Electro-Inactive Compounds

Measurements of electrode impedance offer an extra bonus an electrode placed in an ionic solution is surrounded by the electrical double layer having the corresponding double-layer capacity that contributes to the overall electrode impedance. The value of the double-layer capacity sensitively reflects the interfacial properties of substances present in the solution and therefore the impedance technique is suitable for the investigation of adsorption at the interface, the phase transition in monolayers, the interaction of biosurfactants with counter ions, the inhibition properties of polymers, the analysis of electro-inactive compounds on the basis of adsoprtion effects, and other topics. The theory of electrode impedance has been well formulated and a complete set of diagnostic criteria for the elucidation of electrochemical processes is available. With the increasing availability of ready-made instrumentation an increased number of applications in biochemical studies is also to be expected. 

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 equilibrium between the electro-inactive form A that is transported from the bulk of the solution towards the surface of the electrode and the electroactive form C is established at a finite rate. To simplify the treatment, the experimental conditions are usually chosen so that the compound B (usually a component of the reaction medium) is present in excess so that its concentration can be considered constant. Equation (20) is then simplified to (21) ... 

A classical example of hydration-dehydration equilibria is the behaviour of formaldehyde. This compound predominates in the solution in the electro-inactive methyleneglycol form, from which the... 

However, the potential range within which the spin trap is electro-inactive is considerably reduced. This, the commonly used trap nitroso-t.-butane is reduced at a potential -0.98 V (vs. Ag/AgI) at mercury in dimethyl-formamide. The range may be extended by substitution of appropriate aryl groups for butane [114]. Nitroso compounds also present a problem in that some compounds have been shown to undergo a monomer/dimer equilibrium in solution in which only the monomeric form acts as a radical trap. Hence, for nitroso compounds, it is necessary to understand this equilibrium, as well as the electrochemical properties of the trap, before it may be used in the investigation of electrode reactions. 

The most difficult condition to meet is making the solutions used for constructing calibration curves identical to those used for sample analysis. Frequently the calibration curves are recorded in solutions containing only the studied compound and supporting electrolyte however, the preparation of the sample solution often introduces other substances. It can usually be assumed that these substances wfll have a negligible effect on the waves of the compound to be determined, but sometimes such electro-inactive components of the sample can affect the height of the measured wave. 

These waves are controlled by the rate of some chemical reaction preceding the electron transfer. A good example of such a system is provided by the behaviour of formaldehyde. This compound exists in aqueous solution largely as the hydrate which is electro-inactive and produces no reduction wave. The anhydrous molecule, which is reducible is formed from the hydrate only slowly. The overall reaction may be represented by... 

The development of an electrode reaction is highly dependent on the nature of the electrode-solution interface. Most electrochemical processes are still performed using classical metallic or carbon electrodes. However, modifications brought to the surface of the working electrode may result in the enhancement of particular properties which can be exploited in electroanalytical chemistry. In such a way, many electrode materials, such as metals, metal oxides but also carbon based electrodes have been submitted to chemical or non-chemical modifications. Most of the immobilized compounds are electroactive, the electron transfer having to be reversible. They are mainly used as catalysts for electrochemical reactions which cannot be performed at conventional electrodes. In addition, a judicious choice of a catalyst exhibiting a particular structure may also provide more or less specificity towards certain molecules or ions. Electro-inactive substances may also be immobilized and act as intermediates for... 

While we have not completed any bulk electro-synthetic reactions to ascertain what comprises the products of the oxidative process, we have arrived at some conclusions regarding the requisite structural features for electroactivity. Although the carbohydrates will exist in their open form under the basic conditions of the analysis, we could eliminate the requirement of a carbonyl function due to the activity of some of the alcohols such as glycerol and inositol. The requirement for a polyhydroxy functionality is evident form the inactivity of methanol and the 1,2-dihydroxy pentanes, hexanes and cis- and trans-cyclohexanes. The exception here would appear to be 2-deoxyribose, which is inactive, albite a polyhydroxy compound. Indeed, it would appear that regardless of what oxidative process is occuring, there is a requirement for three hydroxy groups in a 1,2,3-trihydroxy configuration. This is consistent... 

---