Complex ions, deposition potentials

The deposition-reduction (DR) method is based on the weak electrostatic interactions of polymer surfaces with the oppositely charged Au(III) complex ions, leading to the reduction of Au(III) exclusively on the polymer surfaces. Appropriate anionic or cationic Au(III) precursors are chosen based on the zeta potentials of polymer supports (Figure 3.6) [43]. 

The deposition takes place from HTeOs and cadmium-EDTA complex solutions at a potential whereat, whilst Te is deposited from HTeOs under a diffusion-limited condition, the Cd-EDTA complex ion is not reduced to metallic Cd. The first step is the dark deposition of one monolayer of elemental Te on the p-Si substrate (Fig. 4.11a, i). After completion of this step, as specified by measuring the charge passed, the electrode is illuminated by light with energy higher than the band gap energy of silicon for a limited time. Then conduction band electrons are... 

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. 

We now return to the case of codeposition of metals whose standard electrode potentials are wide apart. As stated, the deposition potentials [Eq. (11.2)] are brought together by complexing the more noble metal ions, as illustrated below for the case of the codeposition of copper and zinc as brass. 

Fortunately, the deposition potentials of these metals can be brought together by adjustment of their ionic concentrations. Thus, if KCN is added to the solution of the salts of these metals, it binds the Cu and Zn ions as rather stable Cu(CN)4 and Zn(CN)4 complexes, respectively. In solution, the copper complex dissociates to cuprous and cyanide ions according to... 

High concentrations of electrolyte are needed to ensure high conductivity of the solution and hence a more uniform deposition potential over the surface of the specimen. At the same time, however, adatom formation (b) may be too rapid unless the free metal ion concentration is kept low by complexing it with appropriate ligands, such as silver ion with cyanide ... 

Let us try and understand this. As stated, Ni plating baths (as well as other acidic baths such as those of Cu and Zn) show poor throwing power. This is so because their CE values are =100% at the low and high current density values, and so macroscopic irregularities on a cathode will lead to nonuniform deposits. Alkaline baths, on the other hand, have a better macro throwing power. This is the case since, in order to remain in solution in such a bath, the metal ion, to be deposited, must be present in complex ions. These ions, in turn, encounter high concentration polarization. Also, in most complex baths the deposition potentials are amenable to hydrogen evolution, which competes with metal deposition such that CE falls as current density is increased. That kind of behavior results in a more uniform deposit on... 

The electro-chemical response (i.e. peak area values) reflects the hydrated metal ion content, and contributions from any labile complexes present. The size of the peak can be affected by experimental parameters such as the deposition potential used, the rate of stirring, the mercury drop diameter, the pulse frequency, the stripping solution composition, the system pH and the temperature. The size and position of the ASV peak can also be influenced by the chemical nature of the original test sample through processes such as... 

Another example of the effect of a change of concentration upon the cathodic process can be found in electrolysis of a solution of salts of copper and bismuth. As the respective deposition potentials, which practically equal the equilibrium potentials are fairly close (7c( — 0.34 V, 71 it, = 0.23 V) the two metals cannot he separated from each other electrolytically. On the addition of cyanide, however, Cu++ ions are converted into cupricyanide ions from which copper cannot be deposited prior the cathode reaches the potential Ttt u equalling to about — 1.0 V. As bismuth does not form cyanide complexes the resulting difference in potentials, 7ti — 7Cou — 1.23 V is a sufficient guarantee that during electrolysis only bismuth will be, preferentially deposited. 

Electroraffination — (see also electrorefining) Purification of metals by means of dissolution and subsequent electrodeposition. Common method in - electrometallurgy for the removal of impurities from raw metals. Upon anodic dissolution the metallic constituents of the anode are dissolved as cations, oxyanions, or complex ions. All impurities - whether metallic or not - are also dissolved or will fall to the bottom of the cell. At the cathode set to a suitable potential (in most cases only fractions of one volt are needed) the desired metal is deposited. Less noble metals stay in solution, they can be recovered by processing the electrode solution. Metals more noble than the metal under consideration are in most cases not dissolved anodically, instead they settle in the solid deposit at the cell bottom. From this residue they can be recovered. 

If two metals normally have similar discharge potentials, the conditions can be altered to make them sufficiently different for separation to be possible. For example, in the case of nickel and zinc in ammoniacal solution, to which reference was made previously, the deposition potentials are similar at 20 , but differ at 90 . The two metals can thus be separated satisfactorily at the higher temperature, but not at the lower. Another illustration is provided by the copper-bismuth system, in which simultaneous deposition takes place from simple salt solutions if cyanide is added, however, the copper ions form the complex cuprocyanide and the discharge potential becomes more negative (cf. Table LXXXIII). If citric or tartaric acid is present to keep the bismuth in solution, the addition of cyanide hardly affects the deposition potential of this metal quantitative separation from copper is then possible. 

When a metal ion is complexed, its standard potential is shifted cathodically by (2.3RT/nF)logK, where K is the stability constant of the complex formed. As a result, hydrogen evolution can occur along with metal deposition. The faradaic efficiency, which is the fraction of the current consumed to deposit the metal, may decrease. This quantity is defined by... 

For each atom of copper deposited, three cyanide ions are released at the electrode surface. The concentration of free (CN) ions at the electrode surface is thus higher than its bulk concentration. This shifts the potential on the solution side of the double layer in the negative direction, lowering the concentration of the complex ions, hence lowering the rale of reaction. A typical copper cyanide bath is composed of 0.3 M CuCNandO.7 M KCN. The equilibrium constant in the reaction... 

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