Stoichiometry of electrolytic processes

Now we will consider the stoichiometry of electrolytic processes—that is, how much chemical change occurs with the flow of a given current for a specified time. Suppose we wish to determine the mass of copper that is plated... 

Knowledge Required (1) The stoichiometry of electrolytic processes. (2) The relationships among coulombs, faradays, and moles of electrons. 

The acidity of the aqueous strip solution is a very strong variable whose value should be carefully selected in order to effect a compromise among the iron removal, the zinc concentration in the iron product, the stoichiometry of the process, the process rate and the iron-zinc separation index. As demonstrated, it is possible to separate iron from zinc sulfate electrolyte and recover a concentrated, potentially usable form of iron. It is interesting to contemplate the use of galvanic stripping for the removal of other less concentrated impurities or valuable by-products from zinc processing solutions or from other hydrometallurgical streams, eith in conjunction with iron removal or separately. 

This equation provides the link between electrical measurements and amount of electrons, and the balanced half-reactions for the electrolytic process provide the link between the amount of electrons and amounts of chemical substances. Example shows a calculation regarding electrolytic stoichiometry. 

The conceptual treatment of this topic is rather simple. In the previous sections, we looked at the number of electrons that are required to complete an electrolytic process. This was accomplished through the stoichiometry of the half-reactions. For instance, in the half-reaction ... 

In coulometry the stoichiometry of the electrode process should be known and should proceed with 100% current efficiency, and the product of reaction at any other electrode must not interfere with the reaction at the electrode of interest. If there are intermediate reactions, they too must proceed with the desired accuracy. In practice the electrolytic cell is designed to include isolation chambers. Losses of solute through diffusion, through ionic or electrical migration, and simply through bulk transfer must be minimal. Finally, the end point has to be determined by one of the many techniques used in titrations generally, whether coulometric or not. Both indeterminate and determinate end-point errors limit the overall accuracy achieved. Cooper and Quayle critically examined errors in coulometry, and Lewis reviewed coulometric techniques. 

The stoichiometry of a half-reaction shows how many electrons are needed to achieve an electrolytic process. For example, the reduction of Na to Na is a one-electron process ... 

Stoichiometry of the oxidation process Table 2.5 Dependence of on electrolyte concentration ... 

The above-cited example on Cd/hematite indicates that some groups perform titrations in the presence of solutes different from innocent electrolytes. Such titrations may yield important macroscopic information on the proton balance of the suspension in the presence of such a solute (Table 2). However, the exact proton stoichiometry of some surface complex can rarely be inferred, because this would require that only one complex exists and that the protonation states of the surface groups, which are not contributing to that particular surface complex, are not affected by the adsorption process. This can, at best, be assumed in a quaUtative interpretation but can be quantitatively handled with the mean field approximation and the corresponding assumptions inherent to the respective computer programs. In fitting some models to adsorption data, proton data will constitute an independent and very valuable dataset representative of the system however, they may be restricted to sufficiently high solute to sorbent ratios. 

Transition metals tend to form surface oxides upon contact with aqueous electrolytic conditions under electrochemical oxidizing conditions. For the specific case of noble metals such as Pt and Au, these oxides are chemically reversible, that is, they can be formed through anodization and then reduced by switching the potential to an appropriate cathodic value while other changes may accompany this process such as metal dissolution or roughening of the surface, the initial conditions of the surface are largely restored. The exact stoichiometry of the surface oxides of Au and Pt is still a matter of debate due to the coexistence of different species however, to a first approximation, one can talk about the formation of a few (or sub-) monolayers of AU2O3 or PtO. 

At present about 77% of the industrial hydrogen produced is from petrochemicals, 18% from coal, 4% by electrolysis of aqueous solutions and at most 1% from other sources. Thus, hydrogen is produced as a byproduct of the brine electrolysis process for the manufacture of chlorine and sodium hydroxide (p. 798). The ratio of H2 Cl2 NaOH is, of course, fixed by stoichiometry and this is an economic determinant since bulk transport of the byproduct hydrogen is expensive. To illustrate the scde of the problem the total world chlorine production capacity is about 38 million tonnes per year which corresponds to 105000 toimes of hydrogen (1.3 x I0 m ). Plants designed specifically for the electrolytic manufacture of hydrogen as the main product, use steel cells and aqueous potassium hydroxide as electrolyte. The cells may be operated at atmospheric pressure (Knowles cells) or at 30 atm (Lonza cells). 

In the non-steady state, changes of stoichiometry in the bulk or at the oxide surface can be detected by comparison of transient total and partial ionic currents [32], Because of the stability of the surface charge at oxide electrodes at a given pH, oxidation of oxide surface cations under applied potential would produce simultaneous injection of protons into the solution or uptake of hydroxide ions by the surface, resulting in ionic transient currents [10]. It has also been observed that, after the applied potential is removed from the oxide electrode, the surface composition equilibrates slowly with the electrolyte, and proton (or hydroxide ion) fluxes across the Helmholtz layer can be detected with the rotating ring disk electrode in the potentiometric-pH mode [47]. This pseudo-capacitive process would also result in a drift of the electrode potential, but its interpretation may be difficult if the relative relaxation of the potential distribution in the oxide space charge and across the Helmholtz double layer is not known [48]. 

This reduction process will occur at the cathode of the electrolytic cell. To solve this stoichiometry problem, we need the following steps ... 

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