Electroanalytical technique

Electroanalytical techniques, as seen from the liquid chromatographer s view-2 3 

It is therefore necessary to elucidate some basic principles and methods used, 

In voltammetry, the potential is controlled and the dependence of the current on it is observed. From the resulting plot (the voltaimnogram), both qualitative and quantitative information can be obtained. When voltammetry is carried out with a dropping mercury electrode it is called polarography. 

When the potential is controlled at a constant d.c. value and the current is simply measured (thus fore-going the qualitative information), the technique is termed amperometry, irrespective of whether a dropping mercury electrode or another 

We measure the current through the interface of the working electrode as a function of the potential difference at it. This current is either a displacement current or a real current. The displacement current, which is an undesirable effect in nearly all electroanalytical work, can be described as a charging of a capacitor, located at the interface, and one speaks about the capacitive current. The other, more important, part is due to electrochemical processes, in which ions or electrons are transferred from the electrode to the solution or vice versa. As these processes are governed by Faraday s law, one speaks of faradaic currents. Faraday s law states that the electrochemical conversion of m moles yields an amount of electricity of mnP coulombs, where n is the number of electrons released or taken up in the reaction and F the Faraday constant, with a value of about 10 coulombs/mole. This high value of the electrochemical equivalent is, of course, very attractive from the analytical point of view. The measurement of picocoulombs of electricity is extremely simple nowadays and detection limits of 10 mole could be expected from this simple calculation. 

Suppose we dip Cu and Pt electrodes into a solution of Cu2+ and force electric current through to deposit copper metal at the cathode and to liberate 02 at the anode. 

Electiolytic production of aluminum by the Hali-Heroulf process consumes -5% of fhe electrical output of the United States Al34 in a molten solution of Al203 and cryolite (Na3AIF6) is reduced to Al at the cathode of a cell that typically draws 250 kA. This process was invented by Charles Hall in 1886 when he was 22 years old, just after graduating form Oberlin College.2 

Charles Martin Hall. [Photo courtesy of Alcoa.] 

If a current I flows for a time t, the charge q passing any point in the circuit is 

Approximately 7% of electric power in the United States goes into electrolytic chemical production. The electrolysis apparatus pictured here consists of a sheet of A1 foil taped or cemented to a wood surface. Any size will work, but an area about 15 cm on a side is convenient for a classroom demonstration. Tape to the metal foil (at one edge only) a sandwich consisting of filter paper, 

Reference electrodes are inherently unstable. These electrodes drift, leak, become foul or plugged, and frequendy need to be replaced. 

Electrochemical cells may be used in either active or passive modes, depending on whether or not a signal, typically a current or voltage, must be actively appHed to the cell in order to evoke an analytically usehil response. Electroanalytical techniques have also been divided into two broad categories, static and dynamic, depending on whether or not current dows in the external circuit (1). In the static case, the system is assumed to be at equilibrium. The term dynamic indicates that the system has been disturbed and is not at equilibrium when the measurement is made. These definitions are often inappropriate because active measurements can be made that hardly disturb the system and passive measurements can be made on systems that are far from equilibrium. The terms static and dynamic also imply some sort of artificial time constraints on the measurement. Active and passive are terms that nonelectrochemists seem to understand more readily than static and dynamic. 

The question of what happens when an electrical signal is appHed to an electrochemical ceU needs to be answered with respect to the three components of the ceU the working electrode, the reference electrode, and the sample itself. 

rging Current. In most cases, appHcation of a voltage to an electrode is iatended to produce an analytically useful current that depends solely on the concentration of the analyte. Unfortunately, current flows even ia the complete absence of the analyte. Thus, the current may have nothing to do with the electroactive species ia the sample. This charging current must be circumvented or otherwise compensated. 

Kirk-Othmer Encyclopedia of Chemical Technology (4th Edition) 

Principles and Characteristics A substantial percentage of chemical analyses are based on electrochemistry, although this is less evident for polymer/additive analysis. In its application to analytical chemistry, electrochemistry involves the measurement of some electrical property in relation to the concentration of a particular chemical species. The electrical properties that are most commonly measured are potential or voltage, current, resistance or conductance charge or capacity, or combinations of these. Often, a material conversion is involved and therefore so are separation processes, which take place when electrons participate on the surface of electrodes, such as in polarography. Electrochemical analysis also comprises currentless methods, such as potentiometry, including the use of ion-selective electrodes. 

Analytical methods based upon oxidation/reduction reactions include oxidation/reduction titrimetry, potentiometry, coulometry, electrogravimetry and voltammetry. Faradaic oxidation/reduction equilibria are conveniently studied by measuring the potentials of electrochemical cells in which the two half-reactions making up the equilibrium are participants. Electrochemical cells, which are galvanic or electrolytic, reversible or irreversible, consist of two conductors called electrodes, each of which is immersed in an electrolyte solution. In most of the cells, the two electrodes are different and must be separated (by a salt bridge) to avoid direct reaction between the reactants. 

