Generation of reagents

An advantage of the technique is the use of an electrical standard to replace chemical standards and the problems associated with their preparation and stability. The coulometric titration also permits the generation of reagents such as copper(I) or bromine, which are difficult to employ as standard solution, or others such as silver(II) or chlorine, which are virtually impossible to use in any other way. A disadvantage of the coulometric titration is its lack of specificity. 

In recent years a second generation of reagents for electrophilic fluori-nation has been developed. These reagents contain N—F bonds. They are more stable, easier to handle, and often more selective. These reagents could be useful in the synthesis of bioactive molecules where selectivity and mild reaction conditions are essential. 

Experimentally, then, the two operational standards are intimately related. Silver of high purity (99.999-1-%) is obtainable at the present time. Also available are techniques and equipment for measuring current and time to high precision as well as conditions for generation of reagents at electrodes with high current efficiency. Thus either silver or the coulomb can be used as the operational standard with justification. 

The development of new ionization strategies usually opens the way to new fields of application. In particular, soft ionization techniques such as atmospheric pressure chenaical ionization (APCI) are interesting analytical tools for inorganic/organic MS. The mechanisms for the generation of reagent ions and the ionization pathways are similar for the most common APCI sources. First, the buffer gas is ionized by a beam of electrons accelerated in a high electric field. Then, a series of reactions. 

Figure 3. Cell for coulometric titration with external generation of reagent. (Reproduced from Ref. 28.

Fig. 7 Micropatteming. a Direct mode where the microelectrode acts as counter electrode, b Local generation of reagents which corrode locally the sample surface, c coupling of homogenous and heterogeneous reactions in which the species generated at the tip undergoes a homogenous chemical reaction locally enabling a heterogeneous electrochemical reaction at the sample surface. Adapted from [2] with permission of Wiley...

Although all of the work reported here has involved either pH or electrometric titrations, much work has been done on coulometric generation of reagents and colorimetric determination of end points. This work is still in progress. Thus far none of the industrial problems encountered have required these techniques. 

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