The electrolyte

Other electrolytes, such as sodium hydroxide, can be used, but they are not as conductive as KOH. Acids such as sulphuric acid can used as electrolytes, but this is more corrosive to the electrodes, and the added wear and tear on various components does not justify its use. 

KOH can be purchased, or made if you have a source of hardwood ashes. The making of lye used to be a common chore in most households. If you are interested in making your own KOH my book Build Your Own Fuel Cells contains complete illustrated instructions. If you buy KOH from a chemical supply house you will find the following table helpful. 

Specific Gravity Percent KOH Lbs. per US Gallon Specific Gravity Percent KOH Lbs. per US Gallon 

If you make your own KOH, use this handy table to determine the specific gravity of the solution when you take a hydrometer reading. This will indicate whether to boil the solution down more to strengthen it, or add more distilled water to weaken it. 

I make my own KOH and do not bother to boil it down. I simply refill my reservoir with more KOH solution rather than distilled water. The KOH, when I first make it, comes out at about a 12% solution. In the electrolysis process, the solution becomes stronger as the water disassociates and is used up, leaving the KOH behind. Once the solution in the electrolyzer reservoir is at the right specific gravity, all that is necessary is to add distilled water as the water in the electrolyte disassociates and is used up. 

The purpose of the electrolyte is to provide a conducting medium, and at the same time, it must not corrode the cathode tool. The cheapest material commonly used is sodium chloride (NaCl) at about 30% by weight. In some cases, additives such as alcohols, amines, thiols, and aldehydes are used to inhibit stray currents, which results in overcuts. Other electrolytes such as Na2Cr207, NaN03, and NaClOs at 50-250 g/L have also been used, but the choice is limited primarily by cost. 

The electrolyte is usually recirculated with the metal products removed or reduced before being reused. This minimizes cost and pollution and prevents the formation of a precipitate in the electrolysis gap. 

The effect of electrolyte on surface roughness, H, was studied by Y. Sugie (1978) for five different iron alloys (characterized in Table 9.3) and is shown in Fig. 9.3. The roughness depends on electrolyte and alloy. 

The accuracy of the machining, Figs. 9.4 and 9.5, was determined for the five alloys by measuring the overcut and machined angle (shown in Fig. 9.6) for the various electrolytes. Based on the results, it was concluded that NaClOs was most suitable for the low alloy steel and that Na2S04 was best for the high nickel alloy steels. 

Intergranular corrosion observed for some alloys can be reduced by using mixed electrolytes such as 15% NaCl with 20% NaClOa. 

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See chemical equivalent, equivalent conductivity The specific conductance multiplied by the volume (ml) which contains 1 g equivalent of the electrolyte. 

NH2-C0-NH NH2,CH5N30. Colourless crystalline substance m.p. 96" C. Prepared by the electrolytic reduction of nitrourea in 20% sulphuric acid at 10 "C. Forms crystalline salts with acids. Reacts with aldehydes and ketones to give semicarbazones. Used for the isolation and identification of aldehydes and ketones. 

Derive the equation of state, that is, the relationship between t and a, of the adsorbed film for the case of a surface active electrolyte. Assume that the activity coefficient for the electrolyte is unity, that the solution is dilute enough so that surface tension is a linear function of the concentration of the electrolyte, and that the electrolyte itself (and not some hydrolyzed form) is the surface-adsorbed species. Do this for the case of a strong 1 1 electrolyte and a strong 1 3 electrolyte. 

Repeat the calculation of Problem 1, assuming all conditions to be the same except that the electrolyte is di-divalent (e.g., MgS04). 

The interaction of an electrolyte with an adsorbent may take one of several forms. Several of these are discussed, albeit briefly, in what follows. The electrolyte may be adsorbed in toto, in which case the situation is similar to that for molecular adsorption. It is more often true, however, that ions of one sign are held more strongly, with those of the opposite sign forming a diffuse or secondary layer. The surface may be polar, with a potential l/, so that primary adsorption can be treated in terms of the Stem model (Section V-3), or the adsorption of interest may involve exchange of ions in the diffuse layer. 

