Anodic polarization of silicon electrodes in water solutions of NaNO3

The polarization curves of silicon electrodes in sodium nitrate, ferric chloride, ferric sulphate and ferric nitrate obtained experimentally are presented. The curves calculated theoretically assuming the anodic passivation and the carrier generation in the space charge layer are compared with an empirical one derived from the equation of the generation in the space charge layer. The experimental results are in excellent agreement with the empirical curve providing that a suitable coefficient of the current multiplication is introduced.     

The dehydration kinetics of calcium sulphate dihydrate influence of the gaseous atmosphere and the temperature

The dehydration reaction rate of calcium sulphate dihydrate decreases monotonically with the water vapour pressure. The kinetics curves, of sigmoidal form, may be described, beyond the point of inflexion by a process of interface advance. The influence of foreign gases provide evidence that it is necessary for crystallizing water to pass through an adsorbed state.     

Anodic behaviour of pyrite in acid solutions

In acid solution, pyrite gives reproducible anodic current3v?oltage curves with Tafel slopes 95 ± 10 mV. There is no influence of pyrite semiconducting properties on the kinetics of the anode process. This process, as determined by coulometry at constant potential and product analysis, is a combination of the reactions

and

The sulphate route dominates over the potential range accessible at ambient temperature, and the sulphate yield increases linearly with potential. Data from other sources suggest that sulphate yield at fixed potential may be independent of temperature. Evidence from studies of the electrochemical behaviour of sulphur and its compounds leads to the conclusion that elemental sulphur is not an intermediate in the sulphate route. A mechanism for sulphate formation involving adsorbed oxygen-containing intermediates is postulated and the kinetics analysed under Temkin adsorption conditions. The Tafel slope and pH dependence suggest that the second electron transfer step is rate-determining.     

Some quantitative studies on light fading of after-treated direct dyes

Quantitative measurements of increase in lightfastness of direct dyes on cellophane on after-treatment with different metallic salts, e.g. sulphates of copper, cobalt, nickel, chromium and (ferrous) iron have been carried out. Copper sulphate has been found to be the best of all these salts. About 2% copper sulphate is found to cause considerable increase in the lightfastness. Measurements of y/x ratios of dyed and after-treated cellophane and also slopes of CF curves have shown that the higher fastness is due to higher aggregation of the dye in the fibre on after-treatment with copper sulphate. Metal complex formation with dye structures of the type o-hydroxy-o′-methoxy (or o′-hydroxy or ethoxy) azo has been found to be more useful for copper after-treatment than complex formation with dyes with a salicylic acid residue.     

The influence of water vapour on the transformation of γ calcium sulphate anhydrite to β anhydrite

The isothermal kinetics of the transformation of γ calcium sulphate anhydrite to β anhydrite have been determined. All the curves obtained have a sigmoidal form. We have also observed an influence of the water vapour pressure on the rate of formation of β anhydrite. On each plot of rate vs. water vapour pressure one observes a maximum whose position shifts to lower values of water vapour pressure when the temperature decreases. The formation of β anhydrite a low temperatures may be explained by the geometry of the water vapour pressure-temperature diagram of the hydrated forms of calcium sulphate, and by the “mineralizing” action of water vapour at or close to saturation.     

Electrodeposition of iron in sulphate solutions

The kinetics of iron electrodeposition from acid sulphate solutions onto a platinum electrode was investigated by means of stationary polarisation curves and electrochemical impedance spectroscopy. Together with interfacial pH data previously obtained, the effect of pH was analysed. The formation of at least three adsorbed intermediates at the cathode surface was evidenced in all pH values. The relative rate of their formation and its surface concentration depend on the solution pH as well as on the electrode potential. It is suggested that two of these species catalyses the H⁺ reduction whereas the other one may have a blocking effect on this reaction.     

Influence of tartaric acid on zinc electrodeposition from sulphate bath

The effect of tartaric acid on zinc electrodeposition from sulphate plating bath was investigated by electrochemical impedance spectroscopy (EIS), stationary polarization curves, X-ray diffraction and SEM imagery. The study shows that it is possible to obtain homogeneous, compact and dendrites-free zinc deposits from sulphate solutions containing tartaric acid. For various hydrodynamic methods (rotating disc electrode and static vertical or horizontal electrode), the results indicate that, the presence of only small quantity of tartaric acid, may induce significant changes on deposition rates and deposit quality.     

Insoluble anode of porous lead dioxide for electrosynthesis: preparation and characterization

Preparation of a novel type of titanium-substrate lead dioxide anode with enhanced electrocatalytic activity for electrosynthesis is described. It has been demonstrated that in the presence of a suitable surfactant in the coating solution, an adherent and mainly tetragonal form of lead dioxide is deposited on a platinized titanium surface such that the solution side of the coating is porous while the substrate side is compact. By an analysis of anodic charging curves and steady-state Tafel plots with such porous electrodes in contact with sodium sulphate solution, it has been proved that the electrochemically active area of these anodes is higher by more than an order of magnitude when compared to the area of conventional titanium-substrate lead dioxide anodes. The electrocatalytic activity is also thereby enhanced to a significant degree.     

