Potential deposition

Blockage of piping, valves or flame arresters due to solid deposition. Potential for system overpressure. 

Table 12.2 Corrosion potentials of substrates of coppter and steel, plated and unplated in same plating solutions. Deposition potential is accompanied by current density (A/m ) in parentheses the plated substrate s coating thickness was 2 5 fim. The final column gives the potential below which hydrogen evolution is possible only in the cuprocyanide is it observed...

It follows that if the original solution contains two cations whose deposition potentials differ by about 0.25 V, then the cation of higher deposition potential should be deposited without any contamination by the ion of lower deposition potential. In practice, in may be necessary to take steps to ensure that the cathode potential is unable to fall to a level where deposition of the second ion may occur (see Section 12.6). 

The separations summarised in Table 12.2 can be readily accomplished electrolytically the first is dependent upon a large difference in deposition potentials (Section 12.4), and the second upon the fact that Pb2+ can be anodically deposited as Pb02. 

Make the connections to the polarographic analyser and adjust the applied voltage to —0.8 V, i.e. a value well in excess of the deposition potential of lead ions. Set the stirrer in motion noting the setting of the speed controller, and after 15-20 seconds, switch on the electrolysis current and at the same time start a stopclock allow electrolysis to proceed for 5 minutes. On completion of the electrolysis time, turn off the stirrer, but leave the electrolysis potential applied to the cell. After 30 seconds to allow the liquid to become quiescent, replace the electrolysis current by the pulsed stripping potential and set the chart recorder in motion. When the lead peak at ca 0.5 V has been passed, turn... 

Anodic shipping voltammetry (ASV) is the most widely used form of stripping analysis, hi this case, the metals are preconcenhated by elechodeposition into a small-volume mercury electrode (a tiiin mercury film or a hanging mercury drop). The preconcenhation is done by catiiodic deposition at a controlled tune and potential. The deposition potential is usually 0.3-0.5 V more negative than E° for the least easily reduced metal ion to be determined. The metal ions reach die mercury electrode by diffusion and convection, where diey are reduced and concentrated as amalgams ... 

Fig. 3.3 Zinc blende (111) XRD reflections vs. deposition potential, for 2 xm thick CdSe films electrodeposited from a pH 2 bath on Ni cathode, at the indicated potentials within the diffusion-limited region. (With kind permission from Springer Science+Business Media [60])...

Cu9ln4 and Cu2Se. They performed electrodeposition potentiostatically at room temperature on Ti or Ni rotating disk electrodes from acidic, citrate-buffered solutions. It was shown that the formation of crystalline definite compounds is correlated with a slow surface process, which induced a plateau on the polarization curves. The use of citrate ions was found to shift the copper deposition potential in the negative direction, lower the plateau current, and slow down the interfacial reactions. 

By electrodeposition of CuInSe2 thin films on glassy carbon disk substrates in acidic (pH 2) baths of cupric ions and sodium citrate, under potentiostatic conditions [176], it was established that the formation of tetragonal chalcopyrite CIS is entirely prevalent in the deposition potential interval -0.7 to -0.9 V vs. SCE. Through analysis of potentiostatic current transients, it was concluded that electrocrystallization of the compound proceeds according to a 3D progressive nucleation-growth model with diffusion control. 

In searching to formulate a mechanism of CuInSc2 phase formation by one-step electrodeposition from acid (pH 1-3) aqueous solutions containing millimolar concentrations of selenous acid and indium and copper sulfates, Kois et al. [178] considered a number of consecutive reactions involving the formation of Se, CuSe, and Cu2Se phases as a pre-requisite for the formation of CIS (Table 3.2). Thermodynamic and kinetic analyses on this basis were used to calculate a potential-pH diagram (Fig. 3.10) for the aqueous Cu+In-i-Se system and construct a distribution diagram of the final products in terms of deposition potential and composition ratio of Se(lV)/Cu(ll) in solution. 

