Reduction Of Hypochlorite Research Articles

Cerium Oxide on a Fluorinated Carbon-Based Electrode as a Promising Catalyst for Hypochlorite Production.

Sodium hypochlorite (NaOCl) is widely used as a disinfectant agent for water treatment and surface cleaning. A straightforward way to produce NaOCl is by the electrolysis of an aqueous sodium chloride (NaCl) solution. This process presents several side reactions decreasing its efficiency with hypochlorite reduction on the cathode surface being one of the main detrimental reactions. In this work, we have studied carbon-based electrodes modified with cerium oxide (CeO2), fluorine, and platinum nanoparticles as cathodes for hypochlorite production. Fluorination was carried out electrochemically; the polyol method was used to synthesize platinum nanoparticles; and the hydrothermal process was applied to form a CeO2 layer. Scanning electron microscopy, FTIR, and inductively coupled plasma (ICP) indicated the presence of cerium oxide as a film, fluorine groups on the substrate, and a load of 3.2 mg/cm2 of platinum nanoparticles and 2.7 mg/cm2 of CeO2. From electrochemical impedance spectroscopy, it was possible to demonstrate that incorporating platinum and fluorine decreases the charge transfer resistance by 16% and 28%, respectively. Linear sweep voltammetry showed a significant decrease in hypochlorite reduction when the substrate was doped with fluorine from −16.6 mA/cm2 at −0.6 V to −9.64 mA/cm2 that further reduced to −8.78 mA/cm2 with cerium oxide covered fluorinated electrodes. The performance of the cathode materials during hypochlorite production improved by 80% compared with pristine activated carbon cloth (ACC) electrodes. The improvement toward hindering NaOCl reduction is probably caused by the incorporation of a partial negative charge upon doping with fluorine.

Mixed Chromate and Molybdate Additives for Cathodic Enhancement in the Chlorate Process

The economic viability of the electrochemical chlorate process depends on toxic chromate to induce cathodic selectivity to hydrogen and mitigate reduction of hypochlorite or chlorate. In this study, it is shown that performance of a pilot plant for chlorate production can be sustained when a 1000-fold reduction in chromate concentration is compensated by addition of molybdate. Laboratory measurements employing a Quartz Crystal Microbalance suggest growth of a nanometre-thick hybrid Mo–Cr-oxide film to induce cathodic selectivity. An optimized energy efficiency for pilot plant operation was obtained using 0.8 mM molybdate and 27 μM chromate, balancing formation of an effective oxide layer and undesired Mo-induced decomposition of hypochlorite to oxygen in solution. Refinement at the pilot scale level is expected to further optimize the energy consumption, thereby increasing safety aspects and the economic viability of chlorate production.Graphical

Selective electrochemical hydrogen evolution on cerium oxide protected catalyst surfaces

To date the only known solution to avoid the unwanted electrochemical reduction of hypochlorite and chlorate in industrial chlorate production, performed in undivided cells, is the addition of dichromate to the chlorate electrolyte. Because of the toxicity of this compound its use is restricted within the European Union to time limited authorization by REACH. Therefore, an alternative to sodium dichromate is essential to maintain, or even increase the process efficiency.The addition of cerium (III) salts to a hypochlorite solution increases the cathodic selectivity towards hydrogen evolution (HER), the preferred cathode process in industrial chlorate production. This is attributed to the deposition of a thin cerium oxide/hydroxide coating on the cathode, induced by the increased local alkalinity during electrolysis.Performing the electrodeposition of such protective coating ex situ, well-controlled coating thickness can be achieved. Optimizing the deposition conditions (time, current density), a coherent and stable coating is formed on the electrode surface. On this protected electrode surface the electrochemical reduction of hypochlorite is suppressed by ca. 90% compared to the bare Pt electrode, while the HER proceeds with high selectivity and unchanged kinetics. Interestingly, other electrochemical reactions (O2 reduction, H2O2 reduction and oxidation) are also suppressed by the protective coating, suggesting that the deposited layer acts as an inorganic membrane on the electrode surface.