Kinetics of the thermal decomposition of alkyl hydrogen sulphates

First order rate constants and Arrhenius parameters have been obtained for the thermal decomposition of 1-hexadecyl hydrogen sulphate. From published data on thermal decomposition of lauryl hydrogen sulphate and lauryl ether hydrogen sulphate, first order rate constants and Arrhenius parameters have been obtained. The agreement between the two sets of data for the two alkyl hydrogen sulphates is within the 95% confidence limits, a combined Arrhenius plot giving an activation energy of 12.69 ± 1.97 K cal mol⁻¹ and pre-exponential factor of 10^{(4.68 ± 1.24)} sec⁻¹. For lauryl ether hydrogen sulphate, a 3-point Arrhenius plot gives an activation energy of 9.4 K cal. mol⁻¹ and a pre-exponential factor of 10² sec⁻¹.     

The discharge behaviour of the porous lead electrode in the lead-acid battery. II. Mathematical model

A mathematical model for the porous lead electrode in the lead-acid battery has been derived in order to explain the limited discharge capacity at high rates of discharge. By comparison with experimental discharge curves the model predicts that the discharge process is ended by a coverage of the electroactive surface of lead by lead sulphate, even at current densities around 1000 A m^–2. Only at concentrations lower than the normal (4–5 M) and very high current densities will depletion of sulphuric acid become a limitation. By using an expression which relates, the local maximum utilization of the active material to the time integral of the local current density in a way which resembles the empirical Peukert equation, final lead sulphate distributions can be predicted with a maximum in the centre of the electrode, which is in agreement, with measurements.     

Electrochemical behaviour of heat-treated AlZnMg alloys in chloride solutions containing sulphate

The electrochemical corrosion and passivation of Al5Zn1.7Mg0.23Cu0.053Nb alloys, submitted to different heat treatments (cold-rolled, annealed, quenched and aged, and quenched in two steps and aged), in sulphate-containing chloride solutions, has been studied by means of cyclic polarization, electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM), energy-dispersive X-ray (EDX), and X-ray photoelectron spectroscopy (XPS). The cyclic polarization curves showed that sulphate addition to the chloride solution produced a poor reproducible shift of the breakdown potential to more positive potentials. The repassivation potentials, much more reproducible, and practically separating the passive from the pitting potential region, were slightly displaced in the negative direction with that addition. When the alloys were potentiodynamically polarized in the passive potential region, sulphate was incorporated in the oxide film, thus precluding chloride ingress. In addition, Zn depletion was favoured, whereas Mg losses were avoided. Different equivalent circuits corresponding to different alloys and potentials in the passive and pitting regions were employed to account for the electrochemical processes taking place in each condition. This work shows that sulphate makes these alloys more sensitive to corrosion, increasing the fracture properties of the surface layer and favouring the pitting attack over greater areas than chloride alone.     

Hydrogen consuming anodes for energy saving in sodium sulphate electrolysis

Gas diffusion electrodes, made of PTFE-bonded carbon with precious metal catalysts, were investigated as hydrogen consuming anodes in sodium sulphate electrolysis. The catalysts used were platinum and palladium and mixtures of both metals, prepared by two different methods. Various metal meshes were used as current collector. The electrodes performed well in pure sulphuric acid (5–15 wt-%) and in mixtures with sodium sulphate (10 wt-%) at temperatures of 30 to 70°C and current densities up to 5 kA/m². In long-term experiments, at a current density of 2.6 kA/m², the electrodes were stable over three months. The electrodes were characterised by stationary current density/potential curves and by galvanostatic current interruption measurements.     

Electrodeposition of Zn in acid sulphate solutions: pH effects

The kinetics of zinc electrodeposition from acid sulphate solution on a platinum electrode was investigated by means of stationary polarization curves, interfacial pH measurement and electrochemical impedance spectroscopy. The effect of pH, namely pH 2, 3 and 4, was analyzed. A significant dependence of Zn electrodeposition with solution pH was verified. The results obtained cannot be predicted by the available models for Zn electrodeposition. A reaction model is then proposed based on the predominant steps as a function of the potential and the electrode surface nature.     

