Coulometry, determination

The situation is seldom favourable enough to enable the use of a single method for establishing a link to the SI system. The information on the content of impurities (by means of relative methods) is needed in most cases. Except for coulometry, determination of molar weight... 

E630 Jacobs, J.C. and Alexander, N.M. (1990). Colorimetry and constant-potential coulometry determinations of transferrin-bound iron, total iron-binding capacity, and total iron in serum containing iron-dextran, with use of sodium di-thionite and alumina columns. Clin. Chem. 36, 1803-1807. 

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. 

In coulometry, current and time are measured, and equation 11.24 or equation 11.25 is used to calculate Q. Equation 11.23 is then used to determine the moles of analyte. To obtain an accurate value for N, therefore, all the current must result in the analyte s oxidation or reduction. In other words, coulometry requires 100% current efficiency (or an accurately measured current efficiency established using a standard), a factor that must be considered in designing a coulometric method of analysis. 

Minimizing Electrolysis Time The current-time curve for controlled-potential coulometry in Figure 11.20 shows that the current decreases continuously throughout electrolysis. An exhaustive electrolysis, therefore, may require a long time. Since time is an important consideration in choosing and designing analytical methods, the factors that determine the analysis time need to be considered. 

The other necessary instrumental component for controlled-current coulometry is an accurate clock for measuring the electrolysis time, fe, and a switch for starting and stopping the electrolysis. Analog clocks can read time to the nearest +0.01 s, but the need to frequently stop and start the electrolysis near the end point leads to a net uncertainty of +0.1 s. Digital clocks provide a more accurate measurement of time, with errors of+1 ms being possible. The switch must control the flow of current and the clock, so that an accurate determination of the electrolysis time is possible. 

Control led-Potential Coulometry The majority of controlled-potential coulometric analyses involve the determination of inorganic cations and anions, including trace metals and halides. Table 11.8 provides a summary of several of these methods. 

Another area where controlled-potential coulometry has found application is in nuclear chemistry, in which elements such as uranium and polonium can be determined at trace levels. Eor example, microgram quantities of uranium in a medium of H2SO4 can be determined by reducing U(VI) to U(IV) at a mercury working electrode. 

Controllcd-Currcnt Coulomctry The use of a mediator makes controlled-current coulometry a more versatile analytical method than controlled-potential coulome-try. For example, the direct oxidation or reduction of a protein at the working electrode in controlled-potential coulometry is difficult if the protein s active redox site lies deep within its structure. The controlled-current coulometric analysis of the protein is made possible, however, by coupling its oxidation or reduction to a mediator that is reduced or oxidized at the working electrode. Controlled-current coulometric methods have been developed for many of the same analytes that may be determined by conventional redox titrimetry. These methods, several of which are summarized in Table 11.9, also are called coulometric redox titrations. 

Studies aimed at characterizing the mechanisms of electrode reactions often make use of coulometry for determining the number of electrons involved in the reaction. To make such measurements a known amount of a pure compound is subject to a controlled-potential electrolysis. The coulombs of charge needed to complete the electrolysis are used to determine the value of n using Faraday s law (equation 11.23). 

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. 

In controlled-potential coulometry, accuracy is determined by current efficiency and the determination of charge. Provided that no interferents are present that are easier to oxidize or reduce than the analyte, current efficiencies of greater than 99.9% are easily obtained. When interferents are present, however, they can often be eliminated by applying a potential such that the exhaustive electrolysis of the interferents is possible without the simultaneous electrolysis of the analyte. Once the interferents have been removed the potential can be switched to a level at... 

Precision Precision is determined by the uncertainties of measuring current, time, and the end point in controlled-current coulometry and of measuring charge in controlled-potential coulometry. Precisions of +0.1-0.3% are routinely obtained for coulometric titrations, and precisions of +0.5% are typical for controlled-potential coulometry. 

The purity of a sample of picric acid, C6H3N3O7, is determined by controlled-potential coulometry, converting the picric acid to triaminophenol, C6H9N3O. A 0.2917-g sample of picric acid is placed in a 1000-mL volumetric flask... 

