Electrochemical cells coulometric

In potentiometry, the potential of an electrochemical cell under static conditions is used to determine an analyte s concentration. As seen in the preceding section, potentiometry is an important and frequently used quantitative method of analysis. Dynamic electrochemical methods, such as coulometry, voltammetry, and amper-ometry, in which current passes through the electrochemical cell, also are important analytical techniques. In this section we consider coulometric methods of analysis. Voltammetry and amperometry are covered in Section 1 ID. 

Coulometric methods of analysis are based on an exhaustive electrolysis of the analyte. By exhaustive we mean that the analyte is quantitatively oxidized or reduced at the working electrode or reacts quantitatively with a reagent generated at the working electrode. There are two forms of coulometry controlled-potential coulometry, in which a constant potential is applied to the electrochemical cell, and controlled-current coulometry, in which a constant current is passed through the electrochemical cell. 

The purity of a sample of Na2S203 was determined by a coulometric redox titration using as a mediator, and as the titrant. A sample weighing 0.1342 g is transferred to a 100-mL volumetric flask and diluted to volume with distilled water. A 10.00-mL portion is transferred to an electrochemical cell along with 25 mL of 1 M KI, 75 mL of a pH 7.0 phosphate buffer, and several drops of a starch indicator solution. Electrolysis at a constant current of 36.45 mA required 221.8 s to reach the starch indicator end point. Determine the purity of the sample. 

Scale of Operation Coulometric methods of analysis can be used to analyze small absolute amounts of analyte. In controlled-current coulometry, for example, the moles of analyte consumed during an exhaustive electrolysis is given by equation 11.32. An electrolysis carried out with a constant current of 100 pA for 100 s, therefore, consumes only 1 X 10 mol of analyte if = 1. For an analyte with a molecular weight of 100 g/mol, 1 X 10 mol corresponds to only 10 pg. The concentration of analyte in the electrochemical cell, however, must be sufficient to allow an accurate determination of the end point. When using visual end points, coulometric titrations require solution concentrations greater than 10 M and, as with conventional titrations, are limited to major and minor analytes. A coulometric titration to a preset potentiometric end point is feasible even with solution concentrations of 10 M, making possible the analysis of trace analytes. 

Coulometric techniques involve the determination of the quantity of material electrolyzed from the amount of charge passed through an electrochemical cell during electrolysis. Faraday s law relates the measured charge to the amount of material electrolyzed,... 

An exhaustive coulometric technique can be used both as an analytical tool and as a preparative tool. These two applications often require different cell designs. The present discussion is restricted to analytical coulometric cells. The design of electrochemical cells for preparative electrolysis has been treated in Chapter 22 and elsewhere [10]. 

The cell shown schematically in Figure 29.20b permits external generation, followed by EPR detection. The solution can either be recirculated to the electrolysis cell or discarded after observation. Umemoto [36] used a similar apparatus to generate moderately stable radicals coulometrically, followed by stopped-flow measurements of the decay kinetics. Forno [37] used a more elaborate recirculating system with two electrochemical cells in series. The unstable product of the first electrolysis was pumped to the second electrolysis cell, where it was converted to a free radical and thence to the cavity for observation (Sec. VI.A). 

The method of determination of the exchange rate is particularly suited to in situ electrogeneration methods, since one can add different, high concentrations of parent material to the cell, coulometrically generate a small amount of radical ion, and determine the exchange-narrowed line width. The occurrence of exchange reactions (Eq. 29.25) can also be a problem in routine electrochemical EPR studies, for unless the radical generation step is essentially complete, the spectral resolution will be less than ideal. 

The three most common modes of operation of electrochemical detection are amperometric, coulometric, and potentiometric. An amperometric detector is an electrochemical cell that produces a signal proportional to the analyte concentration usually the percentage of the analyte that undergoes the redox reaction is very low, about 5%. 

Both techniques take place within an electrochemical cell. A coulometric titrator equipped with a working electrode of large surface area and an auxiliary electrode separated from the reaction compartment by a diaphragm. This protection of the second electrode provides a barrier between the species formed in contact with it and those formed at the working electrode thus avoiding an eventual reaction (see Figure 20.14). 

At first, the relation of the emf E of this cell and the stoichiometric composition of the wustite sample was measured by coulometric titration. Then galvanostatic or potentiostatic measurements of the relaxation curves for oxidation or reduction of the wustite samples were obtained and the chemical diffusivities determined. The values are consistent with those obtained from tracer diffusion measurements, considering thermodynamic and correlation factors. As already mentioned, surface compositions of wustite were also measured by AES, using an electrochemical cell to establish the oxygen activities [28]... 

Bill Heineman s group developed an elegant indirect titration method in the 1970s [15], Indirect coulometric titrations and optically transparent thin-layer electrochemical cells were combined to provide a simple and quick means of making formal potential measurements on electron transfer proteins. Moreover, the amount of sample required for this measurement was quite small. This is now a routine method for measuring the formal potential of electron transfer proteins, which is used by a wide range of non-electrochemical scientists. Use of this method in our laboratory, greatly facilitated by help from the Heineman laboratory, led to our later efforts to develop direct electfochemical methods for protein and enzyme studies. 

An analytical technique in which the amount of electricity passing between two electrodes in an electrochemical cell is measured. The chloride meter is an example of a coulometric technique. 

Zhang s group [82] recently presented a novel CL sensor combined with FIA for ammonium ion determination. It is based on reaction between luminol, immobilized electrostatically on an anion-exchange column, and chlorine, electrochem-ically generated online via a Pt electrode from hydrochloric acid in a coulometric cell. Ammonium ion reacts with the chlorine and decreases the produced CL intensity. The system responds linearly to ammonium ion concentration in a range of 1.0-100 pM, with a detection limit of 0.4 pM. A complete analysis can be performed in 1 min, being satisfactorily applied to the analysis of rainwater. 

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