The electrochemistry of chalcopyrite in alkaline solutions

This paper summarizes the results of a study of the anodic characteristics of chalcopyrite in alkaline hydroxide and carbonate solutions. The cathodic reduction of oxygen, peroxide and hypochlorite have also been investigated as possible oxidants in a two-stage process for the heap leaching of chalcopyrite under alkaline conditions.Mixed potential measurements of chalcopyrite in alkaline solutions showed that the reactivity of the common oxidants is in decreasing order hypochlorite > peroxide > oxygen >> chlorate.Anodic oxidation of chalcopyrite involves active oxidation at low potentials followed by a passivation peak and transpassive oxidation at higher potentials. The kinetics of oxidation is favoured by increasing pH in both hydroxide and carbonate solutions. The formation of solid products of oxidation does not prevent continued oxidation for up to 18 h at potentials below the peak. Coulometric measurements suggest that both thiosulfate and sulfate ions are produced by oxidation in sodium hydroxide solutions while thiosulfate is the main product in carbonate and bicarbonate solutions.At potentials close to the mixed potentials, the rate of reduction of hypochlorite is approximately first-order in hypochlorite concentration and is greater at low hydroxide ion concentrations. The rate of the cathodic reduction of peroxide is also greater at lower pH values but is complicated by the surface catalysed disproportionation of peroxide to water and oxygen.Rates of oxidation of chalcopyrite by both hypochlorite and peroxide in sodium hydroxide solutions have been estimated. The rates in hypochlorite are an order of magnitude greater than those in peroxide solutions and are of the same order of magnitude as those previously measured for the dissolution of chalcopyrite in ammoniacal solutions with copper(II) ions as the oxidant.

Understanding Selectivity in the Chlorate Process: A Step towards Efficient Hydrogen Production

AbstractChlorate production is a highly energy demanding industrial process, where chlorate formation is accompanied with hydrogen formation on the cathode. To ensure a high cathodic current efficiency, sodium dichromate is added to the chlorate electrolyte to avoid reduction of hypochlorite formed as a reaction intermediate in the process. However, chromate is highly toxic to humans and environment, and therefore a replacement is desired. A model system with ex situ formed chromium oxide/hydroxide films were used to study hypochlorite reduction and hydrogen evolution. The experimental results demonstrate that the hypochlorite reduction is fully blocked while hydrogen evolution readily occurs. However, in the presence of hypochlorite the hydrogen evolution reaction is inhibited. By combining experimental findings with density functional theory (DFT) calculations, the mechanism of hypochlorite reduction was revealed and the reason for inhibition by the deposited chromium(III) film was demonstrated. Based on these results possible replacements for chromate are suggested.

Alternative Solution Additives for the Sustainable Electrolytic Production of Sodium Chlorate

Currently the dominating bleaching technology for kraft pulp is the usage of chlorine dioxide. This is formed on site from sodium chlorate, dominantly produced by the electrolysis of concentrated brine solutions. This latter process is very energy intensive and, as large amounts are produced, the annual electricity needs worldwide add up to about 20 TWh, which is comparable to the electricity consumption of a smaller European country or the State of Idaho. The losses caused by parasitic cathodic reactions in the electrochemical process are minimized by the addition of sodium dichromate to the electrolyte. Due to health concerns, the European Commission urge the replacement of this compound, and it is therefore vital to find alternative solutions for the sustainable production of sodium chlorate. The chlorate synthesis is performed in undivided electrochemical cells, where reduction of the hypochlorite intermediate and the chlorate product lead to significant efficiency losses in the absence of dichromate. In this work we present the effect of several transition metal compound additives on the electrochemical reduction of hypochlorite using model electrode surfaces. Most importantly in sodium permanganate containing solutions the hypochlorite reduction is effectively suppressed. This is related to the continuous build-up of an amorphous manganese oxide layer during the electrolysis. At the same time, the hydrogen evolution reaction (HER) proceeds on this electrode without significantly altered kinetics. The effect of the sodium permanganate additive was quantified by mass spectrometric measurements, where a significantly increased Faraday efficiency was observed compared to the addition free case (Figure 1). Figure 1 . H2 production rate measured during electrolysis with platinum counter and working electrodes at j = 100, 200, 300 mA cm− 2 in an 80 mM NaOCl + 2 M NaCl solution, pH = 12 and T = 80°C. The horizontal lines show the maximum H2 production rates at the different current densities, assuming 100 % current efficiency for H2 production. To circumvent the unlimited growth of the manganese oxide film, ex situ prepared electrodes of similar composition were also tested in the process. Using MnO2/Ti electrodes, formed by thermal decomposition, a high selectivity towards HER is achieved in hypochlorite containing solutions. The HER selectivity depends on the composition, morphology and thickness of these electrodes. The effect of these parameters is addressed in our experiments. Another very important sub-step of the chlorate production process is the disproportion of the hypochlorite intermediate to chlorate. As a parallel reaction, oxygen formation from the hypochlorite decomposition also occurs, which is catalyzed by different metal ions. To fully explore the effect of the promising solution additives, we also studied the effect of these compounds on the selectivity between chlorate and oxygen formation from the decomposition of hypochlorite. Figure 1

