Decreasing Electrode Potential Research Articles

Electrocatalytic Selenate Transformation to Tunable Products Using Affordable Catalysts

Anthropogenic selenium (Se) pollution has profound ecosystem and human health impacts. Compared to selenite or Se(IV), selenate or Se(VI) is chemically inert and more challenging for selective removal from the complex aquatic environment.1 Current Se(VI) removal efforts seek intensive resource and energy input to drive Se(VI) transformation, and are subject to environmental and operating conditions.2 The potential solution for sustainable Se(VI) transformation involves catalytic reduction of Se(VI). In current catalytic approaches, the reduction of Se(VI) is achieved using either abiotic photocatalysts (e.g., TiO2) or biotic enzymes (e.g., nitrate reductase).3 Here, we explored the opportunities of electrified Se(VI) transformation with functionalized catalysts to manage aquatic selenate pollution. Se(VI) can be removed through phase transformation into solid Se(0) that could cover reactive surfaces and prevent further catalysis, or gaseous Se(-II) that is toxic and needs to be captured. Alternatively, the reduction of Se(VI) into reactive Se(IV) allows exposure of active interfaces for continuous electrochemcial transformation. Therefore, a controlled reduction of Se(VI) to Se(IV) would be ideal in aquatic Se(VI) management and potential Se recovery. To the best of our knowledge, controlled Se(VI) transformation has not been fully achieved in the literature. In this study, we evaluated five affordable catalysts on graphite cathodes, including TiO2,3 underpotentially deposited Cu (UPD Cu),4 underpotentially deposited Cd (UPD Cd), Co, and CuFe nanoarrays.5 Among these five candidates, characteristic Se(VI) reduction peaks were identified for TiO2, UPD Cu, and UPD Cd through cyclic voltammetry. Less than 5% of 1-mM Se was removed using UPD Cu and UPD Cd, based on 24-hour chronoamperometry tests at corresponding potentials. Unfortunately, ppm-level Cu or Cd were detected in the treated effluent. In contrast, more than 90% of Se(VI) was removed using TiO2 as the catalyst after 24 hours. TiO2 was more robust and stable in electrocatalytic reactions with minimal catalyst dissolution and, therefore, was selected for further electrocatalytic Se(VI) reduction studies.During the TiO2-catalyzed reduction, Se(VI) would first transform into Se(IV) at a relatively positive potential. A maximum of 97% Se(VI) was found to be converted to Se(IV) in a 6-hour electrocatalytic Se(VI) reduction at -0.18 V. Decreasing electrode potential to -0.35 V, solid deposits of Se(0) appeared on a TiO2-modified graphite cathode. With a more negative electrode potential, further Se(0) reduction to Se(-II) was identified. The findings spotlight the opportunities to control Se(VI) transformation and harvest tunable products per removal demand and strategies. The effect of operational conditions was investigated at various TiO2 loading and solution pH. Higher TiO2 loading and lower solution pH favored the electrocatalytic Se(VI) transformation. The removal kinetics and performance were then studied under optimal operational conditions at -0.45 V, where Se(0) was dominant among Se(VI) reduction products, and -0.70 V, where Se(-II) was abundant. Kinetics analysis revealed that electrocatalytic Se(VI) reduction at -0.70 V was roughly four times faster as compared to that at -0.45 V. A total of 88.7 ± 2.3% of Se(VI) was removed after 24 hours of electrocatalysis at -0.70 V. In contrast, 48.2 ± 9.3% of Se(VI) was removed after 24 hours of electrocatalysis at -0.45 V. These results warrant further evaluation of the regenerability and lifespan of TiO2-modified graphite electrodes, an optimized electrode/reactor design, and an integrated Se removal/recovery system.