On the other hand, Xiao et al. [215] reported that smooth, dense, and erystalline PbTe films with nearly stoichiometric composition could be obtained by an optimized electrodeposition process from highly acidic (pH 0) tellurite solutions of uncomplexed Pb(II), on Au-coated silicon wafers. The results from electroanalyti-cal studies on Te, Pb, and PbTe deposition with a Pt rde at various temperatures and solution compositions supported the induced co-deposition scheme. The microstructure and preferred orientation of PbTe films was found to change significantly with the deposition potential and electrolyte concentration. At -0.12 V vs. Ag/AgCl(sat. KCl), the film was granular and oriented preferentially in the [100] direction. At potentials more negative than -0.15 V, the film was dendritic and oriented preferentially in the [211] direction (Pig. 3.13). 

II. Calculated current density and stoichiometry vs. deposition potential curves for parameter values representative of CdTe and with one partial current density diffusion limited. J Electrochem Soc 132 2910-2919... 

Fig. 4.2 (Left) XRD patterns (CuK ) of equally thick ZnSe (ca. 1 xm) deposits prepared on H (a) and on CdSe/Ni (b), at a deposition potential of -0.7 V vs. SHE from a typical acidic (pH 3) solution. The reflection intensities for ZnSe(l 11) are denoted in the figure (right) XRD patterns within the low-angle region showing the (111) reflections of ZnSe/CdSe heterostructures (A). Overlayers of ZnSe were deposited on CdSe films of various (111) texture intensities. The substrate features are shown in (B) in full-intensity scale. Evidently, the preferential orientation of the ZnSe crystallites increases with that of the CdSe substrate, up to a constant limiting value. (Reprinted from [16], Copyright 2009, with permission from Elsevier)...

Plyasova LM, Molina lY, Gavrilov AN, Cherepanova SV, Cherstiouk OV, Rudina NA, Savinova ER, Tsirlina GA. 2006. Electrodeposited platinum revisited tuning nanostructure via the deposition potential. Electrochim Acta 51 4477-4488. 

Thus, co-deposition of silver and copper can take place only when the silver concentration in the electrolyte falls to a very low level. This clearly indicates that the electrolytic process can, instead, be used for separating copper from silver. When both silver and copper ions are present, the initial deposition will mainly be of silver and the deposition of copper will take place only when the concentration of silver becomes very low. Another example worth considering here is the co-deposition of copper and zinc. Under normal conditions, the co-deposition of copper and zinc from an electrolyte containing copper and zinc sulfates is not feasible because of the large difference in the electrode potentials. If, however, an excess of alkali cyanides is added to the solution, both the metals form complex cyanides the cuprocyanide complex is much more stable than the zinc cyanide complex and thus the concentration of the free copper ions available for deposition is considerably reduced. As a result of this, the deposition potentials for copper and zinc become very close and their co-deposition can take place to form alloys. 

As mentioned earlier, the current efficiency also depends on the presence of additives and/or of impurities which may co-deposit or may influence the electrochemical reaction or may affect the overvoltages of the desirable and the undesirable reactions. The impurities which are more noble would be deposited this would not only contaminate the metal but would also consume charge for undesirable reactions. Additives may be deliberately added when depositing alloys, so that the deposition potentials of the different metals involved could be brought closer however, in most other cases these are considered as harmful impurities. The electrolyte, therefore, needs to be purified with respect to such impurities in order to improve the current efficiency. 

Underpotential deposition of heavy metals on H2 evolving electrodes is a well known problem [133], The existence of a direct correlation between H2 evolution activity and metal work function, makes UPD very likely on high work function electrodes like Pt or Ni. Cathode poisoning for H2 evolution is aggravated by UPD for two reasons. First, deposition potentials of UPD metals are shifted to more anodic values (by definition), and second, UPD favors a monolayer by monolayer growth causing a complete coverage of the cathode [100]. Thus H2 evolution may be poisoned by one monolayer of cadmium for example, the reversible bulk deposition potential of which is cathodic to the H2 evolution potential. 

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