Electrochemical Investigation of the Hydrogen Evolution Reaction on Electrodeposited Films of Cr(OH)3 and Cr2O3 in Mild Alkaline Solutions

The hydrogen evolution reaction (HER) from water reduction is the main cathodic reaction in the sodium chlorate process. The reaction typically takes place on electrodes covered with a Cr(III) oxide-like film formed in situ by reduction of sodium dichromate in order to avoid reduction of hypochlorite and thereby increase the selectivity for the HER. However, the chemical structure of the Cr(III) oxide-like film is still under debate. In the present work, the kinetics of the HER were studied using titanium electrodes covered with electrodeposited Cr(OH)3 or Cr2O3, which were characterized by means of scanning electron microscopy (SEM), energy-dispersive x-ray spectroscopy (EDX), x-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. A clear difference in the morphology of the deposited surfaces was obtained, and the structure could be revealed with Raman spectroscopy. The kinetics for the HER were investigated using potentiodynamic and potentiostatic techniques. The results show that the first electron transfer is rate limiting and that the activity decreases in the order Cr2O3@Ti > bare Ti > Cr(OH)3@Ti. The low activity obtained for Cr(OH)3@Ti is discussed in terms of the involvement of structural water in the HER and the slow ligand exchange rate for water in Cr(III) complexes, while the high activity obtained for Cr2O3@Ti is rationalized by a surface area effect in combination with reduction of surface water and water in solution.Graphical ᅟ

Exploring the suitability of different electrode materials for hypochlorite quantification at high concentration in alkaline solutions

A comparison of the electrochemical performance of noble metal, sp2- and sp3-carbon electrodes towards the reduction of concentrated hypochlorite (ClO−; 0.02M–1.88M), an alkaline (pH ~13) and chemically oxidising solution, is presented. Boron-doped diamond (BDD) gave no discernible electrochemical response whilst all other electrodes showed an electrochemically irreversible cathodic response in the following order (most facile electron transfer): Pt>Au>edge-plane pyrolytic graphite (EPPG)≈sp2-carbon containing BDD>basal plane pyrolytic graphite>glassy carbon≈highly-ordered pyrolytic graphite. Reduction at sp2-BDD occurred at a similar potential to EPPG, but with much lower currents, suggesting edge-plane-like sites are the active sp2 sites in this sp2-BDD, but at much lower surface coverage than in EPPG. Finally, both EPPG and sp2-BDD showed linear current versus [ClO−] responses over the concentration range 0.02M–1.41M (R2>0.995).

Study of Hypochlorite Reduction Related to the Sodium Chlorate Process

Reduction of hypochlorite is the most important side reaction in the sodium chlorate reactor leading to high energy losses. Today chromate is added to the reactor solution to minimize the hypochlorite reduction but a replacement is necessary due to health and environmental risks with chromate. In order to understand the effect of different substrates on the hypochlorite reduction, α-FeOOH, γ-FeOOH, Cr2O3 and CrOH3 were electrodeposited on titanium and subjected to electrochemical investigations. These substances are commonly found on cathodes in the chlorate process and can serve as model substances for the experimental investigation. The mechanism of hypochlorite reduction was also studied using DFT calculations in which the reaction at Fe(III) and Cr(III) surface sites were considered in order to single out the electrocatalytic properties. The experimental results clearly demonstrated that the chromium films completely block the reduction of hypochlorite, while for the iron oxyhydroxides the process can readily occur. Since the electrocatalytic properties per se were shown by the DFT calculations to be very similar for Fe(III) and Cr(III) sites in the oxide matrix, other explanations for the blocking ability of chromium films are addressed and discussed in the context of surface charging, reduction of anions and conduction in the deposited films. The main conclusion is that the combined effect of electronic properties and reduction of negatively charged ions can explain the reduction kinetics of hypochlorite and the effect of chromate in the chlorate process.