Understanding surface charge effects in electrocatalysis. Part 2: Hydrogen peroxide reactions at platinum

Electrocatalytic activity is influenced by the surface charge on the solid catalyst. Conventionally, our attention has been focused on how the surface charge shapes the electric potential and concentration of ionic reactant(s) in the local reaction zone. Taking H2O2 redox reactions at Pt(111) as a model system, we reveal a peculiar surface charge effect using ab initio molecular dynamics simulations of electrified Pt(111)-water interfaces. In this scenario, the negative surface charge on Pt(111) repels the O–O bond of the reactant (H2O2) farther away from the electrode surface. This leads to a higher activation barrier for breaking the O–O bond. Incorporating this microscopic mechanism into a microkinetic-double-layer model, we are able to semi-quantitatively interpret the pH-dependent activity of H2O2 redox reactions at Pt(111), especially the anomalously suppressed activity of H2O2 reduction with decreasing electrode potential. The relevance of the present surface charge effect is also examined in wider scenarios with different electrolyte cations, solution pHs, crystal facets of the catalyst, and model parameters. In contrast with previous mechanisms focusing on how surface charge influences the local reaction condition at a fixed reaction plane, the present work gives an example in which the location of the reaction plane is adjusted by the surface charge.

Effect of applied potential on stress corrosion cracking behavior of X80 steel in alkaline soil simulated solution with sulfate-reducing bacteria

In this study, the effect of applied potential on stress corrosion behavior of X80 pipeline steel in Dagang soil containing sulfate-reducing bacteria was comprehensively analyzed by means of the AC impedance technique and slow strain-rate tensile tests and scanning electron microscope observation of fracture morphology. The results indicate that, with decreasing electrode potential, the corrosion degree of the sample first decreases and then increases, and the stress corrosion cracking sensitivity exhibits a certain rule. Under the Ecorr and a potential of −725 mV(vs.CSE), the stress corrosion cracking mechanism of metal is anodic dissolution. The synergistic effect of sulfate-reducing bacteria and stress results in the initiation and propagation of cracks on the surface of the metal electrode. At −1275 mV(vs.CSE), the mechanism of stress corrosion cracking is hydrogen-induced cracking. The sulfate-reducing bacteria exacerbates the cathodic hydrogen evolution at this potential. Under the combined effects of hydrogen and stress, stress corrosion cracking behavior occurs in metals.

Effects of Soil Type of Shanghai and Electrode Potential on Near-Neutral pH Stress Corrosion Cracking of Pipeline Steel X60

The effects of soil type of Shanghai and electrode potential from cathodic protection point on near-neutral pH stress corrosion cracking (SCC) behavior of pipeline steel X60 has been investigated for structural integrity assessment. The SCC behavior was evaluated through slow strain rate tests (SSRT) at various electrode potentials in three solutions containing different typical soils. Results showed that the SCC susceptibility generally increased with decreasing electrode potential in all the three environments, with quick increase happening in the range from-1500 to-700mV(SCE). It is suggested that hydrogen induced cracking should dominate the SCC at low potentials. In cathodic potential range, the rank of the SCC susceptibility for soil type was Near-city soil > Cyan-purple soil > Cyan-yellow soil. In anodic potential range, SCC susceptibility curves mixed each other in the three solutions.

Lithium intercalation into single-walled carbon nanotubes network electrode: Storage mechanisms and impurity effects

This is a detailed study of how impurities can affect the mechanisms of lithium storage in composite electrodes consisting of a three-dimensional single-walled carbon nanotube (SWCNT) bundles network. To remove impurities such as fullerenes, amorphous carbon, catalyst, and nanographite, we submitted the SWCNT bundles to an appropriate chemical treatment before using them to prepare the electrode. Then, we analyzed how this treatment influenced electrode potential, fading capacity, and specific capacity. Additionally, we evaluated the electrode prepared with high-purity SWNCT bundles by galvanostatic intermittent titration, to obtain lithium transport parameters under thermodynamic conditions. We achieved an intrinsic specific capacity of 400 mAh g−1 for the purified SWCNT bundles prepared by an arc-reactor method. The transport parameters revealed that the electrode underwent electronic transition of the semiconducting-metal type. The chemical diffusion coefficient ranged from 10−4 to 10−18 cm2 s−1 with decreasing electrode potential.

