Yields current

Because the water reacts with half of the sodium produced by the electrolysis, the current yield can never be more than 50% of theoretical. Other reactions in the cell lower this yield still more. 

Bromine electrogeneration was carried out from tetrabutylammonium bromide in 0,1 M NaClO acetonitrile solution with 100 % current yield. 

The anodes are generally not of pure metals but of alloys. Certain alloying elements serve to give a fine-grained structure, leading to a relatively uniform metal loss from the surface. Others serve to reduce the self-corrosion and raise the current yield. Finally, alloying elements can prevent or reduce the tendency to surface film formation or passivation. Such activating additions are necessary with aluminum. 

Pure aluminum cannot be used as an anode material on account of its easy passivatability. For galvanic anodes, aluminum alloys are employed that contain activating alloying elements that hinder or prevent the formation of surface films. These are usually up to 8% Zn and/or 5% Mg. In addition, metals such as Cd, Ga, In, Hg and T1 are added as so-called lattice expanders, these maintain the longterm activity of the anode. Activation naturally also encourages self-corrosion of the anode. In order to optimize the current yield, so-called lattice contractors are added that include Mn, Si and Ti. 

Low pH values favor self-corrosion, displace the rest potential to more negative values, reduce polarization, and lead to uniform material consumption pH values above 10.5 act opposite to this. Below pH 5.5 to 5.0, the current yield is so low that their use is impracticable. 

Fig. 6-10 (right) Specimen holder for measuring current yield. 

To estimate the current yield of magnesium alloys, the weight loss is determined indirectly over the volumetric measurement of the evolved hydrogen in the apparatus in Fig. 6-9 in Section 6.2.4. The mass balance for oxygen-free media follows from Eqs. (6-1) and (6-5a,b) or (6-14) ... 

Anodes for boilers can be tested by such methods. Good-quality magnesium anodes have a mass loss rate per unit area < 30 g m d", corresponding to a current yield of >18% under galvanostatic anode loading of 50 /xA cm" in 10 M NaCl at 60°C. In 10 M NaCl at 60°C, the potential should not be more positive than t/jj = -0.9 V for the same polarization conditions [27],... 

The current yield of aluminum depends on the composition of the water and the operating conditions it usually lies between a = 0.8 and 0.9 (see Section 6.2.3). Self-corrosion occurs, as with Mg, with hydrogen evolution. 

In aqueous solution especially, the current yield is distinctly lower furthermore, solubility problems can occur when the salt-deficit method is used. In aqueous solution, a-amino- or a-phenyl substituted carboxylates lead mainly to decomposition products, whilst in dry methanol or methanol-pyridine, coupling products were obtained with a-phenyl- and a-acetylaminocarboxylates [49]. 

Dimethylformamide is also a suitable solvent [50], it has, however, the disadvantage of being oxidized at fairly low potentials to A-acyloxy-iV-methyl formamide [51]. The influence of the composition of the ternary system water/methanol/dimethyl-formamide on the material and current yield has been systematically studied in the electrolysis of co-acetoxy or -acetamido substituted carboxylates [32]. Acetonitrile can also be used, when some water is.added [52]. The influence of various solvents on the ratio of Kolbe to non-Kolbe products is shown in Table 1 [53]. 

The current yield g is a useful parameter for the quantitative characterization of parallel reactions. This is the ratio of the partial CD, consumed in a given reaction n, to the total CD ... 

FIGURE 15.7 Polarization curves for anodic chlorine (1) and oxygen (2) evolution at a graphite electrode, and the current yields of chlorine as a function of potential (3). 

Perchlorates are also produced electrochemicaUy. The oxidation of chlorate to perchlorate ions occurs at a higher positive potential (above 2.0 V vs. SHE) than chloride ion oxidation. The current yield of perchlorate is lower when chloride ions are present in the solution hence, in perchlorate production concentrated pure chlorate solutions free of chlorides are used. Materials stable in this potential range are used as the anodes primarily, these include smooth platinum, platinum on titanium, and lead dioxide. 

Methods have been developed for perchloric acid synthesis which involve the electrolysis of solutions containing hydrogen chloride or molecular chlorine. These processes occur at high anode potentials (2.8 to 3.0 V vs. SHE), when oxygen is evolved at the anode in parallel with perchloric acid formation. The current yields of perchloric acid will increase considerably when the reaction is conducted at low temperatures (e.g., 20°C). 

The reaction proceeds in a solution of [N(C2H5)4]Br in acetonitrile at lead cathodes. The current yield is about 70%. 

In Russia in the 1960s, an industrial production of sebacic acid, HCOOC (CH2)4 COOH (an important intermediate for different plastics), was started which involves the anodic condensation of monomethyl adipate CH300C(CH2)4C00 . Dimethyl sebacate is obtained via the scheme of (15.58), and then hydrolyzed in autoclaves to the final product. Methanol is used as a solvent to lower the rates of side reactions. The reaction occurs with current yields attaining 75% and with chemical yields (degrees of utilization of the original adipate) of 82 to 84% (Vassihev et al., 1982). Upon introduction of this process it was no longer necessary to use castor oil, an expensive raw material that was needed to produce sebacic acid by the chemical process. 

In this case, one magnesium atom is consumed for the formation of one Mg + ion, but the current yield of magnesium again has doubled, because every other magnesium atom is spent not for electron generation but for hydrogen evolution. 

For coulometric analysis, the substance being examined must react in 100% current yields [i.e., other (secondary) reactions must be entirely absent]. In efforts to avoid side reactions, coulometry most often is performed potentiostatically (amperometrically) (i.e., the electrode potential is kept constant during the experiment), and the current consumed at the electrode is measured. The current is highest at the start of the... 

Oxides of Platinum Metals Anodes of platinum (and more rarely of other platinum metals) are used in the laboratory for studies of oxygen and chlorine evolution and in industry for the synthesis of peroxo compounds (such as persulfuric acid, H2S2O8) and organic additive dimerization products (such as sebacic acid see Section 15.6). The selectivity of the catalyst is important for all these reactions. It governs the fraction of the current consumed for chlorine evolution relative to that consumed in oxygen evolution as a possible parallel reaction it also governs the current yields and chemical yields in synthetic electrochemical reactions. 

The current yields of persulfate ion synthesis (for which oxygen evolution is a competing reaction) also strongly depend on the character of the oxide layer. These yields are high (about 70%) at oxide layers having a compact stracture and marked electronic conductivity. They are perceptibly lower at the partly hydrated, thicker layers having higher ionic conductivity. 

The —SO2 group is linked to an aliphatic carbon. Low current yields could be due to hydrogen evolution. TEAOT, Et N tosylate TEAI, Et NI. 

Now, if we assume throughout the entire concentration step a current yield of 100% and no perceptible alteration of the bulk concentration, in view of its function as an indicator in the stripping step the electrode area cannot be taken as large, Faraday s law can be applied to calculate the final amalgam concentration according to... 

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