An electrochemical method to study scaling by calcium sulphate of a heat transfer surface

The main purpose of this investigation was to devise an electrochemistry-based method, using the assessment of the oxygen-reduction cathodic reaction, in order to evaluate the intensity of calcium sulphate scale formation on a heat transfer surface. For this purpose, an experimental device, permitting the simultaneous measurement of the surface temperature and the current intensity relative to electrochemical oxygen-reduction on the heat transfer surface during the fouling process, was built. From the chronoamperometric curves, it is possible to deduce the induction time. The results were confirmed by the simultaneous direct measurement of the surface temperature. This coupling of thermal and electrochemical measurement gave additional information regarding the growth and the porosity of the formed scale layer. By this chronoamperometric method, it was shown that the supersaturation coefficient of a saturated calcium sulphate solution must exceed a critical value Ω_c of the order of 4 to observe the crystallization of the gypsum layer. For the same saturation level, the salt concentration has a stronger effect than the temperature on the precipitation process.     

Optimal operation of a batch cooling crystallizer

A theory of programmed cooling crystallization is presented based on the moment transformation of the population balance. Numerical predictions for the behaviour of a batch crystallizer agree closely with those from an alternative theory based on the discrete-supersaturation balance. A potential advantage of the present approach is shown to lie in the application of the Continuous Maximum Principle in optimal control theory to facilitate the numerical computation of optimal cooling curves.The transient behavior of a computed “size-optimal” operating policy which maximises the terminal size of the largest crystals is shown to be significantly different from either natural of linear cooling or from crontrolled cooling at a constant nucleation rate. A peak in the nucleation rate towards the end of the operation and an increased terminal crystal size is predicted for the conditions considered and this is supported by preliminary experimental work with potassium sulphate solutions in a laboratory-scale crystallizer.     

Thermodynamic analysis of (bi)sulphate adsorption on a Pt(1 1 1) electrode as a function of pH

A complete thermodynamic study of (bi)sulphate adsorption on Pt(1 1 1) electrodes from solutions at four different pHs (pH 0.43, 2.1, 3.1 and 4.1) is reported. The effect of pH on the sum of the Gibbs excesses of sulphate and bisulphate species, standard Gibbs energies of adsorption and formal partial charge numbers is analyzed. The results provide relevant information on the nature of species involved in the different voltammetric features. The experiments at pH 0.43 were performed in a higher base electrolyte concentration (0.5 M), that allows the study of (bi)sulphate adsorption in a broader range of concentrations. Under these conditions, two adsorption steps are clearly defined, associated to two different voltammetric features, between 0.30 and 0.60 V and between 0.65 and 0.90 V (standard hydrogen scale, SHE). Once the pH is increased, a marked decrease in absolute value of the (bi)sulphate adsorption Gibbs energy is observed, concomitant with an increasing amount of OH co-adsorption.     

Adsorption of pollutants on to activated carbon in fixed beds

The adsorption of certain pollutants, namely phenol, p-chlorophenol, sodium dodecyl sulphate and mercuric ions, on to activated carbon has been studied using fixed bed systems. There are three main methods of contacting in solid/liquid adsorption systems, namely batch, fixed bed and fluidized bed systems. In fixed bed adsorption the adsorption rate is determined on the basis of adsorption equilibrium (unfavourable, linear, favourable or completely irreversible) and the controlling mechanism (external film mass transport, internal pore diffusion, internal solid phase diffusion or longitudinal diffusion). One or more of the previous transport mechanisms may be rate controlling depending on the solute-adsorbent system. For an adsorbent like activated carbon which is highly porous both external transport and pore diffusion will be very important. An adsorption model, based on external mass transport and internal pore diffusion, has been applied to the systems to predict theoretical breakthrough curves. These curves have then been compared with experimental data and using a ‘best fit’ technique, an effective pore diffusion coefficient can be determined for each sorbate–carbon system.     

Preparation of polycrystalline thin films of CuInSe2 by electrodeposition

CuInSe₂ thin films with the chalcopyrite structure were electrodeposited on titanium substrates from an acidic sulphate solution containing citrate ions as a complexing agent. These layers were characterized by X-ray diffraction, SEM, X-ray energy dispersive analysis and spectrophotometry of reflected light. The best deposits were obtained atE=–0.97V vs SSE, in a solution adjusted to pH 3.3. The deposition of the binary alloys was also investigated. It was shown that the formation of smooth layers of crystalline definite compounds is correlated with a slow surface process which induces a plateau on the polarization curves.     

The influence of sulphates on chloride binding and pore solution chemistry

Ordinary Portland cement (OPC) and OPC/ground granulated blastfurnace slag (GGBS) 65% cements containing 2.0 to 9.0% sulphates derived from sodium sulphate and calcium sulphate were investigated in respect to their chloride binding properties and the concentrations of chloride and hydroxyl ions in the pore solutions. Chlorides derived from sodium and calcium chlorides were introduced at the time of mixing. The results indicate that calcium sulphate has a different effect on chloride binding and the pore solution chemistry than sodium sulphate. The slag cement has higher chloride binding capacities as a result of simple replacement for OPC, but at the same sulphate contents, the slag cement does not give the expected higher binding capacities, suggesting that the difference in sulphate content between the two cements may be the main reason for their different chloride binding behaviour.