The main electroanalytical techniques are electrogravimetry, potentiometry (including potentiometric titrations), conductometry, voltammetry/polarography, coulometry and electrochemical detection. Some electroanalytical techniques have become very widely accepted others, such as polarography/voltammetry, less so. Table 8.74 compares the main electroanalytical methods. 

For polymer/additive analysis, electrogravimetry, potentiometry, conductometry and voltammetry have never played a major role. Because of many complications, which can arise by the use of conductometry for complicated matrices (such as most polymeric compounds), the technique is not extensively applied in this field. Conductometric measurements are mostly 

Parameter Potentiometry Voltammetric methods Coulometric methods 

Electrochemical methods are used to quantify a broad range of different molecules, including ions, gases, metabolites and drugs. 

The basis of all electrochemical analysis is the transfer of electrons from one atom or molecule to another atom or molecule in an obligately coupled oxidation-reduction reaction (a redox reaction). 

It is convenient to separate such redox reactions into two half-reactions and, by convention, each is written as  

You should note that the half-reaction is reversible by applying suitable conditions, reduction or oxidation can take place. As an example, a simple redox reaction occurs when metallic zinc (Zn) is placed in a solution containing copper ions (Cu- ), as follows  

The half-reactions are (i) Cu + -I- 2e —s- Cu and (ii) Zn + -1- 2e Zn. The oxidizing power of (i) is greater than that of (ii), so in a coupled system, the latter half-reaction proceeds in the opposite direction to that shown above, i.e. as Zn — 2e — Zn. When Zn and Cu electrodes are placed in separate solutions containing their ions, and connected electrically (Fig. 34.1), electrons will flow from the Zn electrode to Cu-+ via the Cu electrode owing to the difference in oxidizing power of the two half-reactions. 

Oxidation - loss of electrons by an atom or molecule (or gain of O atoms, loss of H atoms, increase in positive charge, or decrease in negative charge). 

Galvanic cell - an electrochemical cell in which reactions occur spontaneously at the electrodes when they are connected externally by a conductor, producing electrical energy. 

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Wang, J. Electroanalytical Techniques in Clinical Chemistry and Eaboratory Medicine. VCH New York, 1998. 

Ion Selective Electrodes Technique. Ion selective (ISE) methods, based on a direct potentiometric technique (7) (see Electroanalytical techniques), are routinely used in clinical chemistry to measure pH, sodium, potassium, carbon dioxide, calcium, lithium, and chloride levels in biological fluids. 

Active electrochemical techniques are not confined to pulse and linear sweep waveforms, which are considered large ampHtude methods. A-C voltammetry, considered a small ampHtude method because an alternating voltage <10 mV is appHed to actively couple through the double-layer capacitance, can also be used (15). An excellent source of additional information concerning active electroanalytical techniques can be found in References 16—18. Reference 18, although directed toward clinical chemistry and medicine, also contains an excellent review of electroanalytical techniques (see also... 

Perhaps the most precise, reHable, accurate, convenient, selective, inexpensive, and commercially successful electroanalytical techniques are the passive techniques, which include only potentiometry and use of ion-selective electrodes, either direcdy or in potentiometric titrations. Whereas these techniques receive only cursory or no treatment in electrochemistry textbooks, the subject is regularly reviewed and treated (19—22). Reference 22 is especially recommended for novices in the field. Additionally, there is a journal, Ion-Selective Electrode Reviews, devoted solely to the use of ion-selective electrodes. 

J. Wang, Electroanalytical Techniques in Clinical Chemistry andEaboratoy Medicine, VCH, New York, 1988. 

Polarography, a well known electroanalytical technique, is currently being used to detn the purity of Tetracene as well as other compds contg nitrate and nitroso groups (Ref 38). 

Select an electroanalytical technique most suitable for detecting trace levels of nickel in ground water. Justify this choice. 

While electroanalytical techniques are inherently quite sensitive, the resolution of a mixture of electroactive compounds is very difficult. Practical considerations limit the usable potential window to no more than 3 V and typically around 1.5 V. This is because at more extreme potentials the medium or the electrode itself begin to oxidize or reduce. In addition, the electrochemical response of compounds as a function of applied potential is fairly broad so that at least a 200-400 mV difference in half-wave potentials is required for adequate resolution. This typically limits electrochemical resolution of mixtures to no more than three or four electroactive compounds. 

