Effect of the Phase Volume Ratio on the Potential of a Liquid-Membrane Ion-Selective Electrode

Two-phase liquid system IA(w)jIX(o) comprising the interface between the aqueous solution (w) of uni-univalent electrolyte IA and an organic solvent solution (o) of a uni-univalent electrolyte IX with the common cation I+ is considered as a simple model of a liquid-membrane ion-selective electrode (ISE). Taking into account the electroneutrality and mass balance conditions, the equilibrium Galvani potential difference (pd) between the aqueous and organic phases, ¢o wO ) O(w) - O(o), is calculated numerically as a function of the ratio of the initial electrolyte concentrations, x ) cIA 0 /cIX 0 ) 10-4- 104, for the selected values of the phase volume ratio r ) V(o)/V(w) ) 10-3, 1, and 103, and the standard ion transfer potentials of the present ions ranging from -0.5 to 0.5 V. Numeric results corroborate the symbolic expressions derived for the cases when X- and A- are extremely lipophilic and hydrophilic ions, respectively, or when the concentration ratio x is extremely large or small. In contrast to the extraction system, where both electrolytes are initially present in the aqueous phase, the effect of the phase volume ratio on the equilibrium pd in the ISE model is rather weak, unless the counterions X- and A- differ little in their lipophilicity from the target ion I+. It is shown that both the ISE and extraction model exhibit the Nernstian behavior only in a limited range of the concentration ratio x depending on the value of the standard ion transfer potentials of the counterions. When this ratio is extremely large or small, equilibrium pd approaches the limiting value given by the distribution potential of the electrolyte IA or IX, respectively. Similar conclusions can be drawn for the two-phase liquid system AI(w)jXI(o) with the common anion I-

306 - Nucleotides and related substances: Conformation in solution and at solution|electrode interfaces

The problems involved in inferring conformation and orientation from electrochemical measurements are considered as are the implications of extrapolating the results for relatively simple nucleotides to biopolymers. The determination of conformation, e.g., shape in solution -- more particularly, when approaching the electrode -- largely depends on estimation of the effective molecular cross-section as reflected in the experimentally measured diffusion coefficient, D; for example, formation of associated species as in base stacking is usually reflected in a variation in D and, often, in redox potential. The determination of conformation at the solution|electrode interface is often intimately connected with the state and orientation of an adsorbed species -- more particularly of its electroactive and adsorption sites -- relative to the electrode surface. Current trends in inferring such interfacial conformation for DNA and derived large nucleic acid species are summarized; the adsorption pattern seen on oxidation of NADH at carbon electrodes is reviewed.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/23299/1/0000237.pd

NAD/NADH as a model redox system: Mechanism, mediation, modification by the environment

The biologically important redox couple, [beta]-nicotinamide adenine dinucleotide/1,4,[beta]-dihydronicotinamide adenine dinucleotide, provides a grossly reversible prototype system for an overall electrode reaction consisting of two successive one-electron (1 e-) transfer steps coupled with (a) dimerization of an intermediate free radical product, (b) protonation-deprotonation of an intermediate product, (c) other chemical reactions, (d) adsorption of reactant, intermediate and product species, and (e) mediation by electrode surface species. Cathodic reduction of NAD+ proceeds through two 1 e- steps well separated in potential; protonation of the free radical produced on the first step occurs prior to the second electron-transfer; a first-order chemical reaction coupled to the latter may involve rearrangement of an initial dihydro product to 1,4-NADH (and some 1,6-NADH). In the apparently single stage 2 e- anodic oxidation of NADH, the initial step is an irreversible heterogeneous electron transfer, which proceeds to at least some extent through mediator redox systems located close to the electrode surface; the resulting cation radical, NADH+[middle dot], loses a proton (first order reaction) to form a neutral radical, NAD[middle dot], which may participate in a second heterogeneous electron transfer (ECE mechanism) or may react with NADH+[middle dot] (disproportionation mechanism DISP 1 or half-regeneration mechanism) to yield NAD+.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/23943/1/0000190.pd

Fine tuning of the catalytic effect of a metal-free porphyrin on the homogeneous oxygen reduction

The catalytic effect of tetraphenylporphyrin on the oxygen reduction with ferrocene in 1,2-dichloroethane can be finely tuned by varying the molar ratio of the acid to the catalyst present in the solution. The mechanism involves binding of molecular oxygen to the protonated free porphyrin base, in competition with ion pairing between the protonated base and the acid anion present

Ionic partition diagram of tetraphenylporphyrin at the water|1,2-dichloroethane interface

diagram of 5,10,15,20-tetraphenyl-21H,23H-porphine (H2TPP) at the water|1,2-dichloroethane interface using a simple Born solvation model. This zone diagram shows under which form this porphyrin is present, i.e. neutral, monoprotonated or diprotonated, and in which phase i.e. either in the aqueous or the organic phase as a function of the aqueous pH and the interface polarisation that can be controlled externally or by the distribution of supporting electrolytes. This diagram explains why the monoprotonated form has been difficult to observe when doing biphasic pH titration

Evidence of tetraphenylporphyrin monoacids by ion-transfer voltammetry at polarized liquid|liquid interfaces

We present a simple methodology to illustrate the existence of tetraphenylporphyrin monoacid based on ion-transfer voltammetry at a polarized water|1,2-dichloroethane interface and organic pK values are also estimated

Oxygen and proton reduction by decamethylferrocene in non-aqueous acidic media

Experimental studies and density functional theory (DFT) computations suggest that oxygen and proton reduction by decamethylferrocene (DMFc) in 1,2-dichloroethane involves protonated DMFc, DMFcH+, as an active intermediate species, producing hydrogen peroxide and hydrogen in aerobic and anaerobic conditions, respectively

Oxygen reduction by decamethylferrocene at liquid/liquid interfaces catalyzed by dodecylaniline

Molecular oxygen (O2) reduction by decamethylferrocene (DMFc) was investigated at a polarized water/ 1,2-dichloroethane (DCE) interface. Electrochemical results point to a mechanism similar to the EC type reaction at the conventional electrode/solution interface, in which an assisted proton transfer (APT) by DMFc across the water/DCE interface via the formation of DMFcH+ corresponds to the electrochemical step and O2 reduction to hydrogen peroxide (H2O2) represents the chemical step. The proton transfer step can also be driven using lipophilic bases such as 4-dodecylaniline. Finally, voltammetric data shows that lipophilic DMFc can also be extracted to the aqueous acidic phase to react homogeneously with oxygen

H2O2 generation by decamethylferrocene at a liquid | liquid interface

Hydrogen peroxide generation at a liquid|liquid interface occurs with a yield of 20 % with respect to the concentration of reducing agent (decamethylferrocene). The liquid|liquid interface supplies electrons from the reducing agent and protons from the aqueous phase to drive the reduction of O2 into H2O2, which is extracted into the aqueous phase during the course of reaction (see picture; DCE=1,2-dichloroethane)

Dioxygen Reduction by Cobalt(II) Octaethylporphyrin at Liquid / Liquid Interfaces

Oxygen reduction catalyzed by cobalt(II) (2,3,7,8,12,13,17,18-octaethylporphyrin) [Co(OEP)] at soft interfaces is studied by voltammetry and biphasic reactions. When Co(OEP) is present in a solution of 1,2-dichloroethane in contact with an aqueous acidic solution, oxygen is reduced if the interface is positively polarized (water phase versus organic phase). This reduction reaction is facilitated when an additional electron donor, here ferrocene, is present in excess in the organic phase