Electrolysis time

Minimizing Electrolysis Time The current-time curve for controlled-potential coulometry in Figure 11.20 shows that the current decreases continuously throughout electrolysis. An exhaustive electrolysis, therefore, may require a long time. Since time is an important consideration in choosing and designing analytical methods, the factors that determine the analysis time need to be considered. 

The other necessary instrumental component for controlled-current coulometry is an accurate clock for measuring the electrolysis time, fe, and a switch for starting and stopping the electrolysis. Analog clocks can read time to the nearest +0.01 s, but the need to frequently stop and start the electrolysis near the end point leads to a net uncertainty of +0.1 s. Digital clocks provide a more accurate measurement of time, with errors of+1 ms being possible. The switch must control the flow of current and the clock, so that an accurate determination of the electrolysis time is possible. 

In order to find optimal conditions for the soluble copper determination we examined the influence of electrolysis potential, electrolysis time, and the solution stirring rate on the accuracy and sensitivity of determination. We found that the optimal parameters for PSA determination of copper were electrolysis potential of -0.9 V vs. 3.5 mol/dm Ag/AgCl, electrolysis time of 300 s, and solution stirring rate of 4000 rpm. The soluble copper content in samples investigated in this study varied from 1.85 to 4.85 ppm. Very good correlation between the copper content determined by PSA and AAS indicated that PSA could be successfully applied for the soluble copper content determination in various dental materials. 

This corresponds to 12 amp.-hours (theoretical value is 10.6 amp.-hours). Longer electrolysis times did not significantly increase the yield of product. 

By virtue of its inherent accuracy, coulometric titration is very suitable for the determination of substances present in small amount, and quantities of the order of 10 7-1(U5 mole are typical. Larger amounts of material require very long electrolysis times unless an amperostat capable of delivering relatively large currents (up to 2 A) is available. In such cases, a common procedure is to start the electrolysis with a large current, and then to switch to a much lower output as the end point is approached. 

Make the connections to the polarographic analyser and adjust the applied voltage to —0.8 V, i.e. a value well in excess of the deposition potential of lead ions. Set the stirrer in motion noting the setting of the speed controller, and after 15-20 seconds, switch on the electrolysis current and at the same time start a stopclock allow electrolysis to proceed for 5 minutes. On completion of the electrolysis time, turn off the stirrer, but leave the electrolysis potential applied to the cell. After 30 seconds to allow the liquid to become quiescent, replace the electrolysis current by the pulsed stripping potential and set the chart recorder in motion. When the lead peak at ca 0.5 V has been passed, turn... 

Optimum conditions for the formation of CdS by the acidic method on metallic A1 substrate at 25 °C have been reported as follows pH 2.3, potential -1 V vs. SCE, and electrolysis time > 2 h [44]. Thermal treatment improved the characteristics of the films and their photovoltaic properties, which were evaluated by evaporating a CU2S layer on the CdS/Al film, to form a heterojunction cell. The influence of the deposition substrate on the formation and morphology of CdS was found to be important. The aluminum substrates gave the best results among Pt, Mo, and Al. In the case of molybdenum, surface blocking by adsorbed sulfur was considered. 

The volume of the metal produced depends on the current density, the electrolysis time, the cathode area, and the electrochemical equivalent. Yt is usually expressed as a function of the current density. The interrelationship among Yt, the specific area of the cathode, As (m2/m3), i.e., the area of the cathode per unitvolume of the reactor (cathode area/volume of the cell), the fractional current efficiency, T, and the current density, I, is given by ... 

Expt. Medium composition Electrolysis time (h) Volume of CO produced (ml) Coulombs consumed Average current efficiency (%)c... 

