Coulometric analysis, procedures

If the initial concentration of Cu + is 1.00 X 10 M, for example, then the cathode s potential must be more negative than -1-0.105 V versus the SHE (-0.139 V versus the SCE) to achieve a quantitative reduction of Cu + to Cu. Note that at this potential H3O+ is not reduced to H2, maintaining a 100% current efficiency. Many of the published procedures for the controlled-potential coulometric analysis of Cu + call for potentials that are more negative than that shown for the reduction of H3O+ in Figure 11.21. Such potentials can be used, however, because the slow kinetics for reducing H3O+ results in a significant overpotential that shifts the potential of the H3O+/H2 redox couple to more negative potentials. 

Clement and Paris [17] have devised a pair of methods for the determination of cobalt in steels, especially materials encountered in the nuclear industry. In the first technique, suitable for the analysis of solutions containing 8 to 160 mM cobalt(II), iron(III) is used to oxidize cobalt(II) in a picolinic acid medium, after which the resulting iron(II) is titrated po-tentiometrically with a standard solution of cerium(IV). An alternative procedure, for concentrations of cobalt(II) below 8 mM, involves a constant-current coulometric titration with electrogenerated cerium (IV) to measure the iron(II) that arises from the original reaction between cobalt(II) and iron(III). 

A round robin study of accuracy and precision of the coulometric procedure identified sources of systematic error.33 In some labs, either the instruments were inaccurate or workers did not measure the quantity of standards accurately. In other cases, the solvent was not appropriate. Commercial reagents are designed for Karl Fischer analysis. Reagents recommended by the instrument manufacturer should be used with each instrument. 

Here is a coulometric procedure for analysis of total sulfite in white wine. Total sulfite means all species in Reaction (A) and the adduct in Reaction (B). We use white wine so that we can see the color of a starch-iodine end point. 

No practical apphcations are given in this area to cover food analysis, although the reader is referred to the References section, "additional recent references" portion, for some suggestions. The paper by Kalbus and Lieu represents, for example, a laboratory experiment in the coulometric titration determination of the iodine number of fats and oils. For a practical experiment the following determination of arsenic by coulometric titration is suggested. It is of interest since it presents to alternatives in the final estimation procedure. 

A coulometric titration, like a more conventional volumetric procedure, requires some means of detecting the point of chemical equivalence. Most of the end-point detection methods applicable to volumetric analysis are equally saiisfactory here. Visual observations of color changes of indicators, as well as poicn-tionieiric, amperometric, and photometric measurements have all been used successfully. 

To summarize, then, the eurrent-time measurements required for a coulometric titration are inherently as accurate as or more accurate than the comparable volume-molarity measurements of a classical volumetric analysis, particularly where small quantities of reagent are involved. Often, however, the accuracy of a titration is not limited by these measuremenis hut by the sensitivity of the end point in this respect, the two procedures are equivalent. 

The development of electrochemical applications in pharmaceutical analysis (and to a somewhat lesser degree in food control) was chiefly based on potentiometric titrations, but chiefly on DC polarography and, soemtimes, on coulometric procedures. This phase culminated toward the end of the 1950 s when it became evident that the DC polarographic procedures are not sensitive and selective enough to be applied to all the following problems of pharmaceutical analysis[5] ... 

In our study (Lebiedzihska et al. 2007), the extraction procedure used for determination of the vitamers Bg and B12 was based on a combination of acid digestion and enzymatic hydrolysis according to a study by Esteve et al. (1998) and recommended by AOAC s Ojficial Methods of Analysis to release vitamins from food followed by FIPLC analysis. Vitamins Bg and B12 were detected following HPLC separation with coulometric detection. Our experiments performed with standards i.e. methyl-, hydroxyl-, adenosyl- and cyanocobalamin) clearly indicated the same peak area response at the same potentials and retention times. 

Different methods that are worked out to measure free chlorine concentration mostly take advantage of its strong oxidizing character. A broad scale of volumetric and coulometric titrations with different endpoint detection, as well as voltammetric as colorimetric methods, have been worked out. In practice, water analysis often involves classical titrimetric procedures, such as titration with arsenous acid or an appropriate iodometric approach. 

Ogneva et al. (28) use essentially the same method sample weight of 0.5 g, oxygen stream, 1250°C, coulometric production of reagents. The procedure described by Arsenijevic and Tomasevic (29) is similar too. For the combustion of 0.1 to 1 g titanium an iron and copper flux is used, for the analysis of zirconium 1 g iron and 0.5 g tin are used. The combustion temperature is 1000°C. Carbon dioxide is determined using a differential conductometric method. The sensitivity limit of the method is 5 Mg/g, the repeatability + 10 % relative. 

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