With insoluble redox couples

Fig. 3 Schematic diagram of a two-electrode storage cell. On the left the storage electrode contains an insoluble redox couple, and on the right a soluble redox couple, with storage represented as...

Aqueous systems have been studied by a very large number of investigators. Economy, safety, convenience and quality of product have combined to make this the method of choice for commercial production of copolymers. The industrial importance of such end products as elastomers and acrylic fibers has been a special incentive to related fundamental studies. Furthermore, the relatively high solubility of acrylonitrile monomer in water coupled with insolubility of the polymer make it a convenient test monomer for studies of initiation by redox systems (6, 25, 102). Large numbers of homogeneous chemical initiators and some heterogeneous initiators have been studied as well as initiation by photochemical means, by ultrasonics and by ionizing radiation. It will not be possible here to review the enormous world literature. Several publications (/, 92, 117) refer in some detail to the older papers, and we shall restrict our comments to recent interpretations that have received support from several quarters. 

For those redox couples that involve a metal ion plus the metal, the logical electrode system is the metal itself. In other words, if the measured quantity is to be cupric ion [copper(II)], a practical indicator electrode is a piece of copper metal. All second-class electrodes involve an active metal in combination with an insoluble compound or salt. Thus, the silver/silver chloride electrode actually is a silver/silver ion electrode system that incorporates the means to control the silver ion concentration through the chloride ion concentration [Eq. (2.14)]. A related form of this is the antimony electrode, which involves antimony and its oxide (an adherent film on the surface of the antimony-metal electrode) such that the activity of antimony ion is controlled by... 

Basically, the final choice of the cation has to relate strictly to the application. The presence of cations such as Li+ or Na+ in solutions may lead to precipitation of insoluble surface films or noble metal electrodes and thus interfere with the basic electrochemical behavior of many redox couples on nonactive metal electrodes in polar aprotic solvents [9], The use of tetraalkyl ammonium salts eliminates this problem because the thermodynamics of insoluble salt precipitation on electrodes differs in the presence of these bulky cations from that developed in the presence of cations of alkaline or alkaline earth metals [6-9],... 

Xi and coworkers [135-137] reported on the epoxidation of alkenes performed with (CP)3[P04(W03)4] catalyst. This insoluble catalyst formed soluble active species, (CP)3[P04 W02(02) 4], by the reaction with H202. When H202 was consumed completely, the catalyst became insoluble again. Therefore, the catalyst recovery was simple. When coupled with the 2-ethylanthraquinone/2-ethylanthrahydroquinone redox process for H202 production, 02 could be used as an oxidant for the epoxidation of propylene in 85% yield based on 2-ethylanthrahydro-quinone which was obtained without ary by-products (Figure 13.8). 

It should be mentioned that most natural aldolase enzymes can also be assayed using enzyme-coupled systems relaying the reaction to a redox process with NAD. The formation of NADH by active microbial colonies in expression libraries of mutant enzymes was detected colorimetrically in agar plates using phenazine methosulfate and nitroblue tetrazolium, which forms an insoluble precipitate. The assay was used by Williams et al. [14] and Woodhall et al. [15] for evolving sialic acid aldolases to accept non-natural aldehyde acceptors. 

---