Modified multiwalled carbon nanotube/epoxy amperometric nanocomposite sensors with CuO nanoparticles for electrocatalytic detection of free chlorine

The benefit of using copper (II) oxide nanoparticles (CuO-NPs), which have catalytic activity for the decomposition of hypochlorite solutions, for the amperometric detection of free chlorine is reported. Cyclic voltammetry and electrochemical impedance spectroscopy have been applied for the electrochemical characterization of nanocomposite materials. If the amperometric detection of free chlorine was determined previously using multiwall carbon nanotube (MWCNT) based epoxy nanocomposite sensors, the electrocatalytic reduction of hypochlorite using modified MWCNT/epoxy nanocomposite sensors with CuO-NPs to obtain sensitive devices capable to amperometrically determine traces of free chlorine in water is demonstrated here. The CuO-NPs were incorporated in the nanocomposite electrode in two different ways i) on the MWCNT surface, Route A and ii) in the nanocomposite matrix in powder form, Route B. Both modified-nanocomposite sensors have shown a fast electron transfer exchange, high electroactive area and an enhancement on the electroanalytical signal. Accordingly, a greater sensitivity compared to raw MWCNT/epoxy nanocomposite sensors has been observed, obtaining the lowest limit of detection for the CuO-NPs modified nanocomposite sensors obtained by Route B.

Modelling current efficiency in an electrochemical hypochlorite reactor

A mathematical model was set up for anodic, cathodic and overall current efficiencies of an electrochemical cell for hypochlorite production acting as an ideal stirred reactor. A 0.06–0.20moldm−3 NaClO hypochlorite solution was obtained in the cell by the electrolysis of 0.25–0.50moldm−3 sodium chloride solution at a temperature of 20°C, at a current density of 100mAcm−2 and at 8.3<pH<8.7. Anodic current losses occur through hypochlorite and water oxidation at the DSA anode, and cathodic current losses result from hypochlorite reduction at the titanium cathode.Theoretical dependences of the anodic current efficiency on the overall anodic current density, hypochlorite concentration and partial current density for water oxidation were established. Theoretically derived relations and the corresponding experimental results showed that oxygen evolution due to water and hypochlorite oxidation leads to reduced thickness of the anodic diffusion layer and increases the limiting diffusion current for hypochlorite oxidation. Mathematical models for the dependence of the cathodic current efficiency on the cathodic current density and hypochlorite concentration in the bulk solution were also established. The expression for the overall current efficiency of the electrochemical cell for hypochlorite production was derived on the basis of both the anodic and the cathodic current efficiency. The good agreement between experimental results and the values provided by the mathematical models has confirmed the correctness of the proposed models, suggesting that the model can be used to optimise electrolysis parameters. (The term hypochlorite is used to include both hypochlorite and undissociated hypochlorous acid.)The concentration of dissolved elementary chlorine in the slightly alkaline environment is negligible compared to hypochlorite concentration.

Novel electrodialysis–electrochlorination integrated process for the reclamation of treated wastewaters

This work presents a novel integrated process for the reclamation of wastewaters. This process combines, in the same cell, the reduction in the effluent conductivity and TOC by electrodialysis together with the production of a value added stream, which can be used for disinfection purposes. In the diluate compartment, the solution is desalted whereas hypochlorite is electrochemically synthesized in the anolyte. The treatment of actual effluents from the Waste Water Treatment Plant (WWTP) of Ciudad Real (Spain) was confronted. It was observed that the concentration of hypochlorite synthesized in the anolyte increased when anolyte and catholyte are separated into different circuits, avoiding the reduction of hypochlorite on the cathode surface. Furthermore, a higher voltage to cell pair ratio also enhances the production of hypochlorite. Finally, it was checked the disinfectant potential of the final anolyte, being possible to produce a desalted and disinfected final stream with a total electrical consumption of 1.03Whdm−3 and dosing a volumetric ratio anolyte:diluate of 4:96.

Color Stability of White Mineral Trioxide Aggregate in Contact with Hypochlorite Solution

IntroductionOne of the uses of white mineral trioxide aggregate (MTA) is as an apical barrier in immature teeth. Although this treatment has been reported to have high success rates, a number of cases of discoloration have been noted. The aim of this research was to investigate the color stability of white MTA in contact with various solutions used in endodontics. MethodsThe change in color of white MTA after immersion in water, sodium hypochlorite, or hydrogen peroxide was assessed by viewing the color change on digital photographs and also by using a spectrophotometer. White MTA, white Portland cement, and bismuth oxide were assessed. The changes in the material after immersion in the different solutions were assessed by x-ray diffraction analysis and Fourier transform infrared spectroscopy. ResultsImmersion of white MTA and bismuth oxide in sodium hypochlorite resulted in the formation of a dark brown discoloration. This change was not observed in Portland cement. X-ray diffraction analysis and Fourier transform infrared analysis displayed the reduction of sodium hypochlorite in contact with bismuth oxide and MTA to sodium chloride. ConclusionsContact of white MTA and other bismuth-containing materials with sodium hypochlorite solution should be avoided.

