Current efficiency, bulk electrolysis

The concentration of sodium hydroxide at the cathode surface is higher than that of the bulk solution in electrolysis of sodium chloride because 1 mol of water decomposes at the cathode surface per Faraday. When the solution at the cathode surface is separated from the bulk solution with a suitable separator, sodium hydroxide of higher concentration can be obtained from the cathode surface.171 The concentration of caustic soda produced from an electrolyzer is generally about 32-35%, of which 42-54% is directly and economically produced from an electrolyzer by forming a specific, thin membrane layer on the cathode side of the membrane.172 The current efficiency for caustic soda production is more than 95% in commercial production. 

Bulk electrolysis methods are also classified according to purpose. For example, one form of analysis involves determination of the weight of a deposit on the electrode (electrogravimetry). In this case 100% current efficiency is not required, but the substance of interest must be deposited in a pure, known form. In coulometry, the total quantity of electricity required to carry out an exhaustive electrolysis is determined. The quantity of material or number of electrons involved in the electrode reaction can then be determined by Faraday s laws, if the reaction occurred with 100% current efficiency. For electroseparations, electrolysis is used to remove, selectively, constituents from the solution. 

Electrolysis at controlled potential is the most efficient method of carrying out a bulk electrolysis, because the current is always maintained at the maximum value (for given cell conditions) consistent with 100% current efficiency. Note that the rate of electrolysis is independent of Cq(0), so that electrolysis of a 0.1 M solution of O and a 10 M solution of O should require the same amount of time, given the same values of E, A, V, and thq. 

The course of a bulk electrolysis under controlled-current conditions can be ascertained from consideration of i-E curves like those in Figure 11.4.1. As long as the applied current /app is less than the limiting current at a given bulk concentration //(f), the electrode reaction proceeds with 100% current efficiency. As the electrolysis proceeds, the bulk concentration of O, Cq(0j decreases and i/(0 decreases (linearly with time). When... 

In a similar study, Cu, Ni, and unmetallated tetrakis ortho or para-aminophenyl) porphyrin were polymerized onto Sn02 F on glass electrodes to study the reduction of nitrate. After 6 hr of bulk electrolysis the Cu and Ni ortho polymer produced nitrite and nitrous oxide while the other four polymers only produced nitrite. Nyokong and coworkers also studied nitrate reduction on Cu, Fe, Ni, Co, Mn, and Zn phthalocyanin adsorbed onto a GCE in basic solutions . The main product was ammonia without any detectable nitrite how-ever,the current efficiency is lower than the GCE alone with only the Ni and Cophthalocyanin curbing the overpotential required to reduce nitrate. 

The ability of the Fe state as a catalyst was also demonstrated in the reduction of nitromethane, CH3N02 . Voltammetry of Mb/DDAB under solutions of nitromethane at pH 5.5, obtained a catalytic current at the formal Fe / couple, without an observable shift at the Fe 4/ couple. Mass spectrometry analysis of aqueous products from bulk electrolysis confirmed production of methylhy-droxylamine, CH3NHOH. The catalytic efficiency, Ic/h current ratio, decreased significantly with increased pH or scan rate, indicative of a strongly proton dependant process. The slope of the catalytic potential was ca. 30mV/pH unit, in agreement with the proposed le /2 H+ limiting step, Eqs. (4.49)-(4.51). 

Electrochemical oxidation of complex 1 in die presence of methanol leads to considerable enhancement of the oxidative currents (Figure 2), consistent with an electrocatalytic oxidation process. The onset of this catalytic current coincides with the irreversible Pt(II/IV) oxidative wave at 1.70 V. The bulk electrolysis of 1 and dry methanol were performed at 1.70 V (onset of catalytic current) in 0.7 M TBAT/DCE. Gas chromatographic analysis of the solution indicated that dimethoxymethane (DMM, formaldehyde dimethyl acetal) and methyl formate (MF) are formed (Scheme 1). This result is consistent with the electrooxidation of dry methanol on PtRu anodes, which yields DMM after acid-catalyzed condensation of the formaldehyde product with excess methanol (36). Bulk electrolysis of methanol in the presence of heterobimetallic complex 1 resulted in higher current efficiencies than those obtained from the mononuclear model compound CpRu(PPh3)2Cl (2) (Table II), No oxidation products were found when the electrolysis was performed at 1.70 V in the absence of a Ru complex or in the presence of the Pt model compound (ri -dppm)PtCl2. These results suggest that Pt enhances the catalytic activity of the Ru metal center. 

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