Electrochemical deposition precipitation reactions

An internal electrochemical mechanism was proposed long ago for deposition on certain metal substrates, since the rate of deposition sometimes depended on the nature of the substrate [11].) The standard potential of Reaction (5.3) is -l- 0.08 V, considerably more positive than the rednction potential of S to (-0.45 V). Free sulphide, if formed, would be in a very low concentration, since it will be removed continually by precipitation of PbS this will move the S rednction potential strongly positive according to the Nemst equation [Eq. (1.32)]. This positive shift will be even greater than normal because of the non-Nemstian behavior of the S /S couple when [S] > [S ] (at least in alkaline solntion) [12]. In opposition to this, the solubility of S in the (slightly acidic) aqneons solntions is very low, which will move the potential in the opposite direction. Add to this the very small concentration of S in acid solution [Eq. (1.15)], and it becomes clear that it is not trivial to estimate the feasibility of the formation of PbS by free snlphide. The non-Nemstian behavior of the sulphur-rich S /S couple and the lack of knowledge of the solnbility of free S in the deposition solntion are the two factors that complicate what would have been a tractable thermodynamic calcnlation. 

Own experiments in divided cells using Nation membrane separators and hypochlorite solutions in the ppm range of concentration resulted in current efficiency values for active chlorine reduction of a few percent. Shifting the pH to higher values complicated the experiments. A buffer stabilised the pH but the relatively high concentration of buffer ions hindered the electrochemical reaction. Thus, quantification is difficult. Kuhn et al. (1980) showed reduction inhibition when calcareous deposits were precipitated on the cathode, but practical experiments showed the decrease of chlorine production in this case. 

Where might this be important As discussed above, biological activity can result in the simultaneous precipitation of mixtures of nanoscale sulfide minerals under certain conditions. Each mineral will exhibit a particular particle size distribution, dependent on the solution composition, bacterial activity, rate of crystal growth, and the nature of electrochemical interactions between the particles. These electrochemical reactions could lead to oxidation of one type of nanophase sulfide mineral of a certain size, and reduction of another type of nanophase sulfide particle or other species in the solution. In this way, a tremendous number of mineral-solution-mineral galvanic cells could develop, with potentially significant impact on dissolution kinetics, growth kinetics, and the mixture of phases observed. In addition to environmental relevance, these processes may shape the mineralogy of low-temperature ore deposits. 

Mechanisms for the electrochemical processes at mercury electrodes in solutions of [Ni(cyclam)] + and CO2 have been proposed (see Scheme 5.1 ). Scheme 5.1 shows the formation of a carbon-bonded Ni(II) complex by reaction of CO2 with Ni(cyclam)+. The formation of such a complex is considered to be a fundamental step in the mechanism of the [Ni(cyclam)] +-catalyzed electrochemical reaction. The overall process for the transformation of CO2 into CO also involves inner-sphere reorganization. Scheme 5.1 includes the formation of sparingly soluble complex containing Ni(0), cyclam and CO which is a product of the reduction of [Ni(cyclam)] + under CO. Depositation of a precipitate of the Ni(0) complex on the mercury electrodes inhibits catalysis and removes the catalyst from the cycle. The potential at which the [Ni L-C02H] + intermediate (see lower left hand of Scheme 5.1) accepts electrons from the electrode. This potential is not affected by substitution on the cyclam ring, as shown by comparison of [Ni(cyclam)] + and [Ni(TMC)] " (TMC = tefra-iV-methylcyclam)... 

Liquid-phase deposition is a method for the non-electrochemical production of polycrystalline ceramic films at low temperatures, along with other aqueous solution methods [chemical bath deposition (CBD), successive ion layer adsorption and reaction (SILAR), and electroless deposition (ED) with catalyst] has been developed as a potential substitute for vapor-phase and chemical-precursor systems. The method involves immersion of a substrate in an aqueous solution containing a precursor species (commonly a fluoro-anion) which hydrolyzes slowly to produce a supersaturated solution of the desired oxide, which then precipitates preferentially on the substrate surface, producing a conformal coating... 

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