Ti substrate coated with composite Cr–MoO2 coatings as highly selective cathode materials in hypochlorite production

The aim of this work was to investigate the possibility of preparation of the composite Cr–MoO2 coatings onto steel and titanium substrates as cathode materials with high selective properties which imply the suppression of hypochlorite reduction as a side reaction during hypochlorite commercial production. The electrodes were prepared by simultaneous deposition of chromium and suspended MoO2 particles on titanium substrate from acid chromium (VI) bath. The current efficiency for electrodeposition of the composite coatings did not vary significantly with the concentration of suspended MoO2 particles. The content of molybdenum in the deposits was relatively low (0.2–1.5at.%) and increased with increasing the concentration of suspended MoO2 particles in the bath, in the range from 0 to 10gdm−3. With further increase in the concentration of MoO2, the content of molybdenum in the coating varied insignificantly.X-ray photoelectron spectroscopy-XPS and EDS analysis were applied to analyze elemental composition and chemical bonding of elements on the surface and in the sub-surface region of obtained coatings. When the concentration of MoO2 particles in the bath was raised above 5gdm−3, the appearance of the coating changed from the typical pure chromium deposit to needle-like deposit with the appearance of black inclusions on the surface. XPS analysis and corresponding Cr 2p spectra showed the presence of chromium oxide, probably Cr2O3 with Cr(3+) valence state on the surface and in the sub-surface region of Cr–MoO2 coatings.Investigation of the current efficiency for the hydrogen evolution reaction (HER) on Cr–MoO2 cathodes showed that it increased with the increase of the content of MoO2 particles in the coating, exceeding the value of 97% in the solution with the hypochlorite concentration of 0.21moldm−3. Under the same conditions, the current efficiency for the HER on Ti and Ti/Cr cathodes is very low (≈20%), and corresponding polarization curves confirmed the fact that the side reaction of hypochlorite reduction takes place in the diffusion-controlled regime on these electrodes. The high selectivity of Cr–MoO2 coating is probably caused by the presence of chromium oxide (hydroxide) formed at the surface of the coating during co-deposition of Cr and MoO2 particles, which prevents hypochlorite reduction on the cathode during the HER.

On the suppression of cathodic hypochlorite reduction by electrolyte additions of molybdate and chromate ions

The goal of this study was to gain a better understanding of the feasibility of replacing Cr(VI) in the chlorate process by Mo(VI), focusing on the cathode reaction selectivity for hydrogen evoluti ...

Rare earth metal salts as potential alternatives to Cr(VI) in the chlorate process

Chromate is today added to industrial chlorate electrolyte, where it forms a thin cathode film of chromium hydroxide that hinders unwanted reduction of hypochlorite and chlorate. The aim of this study was to investigate rare earth metal (REM) ions as an environmentally friendly alternative to the toxic chromate addition. Potential sweeps and iR-corrected polarisation curves were recorded using rotating disc electrodes of iron and gold. Addition of Y(III), La(III) or Sm(III) to 5 M NaCl at 70 °C suppressed hypochlorite reduction. Activation of hydrogen evolution by REM ion addition to 0.5 M NaCl was more significant at 25 °C than at 50 and 70 °C. Increasing the chloride concentration to 5 M had a detrimental effect on this activation. The major problem in replacing chromate with REM salts is the poor solubility of REM ions at normal chlorate process conditions, and therefore REM salts are not a realistic alternative to chromate addition.