Measurements of the adsorption of inhibitors on corroding metals are best carried out using the direct methods of radio-tracer detection and solution depletion measurements . These methods provide unambiguous information on uptake, whereas the corrosion reactions may interfere with the indirect methods of adsorption determination, such as double layer capacity measurements", coulometry", ellipsometry and reflectivity Nevertheless, double layer capacity measurements have been widely used for the determination of inhibitor adsorption on corroding metals, with apparently consistent results, though the interpretation may not be straightforward in some cases. 

Electrical methods of analysis (apart from electrogravimetry referred to above) involve the measurement of current, voltage or resistance in relation to the concentration of a certain species in solution. Techniques which can be included under this general heading are (i) voltammetry (measurement of current at a micro-electrode at a specified voltage) (ii) coulometry (measurement of current and time needed to complete an electrochemical reaction or to generate sufficient material to react completely with a specified reagent) (iii) potentiometry (measurement of the potential of an electrode in equilibrium with an ion to be determined) (iv) conductimetry (measurement of the electrical conductivity of a solution). 

It is also possible to reduce the time required for conventional controlled-potential coulometry by adopting the procedure of predictive coulometry.27 A given determination will need a certain number ofcoulombs(Qx)for completion, and if at time t, Q, coulombs have been passed, then QR further coulombs will be required to complete the determination, and <2 = Qx - Qv By choosing a number of times tl,t2,t 3 separated by a common interval (say 10 seconds) and measuring the corresponding numbers of coulombs passed Ql,Q2,Q3, it can be shown that... 

Lingane and Niedrach have claimed that the h-VI states of tellurium (or selenium) are not reduced at the dropping electrode under any of the conditions of then-investigation however, Norton et al. [42] showed that under a variety of conditions, samples of telluric acid prepared by several different procedures do exhibit well-defined (though irreversible) waves, suitable for the analytical determination of the element. The reduction of Te(H-VI) at the dropping electrode was found coulometri-cally to proceed to the -II state (whereas selenate, Se(-i-VI), was not reduced at the dropping electrode in any of the media reported). 

Coulometry can be regarded as an analog of titration where the substance being examined is quantitatively converted to a reaction product not by the addition of titrant, but by a certain amount of electric charge Q. As in titration, the endpoint must be determined. To determine the endpoint during current flow, one combines coulometry with another of the electrochemical methods described, and accordingly is concerned with conductometric, potentiometric, or amperometric coulometry. 

The techniques of voltammetry/polarography, atomic absorption, ICP, etc., have in most cases supplanted the coulometric approach for the determination of inorganic analytes. Coulometry and the use of coulometry in food analysis have recently been reviewed [473,476]. 

Now returning to the coulometric analysis proper we can. say that any determination that can be carried out by voltammetry is also possible by coulometry whether it should be done by means of the controlled-potential or the titration (constant-current) method much depends on the electrochemical properties of the analyte itself and on additional circumstances both methods, because they are based on bulk electrolysis, require continuous stirring. 

The electronic adsorption spectra for the complexes [Ir(OH)6]", where n = 0-2, have been resolved and peak maxima locations, molar extinction coefficients, oscillator strengths, and band half-widths calculated.44 Bands have been assigned in the main part to be one-electron MLCT transitions. Spectrophotometrically determined rate constants for the OH reduction of the IrVI and Irv complexes at 25 °C in 3M NaOH are (2.59 0.09) x 10—3 s—1 and (1.53 0.05) x 10 4 s 1 respectively. The activation energy for the reduction, Irv—>IrIV, is nAkcalmoC1. Cyclic voltammetry and potentiostatic coulometry of [Ir(OEI )r,]2 in 3M NaOH on a Pt electrode show that during the electro-oxidation compounds of Irv and IrVI are formed.45... 

Before in situ external reflectance FTIR can be employed quantitatively to the study of near-electrode processes, one final experimental problem must be overcome the determination of the thickness of the thin layer between electrode and window. This is a fundamental aspect of the application of this increasingly important technique, marking an obstacle that must be overcome if it is to attain its true potential, due to the dearth of extinction coefficients in the IR available in the literature. In the study of adsorbed species this determination is unimportant, as the extinction coefficients of the absorption bands of the surface species can be determined via coulometry. 

---