Eref  E j determining the concentration rather than the activity of an analyte. If the ionic strength of all 2.303RT   R cR  solutions is held constant then the E  E 0  log    ind ind activity coefficient nF  O Oc of the analyte will be constant 2.303RT    2.303RT c   Eind 0  log  R   log  R  and activities may be replaced by nF  O  nF  cO  concentrations in the Nernst equation. 46 93 The cell response in the Ecell Potentiometric measurement takes the form S 2.303RT nF 2.303RT c  Ecell  k   log  R  nF  cO  2.303RT  R  k  E 0  Eref  Ej  log    O  ind nF log c Practical examples Since the ionic strength of an unknown of potentiometric analyte solution is usually not known, a chemical sensor high concentration of supporting systems include the electrolyte is added to both the standards pH electrode where Ecell which mainly reflects and the samples to ensure that the same a membrane potential, ionic strength is maintained. is proportional to log aH+ , and ion selective Electrodes where E is Proportional to log aion. 47 94  Ion selective electrodes. The glass electrode used to measure solution pH is the most Common example of an ion selective electrode. Ideally, an ion selective electrode responds only to one target ion And is unaffected by the presence of other ions in the test solution. In practice there is always some interference by other ions. The operation of ISE devices does not depend on redox processes. The key feature of an ISE is a thin selective membrane across which only the target ion can migrate. Other ions cannot Reference Test cross the membrane. The Membrane contains a binding agent solution E M solution which assists target ion transport across the membrane. The membrane divides two solutions. One is the inner reference solution which contains a low concentration of the target ion species, and the other is an outer test solution containing a higher concentration of the target ion. Selective Binding membrane ligand 48 95  Glass pH Electrodes The glass pH electrode is used almost universally for pH measurements and can be found in a range of environments including hospitals, chemical plants, pH   log10 aH  and forensic laboratories. Its attraction lies in its rapid responses, wide pH range, functions well in physiological systems and is not affected by the presence of oxidising or reducing species. A typical pH electrode and pH meter are shown below. Typical pH meter Typical commercial Glass electrode 96 When a molecule or ion diffuses (which  a1  is facilitated via  G   RT ln   binding to a mobile ligand  a2  which is soluble in the membrane) from a region of high activity a1 to a region of low activity a2 the free energy change is given by the expression across. This diffusional process causes a charge imbalance across the membrane and a potential gradient or membrane potential a   RT ln  1   nFEM is developed across the membrane  a2  thickness which inhibits further diffusion of target ion. In the steady RT  a1  EM  ln   state, the free energy decrease due to nF  a2  diffusion is balanced by the free energy increase due to coulombic repulsion as outlined across. Membrane potential If the ion activity a2 in the reference solution is known then the membrane potential is logarithmically dependent on the activity of the target ion. 97 In an ISE device a local equilibrium is set up at the sensor/ test solution interface and a membrane potential is measured. Many ISE materials have been developed utilising both liquid and solid state membranes. The potentiometric response in all cases is determined by ion exchange reactions at the membrane/solution interface and ion conduction processes within the bulk membrane. The most important difficulty with ISE systems is the interference from ionic species in solution other than the target analyte. In general the response of an ISE to both the primary target ion and interferent ion species is given by the Nikolskii-Eisenmann equation. In the latter expression kij denotes the potentiometric selectivity  zi / z j  Primary & coefficient of the E  Ecell  S log ai   kij a 0  Interferent electrode against the j th  j  ion interfering ion. valencies Electrochemical Primary ion Mobility of ions activity Selectivity uj i and j Nernst slope S coefficient kij kij  K ij Equilibrium constant for process ui must be j(s) + i (m) -> j (m) + i (s) minimized 51 98 99 100 3D network of silicate groups. There are sufficient cations within the interstices of this structure to balance the negative charge of silicate groups. Singly charged cations such as sodium are mobile in the lattice and are responsible for electrical conductance within the membrane. 