A simple method for quantifying the humic content of commercial products

A method based on an analytical technique, initially developed for quantifying aquatic refractory organic matter (often called humics), has been applied to commercial samples claiming to contain humic-type substances. At present, no method exists for quantifying the humic content on this type of sample. The analytical method is based on measuring the peak current obtained by adsorptive stripping voltammetry of the complex formed by refractory organic matter in the presence of trace amounts of Mo(VI). The quantification procedure requires the response obtained for the unknown sample to be compared with the response obtained with International Humic Substance Society (IHSS) reference humic substances. A very simple procedure that enables the humic content of any sample to be expressed as IHSS standard equivalents is described in detail. The method is highly selective, reproducible and suitable for routine analysi

Quantification of refractory organic substances in freshwaters: further insight into the response of the voltammetric method

A recently published method for quantifying refractory organic matter (often referred to as humic substances) in freshwaters was applied to a wide range of International Humic Substance Society (IHSS) humic compounds in order to (i) gain a better understanding of the mechanism of the voltammetric response which is the basis of the analytical method and (ii) provide guidance on choosing the optimal standard to be used. At the same time, the sensitivity of the technique has been increased by switching from the pulse mode initially proposed to the square-wave mode. The results obtained show that (i) differences in adsorption onto the electrode rather than differences in complexation strength are responsible for the differences in the intensity of the signal obtained for the different humic compounds, (ii) carboxylate, N- and S-containing groups do not play a role in the voltammetric signa

The lambda-dimension of commutative arithmetic rings

It is shown that every commutative arithmetic ring $R$ has $lambda$-dimension $leq 3$. An example of a commutative Kaplansky ring with $lambda$-dimension 3 is given. If $R$ satisfies an additional condition then $lambda$-dim($R$

Characterization of the Surface Electrode Reaction in the Presence of Uniform Interaction: The Case of Mo(VI) Reduction in the Presence of Phenanthroline and an Excess of Fulvic Acids

cited By 1International audienceSurface electrode reactions involving lateral uniform interactions between adsorbed species is studied by means of square-wave voltammetry (SWV). Interactions are represented by the interaction product aθ, were a is the Frumkin interaction parameter (a is positive for attraction and negative for repulsion forces) and θ is the surface coverage. The properties of the SW voltammetric response enable detection of interactions and recognition of the type of interaction forces by a simple procedure. The influence of the interactions on the apparent electrochemical reversibility of the surface electrode reaction is studied in detail. Utilizing "quasireversible maximum" the simple methodology for estimation of the standard redox rate constant without knowing the exact value of the interaction product aθ is developed. Theoretical predictions are illustrated and confirmed by experiments with Mo(VI) in the presence of phenantroline and an excess of fulvic acids

Catalytic adsorptive stripping voltammetry of molybdenum: Redox kinetic measurements

cited By 10International audienceThe electrode mechanism of Mo(VI) reduction was studied under catalytic adsorptive stripping mode by means of square-wave voltammetry (SWV). Mo(VI) creates a stable surface active complex with mandelic acid. The electrode reaction of Mo(VI)-mandelic acid system undergoes as one-electron reduction, exhibiting properties of a surface electrode process. In the presence of chlorate, bromate, and hydrogen peroxide, the electrode reaction is transposed into a catalytic mechanism. The experimental results are compared with the recent theory for surface catalytic reaction, enabling qualitative characterization of the electrode mechanism in the presence of different catalytic agents. Utilizing both the method of "split SW peaks" and "quasireversible maximum" the standard redox rate constant of Mo(VI)-mandelic acid system was estimates as ks = 150 ± 5 s-1. By fitting the experimental and theoretical results, the following catalytic rate constants have been estimated: (8.0 ± 0.5) × 104 dm3 s-1, (1.0 ± 0.1) × 105 mol-1 dm3 s-1, and (3.2 ± 0.1) × 106 mol-1 dm3 s -1, for hydrogen peroxide, chlorate, and bromate, respectively