Evaluation of Biofuel Cells with Hemoglobin as Cathodic Electrocatalysts for Hydrogen Peroxide Reduction on Bare Indium-Tin-Oxide Electrodes

A biofuel cell (BFC) cathode has been developed based on direct electron transfer (DET) of hemoglobin (Hb) molecules with an indium-tin-oxide (ITO) electrode and their electrocatalysis for reduction of hydrogen peroxide (H2O2). In this study, the ITO-coated glass plates or porous glasses were prepared by using a chemical vapor deposition (CVD) method and examined the electrochemical characteristics of the formed ITO in pH 7.4 of phosphate buffered saline (PBS) solutions containing and not containing Hb. In half-cell measurements, the reduction current of H2O2 due to the electrocatalytic activity of Hb increased with decreasing electrode potential from around 0.1 V versus Ag|AgCl|KCl(satd.) in the PBS solution. The practical open-circuit voltage (OCV) on BFCs utilizing H2O2 reduction at the Hb-ITO cathode with a hydrogen (H2) oxidation anode at a platinum (Pt) electrode was expected to be at least 0.74 V from the theoretical H2 oxidation potential of −0.64 V versus Ag|AgCl|KCl(satd.) in pH 7.4. The assembled single cell using the ITO-coated glass plate showed the OCV of 0.72 V and the maximum power density of 3.1 µW cm−2. The maximum power per single cell was recorded at 21.5 µW by using the ITO-coated porous glass.

Effects of water chemistry and loading conditions on stress corrosion cracking of cold-rolled 316NG stainless steel in high temperature water

The effects of electrode potential, stress intensity factor and loading history on stress corrosion cracking growth of a cold-rolled 316NG stainless steel in 288 °C pure water were investigated. Crack branching and intergranular stress corrosion cracking along random grain boundaries were observed by electron-back scattering diffraction. A strong dependence of crack growth rate on stress intensity factor is observed. A single-cycle overloading produced a retarded transient cracking growth period. The mild inhibiting effect of decreasing electrode potential on crack growth of cold-rolled 316NG SS is analyzed based on the interaction between crack tip mechanics and crack tip oxidation kinetics.

Effect of Electrode Potential on Solution Chemistry Inside a Crevice

A mathematical model to calculate the solution chemistry inside a corroding crevice was developed based on mass transfer. Distributions of concentrations of ions, pH and electrode potential were obtained for Type 304 and 316 stainless steels in 3% NaCl solution at 80{degree sign}C. The highest pH values at corroding areas did not depend on electrode potential, and were equal to the depassivation pH, pHd. On the other hand, the lowest pH values, pHmin, increased with decreasing electrode potential, and were equal to pHd at the repassivation potential, ER. So, it was confirmed that ER is the critical potential for crevice corrosion, because below this potential, pH values inside crevices are higher than pHd and crevice corrosion cannot continue to progress.

Feasibility study for reductive destruction of carbon tetrachloride using bare and polymer coated nickel electrodes

