Complex-formation coulometric titrations

Precipitation and Complex-Formation Titrations A variety of coulometric titrations involving anodi-cally generated silver ions have been developed (sec Table 24-1). A cell, such as that shown in Figure 24-9, can be used with a generator electrode constructed from a length of heavy silver wire. End points are detected potentiometrically or with chemical indicators. Similar analyses, based on the generation of mer-cury(l) ion at a mercury anode, have been described. 

Coulometric titration. For this technique, often designated controlled-current or amperostatic coulometry, it is useful to distinguish between redox, complex-formation and precipitation titrations on the one hand and acid-base titrations on the other and to discuss each group separately. 

Other typical reagents generated for coulometric titrations are hydrogen and hydroxyl ions, redox reagents such as ceric, cuprous, ferrous, chromate, ferric, manganic, stannous, and titanous ions, precipitation reagents such as silver, mercurous, mercuric, and sulfate ions, and complex-formation reagents such as cyanide ion and EDTA [8-10]. 

A further advantage of the coulometric procedure is that a single constant-current source provides reagents for precipitation, complex formation, neutralization, or oxidation/reduction titrations. Finally, coulometric titrations are more readily automated, since it is easier to control electrical current than liquid flow. 

Summary of Coulometric Titrations Involving Neutralization, Precipitation, and Complex-Formation Reactions... 

A single constant current source can be used to generate precipitation, complex formation, oxidation-reduction, or neutralization reageni.s. Furthermore, the coulometric method adapts easily to automatic titrations, because current can be controlled quite easily. 

The coulometric titration shown in Fig. 9(a) was performed at an HD-CoA concentration such that 95% of the MCAD was complexed. The titration consisted of two reductive events. The absorbance at 456 nm decreased sharply (AA = —0.20) during the first part of the titration because the reduction of flavin (see inset). An increase in absorbance at 570 nm accompanied flavin reduction, consistent with the formation of a charge transfer complex between the dienoyl-CoA hgand and the reduced enzyme. The ahsorhance at 570 nm reached a maximum between n = 2.6 and 3 reducing equivalents, with the majority of the 456 nm decrease in absorbance occurring during this part of the titration. From n = 3 to 11.8, the absorbance of... 

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