Monitoring enzyme catalyzed reactions by voltammetry and amperometry is an extremely active area of bioelectrochemical interest. Whereas liquid chromatography provides selectivity, the use of enzymes to generate electroactive products provides specificity to electroanalytical techniques. In essence, enzymes are used as a derivatiz-ing agent to convert a nonelectroactive species into an electroactive species. Alternatively, electrochemistry has been used as a sensitive method to follow enzymatic reactions and to determine enzyme activity. Enzyme-linked immunoassays with electrochemical detection have been reported to provide even greater specificity and sensitivity than other enzyme linked electrochemical techniques. 

Anodic stripping voltammetry (ASV) has been used extensively for the determination of heavy metals in samples of biological origin, such as lead in blood. ASV has the lowest detection limit of the commonly used electroanalytical techniques. Analyte concentrations as low as 10 M have been determined. Figure 16 illustrates ASV for the determination of Pb at a mercury electrode. The technique consists of two steps. The potential of the electrode is first held at a negative value for several minutes to concentrate some of the Pb " from the solution into the mercury electrode as Pb. The electrode process is... 

In the first two contributions electroanalytical techniques are described for application in bioanalysis and medicine. The increasing interest in this field is mainly due to the excellent selectivities and detection limits. In addition, the possibilities of miniaturization allow the development of in vivo analysis. 

Electrochemical methods such as potentiometry allow analyses up to p,gL quantities, or, with methods such as voltammetry, they extend into the micro-trace range. Table 8.74 compares potentiometry to other electroanalytical techniques. Potentiometry and ion-selective electrodes are described in various books [476-480],... 

Before treating specific faradaic electroanalytical techniques in detail, we shall consider the theory of electrolysis more generally and along two different lines, viz., (a) a pragmatic, quasi-static treatment, based on the establishment of reversible electrode processes, which thermodynamically find expression in the Nernst equation, and (b) a kinetic, more dynamic treatment, starting from passage of a current, so that both reversible and non-reversible processes are taken into account. 

Electrogravimetry, which is the oldest electroanalytical technique, involves the plating of a metal onto one electrode of an electrolysis cell and weighing the deposit. Conditions are controlled so as to produce a uniformly smooth and adherent deposit in as short a time as possible. In practice, solutions are usually stirred and heated and the metal is often complexed to improve the quality of the deposit. The simplest and most rapid procedures are those in which a fixed applied potential or a constant cell current is employed, but in both cases selectivity is poor and they are generally used when there are... 

Potentiometry is the most widely used electroanalytical technique. It involves the measurement of the potential of a galvanic cell, usually under conditions of zero current, for which purpose potentiometers are used. Measurements may be direct whereby the response of samples and standards are compared, or the change in cell potential during a titration can be monitored. 

Active composting, 25 874 Active dry yeast (ADY), 26 460-461 Active electroanalytical techniques, 9 568-581... 

See also Electroanalytical techniques Electroanalytical cells, 9 567 Electroanalytical techniques, 9 567-590 active, 9 568-581 economic aspects, 9 588 passive, 9 581-586 static and dynamic measurements, 9 586-588... 

Passive dielectric polymers, 22 718 Passive electroanalytical techniques, 9 581-586... 

A thorough discussion of electroanalytical techniques, including polarography, voltammetry, and amper-ometry, is given in Chapter 14. An understanding of these would be useful for understanding the amperometric HPLC detector. 

Electroanalytical techniques are an extension of classical oxidation-reduction chemistry, and indeed oxidation and reduction processes occur at the surface of or within the two electrodes, oxidation at one and reduction at the other. Electrons are consumed by the reduction process at one electrode and generated by the oxidation process at the other. The electrode at which oxidation occurs is termed the anode. The electrode at which reduction occurs is termed the cathode. The complete system, with the anode connected to the cathode via an external conductor, is often called a cell. The individual oxidation and reduction reactions are called half-reactions. The individual electrodes with their half-reactions are called half-cells. As we shall see in this chapter, the half-cells are often in separate containers (mostly to prevent contamination) and are themselves often referred to as electrodes because they are housed in portable glass or plastic tubes. In any case, there must be contact between the half-cells to facilitate ionic diffusion. This contact is called the salt bridge and may take the form of an inverted U-shaped tube filled with an electrolyte solution, as shown in Figure 14.2, or, in most cases, a small fibrous plug at the tip of the portable unit, as we will see later in this chapter. 

As mentioned previously, electroanalytical techniques that measure or monitor electrode potential utilize the galvanic cell concept and come under the general heading of potentiometry. Examples include pH electrodes, ion-selective electrodes, and potentiometric titrations, each of which will be described in this section. In these techniques, a pair of electrodes are immersed, the potential (voltage) of one of the electrodes is measured relative to the other, and the concentration of an analyte in the solution into which the electrodes are dipped is determined. One of the immersed electrodes is called the indicator electrode and the other is called the reference electrode. Often, these two electrodes are housed together in one probe. Such a probe is called a combination electrode. 

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