This research investigated the feasibility of an electrochemical reductive dechlorination method for removing carbon tetrachloride (CT) from contaminated waters. Reaction rates and Faradaic current efficiencies were measured for CT dechlorination in small flow-through reactors utilizing bare and silicone polymer coated nickel cathodes. CT dechlorination resulted in near stoichiometric production of methane. Rates of CT reduction were found to follow a first-order kinetic model for all CT concentrations investigated. CT disappearance was limited by its reaction rate, and the performance of the reactor could be approximated with an ideal plug-flow reactor model. Destruction half-life values for CT with the bare nickel electrode ranged from 3.5 to 5.8 min for electrode potentials ranging from −652 to −852 mV with respect to the standard hydrogen electrode (SHE). The apparent electron transfer coefficient for CT reduction was only 0.06. The low transfer coefficient can be attributed to oxides coating the electrode surface that contributed to mass transfer resistance for CT reduction. Faradaic current efficiencies for CT reduction were found to decline with decreasing electrode potential. This can be attributed to an electron transfer coefficient for water reduction of 0.33 that was significantly greater than that for CT reduction. Faradaic current efficiencies could be increased by 100–360% by coating the electrode with a silicone polymer. In addition to decreasing the rate of water reduction by acting as hydrophobic mass transfer barrier, the polymer coating resulted in small increases in CT reaction rates. The energy cost per volume of water treated was strongly dependent on the electrode potential, but only weakly dependent on the influent CT concentration over the range of practical interest. The energy costs for reductive dechlorination appear to be lower than the carbon costs for adsorptive treatment of CT. This indicates that low current efficiencies at low CT concentrations are not a significant obstacle for developing a practical treatment process. The main impediment to electrochemical treatment for removing CT from water is the slow reaction rate that requires large reactors for obtaining sufficient hydraulic detention time to meet effluent water standards.

Understanding reduction of carbon tetrachloride at nickel surfaces.

Nickel has been found to be an effective cathode material and catalyst for reductive destruction of chlorinated solvents in contaminated water. This study investigated reductive dechlorination of carbon tetrachloride (CT) at a nickel rotating disk electrode using chronoamperometry and electrochemical impedance spectroscopy. Chronoamperometry experiments were performed to determine rates of CT reduction as a function of the electrode potential, pH, CT concentration, and temperature. The reaction products of CT dechlorination were 95 +/- 4% methane and 4.1 +/- 2.5% chloroform. Only trace levels of methylene chloride and chloromethane were produced, indicating that sequential hydrogenolysis was not the predominant pathway for methane production. Electrochemical impedance spectroscopy showed that the rate-limiting step for methane production was the transfer of the first electron to a physically adsorbed CT molecule. The temperature independence of the electron transfer coefficient and the decreasing activation energy with decreasing electrode potential indicated that the rate-limiting step involved an outer-sphere electron transfer. At neutral pH values, oxides inactivated much of the electrode surface for both CT reduction and hydrogen evolution. At lower pH values, oxide dissolution served to increase the electroactive surface area of the disk electrode. Anson analysis and kinetic modeling showed that CT adsorption to electroactive sites was a nonlinear function of the CT concentration and was in equilibrium with the bulk solution. CT dechlorination rates on nickel electrodes were 16 times slower than those on iron electrodes under similar conditions. However, CT reactions at nickel surfaces produced predominantly methane as the first detectable product, while reduction at iron surfaces produced chloroform. These results suggest that, although nickel is not a catalyst for the rate-limiting step for CT dechlorination, it may serve a catalytic role in subsequent reaction steps.

The Nonlinear Optical Response of Pt(111) Electrodes in Perchloric Acid Solution: Implications for the Potential of Zero Charge

Abstract Second harmomic and sum frequency generation (SHG, SFG) are used to investigate the second-order non-linear optical response of a Pt(111) single-crystal electrodes in perchloric acid solution. In the potential window between hydrogen evolution and surface oxidation the SHG signal shows a pronounced minimum at 760 mVRHE and rises linearly with decreasing electrode potential. The potential of the minimum as well as the magnitude of the signal in the potential range of hydrogen adsorption depend on the pH of the electrolyte. The dependence of the SHG signal on excitation frequency in the range of 9000–1200 0cm−1 shows a continuous SHG signal increase to higher frequencies without characteristic surface resonances. The SHG signal is assigned to the excitation of a continuum of electronic levels. The maximum of the signal intensity is observed at potentials close to 0 VRHE, where hydrogen evolution takes place and the surface has a maximum of negative charge. Sum-frequency spectra of CO adsorbed on Pt(111) exhibit the known vibrational signature of terminal and bridge-like coordination and an additional resonant signal. On the same surface, the SHG signal is characterized by a high signal intensity which remains constant up to the CO oxidation potential. The potential dependence of the nonlinear response of the Pt(111)/CO surface as well as of the neat surface in perchloric acid indicates a high sensitivity to the surface charge. As a consequence, a negatively charged surface up to a potential of 600 mVRHE is deduced. Our results are at variance with a value for the potential of zero charge of 0.34 VRHE which was derived from the CO charge displacement method, but in agreement with the value based on the immersion technique (U. W. Hamm et al. , J. Electroanal. Chem. 414 (1996) 85).

