Amperometric method

The amperometric approach to endpoint detection provides considerable latitude in the selection of the best conditions for the most specific and sensitive endpoint response. Furthermore, the response signal is directly proportional to the concentration of the observed species, whereas potentiometric responses are a logarithmic function of the concentration. Another attractive feature of amperometric endpoint detection is that the most important data are obtained prior to and after the equivalence point, whereas in potentiometric titrations the most important data occur at the equivalence point, which is the most unstable condition of the titration. With amperometric titrations an extrapolation of the straight-line portion of the curve, either prior to or after the equivalence point, to an intercept will provide an accurate measure of the equivalence point. 

To have as straight a line as possible, it is necessary to apply a dilution correction to the observed current. This can be simplified by using an extremely high concentration of titrant relative to the concentration of the species being titrated. 

The entire subject of amperometric titrations has been reviewed in a number of monographs on electrochemistry 4-6 a definitive work on this subject also has been published.7 Because the amperometric titration method does not depend on one or more reversible couples associated with the titration reaction, it permits electrochemical detection of the endpoint for a number of systems that are not amenable to potentiometric detection. All that is required is that electrode conditions be adjusted such that either a titrant, a reactant, or a product from the reaction gives a polarographic diffusion current. 

SECM amperometric methods are based on the measurement of electrode currents (tip and substrate, (p and ig, respectively) as a function of various parameters, including tip-substrate distance (d) and tip or substrate potentials (Ej or Eg). Irrespective of the system studied (e.g., an electrode, an inert surface, a catalyst, or a living organism), the probe tip is a necessary component to perform any SECM experiment. In all cases, the amperometric tip is a UME that can be positioned in close proximity to another surface. There are two amperometric operation modes feedback and generation/collection modes. The preparation and characterization of commonly used amperometric tips of different geometries as well as their operation modes will be described in this section. 

The first question that one may ask is what is the reason for carrying out an approach curve The technical aspect of the answer is (a) to evaluate the geometric characteristics of an SECM UME tip, and (b) to know how close the tip can be positioned with respect to the substrate surface for further experiments. Whatever the reason, the procedure is the same and is described in detail below. 

An SECM approach curve (i.e., tip current as a function of distance) allows one to get a very precise value of the tip-substrate distance when the electrode reactions are diffusion controlled. This ability is a consequence of the strong effect that a surface in close proximity to the UME ( 5 radii) has on the diffusion-controlled current. Both positive and negative feedback effects have been theoretically treated and it is possible to correlate the experimental approach curves to analytical equations to determine very accurately the position of the tip with respect to the substrate surface. Furthermore, approach curves recorded over a conducting substrate provide an additional measurement of the effective radius of the UME tip, while those recorded over insulators provide information about the effective RG of the tip. Thus, this type of experiment is very useful for the characterization of SECM tips. 

SECM theory, including amperomettic feedback mode, has been recently reviewed in Chapter 5 of reference (1). Recent progress in the theoretical description of SECM include 

Different strategies to measure approach curves, their theoretical treatment, and the ways to estimate the tip dimensions when using tips having other geometries have been published for hemispherical (21, 38), conical (27), and ring or ring-disk shaped tips (39, 40). 

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These titrations can be done manually using a starch indicator for end point detection or more accurately by amperometric methods. 

When applied on a macro scale — samples of 1 - 5 millimoles — generation rates of 100-500 milliamps are required parasitic currents may be induced in the indicator electrodes at currents in excess of about 10-20 mA consequently precise location of the equivalence point by amperometric methods is not trustworthy. 

An example of amperometric methods used for analytical purposes is the sensor proposed in 1953 by Leland C. Clark, Jr. for determining the concentration of dissolved molecular oxygen in aqueous solutions (chiefly biological fluids). A schematic of the sensor is shown in Fig. 23.1. A cylindrical cap (1) houses the platinum or other indicator electrode (2), the cylindrical auxiliary electrode (3), and an electrolyte (e.g., KCl) solution (4). The internal solution is separated by the polymer... 

Another example of an amperometric method is the method of polarography, which was widely used for analytical purposes some decades ago and which is described in detail in the following section. 

Again for the titration of Ce(IV) with Fe(II) we shall now consider constant-potential amperometry at one Pt indicator electrode and do so on the basis of the voltammetric curves in Fig. 3.71. One can make a choice from three potentials eu e2 and e3, where the curves are virtually horizontal. Fig. 3.74 shows the current changes concerned during titration at e1 there is no deflection at all as it concerns Fe(III) and Fe(II) only at e2 and e3 there is a deflection at A = 1 but only to an extent determined by the ratio of the it values of the Ce and Fe redox couples. The establishment of the deflection point is easiest at e2 as it simply agrees with the intersection with the zero-current abscissa as being the equivalence point in fact, no deflection is needed in order to determine this intersection point, but if there is a deflection, the amperometric method is not useful compared with the non-faradaic potentiometric titration unless the concentration of analyte is too low. 

