Polarographically inactive compounds

In solutions of histidine and histamine containing pyruvic acid, another reaction takes place (9) in addition to the formation of Schiff base. With increasing concentration of these amino acids apart from the formation of the wave of the Schiff base at a more positive potential, a decrease of the total limiting current can be observed (Fig. 5). The decrease corresponds to the formation of a polarographically inactive compound this may be interpreted by reaction (6) ... 

Cyclopentanone and cyclohexanone can also be determined in complex mixtures by using the formation of ketimines, from 20 per cent solutions of hexamethylenediamine, at a pH above j2.(4oi>) The aldehydes, when present, give aldimine waves at potentials about 0-2 V more positive than the ketimine waves. However, moderate concentrations of aldehydes do not interfere in the determination of ketones as (at the concentrations of hexamethylenediamine used) a substantial part of the aldimine is converted into a polarographically inactive compound which, during the course of the reaction, is competitive to the aldimine formation. For the determination of aldehydes, a 2 per cent solution of hexamethylenediamine proved to be the most suitable. 

In indirect methods, as mentioned in Chapter IV, polarographic-ally inactive substances are transformed into compounds showing waves on polarographic curves, or concentration changes of a polarographically active substance which reacts with the electro-inactive compound to be determined, are measured. Finally, polarometric (amperometric) titrations can be included in this group of analytical methods. 

The different protonated forms of an individual compound can vary considerably in their polarographic behaviour. Often only one protonated form is electroactive. In other cases a second protonated form is also electroactive but at a quite different potential. In some cases the electroinactive form can be rapidly protonated or depro-tonated to the electroactive form in a fast step before the electrolytic step, giving rise to the wave but at a shifted potential. In other cases the electroinactive form remains totally inactive and the wave vanishes when the pH is adjusted to a value where this form is dominant. Sometimes the mechanism of the reduction or oxidation can change with pH. 

Nonelectroactive benzene or toluene can be quantitatively converted to electroactive nitroderivatives. The method can be extended to any polarographic-ally inactive but easily nitrated aromatic compounds such as phenols, phenyl-bearing amino acids, and aniline derivatives. Interesting discriminations between similar compounds based on their ability to be nitrated or not can be achieved. Thus, the readily nitrated phenylbarbituric acid (phenobarbital) can be polarographically determined in the presence of a 100-fold excess of diethylbarbituric acid (barbital). The nitration of morphine also provides an excellent route for the polarographic determination of this otherwise practically nonelectroactive alkaloid. 

Electrochemically inactive organic compounds can be converted into an electrochemically active state by nitration, nitrosation, oxidation, hydrolysis, or some other preliminary chemical reaction, and can then be determined polarographically or voltammetrically. Examples of this are polarographic determinations of alkylbenzenesulfonates in natural water samples after extraction and nitration of the aromatic ring [77] and determinations of beta-receptor blocking agents in tablets after conversion to the corresponding V-nitroso derivatives [78]. 

Like other azomethine sugar derivatives, the hydrazones of aldopentoses reveal (under identical conditions) waves with different heights, since their reduction currents arise not only from diffusion of the polarographically active hydrazone form to the electrode but also are limited by the kinetics of conversion of the inactive cyclic form into the hydrazone form. It has been assumed that in these compounds a two-electron reduction of the C = N bond takes place [118]. 

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