Constant current reactions

To a 250-ml not-partitioned electrochemical cell, 135 ml of CH3CN, 15 ml ofHiO, 6.20 g of NaBr and 2.82 g of olefin ( ) is added. The mixture, kept at 2(f C, is electrolysed by using the same electrodes as of Example 1, but with a constant current density of 1.7 A being used,until through the cell 4,000 Coulombs have been passed. The reaction mixture is then processed as described in Example 4.2.56 g is obtained of ketone (III), with a yield of 83.2%, as computed relatively to the olefin (I) used as the starting material. 

Fig. 9. Discharge and charging curves for a sintered iron electrode at a constant current of 0.2 A where the apparent geometrical surface area is 36 cm and porosity is 65%. A and B represent the discharging and charging regions, respectively. Overall electrode reactions, midpoint potentials, and, in parentheses, theoretical potentials at pH 15 ate Al, n-Fe + 2 OH Fe(OH)2 + 2, 0.88 V (1.03 V) B, Fe(OH)2 FeOOH + H+ +, 0.63 V (0.72 V) C,...

A Perkin-Elmer 5000 AAS was used, with an electrically heated quartz tube atomizer. The electrolyte is continuously conveyed by peristaltic pump. The sample solution is introduced into the loop and transported to the electrochemical cell. A constant current is applied to the electrolytic cell. The gaseous reaction products, hydrides and hydrogen, fonued at the cathode, are flowed out of the cell with the carrier stream of argon and separated from the solution in a gas-liquid separator. The hydrides are transported to an electrically heated quartz tube with argon and determined under operating conditions for hydride fonuing elements by AAS. 

Two distinctly different coulometric techniques are available (1) coulometric analysis with controlled potential of the working electrode, and (2) coulometric analysis with constant current. In the former method the substance being determined reacts with 100 per cent current efficiency at a working electrode, the potential of which is controlled. The completion of the reaction is indicated by the current decreasing to practically zero, and the quantity of the substance reacted is obtained from the reading of a coulometer in series with the cell or by means of a current-time integrating device. In method (2) a solution of the substance to be determined is electrolysed with constant current until the reaction is completed (as detected by a visual indicator in the solution or by amperometric, potentiometric, or spectrophotometric methods) and the circuit is then opened. The total quantity of electricity passed is derived from the product current (amperes) x time (seconds) the present practice is to include an electronic integrator in the circuit. 

The principle of coulometric titration. This involves the generation of a titrant by electrolysis and may be illustrated by reference to the titration of iron(II) with electro-generated cerium(IV), A large excess of Ce(III) is added to the solution containing the Fe(II) ion in the presence of, say IM sulphuric acid. Consider what happens at a platinum anode when a solution containing Fe(II) ions alone is electrolysed at constant current. Initially the reaction... 

Since the rate of movement is controlled by the rate of the electrochemical reaction, when we oxidize or reduce the conductins polymer of the device at constant current, we will have a uniform movement with perfect control of the movement rate the movement is stopped by stopping the current flow the movement is reversed by reversing the direction of the current flow. By doubling the current density, we obtain a movement rate that is twice the previous one. Rates and mechanical energy are proportional to the current consumed per mass unit (Fig. 25). 

The same experimental procedure used in Fig. 4.15 is followed here. The Pt surface is initially (t < - 1 min) cleaned from Na via application of a positive potential (Uwr=0.2 V) using the reverse of reaction (4.23). The potentiostat is then disconnected (1=0, t=-lmin) andUWR relaxes to 0 V, i.e. to the value imposed by the gaseous composition and corresponding surface coverages of NO and H. Similar to the steady-state results depicted in Fig. 4.18 this decrease in catalyst potential from 0.2 to 0 V causes a sixfold enhancement in the rate, rN2, of N2 production and a 50% increase in the rate of N20 production. Then at t=0 the galvanostat is used to impose a constant current I=-20 pA Na+ is now pumped to the Pt catalyst surface at a... 

C19-0126. Electrolytic reactions, like other chemical reactions, are not 100% efficient. In a copper purification apparatus depositing Cu from C11SO4 solution, operation for 5.0 hours at constant current of 5.8 A deposits 32 g of Cu metal. What is the efficiency ... 

Chronopotentiometry has been widely used to determine diffusion coefficients in molten salts. Chronopotentiometry is an experimental procedure in which the potential of an electrode is observed as a function of time during the passage of a constant current sufficiently large to produce concentration polarization with respect to the species undergoing electrochemical reaction. 

When, after the attainment of zero surface concentration, a constant current density is maintained artificially from outside, the electrode potential will shift to a value such that a new electrochemical reaction involving other solution components can start (e.g., in aqueous solution, the evolution of hydrogen or oxygen). It follows from Eq. (11.9) that at a given concentration Cy the product is constant and is... 

A particular constant current density is applied to the electrode, and the potential variation is followed as a function of time. When there is no electrode reaction, the entire current is a nonfaradaic charging current According to Eq. (12.13), the slope of the E vs. t curve (Fig. 12.10, curve 1) is determined by the EDL capacitance. 

Heptyl 3-Phenylpropyl Ether [Electrogenerated Acid-Promoted Reduction of an Aldehyde to an Unsymmetrical Ether].333 A mixture of 1-heptanal (1.0 mmol), 3-phenylpropoxytrimethylsilane (1.2 mmol), tetra-n-butylammonium perchlorate (0.1 mmol), and lithium perchlorate (0.1 mmol) was dissolved in CH2CI2 (3 mL) in an undivided cell. The mixture was electrolyzed under constant current (1.67 mA cm-2) with platinum electrodes at ambient temperature. After 5 minutes, dimethylphenylsilane (1.2 mmol) was added drop-wise and the electrolysis was continued (0.06 Faraday/mol). After completion of the reaction, one drop of Et3N was added and the solution was concentrated. The residue was chromatographed on Si02 to give 1-heptyl 3-phenylpropyl... 

Measurement of the quantity of electricity used in an electrochemical reaction at constant potential or constant current. 

A related technique is the current-step method The current is zero for t < 0, and then a constant current density j is applied for a certain time, and the transient of the overpotential 77(f) is recorded. The correction for the IRq drop is trivial, since I is constant, but the charging of the double layer takes longer than in the potential step method, and is never complete because 77 increases continuously. The superposition of the charge-transfer reaction and double-layer charging creates rather complex boundary conditions for the diffusion equation only for the case of a simple redox reaction and the range of small overpotentials 77 [Pg.177]

Even though the effect of moisture on the anode kinetics is well known, interpretation of experimental results on the effect of moisture can be tricky. As Nakagawa et al. [52] pointed out, the measurement of the total cell impedance under the OCV condition is not convincing since the reduction of polarization could as well be due to the availability of H20 for the cathodic reaction. In addition, the measurement of cell performance under the constant voltage or constant current conditions may also lead to wrong conclusions about the effect of water, because the addition of H20 will... 

Figure 7 shows that PS thickness increases linearly with time up to certain thickness.16,17 Such constant growth rate at a constant current density means that the PS formed is uniform in thickness (Effective surface area remains constant assuming reaction kinetics is the same). At a large thickness the growth may deviate from linearity due to the effect of diffusion in the electrolyte within the pores.19,25 It has been found that for a very thick PS layer (150 pm) there is about 20% difference in HF concentration between that at the tips of pores and that in the bulk solution.19... 

FIGURE 2.32. First-order reaction product (C) and second-order product (D) yields for Scheme 2.17 as a function of the competition parameter, a Constant concentration-constant potential and constant-current electrolyses, b Exhaustive constant-potential electrolysis. 

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