Cathodic current efficiency in the chlorate process

Sodium chlorate is produced in undivided electrolysis cells. Hydrogen is evolved on the cathodes, usually made of steel, while chloride ions are oxidised to chlorine on the anodes, usually DSA®s. Parasitic cathodic reactions, lowering the cathodic current efficiency (CE), are the reduction of hypochlorite and chlorate ions. These reactions are suppressed by the addition of Cr(VI) to the electrolyte. In this work the effects that time of the electrolysis, chromate concentration and interruption of the electrolysis process have on CE has been investigated. New steel, as well as steel samples cut from cathodes used in a chlorate plant, were used as cathode material. Laboratory experiments in a divided cell were made to determine the rate of hydrogen production, and thereby indirectly CE, at varying operating conditions. It was found that the chromate concentration is important for the CE in the range 0.5–6 g l−1 Na2Cr2O7. The CE was higher on new steel than on the used steel, which had a more corroded and inhomogeneous surface. When starting the electrolysis the CE was initially low, at a value depending on the operating conditions, but increased with time of polarisation. The time to reach an approximate steady CE was generally in the order of hours. Electrolysis shut downs in the presence of hypochlorite (≤ 3 g l−1 NaClO) resulted in corrosion of iron and a low CE when restarting the process. After one such corrosion shut down the new steel showed as low CE as the used steel. When restarting the electrolysis after a shut down without hypochlorite the CE was higher than before the shut down. Current densities of a simulated bipolar plate during a shut down were measured to 50–150 A m−2, resulting from oxidation of steel and reduction of oxy chlorides on the catalytic DSA® electrode.

A Simple Preconcentration-Storing Technique Based on Sulfide Precipitation for Multipoint Monitoring of Mercury in Rainwater and Snow by Cold Vapor Atomic Absorption Spectrometry

Abstract A preconcentration-storing technique that does not require any special or expensive apparatus and reagents was developed to monitor mercury in rainwater and snow at many points as simultaneously as possible. A commercially available sodium hypochlorite solution was added to the sample solution to prevent a decrease in mercury concentration after sampling. Mercury(II) ion (0.5–50 ng) in 500 mL of the sample solution was concentrated by adding zinc(II) ion and a small amount of sulfide ion and then coagulating/coprecipitating with Zn(OH)2 or ZnS after reduction of residual hypochlorite by addition of hydroxylammonium chloride. The supernatant solution was discarded by decantation, and the solution including the precipitates could be stored for at least 20 days without any loss of mercury. The recoveries of mercury(II) ion from the 500 mL of the sample solutions spiked with mercury(II) ions were 99–109%, with standard deviation ranges of 3–8%, although the recovery of methylmercury(II) ion was 68 ± 5%. Using this technique, the determination of mercury in rainwater and snow was demonstrated at Toyama Prefecture, Japan, from April 2004 to February 2005; mercury levels ranging from 2.7 to 33.7 ng L−1 were found.

Effect of Chromium Additives to Nanocrystalline Ti:Ru:Fe:O on the Reduction of Hypochlorite

Composite materials made of nanocrystalline Ti:Ru:Fe:O (2:1:1:2) mixed with or were prepared by high-energy ballmilling. These materials were cold-pressed in the form of disk electrodes and used to electrolyze a chlorate electrolyte that does not contain dichromate or chromate ions. The concentration in the electrolyte was measured with respect to the electrolysis time for a variety of composite electrodes prepared by varying the milling time, . In all cases, the decomposition rate, , was calculated from the slope of the vs time curve. In the case of nanocrystalline Ti:Ru:Fe:O mixed with , decreases from as increases from . For , the decomposition rate increases to . In the case of nanocrystalline Ti:Ru:Fe:O mixed with , is only marginally affected for , while it reaches a value as high as after of milling. Through a detailed structural and surface analysis performed by using X-ray diffraction, energy-dispersive X-ray analysis, and X-ray photoelectron spectroscopy, it is shown that a chromium hydroxide layer is formed at the surface of the electrodes even if the electrolysis is conducted in a chlorate electrolyte that does not contain dichromate or chromate ions. This arises due to dissolution of Cr species in the electrolyte and their redeposition at the electrode surface under cathodic polarization. The reasons underlying the different behavior of composite electrodes made with instead of are discussed.

Continuous Detection of Hypochlorous Acid/Hypochlorite for Water Quality Monitoring and Control

This work demonstrates the continuous detection of hypochlorite in tap water using bare platinum electrodes. Continuous amperometric operation is possible for several days in spite of Pt oxide formation. Pt oxides are formed both by simple contact of the Pt surface with a hypochlorite solution, and during the electrochemical reduction of hypochlorite ions. Pt oxide and hypochlorite reduction processes may overlap at moderate to high hypochlorite concentrations, resulting in erroneous concentration readings. Passivated cathodes are reactivated by cathodic cycling in supporting electrolyte. Electrode protection using suitable membranes may extend the useful life time of Pt electrodes, particularly in complex samples.