Complex electrolyte systems

Thermodynamic calculations of solutions containing several substances that form electrolyte equilibria require a very careful approach. In contrast to conventional vapor-liquid equilibrium (VLE) calculations, the knowledge about the subsystems does not necessarily lead to a satisfactory representation. Moreover, even new components can be formed which do not occur in the subsystems. 

S imilar reactions take place in the systems CO2 - M E A (monoethanolamine) - H 2 O and CO2-MDEA (methyldiethanolamine)-H20, which are technically important in the field of CO2 removal from flue gases, giving a carbamate ion for the MEA reaction 

---

In this paper we examine the assumptions of our previous modeling approach and present new model calculations which consider alternative assumptions. In addition, we discuss the physicochemical factors which affect the formation of surface complexes at the oxide/water interface, in particular the effect of decreasing dielectric strength of the solvent. Finally, to demonstrate the general applicability of the model we present modeling results for a complex electrolyte system, where adsorption of a metal-ligand complex must be considered. 

In practical experiments, it is hard to obtain a perfect semi-circle. More often two semi-circles are obtained, meaning an electrochemical system contains more than one RC circuit, corresponding to a more complex electrolytic system. Moreover, when the electrochemical system contains a component which is under diffusion control, an oblique line with a slope of -1/2 appears on Niquist s plot and the equivalent circuit is modified adding a diffusion component, W, known as Warburg impedance (Figure 10.11). 

Each immittance highlights different features of an electrolyte system. The impedance, Z plots give prominence to resistive elements whereas the modulus, M plots give prominence to capacitances elements. The values are usually extracted from the Z plots and the C values are extracted from the modulus peaks. Thus, for a complex electrolyte system, it is worthwhile to plot the impedance data in more than one immittance formalisms in order to extract all possible information. 

The holistic thermodynamic approach based on material (charge, concentration and electron) balances is a firm and valuable tool for a choice of the best a priori conditions of chemical analyses performed in electrolytic systems. Such an approach has been already presented in a series of papers issued in recent years, see [1-4] and references cited therein. In this communication, the approach will be exemplified with electrolytic systems, with special emphasis put on the complex systems where all particular types (acid-base, redox, complexation and precipitation) of chemical equilibria occur in parallel and/or sequentially. All attainable physicochemical knowledge can be involved in calculations and none simplifying assumptions are needed. All analytical prescriptions can be followed. The approach enables all possible (from thermodynamic viewpoint) reactions to be included and all effects resulting from activation barrier(s) and incomplete set of equilibrium data presumed can be tested. The problems involved are presented on some examples of analytical systems considered lately, concerning potentiometric titrations in complex titrand + titrant systems. All calculations were done with use of iterative computer programs MATLAB and DELPHI. 

To find the best a priori conditions of analysis, the equilibrium analysis, based on material balances and all physicochemical knowledge involved with an electrolytic system, has been done with use of iterative computer programs. The effects resulting from (a) a buffer chosen, (b) its concentration and (c) complexing properties, (d) pH value established were considered in simulated and experimental titrations. Further effects tested were tolerances in (e) volumes of titrants added in aliquots, (f) pre-assumed pH values on precision and accuracy of concentration measured from intersection of two segments obtained in such titrations. 

This reaction may be followed by others (complex formation and/or precipitation) which are independent of the electrode potential but determined by the nature and concentration of the electrolyte. It is impossible to discuss all the problems relating to zinc electrodes without looking at the electrolyte system and the kind of cell operation (primary or rechargeable). The only way to cover all the possible combinations is by another mode of characterization or categorization, which is used in the subsequent sections ... 

In a complex reaction system occurring in a flow electrolyzer, one of the anode reaction products reacts with the principal constituent of the composite electrolyte, yielding an anionic species whose parasitic reaction at the cathode reduces the cathode efficiency. 

Within the past few years the advances made in hydrocarbon thermodynamics combined wtih increased sophistication in computer software and hardware have made it quite simple for engineers to predict phase equilibria or simulate complex fractionation towers to a high degree of accuracy through software systems such as SSI s PROCESS, Monsanto s FLOWTRAN, and Chemshare s DISTILL among others. This has not beem the case for electrolyte systems. 

From the foregoing discussion we conclude that some sophisticated tools are now available by which the activity coefficient in hydrometal— lurgical systems can be addressed. What is lacking is the actual application of these tools by the industry. The next step in establishing the accuracy of the available approaches lies in providing a broader data base for complex multicomponent systems which can be used for parameter refinement. TTte lack of data is most serious in the weak electrolyte area, but even familiar systems such as those encountered in sulfuric acid leaching need attention. 

Phase behavior 1n concentrated aqueous electrolyte systems is of interest for a variety of applications such as separation processes for complex salts, hydrometal 1urgical extraction of metals, interpretation of geological data and development of high energy density batteries. Our interest in developing simple thermodynamic correlations for concentrated salt systems was motivated by the need to interpret the complex solid-liquid equilibria which occur in the extraction of sodium nitrate from complex salt mixtures which occur in Northern Chile (Chilean saltpeter). However, we believe the thermodynamic approach can also be applied to other areas of technological interest. 

Increase selectivity by increasing the difference in mobilities. This can be achieved by using appropriate electrolyte systems and other additives in the background electrolyte (e.g., complexing agents). 

In summary, these trends in the change of conductivity with m, xec, and T can be consistently interpreted in terms of the change of e and r] with these same variables. Since these factors and their effect on ion conductivity are not unique to the system illustrated, LiPFe/EC/DMC, these trends should provide general guidance as to how ion conductivities of other electrolyte systems with similar compositions would change with these same variables, and they should constitute a useful database for the understanding of more complex systems, such as ternary or quaternary mixtures. 