Electrochemical investigation of the rate-limiting mechanisms for trichloroethylene and carbon tetrachloride reduction at iron surfaces.

The mechanisms involved in reductive dechlorination of carbon tetrachloride (CT) and trichloroethylene (TCE) at iron surfaces were studied to determine if their reaction rates were limited by rates of electron transfer. Chronoamperometry and chronopotentiometry analyses were used to determine the kinetics of CT and TCE reduction by a rotating disk electrode in solutions of constant halocarbon concentration. Rate constants for CT and TCE dechlorination were measured as a function of the electrode potential over a temperature range from 2 to 42 degrees C. Changes in dechlorination rate constants with electrode potential were used to determine the apparent electron-transfer coefficients at each temperature. The transfer coefficient for CT dechlorination was 0.22 +/- 0.02 and was independent of temperature. The temperature independence of the CT transfer coefficient is consistent with a rate-limiting mechanism involving an outer-sphere electron-transfer step. Conversely, the transfer coefficient for TCE was temperature dependent and ranged from 0.06 +/- 0.01 at 2 degrees C to 0.21 +/- 0.02 at 42 degrees C. The temperature-dependent TCE transfer coefficient indicated that its reduction rate was limited by chemical dependent factors and not exclusively by the rate of electron transfer. In accord with a rate-limiting mechanism involving an electron-transfer step, the apparent activation energy (Ea) for CT reduction decreased with decreasing electrode potential and ranged from 33.0 +/- 1.6 to 47.8 +/- 2.0 kJ/mol. In contrast, the Ea for TCE reduction did not decline with decreasing electrode potential and ranged from 29.4 +/- 3.4 to 40.3 +/- 3.9. The absence of a potential dependence for the TCE Ea supports the conclusion that its reaction rate was not limited by an electron-transfer step. The small potential dependence of TCE reaction rates can be explained by a reaction mechanism in which TCE reacts with atomic hydrogen produced from reduction of water.

Impedance spectroscopy of polypyrrole/poly(styrenesulphonate) composites. Simultaneous anion and cation transport

Anodic oxidation of pyrrole in a solution containing both sodium perchlorate and sodium poly(styrenesulphonate) (NaPSS) forms polypyrrole (PPY) that is doped partially with ClO 4 − and partially with PSS. The impedance of PPY/PSS/ClO 4 film coated Pt electrodes in 0.2 M NaClO 4(aq) shows simultaneously the ion transport behaviours of PPY/ClO 4 and PPY/PPS. The fully oxidized form contains ClO − 4 as the major mobile ionic species, and therefore its ionic conductivity decreases with decreasing electrode potential as ClO − 4 ions are expelled. The reduced form contains Na + as the major mobile ionic species. Increasing the potential causes expulsion of these cations, and therefore decreases the film's ionic conductivity. At intermediate oxidation states, both mobile anions and cations are present and contribute to the film's ionic conductivity. Complex impedance plots, capacitance plots and cyclic voltammograms show the separation of two charging processes, indicating that the PSS and ClO 4 − doped parts of the polymer occur as separate domains. The observed impedance responses can be modelled with an electrical circuit containing two parallel transmission lines, representing anion and cation transport, respectively. At low frequencies, both experimental and simulated complex plane impedance plots show a deviation from the ideal capacitive response of a simple transmission line.