Of much greater importance, however, is the amperometric method for precipitation and complex-formation titrations. Here the advantages are as follows ... 

In the indirect amperometric method [560], saturated uranyl zinc acetate solution is added to the sample containing 0.1-10 mg sodium. The solution is heated for 30 minutes at 100 °C to complete precipitation. The solution is filtered and the precipitate washed several times with 2 ml of the reagent and then five times with 99% ethanol saturated with sodium uranyl zinc acetate. The precipitate is dissolved and diluted to a known volume. To an aliquot containing up to 1.7 mg zinc, 1M tartaric acid (2-3 ml) and 3 M ammonium acetate (8-10 ml) are added and the pH adjusted to 7.5-8.0 with 2 M aqueous ammonia. The solution is diluted to 25 ml and an equal volume of ethanol added. It is titrated amperometrically with 0.01 M K4Fe(CN)6 using a platinum electrode. Uranium does not interfere with the determination of sodium. 

Electroanalytical methods involve the measurement of either the electrical current flowing between a pair of electrodes immersed in the solution tested (voltammetric and amperometric methods) or an electrical potential developed between a pair of electrodes immersed in the solution tested (potentio-metric methods). In either case, the measured parameter (current or potential) is proportional to the concentration of analyte. See Figure 14.1. 

A galvanic cell is one in which this current flows (and the redox reaction proceeds) spontaneously because of the strong tendency for the chemical species involved to give and take electrons. An electrolytic cell is one in which the current is not a spontaneous current, but rather is the result of incorporating an external power source, such as a battery, in the circuit to drive the reaction in one direction or the other. Potentiometric methods involve galvanic cells, and voltammetric and amperometric methods involve electrolytic cells. 

Differentiate between potentiometric methods and voltammetric and amperometric methods. 

An amperometric method or amperometry is concerned with the measurement of current under a constant applied voltage and under such experimental parameters the concentration of the analyte exclusively determines the quantum and magnitude of the current. Hence, these measurements may be employed effectively to record the alteration in concentration of an ion in question in the course of a titration, and ultimately the end-point is established. This specific process is commonly referred to as amperometric method or amperometry. 

Salient Features of Amperometric Methods The various salient features of amperometric titrations are enumerated below ... 

The most commonly obtained various kinds of curves encountered in amperometric methods are illustrated in Fig. 17.1 (a) through (d) and each of them shall be discussed briefly as follows ... 

Figure 17.1 (c) It represents an amperometric method wherein the solute as well as the titrating reagent afford diffusion currents and give rise to a sharp V-shaped curve. The end-point may be obtained by extrapolation of the lower-end of the V-shaped portion of the curve as depicted in the above Figure. 

In addition to the above four types of amperometric methods cited, there also exist a plethora of titrations involving neutralization and complex ion formation that have been accomplished successfully, for... 

Amperometric method for the study of precipitation reactions, e.g., salicylaldoxime (or salicylaldehyde oxime), dimethylglyoxime, have been used for such type of studies. 

Discuss the four typical amperometric titration curves obtained in amperometric method of analysis and examine them critically with appropriate examples. 

Part—III exclusively treats Electrochemical Methods invariably and extensively used in the analysis of pharmaceutical substances in the Official Compendia. Two important methods, namely potentiometric methods (Chapter 16) deal with various types of reference electrodes and indicator electrodes, automatic titrator besides typical examples of nitrazepam, allopurinol and clonidine hydrochloride. Amperometric methods (Chapter 17) comprise of titrations involving dropping-mercury electrode, rotating—platinum electrode and twin-polarized microelectrodes (i.e., dead-stop-end-point method). 

Amperometric methods involve the measurement of current t-iat results trom the application ot a fixed voltngi. ... 

A test system that involves extraeting dichlorobenzidine or its metabolite (monoacetyldichlorobenzidine) from urine and reaeting it with Chloramine-T has been developed to screen for dichlorobenzidine exposure in workers (Hatfield et al. 1982). An amperometric method has been developed for the detection of 3,3 -dichlorobenzidine in the urine as a quantitative assay for the biological monitoring of people oeeupationally exposed to this substance or a metabolic precursor such as certain pigments. This method is based on the possibility of two electron oxidation at carbon electrodes by aromatic diamines (Trippel-Sehulte et al. 1986). 

It is now clear that electrochemical measurements can often have significant advantages over the classical spectroscopic approaches. Amperometry can be more specific therefore, lower detection limits are often feasible. Because electrochemical detectors do not require optical carriers, they can be much less expensive than UV absorption or fluorescence detectors. This is especially true when one considers that electrochemical detectors are inherently tunable without the need for such things as monochrometers or filters. On the other hand, there can be significant problems with reliability, and, more often than not, there is a lack of acceptance by chemists weaned on Beer s law. Amperometric methods in biochemistry are just beginning to be commercialized, and it is now almost certain that they will come into widespread use. 

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