Constant-current potentiometry

With a low constant current -1 (see Fig. 3.71) one obtains the same type of curve but its position is slightly higher and the potential falls just beyond the equivalence point (see Fig. 3.72, anodic curve -1). In order to minimize the aforementioned deviations from the equivalence point, I should be taken as low as possible. Now, it will be clear that the zero current line (abscissa) in Fig. 3.71 yields the well known non-faradaic potentiometric titration curve (B B in Fig. 2.22) with the correct equivalence point at 1.107 V this means that, when two electroactive redox systems are involved, there is no real need for constant-current potentiometry, whereas this technique becomes of major advantage... 

Conclusions. A sampling and analytical method for two chlorinated isocyanuric acids, NaDCC and TCCA, in air has been described. Briefly, these acids can be collected from air with DM-5000 (PVC copolymer) filters. The filter samples are extracted with water and titrated against sodium thiosulfate using constant-current potentiometry. The titration method will neither separate or distinguish NaDCC in the presence of TCCA or the reverse. The identity of either compound must be known in the workplace environment along with the identities of any other interfering... 

Constant-current potentiometry seems to have the advantage of error-free operation, as compared to the conventional amperometric endpoint detection in the case of chlorine determinations in water. Barbolani et al. [7] employed 1 jxA DC between two identical platinum electrodes and measured the potential difference between them to detect the endpoint of the titrations. Chlorine was titrated with phenylarsine oxide at pH 7 chlorine and chlorine dioxide were titrated analogously in the presence of iodide ions, and all three components were titrated at pH 2 in the presence of iodide. The method was used for water samples (taken from a water purification plant) containing both chlorine and chlorine dioxide. 

The constant-current potentiometry is another endpoint detection technique that can be used in the titrations of very dilute chloride sample [22-24], In this case, two silver metal or chloride-coated silver indicator electrodes are used dipping into the intensively stirred titration vessel. A well-stabilized constant, low intensity current is forced between the two electrodes and the potential difference is measured between them. The titration endpoint is very sharp in this case even at very low concentration. 

Hendrikse J, Olthuis W, Bergveld P (1999) The EMOSFET as an oxygen sensor constant current potentiometry. Sensors Actuators B Chem 59(1) 35-41. doi 10.1016/S0925-4005(99) 00195-1... 

The reader familiar with controlled-potential methodology will have no trouble understanding a controlled-current apparatus. Figure 6.18 illustrates classical approaches to two- and three-electrode constant-current chronopotentiometric experiments (see Chap. 4). The simplicity of these circuits was for many years an attractive feature of chronopotentiometry. Improvements in potentiostats have been largely responsible for a decline in the popularity of chronopot in recent years. Nevertheless, constant-current experiments are even more important with respect to coulometric titrations and stripping potentiometry (Chap. 24). 

The simulation of other electrochemical experiments will require different electrode boundary conditions. The simulation of potential-step Nernstian behavior will require that the ratio of reactant and product concentrations at the electrode surface be a fixed function of electrode potential. In the simulation of voltammetry, this ratio is no longer fixed it is a function of time. Chrono-potentiometry may be simulated by fixing the slope of the concentration profile in the vicinity of the electrode surface according to the magnitude of the constant current passed. These other techniques are discussed later a model for diffusion-limited semi-infinite linear diffusion is developed immediately. 

Ascorbic acid and Na ascorbate can be determined in an automated constant current coulometric system. Under optimal conditions, an excellent precision of 0.3% was achieved, with 95% probability . Ca ascorbate can be determined by potentiometry (using Ag as indicator electrode) and constant current coulometric methods. Automatic coulom-etry possesses the advantage of speed and, with its satisfactory precision, is well suited to routine pharmaceutical analysis . 

