Absolute Standard Hydrogen Electrode Potential Measured by Reduction of Aqueous Nanodrops in the Gas Phase

In a pervious communication a study has been made of the potential changes which occur during the discharge of small quantities of electricity at metallic cathodes in an acid electrolyte. The electrode potential was, in general, more negative than the reversible hydrogen electrode, and it was found that over this range the potential change was a linear function of the quantity of electricity passed. This quantity was very small, 6 X 10 -7 coulombs per square centimetre causing a change of 100 millivolts in the electrode potential at a mercury surface. This linear relation was found on all the metals investigated, but the quantity varied with the nature and condition of the surface, being greater the rougher the surface. Experiments with amalgams, and platinised mercury surfaces showed that this quantity was, to a first approximation, accessible area of its surface. It was suggested that this change in potential may be regarded as due to the deposition of more hydrogen dipoles to the surface, or alternatively to a flux of electricity across the interface causing a further deformation of the hydrogen dipoles already present on the surface. Although the potential changes accompanying these additions to the surface have been studied, few measurements were made of the quantity of hydrogen initially present on the surface at the reversible hydrogen potential. It was considered probable that this was approximately a monatomic layer but it was of some interest to investigate this point.

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Tactic Response ofShewanella oneidensisMR-1 toward Insoluble Electron Acceptors

mBio ◽

10.1128/mbio.02490-18 ◽

2019 ◽

Vol 10 (1) ◽

Cited By ~ 4

Author(s):

Joseph Oram ◽

Lars J. C. Jeuken

Keyword(s):

Average Velocity ◽

Cell Tracking ◽

Reduction Potential ◽

Electron Acceptors ◽

Electrochemical Potential ◽

Electrochemical Cells ◽

Standard Hydrogen Electrode ◽

Hydrogen Electrode ◽

Exoelectrogenic Bacteria ◽

Insoluble Electron Acceptors

ABSTRACTExoelectrogenic bacteria are defined by their ability to respire on extracellular and insoluble electron acceptors and have applications in bioremediation and microbial electrochemical systems (MESs), while playing important roles in biogeochemical cycling.Shewanella oneidensisMR-1, which has become a model organism for the study of extracellular respiration, is known to display taxis toward insoluble electron acceptors, including electrodes. Multiple mechanisms have been proposed for MR-1’s tactic behavior, and, here, we report on the role of electrochemical potential by video microscopy cell tracking experiments in three-electrode electrochemical cells. MR-1 trajectories were determined using a particle tracking algorithm and validated with Shannon’s entropy method. Tactic response by MR-1 in the electrochemical cell was observed to depend on the applied potential, as indicated by the average velocity and density of motile (>4 µm/s) MR-1 close to the electrode (<50 µm). Tactic behavior was observed at oxidative potentials, with a strong switch between the potentials −0.15 to −0.25 V versus the standard hydrogen electrode (SHE), which coincides with the reduction potential of flavins. The average velocity and density of motile MR-1 close to the electrode increased when riboflavin was added (2 µM), but were completely absent in a ΔmtrC/ΔomcAmutant of MR-1. Besides flavin’s function as an electron mediator to support anaerobic respiration on insoluble electron acceptors, we propose that riboflavin is excreted by MR-1 to sense redox gradients in its environment, aiding taxis toward insoluble electron acceptors, including electrodes in MESs.IMPORTANCEPrevious hypotheses of tactic behavior of exoelectrogenic bacteria are based on techniques that do not accurately control the electrochemical potential, such as chemical-in-plug assays or microscopy tracking experiments in two-electrode cells. Here, we have revisited previous experiments and, for the first time, performed microscopy cell-tracking experiments in three-electrode electrochemical cells, with defined electrode potentials. Based on these experiments, taxis toward electrodes is observed to switch at about −0.2 V versus standard hydrogen electrode (SHE), coinciding with the reduction potential of flavins.

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Absolute Potentials of the Standard Hydrogen Electrode (4.20 V), Standard Reference Electrodes & Aqueous Redox Couples of Elements

ECS Meeting Abstracts ◽

10.1149/ma2016-01/34/1649 ◽

2016 ◽

Keyword(s):

Standard Hydrogen Electrode ◽

Hydrogen Electrode ◽

Reference Electrodes ◽

Redox Couples

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Electrochemical and spectroscopic characterization of the 7Fe form of ferredoxin III from Desulfovibrio africanus

Biochemical Journal ◽

10.1042/bj2640265 ◽

1989 ◽

Vol 264 (1) ◽

pp. 265-273 ◽

Cited By ~ 56

Author(s):

F A Armstrong ◽

S J George ◽

R Cammack ◽

E C Hatchikian ◽

A J Thomson

Keyword(s):

Low Temperature ◽

Pyrolytic Graphite ◽

Spectroscopic Characterization ◽

Carbon Electrode ◽

Standard Hydrogen Electrode ◽

Hydrogen Electrode ◽

Iron Cluster ◽

Desulfovibrio Gigas ◽

Acid Labile

Desulfovibrio africanus ferredoxin III is a monomeric protein (Mr 6585) containing seven cysteine residues and 7-8 iron atoms and 6-8 atoms of acid-labile sulphur. It is shown that reversible unmediated electrochemistry of the two iron-sulphur clusters can be obtained by using a pyrolytic-graphite-‘edge’ carbon electrode in the presence of an appropriate aminoglycoside, neomycin or tobramycin, as promoter. Cyclic voltammetry reveals two well-defined reversible waves with E0′ = -140 +/- 10 mV and -410 +/- 5 mV (standard hydrogen electrode) at 2 degrees C. Bulk reduction confirms that each of these corresponds to a one-electron process. Low-temperature e.p.r. and magnetic-c.d. spectroscopy identify the higher-potential redox couple with a cluster of core [3Fe-4S]1+.0 and the lower with a [4Fe-4S]2+.1+ centre. The low-temperature magnetic-c.d. spectra and magnetization properties of the three-iron cluster show that it is essentially identical with that in Desulfovibrio gigas ferredoxin II. We assign cysteine-11, -17 and -51 as ligands of the [3Fe-4S] core and cysteine-21, -41, -44 and -47 to the [4Fe-4S] centre.

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