Oxidation, of analyte

Excess reagent is destroyed by boiling the solution after oxidation of analyte is complete. 2S20 - + 2H20- >4S0 - + Oz + 4H+... 

Both aliphatic and aromatic aldehydes are easily oxidized compounds (even by atmospheric oxygen). Hence, one of the aims of their derivatization is to prevent the oxidation of analytes with resultant formation of carboxylic acids. 

Fiy. 15.2. Different types of voltammetric curves, (a) Stirred solution or rotated electrode, (b) unstirred solution, and (c) stepwise reduction (or oxidation) of analyte or of a mixture of two electroactive substances (unstirred solution). 

In voltammetry, current is measured while voltage between two electrodes is varied. (In amperometry, we held voltage fixed during the measurement of current.) Consider the apparatus in Figure 17-9 used to measure vitamin C (ascorbic acid) in fruit drinks. Oxidation of analyte takes place at the exposed tip of the graphite working electrode ... 

Masawat and Slater have presented the FIA method for determination of tetracyclines (tetracycline, chlortetracycline, and oxytetracycline) in chicken and shrimp meats and pharmaceutical samples [52]. The electrochemical oxidation of analytes was performed using a gold screen-printed electrode and good detection limits were achieved, 0.96,0.58, and 0.35)imoll for tetracycline, chlortetracycline, and oxytetracycUne, respectively. Moreover, some advantages of the screen-printed electrodes were highlighted such as potential for miniaturization, reliability, portability, low cost, simplicity of construction, and operation. 

Mode II Direct oxide-catalyzed detection—In contrast to Mode 1 detections. Mode 11 detections require the concurrent formation of surface oxide. Hence, Mode 11 detections occur at potentials > ca. +150 mV (Figure 10. lA). Oxidation of preadsorbed analyte is the primary contributor to the analytical signal, however, simultaneous catalytic oxidation of analyte in the diffusion layer is not excluded. The oxidation products may leave the diffusion layer or foul the electrode surface. 

In electrogravimetry the analyte is deposited as a solid film on one electrode in an electrochemical cell. The oxidation of Pb +, and its deposition as Pb02 on a Pt anode is one example of electrogravimetry. Reduction also may be used in electrogravimetry. The electrodeposition of Cu on a Pt cathode, for example, provides a direct analysis for Cu +. 

In this titration the analyte is oxidized from Fe + to Fe +, and the titrant is reduced from CryOy to Cr +. Oxidation of Fe + requires only a single electron. Reducing CryOy, in which chromium is in the +6 oxidation state, requires a total of six electrons. Conservation of electrons for the redox reaction, therefore, requires that... 

The scale of operations, accuracy, precision, sensitivity, time, and cost of methods involving redox titrations are similar to those described earlier in the chapter for acid-base and complexometric titrimetric methods. As with acid-base titrations, redox titrations can be extended to the analysis of mixtures if there is a significant difference in the ease with which the analytes can be oxidized or reduced. Figure 9.40 shows an example of the titration curve for a mixture of Fe + and Sn +, using Ce + as the titrant. The titration of a mixture of analytes whose standard-state potentials or formal potentials differ by at least 200 mV will result in a separate equivalence point for each analyte. 

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. 

Since the current due to the oxidation of H3O+ does not contribute to the oxidation of Fe +, the current efficiency of the analysis is less than 100%. To maintain a 100% current efficiency the products of any competing oxidation reactions must react both rapidly and quantitatively with the remaining Fe +. This may be accomplished, for example, by adding an excess of Ce + to the analytical solution (Figure 11.24b). When the potential of the working electrode shifts to a more positive potential, the first species to be oxidized is Ce +. 

End Point Determination Adding a mediator solves the problem of maintaining 100% current efficiency, but does not solve the problem of determining when the analyte s electrolysis is complete. Using the same example, once all the Fe + has been oxidized current continues to flow as a result of the oxidation of Ce + and, eventually, the oxidation of 1T20. What is needed is a means of indicating when the oxidation of Fe + is complete. In this respect it is convenient to treat a controlled-current coulometric analysis as if electrolysis of the analyte occurs only as a result of its reaction with the mediator. A reaction between an analyte and a mediator, such as that shown in reaction 11.31, is identical to that encountered in a redox titration. Thus, the same end points that are used in redox titrimetry (see Chapter 9), such as visual indicators, and potentiometric and conductometric measurements, may be used to signal the end point of a controlled-current coulometric analysis. For example, ferroin may be used to provide a visual end point for the Ce -mediated coulometric analysis for Fe +. 

Coulometric Titrations Controlled-current coulometric methods commonly are called coulometric titrations because of their similarity to conventional titrations. We already have noted, in discussing the controlled-current coulometric determination of Fe +, that the oxidation of Fe + by Ce + is identical to the reaction used in a redox titration. Other similarities between the two techniques also exist. Combining equations 11.23 and 11.24 and solving for the moles of analyte gives... 

Coupling the mediator s oxidation or reduction to an acid-base, precipitation, or complexation reaction involving the analyte allows for the coulometric titration of analytes that are not easily oxidized or reduced. For example, when using H2O as a mediator, oxidation at the anode produces H3O+... 

Residual Current Even in the absence of analyte, a small current inevitably flows through an electrochemical cell. This current, which is called the residual current, consists of two components a faradaic current due to the oxidation or reduction of trace impurities, and the charging current. Methods for discriminating between the faradaic current due to the analyte and the residual current are discussed later in this chapter. 

In hydrodynamic voltammetry current is measured as a function of the potential applied to a solid working electrode. The same potential profiles used for polarography, such as a linear scan or a differential pulse, are used in hydrodynamic voltammetry. The resulting voltammograms are identical to those for polarography, except for the lack of current oscillations resulting from the growth of the mercury drops. Because hydrodynamic voltammetry is not limited to Hg electrodes, it is useful for the analysis of analytes that are reduced or oxidized at more positive potentials. 

Other routes for hydroxybenzaldehydes are the electrolytic or catalytic reduction of hydroxybenzoic acids (65,66) and the electrolytic or catalytic oxidation of cresols (67,68). (see Salicylic acid and related compounds). Sahcylaldehyde is available in drums and bulk quantities. The normal specification is a freezing point minimum of 1.4°C. 4-Hydroxybenzaldehyde is available in fiber dmms, and has a normal specification requirement of a 114°C initial melting point. More refined analytical methods are used where the appHcation requires more stringent specifications. 

Analytical and Test Methods. Ash in isophthahc acid refers to the residue left after combustion of the sample. Ash consists of oxides of trace metals that are deterrnined individually by atomic absorption or inductively coupled plasma. A Kad Fischer titration is specific for the water content. 

Poly(ethylene oxide) associates in solution with certain electrolytes (48—52). For example, high molecular weight species of poly(ethylene oxide) readily dissolve in methanol that contains 0.5 wt % KI, although the resin does not remain in methanol solution at room temperature. This salting-in effect has been attributed to ion binding, which prevents coagulation in the nonsolvent. Complexes with electrolytes, in particular lithium salts, have received widespread attention on account of the potential for using these materials in a polymeric battery. The performance of soHd electrolytes based on poly(ethylene oxide) in terms of ion transport and conductivity has been discussed (53—58). The use of complexes of poly(ethylene oxide) in analytical chemistry has also been reviewed (59). 

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