Construct a coulometric titration curve of 100.0 mL of a 1 M H2SO4 solution containing Fe(ll) titrated with Ce(lV) generated from 0.075 M Ce(lll). The titration is monitored by potentiometry. The initial amount of Fe(II) present is 0.05182 mmol. A constant current of 20.0 mA is used. Find the time corresponding to the equivalence point. Then, for about 10 values of time before the equivalence point, use the stoichiometry of the reaction to calculate the amount of Fe produced and the amount of Fe + remaining. Use the Nemst equation to find the system potential. Find the equivalence point potential in the usual manner for a redox titration. For about 10 times after the equivalence point, calculate the amount of Ce " produced from the electrolysis and the amount of Ce + remaining. Plot the curve of system potential versus electrolysis time. 

In two-electrode potentiometry the titration curve can be derived by considering that the small constant current applied between two polarizable electrode must be the same at the anode and cathode. The result is a curve of E v. /that shows a peak at the equivalence point. 

Three broad classifications of electrochemical methods are used in this chapter. Po-tentiometric methods include zero-current potentiometry and methods in which current of controlled magnitude is apphed to the working electrode, such as in potentiometric stripping analysis (PSA). Amperometric methods consider all techniques in which current is measured these include constant-potential amper-ometry and amperometric measurements made in response to a variety of applied potential waveforms in voltammetric methods. Impedimetric methods comprise a final classification in these methods, faradaic currents are generally absent, and impedance, conductance, or capacitance is the measured property. 

In this section the theory and methodology of electro-analytical chemistry are explored. Chapter 22 provides a (general foundation for the study of subsequent chapters in this section. Terminology- and conventions of electrochemistry as well as theoretical and practical aspects of the measurement of electrochemical potentials and current s are. presented. Chapter 23 comprises the many methods and applications of potentiometry. and constant-potential coulometry and constant-current coulornetrv are discussed in Chapter 2 4. The many facets of the important and widely used technique of voltammetry are presented in ( hapter 2.5. which concludes the section. 

D. Jagner, L. Renman, and S.H. Stefansdottir. Determination of iron(III) and titanium(IV) as their solochrome violet RS complexes by constant-current stripping potentiometry. Part 1. Automated single-point calibration method for iron(III). Analytica Chimica Acta 281 305-312,1993. J.M. Laporte, J.P. Truchot, F. Ribeyre, and A. Boudou. Combined effects of water pH and salinity on the bioaccumulation of inorganic mercury and methylmercury in the shore crab Carcinus maenas. Marine Pollution Bulletin 34 880-893,1997. 

Fig. 84. The circuit of potentiometry with constant current A a voltage measuring instrument B batteries C cell R resistor G galvanometer.

There are several measurement modes amperometry, potentiometry, constant current, and impedance. The amperometry mode measures the probe current-position curve. The potentiometry mode measures the probe potential-position curve. The constant current mode will adjust the Z-position of the probe to maintain the current constant. A probe height-position curve is recorded. If the sample or substrate is an insulator, the curve can be translated to the topographic information. The impedance mode measures the probe impedance-position curve. If the probe scan is in the X- or T-direction, amperometry, potentiometry, and impedance are under a constant height mode. 

Different electrochemical techniques depend on whether they are bulk methods as in conductometry or interfacial methods. The latter may be static as in potentiometry or dynamic. Dynamic methods are classified on the basis of the current used. Conductometry uses a constant current but controlled current methods include voltametry, amperometry and coulometry. 

Whereas potentiometry uses an experimental approach with a minimum current flow, the voltametric setup involves a galvanostat and measures potential differences with a predefined constant current. In this case electrochemical processes at the electrode surface provide the generation of electric charge thus the working electrodes are basically the same as the ones for voltammetric and amperometric measurements (see below). Depletion of an electroactive species causes the galvanostat to increase the potential to keep up a constant current flow. 

Gil EP, Ostapczuk P (1993) Nickel and cobalt determination by constant current stripping potentiometry 1. Method development 2. Modifications and check of the method. Eresenius J Anal Chem 346 952-956, 957-960... 

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