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118  ISE Applications. Ion-selective electrodes are used in a wide variety of applications for determining the concentrations of various ions in aqueous solutions. The following is a list of some of the main areas in which ISEs have been used. Pollution Monitoring: CN, F, S, Cl, NO3 etc., in effluents, and natural waters. Agriculture: NO3, Cl, NH4, K, Ca, I, CN in soils, plant material, fertilisers and feedstuffs. Food Processing: NO3, NO2 in meat preservatives. Salt content of meat, fish, dairy products, fruit juices, brewing solutions. F in drinking water and other drinks. Ca in dairy products and beer. K in fruit juices and wine making. Corrosive effect of NO3 in canned foods. Detergent Manufacture: Ca, Ba, F for studying effects on water quality. Paper Manufacture: S and Cl in pulping and recovery-cycle liquors. Explosives: F, Cl, NO3 in explosive materials and combustion products. Electroplating: F and Cl in etching baths; S in anodizing baths. 119  ISE Advantages. When compared to many other analytical They are particularly useful in techniques, Ion-Selective Electrodes are biological/medical applications because relatively inexpensive and simple to use they measure the activity of the ion and have an extremely wide range of directly, rather than the concentration. applications and wide concentration range. In applications where interfering ions, pH The most recent plastic-bodied all-solid- levels, or high concentrations are a state or gel-filled models are very robust problem, then many manufacturers can and durable and ideal for use in either supply a library of specialised field or laboratory environments. experimental methods and special Under the most favourable conditions, reagents to overcome many of these when measuring ions in relatively dilute difficulties. aqueous solutions and where interfering With careful use, frequent calibration, ions are not a problem, they can be used and an awareness of the limitations, they very rapidly and easily (e.g. simply dipping can achieve accuracy and precision levels in lakes or rivers, dangling from a bridge of ± 2 or 3% for some ions and thus or dragging behind a boat). compare favourably with analytical They are particularly useful in techniques which require far more applications where only an order of complex and expensive instrumentation. magnitude concentration is required, or it ISEs are one of the few techniques which is only necessary to know that a particular can measure both positive and negative ion is below a certain concentration level. ions. They are invaluable for the continuous They are unaffected by sample colour or monitoring of changes in concentration: turbidity. e.g. in potentiometric titrations or ISEs can be used in aqueous solutions monitoring the uptake of nutrients, or the over a wide temperature range. Crystal consumption of reagents. membranes can operate in the range 0°C to 80°C and plastic membranes from 0°C to 50°C. 120 121 122 123 124 125 126 127 128 129 3 generations of enzyme biosensor electrodes. 1st generation: Charge shuttling via O2/H2O2. 2nd generation : Synthetic electron shuttles (redox mediators) used. 3rd generation : No mediator used , enzyme wiring. 83 130 Enzyme communication with electrodes. 84 131  Homogeneous mediation using substituted ferrocene. Mediator redox couple reasonably insoluble in aqueous solution, hence is located close to electrode. G  FAD GL  FADH 2 FADH 2  2 FeCp 2 FAD  2 FeCp 2  2 H   2 FeCp 2 2 FeCp 2  2e   substrate n e‐ GOx(FADH2) G P Q GL GOx(FAD) product Electrode Solution P,Q represents reduced and oxidised forms of redox mediator (ferrocene and ferricinium); G = glucose, GL = gluconolactone. GOx (FADH2) = reduced form of glucose oxidase; GOx(FAD) = 85 oxidised form of glucose oxidase. 132  SAM SAM Au surface Enzyme S MOX P MRED 5e-5 5e-5 5e-5 1mM ferrocene carboxylic acid, 0.25M 1mM ferrocene carboxylic glucose. Scan rate 0.1Vs-1. 4e-5 4e-5 4e-5 acid, 0.25M glucose. Scan rate 0.1Vs-1. 3e-5 3e-5 3e-5 Current (A) Au/SAM-NH2/GOx Current (A) 2e-5 Au/SAM-CO2H/GOx 2e-5 modified electrode 2e-5 modified electrode 1e-5 1e-5 1e-5 Au/GOx Au/GOx 0 modified electrode 0 0 modified electrode -1e-5 -1e-5 -1e-5 0 200 400 600 800 0 200 400 600 800 Potential (mV) Potential (mV) 86 Glucose bio-sensing via immobilized redox enzyme using self assembled monolayer (SAM) 133  Analysis of immobilized enzyme kinetics. 25 b Enzyme/substrate kinetics well described By Michaelis-Menten model. 20 Current ( A) 15 a 10 5 0 System Vmax /A KM/mM 0 10 20 30 Conc (mM) Au/GOx 19 1.5 Steady state current vs concentration data (a) Au/glucose oxidase, Au/MUA/GO 23 0.8 (b) Au/SAM-CO2H/glucose x oxidase. Data fit to simple Michaelis- Menten kinetic equation iSS v c kec using NLLS fitting program. f   max  c  nFA K M  c K M  c 87 134 Amperometric glucose detection via traditional oxygen mediation. Eappl = + 0.8 V gluconolactone glucose Ping pong mechanism for the oxidation of glucose by oxygen catalysed by glucose GOx GOx oxidase. FADH2 FAD+ Subject to considerable Amperometric detection interference by via oxidation of H2O2 O2 oxidizable substances. Hydrogen peroxide. Detector electrode 88 135  Au/SWCNT/Nafion/GOx modified electrode : amperometric glucose detection at positive potentials. Amperometric detection of H2O2 Produced at detector electrode. 