Current efficiency side reactions

Monofunctional and Polyfunctional Electrodes At monofunctional electrodes, one sole electrode reaction occurs under the conditions specified when current flows. At polyfunctional electrodes, two or more reactions occur simultaneously an example is the zinc electrode in acidic zinc sulfate solution. When the current is cathodic, metallic zinc is deposited at the electrode [reaction (1.21)] and at the same time, hydrogen is evolved [reaction (1.27)]. The relative strengths of the partial currents corresponding to these two reactions depend on the conditions (e.g., the temperature, pH, solution purity). Conditions may change so that a monofunctional electrode becomes polyfunctional, and vice versa. In the case of polyfunctional electrodes secondary (or side) reactions are distinguished from the principal (for the given purpose) reaction (e.g., zinc deposition). In the electrolytic production of substances and in other practical applications, one usually tries to suppress all side reactions so that the principal (desired) reaction will occur with the highest possible efficiency. 

The thionine reduction prodnct TH is anodically reoxidized to thionine, while the Fe ions are cathodically rerednced to Fe ions. Thns, the chemical composition of the system will not change dnring cnrrent flow. The potential difference between the electrodes that can be used to extract electrical energy is 0.2 to 0.4 V under current flow. The conversion factor of Inminous to electrical energy is very low in such cells, about 0.1%. This is due to the numerous side reactions, which drastically lower the overall efficiency. Moreover, the stability of such systems is not high. Therefore, the chances for a practical use are not evident so far. 

The current efficiency of an electrolytic process is a measure of the current or the charge actually used in carrying out the desired electrochemical reaction as compared to the theoretical requirement. It is, therefore, defined as the ratio of the theoretical current requirement to the actual current requirement for the desired reaction alternatively, it may also be expressed as the ratio of amount of material actually deposited at the electrode to that which should have deposited on the basis of Faraday s law, by the passage of the same charge, assuming that no side reactions take place at the electrode. The current efficiency, can be expressed as... 

The current efficiency may range from 25-30% to as high as 90-100%. The loss in efficiency may be due to several factors which influence the chemical and the electrochemical reactions at the electrodes. For example, factors such as the occurrence of side reactions, the passage of current by electronic conduction, and the dislodging of the deposited product from the electrode may substantially reduce the value of T. In general, the current... 

If there are no detrimental organic side reactions, a cell current density in excess of the limiting current density - and as result a loss of current efficiency - may be acceptable for laboratory scale experiments. For example, a hydrogen evolution parallel to an electroorganic cathodic reduction can even be advantageous as it improves the mass transfer by moving gas bubbles and thus enhances the organic cathodic reduction. 

Chemical yields from an electrochemical reaction are expressed in the usual way based on the starting material consumed. Cunent efficiency is determined from the ratio of Coulombs consumed in forming tlie product to the total number of Coulombs passed through the cell. Side reactions, particularly oxygen or hydrogen evolution, decrease the current efficiency. 

Corrosion of the zinc anode is a significant side reaction under acidic conditions because zinc reacts with H+(aq) to give Zn2+(aq) and H2(y). Under basic conditions, however, the cell has a longer life because zinc corrodes more slowly. The alkaline cell also produces higher power and more stable current and voltage because of more efficient ion transport in the alkaline electrolyte. 

Current views on polymerization of acrylonitrile in homogeneous solution are illustrated by a description of the reaction in N,N-dimethyl-formamide (DMF) as initiated by azobisisobutyronitrile (AIBN) at about SO to 60°. Primary radicals from the decomposition of AIBN react with monomer to start a growing chain. About one-half of the primary radicals are effective, the others being lost in side reactions not leading to polymer. Bevington and Eaves (32) estimated initiator efficiency by use of AIBN labelled with C-14, whereas Bamford, Jenkins and Johnson (13) used the FeCls termination technique. Both of these methods require that the rate of AIBN decomposition be known, and the numerical value of this rate has undergone a number of revisions that require recalculation of efficiency results. From recently proposed rate expressions for AIBN decomposition at 60° (22, 136) one calculates an efficiency of about 40% by the tracer technique and 60—65% by the FeCl3 method. 

Reduction of naphthalene under conditions suitable for reduction of isolated benzene rings resulted, as expected, in the formation of some 1,4,5,9-tetrahydronaphthalene (44). It was carried out12) in 15% aqueous (C4H9)4NOH at 80 °C and the yield of the products and the low current efficiency (17% naphthalene remained unreacted in spite of the transfer of large excess charge) were caused by anodic side reactions, as the experiment was conducted in an undivided cell. 