Microscopic and local probe method for studying crevice corrosion and its application to iron and stainless steel

Transparent polyethylene, used as part of a crevice, enables in situ, real time microscopy of the metal crevice wall, while a probe inserted into the crevice measures the electrode potential at the crevice corrosion location on the crevice wall. The current flowing out of the crevice and pH in the crevice are also measured. Application of the technique to iron and, to a lesser extent, ferritic stainless steel shows that the crevice wall in the depth direction has a sharply decreasing electrode potential, with the location of crevice corrosion on the crevice wall corresponding to the active loop of the crevice electrolyte polarization curve. The role of both acidification and chloride build-up in the crevice is to enlarge the active loop so that the IR drop condition for stable crevice corrosion is met more readily. In contrast, the role of sodium chromate inhibitor is to shift the Flade (passivation) potential in the less noble direction, thereby decreasing the size of the active loop, which makes it more difficult to achieve the IR required for crevice corrosion to occur.

Underpotentially deposited thallium on silver (111) byin situsurface x-ray scattering

Using in situ surface x-ray scattering, we have investigated the atomic structure, the stability, and the dependence of the structure on electrode potential for electrochemically deposited Tl monolayers and bilayers on Ag(111). The layers were formed by underpotential deposition (UPD) at electrode potentials positive of the reversible potential for bulk Tl deposition. At potentials between -475 and -680 mV (versus Ag/AgCl), the Tl deposit forms an incommensurate, hexagonal two-dimensional (2D) monolayer that is compressed relative to bulk Tl by 1.4--3.0 % and rotated from the Ag [011\ifmmode\bar\else\textasciimacron\fi{}] direction by \ensuremath{\Omega}=4\ifmmode^\circ\else\textdegree\fi{}--5\ifmmode^\circ\else\textdegree\fi{}. The structure of the monolayer does not change over at least 24 h (the longest we waited). From diffraction scans of the Tl Bragg rods, we find that the in-plane and vertical root-mean-square displacement amplitudes are 0.36\ifmmode\pm\else\textpm\fi{}0.1 and 0.46\ifmmode\pm\else\textpm\fi{}0.1 \AA{}, respectively.The monolayer structure is the same as that of vapor-deposited Tl/Ag(111), and this shows that the interaction between the solvent molecules and the Tl adatoms does not influence the monolayer structure. Since the monolayer has a structure that is about the same as the closest-packed planes of bulk Tl, we deduce that the adatom-adatom interactions are the most important structure-determining forces. The compression of the monolayer (compared to bulk Tl) is explained in terms of effective-medium theory. With decreasing electrode potential, the in-plane spacing between Tl adatoms decreases and this permits a calculation of the 2D compressibility. This decreases with atomic spacing, but has an average value ${\mathrm{\ensuremath{\kappa}}}_{2\mathrm{D}}$=1.54\ifmmode\pm\else\textpm\fi{}0.10 A${\mathrm{\r{}}}^{2}$/eV, which is similar to previously measured compressibilities of UPD monolayers and is in reasonable agreement with theoretical estimates. The rotation angle \ensuremath{\Omega} depends on electrode potential and adatom spacing, but irreversibly decreases with potential cycling (which is possibly due to the adsorption of trace impurities). Despite this irreversibility, the dependence of \ensuremath{\Omega} on adatom spacing qualitatively agrees with theory. We have investigated the structure of monolayers where the deposition potential is reached in either an anodic or cathodic scan and found these structures to be identical. This shows that the monolayer is in thermodynamic equilibrium and that the finite width and offset of the peaks in the deposition curves are due to kinetics, the influence of adsorbed anions, and/or substrate heterogeneity. At potentials between the monolayer region and bulk deposition, Tl forms a bilayer and this also has a hexagonal structure that is incommensurate with the Ag(111) substrate. In the bilayer, the compression is 1.0% (compared to bulk Tl) and the rotation from the Ag [011\ifmmode\bar\else\textasciimacron\fi{}] direction is 3.9\ifmmode^\circ\else\textdegree\fi{}; these are both less than in the monolayer.