Square-wave voltammetry of molybdenum-fulvic acid complex

cited By 34International audienceThe reduction process of molybdenum in the presence of fulvic acids was investigated by square-wave voltammetry (SWV). The influence of the parameters (frequency, amplitude) of the SW on the reduction signal is analyzed according to the theoretical model. The molybdenum-fulvic acid complex is reduced reversibly with adsorption of the reactant

The role of adsorption in the catalytic electrode mechanism studied by means of square-wave voltammetry

cited By 6International audienceA complex electrode mechanism coupled with adsorption of the redox couple and an irreversible homogeneous chemical reaction that regenerates the electroactive reactant is studied both theoretically and experimentally under conditions of square-wave voltammetry. The homogeneous regenerative redox reaction, i.e., the so-called catalytic reaction, takes place as a surface or volume process, depending on whether it includes the dissolved or adsorbed form of the electrode product, respectively. A rigorous theoretical model is presented considering simultaneously all relevant phenomena affecting the voltammetric response, such as mass transport, adsorption equilibria and kinetics of two different catalytic reactions, i.e., volume and surface catalytic reactions. The solutions for the surface concentration of the electroactive species are presented in the form of integral equations, thus they are general and valid for any chronoamperometric and voltammetric technique. The model enables systematic study of the role of adsorption in a complex catalytic electrode mechanism. The properties of the voltammetric response show remarkable discrepancies between the effect of the volume and surface catalytic reactions, thus enabling their recognition and separate characterization. From an analytical point of view it is demonstrated that the catalytic mechanism of moderate adsorption is superior in comparison with the pure volume and surface catalytic mechanisms. The theoretical predictions are confirmed by the voltammetric behaviour of the redox couple azobenzene/hydrazobenzene at the mercury electrode by using a mixture of water + acetonitrile as a solvent. © 2005 Elsevier B.V. All rights reserved

Lutetium bis(tetra-tert-butylphthalocyaninato): A superior redox probe to study the transfer of anions and cations across the water|nitrobenzene interface by means of square-wave voltammetry at the three-phase electrode

cited By 34International audienceThe redox properties of lutetium bis(tetra-tert-butylphthalocyaninato) (LBPC) have been studied in nitrobenzene that is deposited as a microfilm on the surface of highly oriented pyrolytic graphite electrodes. The behavior of the modified electrode, which is immersed in an aqueous electrolyte solution, is typical for the three-phase electrode (Scholz, F.; Komorsky-Lovrić, Š.; Lovrić, M. Electrochem. Comm. 2000, 2, 112-118). LBPC can be both oxidized and reduced in one electron reversible processes. The oxidation and the reduction of LBPC at the graphite|nitrobenzene interface is accompanied by the transfer of anion or cation, respectively, from the aqueous phase into the organic layer. Thus, using LBPC as a redox probe for the three-phase electrode, the transfer of both anions and cations across the water|nitrobenzene interface can be studied in a single experiment. The hydrophobicity of LBPC is so high that it enables inspection of cations and anions with ΔwnbGCat+θ ≤ 43 kJ/mol and Δwnb ≤ 50 kJ/mol, respectively. The direct transfer of Na+ and Li+ from water to nitrobenzene, mutually saturated, is achieved for the first time at a macroscopic water|nitrobenzene interface

Electrochemical hydrogen production in aqueous micellar solution by a diiron benzenedithiolate complex relevant to [FeFe] hydrogenases

International audienceThe diiron hydrogenase model Fe2(bdt)(CO)6 (1, bdt = benzenedithiolate) was dispersed in an aqueous micellar solution prepared from sodium dodecyl sulfate (SDS). The aqueous solution of 1 showed no sign of decomposition when left in contact with air over a period of several days. Current–potential responses recorded at a dropping mercury electrode over pH 7–3 were consistent with reduction of freely diffusing species. Catalysis of proton reduction was observed at pH < 6 with current densities exceeding 0.5 mA cm−2 at an acid-to-catalyst ratio of 17. Bulk electrolysis at −0.66 V vs. SHE of a solution of 1 at pH 3 confirmed the production of hydrogen with Faradaic efficiency close to 100%. A mechanism involving initial reduction of 1 and subsequent proton-coupled electron transfer is proposed