Should any iron(II) reach the anode, it also would be oxidized and thus not require the chemical reaction of Eq. (4.13) to bring about oxidation, but this would not in any way cause an error in the titration. This method is equivalent to the constant-rate addition of titrants from a burette. However, in place of a burette the titrant is electrochemically generated in the solution at a constant rate that is directly proportional to the constant current. For accurate results to be obtained the electrode reaction must occur with 100% current efficiency (i.e., without any side reactions that involve solvent or other materials that would not be effective in the secondary reaction). In the method of coulometric titrations the material that chemically reacts with the sample system is referred to as an electrochemical intermediate [the cerium(III)/cerium(IV) couple is the electrochemical intermediate for the titration of iron(II)]. Because one faraday of electrolysis current is equivalent to one gram-equivalent (g-equiv) of titrant, the coulometric titration method is extremely sensitive relative to conventional titration procedures. This becomes obvious when it is recognized that there are 96,485 coulombs (C) per faraday. Thus, 1 mA of current flowing for 1 second represents approximately 10-8 g-equiv of titrant. 

On the other hand, several experimental parameters have been defined to quantify the destruction of an organic species in aqueous medium by anodic oxidation (38-44). Because the main side reaction is 02 evolution due to water decomposition, the instantaneous current efficiency (ICE) at a given time t for its oxidation can be determined from the 02 flow rate during electrolysis in the absence (V0) and the presence (F,>org) of the selected pollutant as follows ... 

Based only on stoichiometry and assuming no side reactions, in which case will the current efficiency (defined as the charge employed for a given process divided by the total charge passed through the system) for formaldehyde destruction be higher (Ibanez)... 

One more inorganic electrosynthesis, peroxodisulphate production, has been studied [135]. Despite an inevitable side-reaction (oxygen evolution), the current efficiency obtained was as high as 75 %. 

The electrolysis of water proceeds without any side reactions and also the current losses which are caused by dissipation and by the recombination of hydrogen and oxygen dissolved in water are low. The current efficiency at normal pressure is, therefore, very good, reaching about 99 per cent. If electrolysis is carried out at a higher pressure the current efficiency is somewhat lower, as more gas is dissolved in the electrolyte which causes an increased quantity of the recombined hydrogen and oxygen. 

Current efficiency is here not so high as during the electrolysis of water because it is lowered by certain side reactions which occur simultaneously while the main process takes place (see equation XI-9) and (XI-10). First of all, it is chlorine which is not entirely removed from the electrolytic cell. It is partially dissolved in the electrolyte and reacts according to the nature of the medium and gives either a very slightly dissociated hypochlorous acid or a considerably dissociated hypochlorite. 

Data from industrial cells show that the exit gas contains 90-60% C02 and 10-40% CO. The carbon consumption usually ranges from 400 to 550 kg/ton Al, while the theoretical amount at 95% current efficiency is 350 kg/ton A1 for reaction (88) and 700 kg/ton Al for reaction (89). The question is then whether the extra consumption, compared to the theoretical value for reaction (88), is due to simultaneous primary formation of C02 and CO or if it is due to the Boudouard reaction between gaseous C02 and the sides and the interior parts of the anode [141], or carbon dust [200] dispersed in the electrolyte, or air burning of the anode. In any case, it is known [200,201] that in the laboratory a high content of CO is formed at low cds (<0.05 A cnr2) and almost pure C02 is formed at 1 A cm-2. 

In contrast to the case of HMPC, most lipases hydrolyze the racemic acetate of CPBA 9 to give a mixture of the insecticidally active (S)-CPBA 2 and the (R)-acetate 1J). Thus, the desired (S)-CPBA J2 could be separated from the (R)-acetate 10 by means of a continuous counter-current extraction using n-heptane solvent at 80°C. However, it is important to utilize the recovered (R)-acetate W for an efficient process. Fortunately, since the proton of the asymmetric carbon of the cyanohydrin acetate is labile, the antipodal (R)-acetate is easily racemized by treatment with weak organic base such as triethylamine without any side reactions. The racemized acetate J9 thus obtained was recycled as shown in Figure 6. Therefore, all of the racemic acetate 9 was converted to the desired (S)-CPBA 2 in this recycling process. The (S)-CPBA 2 obtained was esterified with (S)-2-(4-chlorophenyl)-3-methylbutyryl chloride to produce the most insecticidally active stereoisomer V2 of fenvalerate, namely esfenvalerate. The relative biocidal activities between... 

The product of the oxidation of Iodide on the film Is lodate. Table IV shows that the yield of lodate Increases when Hyamlne is added to the solution and that current efficiency Increases. The latter fact is caused by the decrease in the side reaction, the electrolysis of water. 

Although it is not as severe in PEVD systems as in aqueous electrochemical systems in which various kinds of mobile ions are present in the electrolytes, it should be pointed out that, in the presence of reactants at the sink electrode surface, other electrochemical reactions might also take place in parallel with the desired one at the sink side. If side reactions exist, usually such parallel reactions contributions to the measured current are not easy to quantify. If it is desired to use current to monitor the reaction and product formation in PEVD, side reactions should be eliminated or at least controlled. Fortunately, only one ionic species is usually mobile in a solid electrochemical cell because of the nature of the solid electrolyte. As long as the vapor phase is properly controlled, usually one electrode reaction is predominant over a wide range of PEVD applied potentials. Virtually 100% current efficiency for product formation can be expected. 

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