40 30 70 ∆Ia / μA 60 Edet 10 mM 50 20 4 mM CURRENT / μA 40 0 mM 30 10 20 [GLUCOSE ] / mM 10 0 0 2 4 6 8 10 12 0 -10 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Amperometric detection POTENTIAL / V (vs. Ag / AgCl) potential fixed at + 0.8 V 136  Amperometric glucose detection via traditional oxygen mediation. Linear Region 0.3-2 mM R2=0.998 (N=9) Detection limit (S/N=3) Detection potential E = 0.8 V = 0.1 mM Hydrogen peroxide oxidation Response time = 17s at SWCNT surface. Sensitivity 0.17 A/mM 0.8 8 6 Lineweaver-Burk 0.6 Plot (i/A)-1 i/A 0.4 4 0.2 Batch amperometry data 2 0.0 0 0 2 4 6 8 10 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 [glucose]/mM ([glucose]/mM)-1 90 137  Davis et al. J. Am. Chem. Soc., 2002, 124, 12664-12665. 91 138  Sensor response to ascorbate injections An example of an EC process is the electrochemical oxidation of ascorbic acid (vitamin C) and its subsequent reaction with water (the solvent) to yield electrochemically inactive dehydroascorbic acid. current direct measure of ascorbate concentration detection potential polymer film electronically conducting at this potential 92 139 140  Further aspects of voltammetry. Depending on the shape of the potential/time perturbation signal and on the mode of the analyte transport, voltammetric techniques can be classified as Linear potential sweep & cyclic voltammetry (LPSV, CV) Potential Step Methods (chrono-amperometry,) Hydrodynamic voltammetry (rotating disc, rotating ring/disc voltammetry, flow injection analysis, wall/jet voltammetry) Stripping Voltammetry (ASV,CSV). Voltammetric techniques are distinct analytical tools for the determination of many inorganic and organic substances which can be reduced or oxidised at indicator electrodes at trace levels. the simultaneous determination of a number of analytes is possible using voltammetric techniques. Usually the volume of the sample solution used is large and the size of the indicator electrode is small, and the measurement time is short, so the bulk concentration of the analyte does not change appreciably during the analysis. Thus repeated measurements can be performed in the same solution. The selectivity of the technique is moderate and can be largely enhanced by the combination of a separation step such as liquid chromatography with electrochemical detection (LCEC). 141 Some practical considerations in voltammetry. Choice of working electrode depends on specific analytical situation. Mercury working electrodes (DME, SMDE, MFE) useful for trace metal analysis in which reduction of metal ions (e.g. Cd2+, Zn2+ etc) occurs at Hg surface. Hg exhibits a large overpotential for H2 gas evolution, and so the potential window available in aqueous solution for metal ion reductions is quite large (-2.7 to ca. + 0.3 V vs SCE). Hg electrodes not useful for oxidations because Hg oxidises at low potentials via 2Hg -> Hg22+ + 2e-. Also there is concern over toxicity of Hg. Instead solid electrodes such as Pt, Au and C (graphite, glassy carbon) are employed for oxidative analysis. These electrodes are electronically conductive and the surface can be readily renewed. The potential window available in analysis will be determined by the solvent adopted as well as on the supporting electrolyte used. The upper potential limit in aqueous media is ca. + 1.5 V (vs SCE) and is set by the onset of O2 evolution. 142  Charging and Faradaic currents. Voltammetric measurements rely CDL on the examination of ET i processes at solid/liquid i  i i F C C RS interfaces. i Analytical uses of voltammetry rely on measuring current flow as a function of analyte concentration. However applying a potential iF programme to an electrode necessitates Electrode RCT Solution the charging of the solid/liquid interface up to the new applied potential. This causes a current to flow which is independent of the concentration of analyte. Hence the observed current is the sum of two contributions, the charging current and the The objective is to maximise iF and Faradaic current. minimise iC. Note that iC is always The Faradaic current iF is present and has a constant residual value. of primary analytical interest As the concentration of analyte decreases since this quantity is iF decreases and will approach the value of directly proportional to the residual value of iC. This places a lower the bulk concentration of the limit on analyte detection and hence on the analyte species of interest. use of voltammetry as an analytical application. 96 143 Charging current and Faradaic current contribution can be computed for various transient electrochemical techniques i EF iF  iC i  iF E  E f  Ei iF (t ) Ei iC (t ) E  t  nFAD1/ 2 c  t iC  exp   iF  Take reading RS  RS C DL   1/ 2t 1/ 2 when iC is small. Potential step technique. A similar quantitative analysis can be done for DC polarography and linear potential sweep voltammetry. 