PHOTOELECTROCHEMICAL EFFECT ON YBa2Cu3O7 SINGLE CRYSTALS

Electrochemical dc and transient measurements in the system YBa 2 Cu 3 O 7/ acetonitrile + tetrabutylammonium-perchlorate (as a supporting electrolyte) at T = 298 K showed a significant photoeffect on single crystal electrodes after irradiation with chopped white or monochromatic light under cathodic polarization conditions. The effect could not be observed on polycrystalline electrodes and on single crystals under anodic polarization conditions. Under potentiostatic control, the photoreduction current increased nearly exponentially with decreasing electrode potential. Obviously, an enhanced reduction of copper oxide components at the surface takes place during illumination. The photoeffect was found to depend on both the photon energy and the adjustment of the (twinned) single crystals and may be explained assuming a significant band gap in c-direction and an intrinsic photo-excitation of electrons from the valence band to the conduction band of the p-type conducting material.

ChemInform Abstract: Electrochemical Behaviour of Aluminium in Aqueous Caustic Solutions

Abstract The electrochemical behaviour of pure aluminium in aqueous KOH solutions has been examined by steady state and non-steady state techniques (impedance, potential pulse, potential scan). A surface free from oxide and hydroxide could not be established at any potential. The fall in the aluminium dissolution rate and the rise in the hydrogen evolution rate with decreasing electrode potential both arise from the thinning and hydration of the oxide layer. Decrease in alkalinity causes the film to thicken and to hydrate. In the non-steady state the relatively slow changes in the oxide film lead to non-uniformity of composition and a consequent temporary surface roughening.

Electrochemical behaviour of aluminium in aqueous caustic solutions

The electrochemical behaviour of pure aluminium in aqueous KOH solutions has been examined by steady state and non-steady state techniques (impedance, potential pulse, potential scan). A surface free from oxide and hydroxide could not be established at any potential. The fall in the aluminium dissolution rate and the rise in the hydrogen evolution rate with decreasing electrode potential both arise from the thinning and hydration of the oxide layer. Decrease in alkalinity causes the film to thicken and to hydrate. In the non-steady state the relatively slow changes in the oxide film lead to non-uniformity of composition and a consequent temporary surface roughening.

Microbial Activity in Lake Sediments with Particular Reference to Electrode Potential Gradients

Microbial activity was determined along electrode potential gradients within sediments, and along sediment surfaces in a stratified eutrophic lake. There was evidence of a change from a tricarboxylic acid cycle-based metabolism to a fermentative one with decreasing electrode potential. A more detailed examination of the stratification of the microbial community showed that the activities of enzymes associated with energy metabolism (in this case electron transport) were highest on electrode potential gradients. This was observed within a sediment core on a millimetredepth scale, as well as over several metres at the surface of sediments, on a transect which ran from oxic littoral muds to the anoxic profundal zone. In contrast, microbial hydrolytic enzymes, such as protease and amylase, were most active at the sediment surface, where the highest concentrations of substrates might be expected.

The kinetics of the cerous—ceric redox reaction at a platinum electrode

The kinetics of the cerous-ceric redox couple at the smooth platinum electrode in acid solution have been investigated using the rotating disc electrode technique. The polarisation curves obtained in sulphuric acid show the retarding effect of the oxidic layer formed on the electrode, upon the ceric reduction kinetics. In perchloric acid, and to a lesser extent in nitric acid, however, some gross yysteresis effects were observed in the polarisation curves which display a region where the reduction current decreases with decreasing electrode potential. This effect cannot be accounted for on the basis of any expected influence of the oxidic layer alone, and it is suggested that the explanation should be sought in the modification of the electrical double layer at the electrode brought about by appreciable coverages with adsorbed cerous ions. The diffusion coefficient of the ceric species in sulphuric acid was measured and found to be in agreement with the value previously measured by means of the anthracene electrode.