Because of the use of various electrolyte systems, pH gradients, and not least an electric field, some complexes would not survive the separation. It is therefore necessary that the species to be separated are both thermodynamically and kineti-cally stable. Recently, Bocek and Foret have reviewed the application of isotachophoresis to the separation of inorganic species. This technique appears to be well-suited for the study of the distribution of metabolites of metal-containing drugs in body fluids. A survey of the application of electrophoretic techniques to biological materials can be found in the book edited by Deyl... 

Jote the greater complexity of defining adsorption here in studies of electric double layers than, e.g., for metal-gas systems. With electric double layers, one is concerned with the whole interphasial region. The total adsorption is the sum of the increases of concentration over a distance, which in dilute solutions may extend for tens of nanometers. Within this total adsorption, there are, as will be seen, various types of adsorptive situations, including one, contact adsorption, which counts only Arose ions in contact with the electronically conducting phase (and is Aren, like the adsorption referred to in metal-gas systems, the particles on Are surface). Metal-gas systems deal with interfaces, one might say, whereas metal-electrolyte systems deal primarily with interphases and only secondarily with interfaces. 

The local composition model (LCM) is an excess Gibbs energy model for electrolyte systems from which activity coefficients can be derived. Chen and co-workers (17-19) presented the original LCM activity coefficient equations for binary and multicomponent systems. The LCM equations were subsequently modified (1, 2) and used in the ASPEN process simulator (Aspen Technology Inc.) as a means of handling chemical processes with electrolytes. The LCM activity coefficient equations are explicit functions, and require computational methods. Due to length and complexity, only the salient features of the LCM equations will be reviewed in this paper. The Aspen Plus Electrolyte Manual (1) and Taylor (21) present the final form of the LCM binary and multicomponent equations used in this work. 

The photocurrent generation in the present system is initiated by photoinduced charge separation from the porphyrin excited singlet state (1H2P /H2P+ = -0.7 V vs. NHE) [78] in the dendrimer to C60 (C60/Cf>0 = -0.2 V vs. NHE) [78] in the porphyrin dendrimer-C60 complex rather than direct electron injection to conduction band of Sn02 (0 V vs. NHE) system [91] The reduced C60 injects electrons into the Sn02 nanocrystallites, whereas the oxidized porphyrin (H2P/H2P+ = 1.2 V vs. NHE) [78] undergoes electron-transfer reduction with iodide (I3 /I = 0.5 V vs. NHE) [78] in the electrolyte system [91]. 

In both polarographic and preparative electrochemistry in aptotic solvents the custom is to use tetraalkylammonium salts as supporting electrolytes. In such solvent-supporting electrolyte systems electrochemical reductions at a mercury cathode can be performed at —2.5 to —2.9 V versus SCE. The reduction potential ultimately is limited by the reduction of the quaternary ammonium cation to form an amalgam, (/ 4N )Hg , n = 12-13. The tetra-n-butyl salts are more difficult to reduce than are the tetraethylammonium salts and are preferred when the maximum cathodic range is needed. On the anodic side the oxidation of mercury occurs at about +0.4 V versus SCE in a supporting electrolyte that does not complex or form a precipitate with the Hg(I) or Hg(II) ions that are formed. 

The impressive electrolytic systems due to Ziegler and co-workers are perhaps most widely known. These are based on research dating back to 1955. There are several patents on the electrolysis of sodium fluoride-trialkylaluminum complexes with a lead anode 334>, and the electrolysis of sodium or potassium tetraalkylaluminum 336>, as in the following examples ... 

From Eqn. (14) it follows that with an exothermic reaction - and this is the case for most reactions in reactive absorption processes - decreases with increasing temperature. The electrolyte solution chemistry involves a variety of chemical reactions in the liquid phase, for example, complete dissociation of strong electrolytes, partial dissociation of weak electrolytes, reactions among ionic species, and complex ion formation. These reactions occur very rapidly, and hence, chemical equilibrium conditions are often assumed. Therefore, for electrolyte systems, chemical equilibrium calculations are of special importance. Concentration or activity-based reaction equilibrium constants as functions of temperature can be found in the literature [50]. 

Using these methods to describe an aqueous electrolyte system with its associated chemical equilibria involves a unique set of highly nonlinear algebraic equations for each set of interest, even if not incorporated within the framework of a complex fractionation program. To overcome this difficulty, Zemaitis and Rafal (8) developed an automatic system, ECES, for finding accurate solutions to the equilibria of electrolyte systems which combines a unified and thermodynamically consistent treatment of electrolyte solution data and theory with computer software capable of automatic program generation from simple user input. 

In addition to these three major classes of electrolyte systems, there are also reports on the successful deposition of aluminum from AlCl3 LiCl solutions in dimethyl sulfone [(CH3)2S02] [478,479], Aluminum is deposited in these systems from the complex A1[(CH3)2S02]33+ [479], The above systems differ from each other in their ionic structure, conductance mechanism, and the mechanism of A1 deposition [480], In the ethereal solutions (class 1) containing A1C13 and... 

It has long been known that liquid electrolyte systems that melted in the low hundreds of degrees were available in systems of metal chlorides and AICI3, and that some tetraalkylammonium salts melted at < 373 K. Hurley and Wier in 1951 showed that a 2 1 mixture of some complex organic chlorides with AICI3 gave liquid electrolytes at room temperatures. The discovery remained undeveloped for more than 25... 

---