97 144  Polarography : voltammetry at mercury electrodes. Polarography uses mercury droplet electrode that is regularly renewed during analysis. Applications: Metal ions (especially heavy metal pollutants) - high sensitivity. Organic species able to be oxidized or reduced at electrodes: quinones, reducing sugars and derivatives, thiol and disulphide compounds, oxidation cofactors (coenzymes etc), vitamins, pharmaceuticals. Alternative when spectroscopic methods fail. 98 145 Jaroslav Heyrovský was the inventor of the polarographic method, and the father of electroanalytical chemistry, for which he was the recipient of the Nobel Prize. His contribution to electroanalytical chemistry can not be overestimated. All modern voltammetric methods used now in electroanalytical chemistry originate from polarography. Swedish king Gustav Adolf VI awards the Nobel Prize to Heyrovský in Stockholm on 10.12.1959 99 146 On February 10, 1922, the "polarograph" was born as Heyrovský recorded the current-voltage curve for a solution of 1 M NaOH. Heyrovský correctly interpreted the current increase between -1.9 and -2.0 V as being due to deposition of Na+ ions, forming an amalgam. Typical polarographic curves (dependence of current I on the voltage E applied to the electrodes. The small oscillations indicate the slow dropping of mercury): lower curve - the supporting solution of ammonium chloride and hydroxide containing small amounts of cadmium, zinc and manganese, upper curve - the same after addition of small amount of thallium. 147  Cd2+ ion reduction i/A [Cd2+] ‐ E/V Cd2+ + 2e-  Cd(Hg) 148 149 Differential pulse voltammetry (DPV) 150  Mercury working electrodes Most common waveforms used in voltammetry. 151  Polarography : voltammetry using Hg as a working electrode. Spherical Hg drop formed at the end of a glass capillary. This is used as a working electrode. Linear potential ramp used as perturbation. Resulting current response examined as function of potential. Using the dropping Electrode schematic. mercury electrode (DME) Typical or the static mercury current/voltage drop electrode (SMDE) polarograms. the drop size and drop lifetime can be accurately controlled. Each data point measured at a new Hg drop ensuring constant surface renewal. 105 152   E New drop t Faradaic current iF Apply sequence of A  F (t ) Drop lifetime  little potential steps Charging current iC to a series of growing t  * Hg drops and measure Drop fall current flowing as a function of New drop drop lifetime. Resulting current response given by the Cottrell equation. iD  nFA(t )c  D1/ 2 1/ 2t 1/ 2 The drop area is a function of time, Net current flowing Best to sample and also the diffusion layer thickness consists of Faradaic current at a time * gets thinner as a result of the current iF of just before the expanding drop. Taking these effects analytical significance end of the drop life into account gives the Ilkovic equation. and a charging current when iF is maximum iC arising from the and iC is minimum  7  3   2/3   1/ 2  2 / 3 1/ 6 electrical properties for optimum i D  4 F  nD c m t of electrode/solution sensitivity.  3  4 Hg    interface. 106 153  i  ic  iD  Kt 1/ 3  K ' t1/ 6 Ilkovich equation Drop time Charging current iD  607 nD1/ 2 m 2 / 3t1/ 6 c  ic  0.00567( E  E pzc )CDL m 2 / 3t 1/ 3 Mass flow rate (gs‐1) 107 154 Heyrovsky-Ilkovich equation. RT iD  i  E  E  E1/ 2  ln nF iE E Slope = RT/nF Can evaluate n and E1/2 via Heyrovsky Ilkovich plot. Ln (iD-i)/i 108 155  Differential pulse voltammetry. Charging current Ordinary voltammetry has a LOD of ca. 1 M. To obtain greater contribution sensitivity (LOD ca. 10-8M) we can develop a more sophisticated minimized : potential waveform such as that used in hence greater differential pulse polarography. sensitivity. Second current E First current sample I sample 5-100 ms Drop birth E = 10-100 mV IP Drop fall t EP  E1/ 2  E EP E 0.5-4 s 2 Apply series of potential pulses of A plot of I vs E is peaked, constant amplitude E with respect the height of which is proportional to a linearly varying base potential E. to the bulk concentration of We plot I = I() – I(’) where ’ denotes analyte. nFAD1/ 2 c  1    the time immediately before application I P  1/ 2   of pulse and  is the time, late in the pulse     1/ 2 1    just before drop is dislodged, as a function  nFE    exp  Of base potential E.  2 RT  109 156 Stripping Voltammetry. Trace and ultratrace determination of analyte species in complex matrices of environmental, clinical or industrial samples pose a significant challenge. Resolution of these analytical problems is often obtained by use of preconcentration techniques. One such technique is stripping voltammetry. This is a two stage technique. 1. Preconcentration or accumulation step. Here the analyte species is collected onto/into the working electrode 2. Measurement step : here a potential waveform is applied to the electrode to remove (strip) the accumulated analyte. SV is the most sensitive of the available electroanalytical techniques and very low detection limits are possible