The repulsion between oil droplets will be more effective in preventing flocculation Ae greater the thickness of the diffuse layer and the greater the value of 0. the surface potential. These two quantities depend oppositely on the electrolyte concentration, however. The total surface potential should increase with electrolyte concentration, since the absolute excess of anions over cations in the oil phase should increase. On the other hand, the half-thickness of the double layer decreases with increasing electrolyte concentration. The plot of emulsion stability versus electrolyte concentration may thus go through a maximum. 

The reference free energy in this case is an upper bound for tlie free energy of the electrolyte. A lower bound for the free energy difference A A between the charged and uncharged RPM system was derived by Onsager... 

Ionic conductors arise whenever there are mobile ions present. In electrolyte solutions, such ions are nonually fonued by the dissolution of an ionic solid. Provided the dissolution leads to the complete separation of the ionic components to fonu essentially independent anions and cations, the electrolyte is tenued strong. By contrast, weak electrolytes, such as organic carboxylic acids, are present mainly in the undissociated fonu in solution, with the total ionic concentration orders of magnitude lower than the fonual concentration of the solute. Ionic conductivity will be treated in some detail below, but we initially concentrate on the equilibrium stmcture of liquids and ionic solutions. 

Figure A2.4.7. Hypothetical structure of the electrolyte double layer. From [15],...

At low currents, the rate of change of die electrode potential with current is associated with the limiting rate of electron transfer across the phase boundary between the electronically conducting electrode and the ionically conducting solution, and is temied the electron transfer overpotential. The electron transfer rate at a given overpotential has been found to depend on the nature of the species participating in the reaction, and the properties of the electrolyte and the electrode itself (such as, for example, the chemical nature of the metal). 

Figure Bl.28.6. (a) Convection within the electrolyte solution, due to rotation of the electrode (b) Nemst diflfiision model for steady state.

Introducing the complex notation enables the impedance relationships to be presented as Argand diagrams in both Cartesian and polar co-ordinates (r,rp). The fomier leads to the Nyquist impedance spectrum, where the real impedance is plotted against the imaginary and the latter to the Bode spectrum, where both the modulus of impedance, r, and the phase angle are plotted as a fiinction of the frequency. In AC impedance tire cell is essentially replaced by a suitable model system in which the properties of the interface and the electrolyte are represented by appropriate electrical analogues and the impedance of the cell is then measured over a wide... 

Figure Bl.28.9. Energetic sitiration for an n-type semiconductor (a) before and (b) after contact with an electrolyte solution. The electrochemical potentials of the two systems reach equilibrium by electron exchange at the interface. Transfer of electrons from the semiconductor to the electrolyte leads to a positive space charge layer, W. is the potential drop in the space-charge layer.

A combination of equation (C2.6.13), equation (C2.6.14), equation (C2.6.15), equation (C2.6.16), equation (C2.6.17), equation (C2.6.18) and equation (C2.6.19) tlien allows us to estimate how low the electrolyte concentration needs to be to provide kinetic stability for a desired lengtli of time. This tlieory successfully accounts for a number of observations on slowly aggregating systems, but two discrepancies are found (see, for instance, [33]). First, tire observed dependence of stability ratio on salt concentration tends to be much weaker tlian predicted. Second, tire variation of tire stability ratio witli particle size is not reproduced experimentally. Recently, however, it was reported that for model particles witli a low surface charge, where tire DL VO tlieory is expected to hold, tire aggregation kinetics do agree witli tire tlieoretical predictions (see [60], and references tlierein). 

The driving force for migration is established by the different electrochemical potentials (AU) that exist at the two interfaces of the oxide. In other words, the electrochemical potential at the outer interface is controlled by the dominant redox species present in the electrolyte (e.g. O2). 

The situation in figure C2.8.5(b) is different in that, in addition to the mechanism in figure C2.8.5(a), reduction of the redox species can occur at the counter-electrode. Thus, electron transfer tlirough the layer may not be needed, as film growth can occur with OH species present in the electrolyte involving a (field-aided) deprotonation of the film. The driving force is provided by the applied voltage, AU. 

The protective quality of the passive film is detennined by the ion transfer tlirough the film as well as the stability of the film with respect to dissolution. The dissolution of passive oxide films can occur either chemically or electrochemically. The latter case takes place if an oxidized or reduced component of the passive film is more soluble in the electrolyte than the original component. An example of this is the oxidative dissolution of CrjO ... 

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