Variation polarographic

Beyond specific polarographic techniques variations of these methods have been developed aiming at simplified and fast evaluation using modified measurement procedures and data processing. Oldham and... 

Figure 3.2. Variation of halt-wave potential (Ek) and diffusion current constant (I) for the two polarographic waves of cyclohexenone in aqueous buffers. Data from ref. [84].

This explanation for the two polarographic waves seen in Figure 3.2 suggests that the region of transition between the two waves will be sensitive to buffer concentration and composition. Such effects are seen in the polarography-pH profiles of steroid enones, some of which [88] show behaviour like that of cyclohexenone while others show only a linear variation of half-wave potential over the whole pH range of 2 — 11 [89, 90]. 

The shape of this wave and the variation with pH are both consistent with fast equ-librium reactions In the pH region lower than the value of pK, for the hydroxyl radical, the reactions of this hydroxyl radical dominate the electrochemical process. Controlled potential reduction at the potential of this first wave indicates a IF process and the principal products are dimers of the hydroxyl radical. The second wave in this acidic region is due to addition of an electron and a proton to the neutral radical. This process competes with dimerization in the mid-pH range where the two polarographic waves merge. Over the pH range 7-9, monohydric alcohol is the principal product. At pH <3 or >12, pinacols are the main products. Unsymmet-rical carbonyl compounds afford mixtures of ( )- and meso-pinacols. Data (Table 10.3) for the ( ) / meso isomer ratio for pinacols from acetophenone and propio-phenone indicate different dimerization mechanisms in acid and in alkaline solutions. 

Several techniques arising from cyclic voltammetry help the interested reader to peer into the future. Derivative polarograph (di/dV against Vt) increases the sharpness of detection of dissolved radicals and molecular fragments. Microelectrodes can be used with potential sweep circuitry. The use of varying electrical wave forms (instead of the linear potential variation) offers much to be learned in the future. Automation and the use of pattern recognition in mechanism evaluations... 

C. Chronopolarography. The term proposed by Barredo (Ref la) for polarographic method of investigating the variation of current with time (recorded oscillographically) of phenomena that occur at electrode-solution interface (Compare with item D)... 

The anodic electrochemical behavior of various 1,4-dihydropyridines (including nimodipine) was studied in acetate-phosphate buffer (pH 8) containing 70% (v/v) ethanol [10]. An anodic peak due to a two-electron oxidation of the dihydropyridine ring to a pyridine derivative was obtained. Recoveries were 91.7-104%, with a RSD being less than 3%. A cathodic polarographic method for nimodipine in tablets was developed. The dependence of peak current on concentration in acetate-phosphate buffer of pH 5 containing 20% ethanol was linear over the range of 1 pM-1 mM, with a coefficient of variation of 0.2% [5]. 

Marangoni streaming — A -> surface tension gradient (due to local temperature or composition variation) induces a convection effect. Marangoni effects [i, ii] occur at liquid-gas or at liquid-liquid interfaces and result in convection or streaming in the liquid adjacent to the interface. See also -> polarographic maximum. 

Use of donicity values as a measure of cation solvation and cation stabilization has been demonstrated by polarographic measurements on alkali and alkaline earth metal ions, and various transition and rare earth metal ions (17,16,34). This is illustrated in Fig. 2 which shows the variation of half-wave potentials for the reduction of Tl -vTF, Zn " " -> Zn°, and Eu -> Eu " as a function of solvent donicities. 

Vlcek showed polarographically that the decomposition of Co2(CO)8 in ethanolic solution in the presence of ligand has, as the rate-determining step, the formation of Co2(CO)7L (703). Where C = Mo(CO)4, Y = 1,5-cyclooctadiene, and Z = bipy or phen, the k term is believed to correspond to bond fission as the rate-determining step (i.e., Because of the value of the entropy of activation and the variation of 2 with Z,... 

Soviet workers have examined the polarographic properties of unsym-metrically substituted pyrylium ions (e.g., 34 and 35). As expected, pyranyl radicals intervene and dimerize. Adsorption phenomena at the dropping mercury electrode are important. The consequences for the electrode and homogeneous processes of variation in substitution, solvent, acidity, etc. were explored. 

An examination of reported reactivity ratios (Table 6) shows that the behaviour rj > 1, r2 1 or vice versa is a common feature of anionic copolymerization. Only in copolymerizations involving the monomers 1,1-diphenylethylene and stilbene, which cannot homopolymerize, do we find <1, r2 <1 [212—215], and hence the alternating tendency so characteristic of many free radical initiated copolymerizations. Normally one monomer is much more reactive to either type of active centre in the order acrylonitrile > methylmethacrylate > styrene > butadiene > isoprene. This is the order of electron affinities of the monomers as measured polarographically in polar solvents [216, 217]. In other words, the reactivity correlates well with the overall thermodynamic stability of the product. Variations of reactivity ratio occur with different solvents and counter-ions but the gross order is predictable. 

Fig. 19. Current-voltage variation associated with a polarographic wave.

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