Electrodes impurities

In many cases, the electrode reactions are not reversible and the decomposition potential is observed to be in excess of the thermodynamically calculated value. The excess voltage, referred to as an overvoltage, is found to vary with the nature and surface area (e.g., roughness) of the electrodes, impurities in the solution, and the actual current density passing through the solution. The relation between current density Id and overvoltage E was investigated by Tafel, who proposed the very successful empirical equation... 

Both zirconium and hafnium metal sponges are generally converted to massive metal by arc melting processes. A consumable-electrode type of furnace is usually preferred, to avoid contamination by electrode impurities. The massive metal may have a Brinell hardness as low as 192. At figures below 300, it can be cut, machined, forged, swaged and rolled. 

Fig. 18.13 Generalized model for electrode-impurity interactions and their relation to degradation (a) chemical phenomena as functions of transported amount of impurities and chemical affinity between electrode materials and (b) three fundamental regions for electrode-impurity interaction...

Sulfur poisoning (both electrodes) Impurities in the fuel stream, especially sulfur, inhibit anode performance. Strong reversible poisoning of the Ni-YSZ anode occurs at feed concentrations of 1 ppm H2S in H2 at 1000°C and as low as 50 ppb H2S in H2 at... 

The coking process produces electrode quality coke from vacuum residues of good quality (low metal and sulfur contents) or coke for fuel in the case of heavy crude or vacuum residue conversion having high impurity levels. 

Despite its electrode potential (p. 98), very pure zinc has little or no reaction with dilute acids. If impurities are present, local electrochemical cells are set up (cf the rusting of iron. p. 398) and the zinc reacts readily evolving hydrogen. Amalgamation of zinc with mercury reduces the reactivity by giving uniformity to the surface. Very pure zinc reacts readily with dilute acids if previously coated with copper by adding copper(II) sulphate ... 

The residual current, in turn, has two sources. One source is a faradaic current due to the oxidation or reduction of trace impurities in the sample, i . The other source is the charging current, ich> that is present whenever the working electrode s potential changes. 

The standard electrode potential for zinc reduction (—0.763 V) is much more cathodic than the potential for hydrogen evolution, and the two reactions proceed simultaneously, thereby reducing the electrochemical yield of zinc. Current efficiencies slightly above 90% are achieved in modem plants by careful purification of the electrolyte to bring the concentration of the most harmful impurities, eg, germanium, arsenic, and antimony, down to ca 0.01 mg/L. Addition of organic surfactants (qv) like glue, improves the quaUty of the deposit and the current efficiency. 

Electrorefining. Electrolytic refining is a purification process in which an impure metal anode is dissolved electrochemicaHy in a solution of a salt of the metal to be refined, and then recovered as a pure cathodic deposit. Electrorefining is a more efficient purification process than other chemical methods because of its selectivity. In particular, for metals such as copper, silver, gold, and lead, which exhibit Htfle irreversibHity, the operating electrode potential is close to the reversible potential, and a sharp separation can be accompHshed, both at the anode where more noble metals do not dissolve and at the cathode where more active metals do not deposit. 

Atmospheric corrosion is electrochemical ia nature and depends on the flow of current between anodic and cathodic areas. The resulting attack is generally localized to particular features of the metallurgical stmcture. Features that contribute to differences ia potential iaclude the iatermetaUic particles and the electrode potentials of the matrix. The electrode potentials of some soHd solutions and iatermetaUic particles are shown ia Table 26. Iron and sUicon impurities ia commercially pure aluminum form iatermetaUic coastitueat particles that are cathodic to alumiaum. Because the oxide film over these coastitueats may be weak, they can promote electrochemical attack of the surrounding aluminum matrix. The superior resistance to corrosion of high purity aluminum is attributed to the small number of these constituents. 

The sodium hydroxide is titrated with HCl. In a thermometric titration (92), the sibcate solution is treated first with hydrochloric acid to measure Na20 and then with hydrofluoric acid to determine precipitated Si02. Lower sibca concentrations are measured with the sibcomolybdate colorimetric method or instmmental techniques. X-ray fluorescence, atomic absorption and plasma emission spectroscopies, ion-selective electrodes, and ion chromatography are utilized to detect principal components as weU as trace cationic and anionic impurities. Eourier transform infrared, ft-nmr, laser Raman, and x-ray... 

Ha2ards encountered with tungsten may be caused by substances associated with the production and use of tungsten, eg. As, Sb, Pb, and other impurities in tungsten ores, Co aerosols and dust in the carbide industry, and thoria used in welding electrodes. Lanthanum is being promoted as a substitute for thoria in this appHcation. 

Brine Preparation. Rock salt and solar salt (see Chemicals frombrine) can be used for preparing sodium chloride solution for electrolysis. These salts contain Ca, Mg, and other impurities that must be removed prior to electrolysis. Otherwise these impurities are deposited on electrodes and increase the energy requirements. The raw brine can be treated by addition of sodium carbonate and hydroxide to reduce calcium and magnesium levels to below 10 ppm. If further reduction in hardness is required, an ion-exchange resin can be used. A typical brine specification for the Huron chlorate ceU design is given in Table 6. 

Tellurium [13494-80-9] M 127.6, m 450 . Purified by zone refining and repeated sublimation to an impurity of less than 1 part in 10 (except for surface contamination by Te02). [Machol and Westrum J Am Chem Soc 80 2950 1958.] Tellurium is volatile at 500°/0.2mm. Also purified by electrode deposition [Mathers and Turner Trans Amer Electrochem Soc 54 293 1928]. 

Equipment. The impurities in this grade of platinum, intended for all normal purposes in chemical plant, electrodes, crucibles, etc. other than for use above 1 000°C, consist only of traces of the other platinum metals. 

Thus the net effect of electrolysis is to transfer copper metal from the impure blister copper used as one electrode to the pure copper sheet used as the other electrode. Electrolytic copper is 99.95% pure. 

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