If the total current can be assumed to be limited by diffusion to the STM tip, Case III is similar to diffusion to a microdisk electrode (one electrode) thin-layer cell (63). Murray and coworkers (66) have shown that for long electrolysis times, diffusion to a planar microdisk electrode TLC can be treated as purely cylindrical diffusion, provided that the layer thickness is much smaller than the disk diameter (66). In contrast to the reversible case discussed above (Case I), the currents in this scenario should decrease gradually with time at a rate that is dependent on the tip radius and the thickness of the interelectrode gap. Thus, for sufficiently narrow tip/sample spacings, diffusion may be constrained sufficiently (ip decayed) at long electrolysis times to permit the imaging of surfaces with STM. 

The current consumed (measured in fara-days [F]) corresponds to F = current (in ampere) multiplied hy electrolysis time (in hours) multiphed hy 0.0374. 

The electrode processes on the voltammetric and the preparative electrolysis time scales may be quite different. The oxidation of enaminone 1 with the hydroxy group in the ortho position under the controlled potential electrolysis gave bichromone 2 in 68% yield (Scheme 4.) with the consumption of 2.4 F/mol [21], The RDE voltammogram of the solution of 1 in CH3CN-O.I mol/1 Et4C104 showed one wave whose current function, ii/co C, was constant with rotation rates in the range from 1(X) to 2700 rpm and showed one-electron behavior by comparison to the values of the current function with that obtained for ferrocene. The LSV analysis was undertaken in order to explain the mechanism of the reaction which involves several steps (e-c-dimerization-p-deamina-tion). The variation of Ep/2 with log v was 30.1 1.8 mV and variation of Ep/2 with logC was zero. Thus, our kinetic data obtained from LSV compare favorably with the theoretical value, 29.6 mV at 298 K, for a first order rate low [15]. This observation ruled out the dimerization of radical cation, for... 

Figure 3. Electrolyses of 50 mM Na2S04 adjusted to pH 2 continuously saturated with O2 at a reticulated vitreous carbon cathode in a flow-cell. Plots of H2O2 formed versus electrolysis time for...

Figure 10. COD evolution as a function of electrolysis time for electrolysis of solutions containing 50 mMNa2S04 + 1 mMFe + dye (a) 0.082 mMDR23, (b) 0.25 mM A07, (c) 0.33 mM Am, (d) 0.1 mM AG25, (e) 0.1 mM Am, (f) 0.17 mM BB9. Electrolyses were carried out in a membrane cell with a reticulated vitreous carbon cathode (5 cm x 5 cm x 1 cm).

Figure 7.16 Variation of activity for O2 evolution as a function of electrolysis time for continuous and intermittent electrolysis [65].

The ferrocenyl dendrimers were electrodeposited in their oxidized forms onto the electrode surfaces (platinum, glassy-caibon, and gold) either by controlled potential electrolysis or by repeated cycling between the appropriate anodic and cathodic potential limits therefore the amount of electroactive material electrode-posited can be controlled with the electrolysis time or the number of scans. The electrochemical behavior of films of the polyfeirocenyl dendrimers was studied by cyclic voltammetry in fresh CH2CI2 and CHjCN solutions containing only supporting electrolyte. 

Although the theoretical studies predict solvent medium breakdown before the onset of actinium electrodeposition, there have been reports of Ac(0) electrodeposition from aqueous solutions utilizing several different methods [8, 9]. One set of studies [8] describes the electrodeposition of actinium from nitric acid solutions, with varying pH values (1.0-4.0) being set to the appropriate level by the addition of sodium hydroxide. The anode and cathode in these studies were platinum metal, and the current density was varied from 50 to 200 mA cm . The authors found that quantitative electrodeposition of actinium could be achieved under various conditions, with the shortest electrolysis time of 1 h being obtained with a current density of200 mA cm and a pH of 2.0. A second study employed a saturated aqueous solution of urea oxalate (ca 6.6% at 30 °C) as an electrolyte for the electrodeposition of Ac onto a nickel foil cathode [9]. The authors of this study found that the yield of electrodeposited Ac increased with time and reached a near quantitative maximum yield of 97% at a current density of 53 mAcm after 2 h. The Ac electrodeposits were suitable for further study using nuclear spectroscopy. 

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