Adsorption of Weak Electrolytes

Adsorption of Weak Electrolytes.—The effect of pH on the adsorption of weak electrolytes by activated carbon has been studied by several workers. The early 

Lopez Gonzalez, C. Valenzuela Calahorro, and A. Jimenez Lopez, An. Edefol. AgrohioL, 1977, 36, 83.5 (Otem. Ahstr., 1978, 88, 1498). 

Bachurin, S. P. Kalinkina, A. A. Kniga, and V. M. Perelygin, Fermentn. Spin. Prom-st., 

Rosene and Manes studied the effect of pH on the total adsorption from aqueous solutions of sodium benzoate + benzoic acid by activated charcoal. They interpreted their data in terms of the Polanyi potential theory applied to bisolute adsorption (see later p. 117), in which the concentrations of neutral benzoic acid and benzoate anions depend on the pH of the solution (activity coefficient corrections were ignored). They confirmed that, at constant total equilibrium concentration, the adsorption dropped from a relatively high plateau for pH 2 down to a small adsorption at pH 10. The analysis of results is somewhat more complex than with essentially non-electrolyte adsorption, and in this case there were additional effects involving chemisorption of benzoate ion by residual ash in the carbon which had, therefore, to be eliminated. Even with ash-extracted carbon there was evidence of some residual chemisorption. The theoretical analysis correlated satisfactorily with the experimental data on the basis that at pH 10 sodium benzoate is not physically adsorbed and that the effect of pH is completely accounted for by its effect on the concentration of free acid. In addition the theory explains successfully the increase in pH (called by the authors hydrolytic adsorption ) when solutions of sodium benzoate are treated with neutral carbon. However, no account is taken in this paper of the effect of pH on the surface charge of the carbon. 

A study of the effect of pH on the adsorption of the lower fatty acids (Ci to Cg) and poly(ethylene glycol)s (PEG) by active carbon is reported by Chudoba, Hrncir, and Remmelzwaal. The experimental isotherms were analysed in terms of the Freundlich isotherm. The constant K increased with increasing 

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A similar argument [7] was presented by Muller et al. [227] in their incisive analysis of adsorption of weak electrolytes from aqueous solution on ACs The solid surface charges in response to solution pH and ionic strength the resulting (smeared) surface electrostatic potential influences the adsorption affinity of the ionized solute. ... 

Chapter 7 of our landmark reference [6] discusses various aspects of the adsorption of weak electrolytes and nonelectrolytes from aqueous solution. In particular an attempt is made to elucidate the mechanism of adsorption of undissociated aromatic compounds from dilute aqueous solutions. As discussed in detail in Section VI, the authors concluded that the aromatic ring of the adsorbate inter-... 

Muller, G., Radke, C.J., and Prausnitz, J.M. (1985). Adsorption of weak electrolytes from dilute aqueous solution onto activated carbon. Part I. Single-solute systems. J. Colloid Interface Sci., 103, 466—83. 

Identical correlations have been observed for albite [372], confirming the validity of the two- and three-dimensional precipitation models in interpreting the mechanism of adsorption of weak-electrolyte-type surfactants on silicates. However, the concentration at which two-dimensional precipitation for albite... 

Quirk JP, Posner AM (1975) Trace element adsorption by soil minerals. In Nicholas DJ, Egan AR (eds) Trace elements in soil plant animal system. Academic Press, New York, pp 95-107 Randall M, Failey CF (1927) The activity coefficient of the undissociated part of weak electrolytes. Chem Rev 4 117-128... 

Adsorption of borate on two types of commercial activated carbons (from deionized water and its mixtures with natural waters) resulted in uptake maximum at pH about 8 [9], Nearly linear decrease in uptake of chromate by a commercial sample from 50% to 10% over the pH range 2-10 (no supporting electrolyte added) was reported [7], In contrast Dobrowolski [5] reported uptake curves of chromate with a maximum at pH about 3 for three de-ashed and modified activated carbons. These two types of adsorption behavior are common in specific adsorption of weak acids on inorganic materials (cf Table 4,2). Activated carbon was found to be an efficient adsorbent of iodides from synthetic clay water at pH as high as 8.5 [10],... 

Recently from the electrochemical point of view the adsorption of weak as well as strong organic electrolytes has been examined by Koopal (1993) who also considered specific electrochemical aspects of adsorbed ionic surfactants. 

An important contribution to the problem is made in a paper by Muller, Radke, and Prausnitz who present a new theoretical model for the adsorption of weak organic electrolytes on activated carbon. Unlike previous models the theory takes into account surface heterogeneity and the effect of pH on surface charge. The solid surface is assumed to consist of three types of adsorption site, neutral, basic, and acidic, the relative proportions of which vary with pH and are characterized by q, the surface charge density per unit area. This is related to the surface potential i/ o by simple diffuse double-layer theory, assuming that the surface charge is balanced only by the counter charge of the double layer. 

The authors aim was to produce a text which critically reviews the available literature on solution adsorption phenomena and offers an interpretation of the surface-related interactions of activated carbons that is consistent for the adsorption of a wide variety of solutes ranging from strong electrolytes to organic non-electrolytes. The seven chapters cover the activation of carbon, surface oxygen functional groups and neutralization of base by acidic surface oxides, spectroscopic methods for molecular structure determinations on surfaces, nature of the electrical double layer, adsorption of electrolytes, and adsorption of weak and non-electrolytes from aqueous solution. 

The electrical double layer has been studied at the interface of acidified (pH = 3) KCIO4 and K2SO4 solutions in contact with an Sn solid drop electrode with an additionally remelted surface (SnDER).616 The E, is independent of ctl as well as of the electrolyte. Weak specific adsorption of CIO4 at SnDER is probable around <7 = 0. This view is supported by the high value of/pz for SnDER/H20 + KCIO4 (fpz = 1 -27). A value of fpz = 0.99 for SnDER/H20 + K2S04 indicates that the surface of SnDER is geometrically smooth and free from components of pseudo-capacitance.616... 

Anodically polished and then cathodically reduced Cd + Pb alloys have been studied by impedance in aqueous electrolyte solutions (NaF, KF, NaC104, NaN02, NaN03).827 For an alloy with 2% Pb at cNap 0.03 M, Emfo = -0.88 V (SCE) and depends on cNaF, which has been explained by weak specific adsorption of F" anions. Surface activity increases in the sequence F" < CIO4 < N02. The Parsons-Zobel plot at E is linear, with /pz = 1.33 and CT° = 0.31 F m"2. Since the electrical double-layer parameters are closer to those for pc-Pb than for pc-Cd, it has been concluded that Pb is the surface-active component in Cd + Pb alloys827 (Pb has a lower interfacial tension in the liquid state). 

The mechanism of anodic oxidation of CO at polycrystalline Au remains uncertain. Several groups have reported that the voltammetry of Au in acidic electrolytes is straightforward, with a well-formed oxidation wave/peak [Stonehart, 1966 Gibbs et al., 1977 Kita et al., 1985 Sun et al., 1999]. There is, however, no voltammetric evidence for the adsorption of CO on the Au surface, and spectroscopic studies indicate only a weak interaction of CO with poly crystalline Au surfaces in acidic solutions [Kunimatsu et al., 1986 Cuesta et al., 2003]. Moreover, there is little evidence for the formation of oxidizing species at the potential where the oxidation process is observed. Certainly, the oxidation of CO occurs at a potential over 500 mV less positive than that where bulk Au oxide is formed, and, indeed, the formation of this oxide strongly... 

Adsorbed carbon monoxide on platinum formed at 455 mV in H2S04 presents a thermal desorption spectrum as shown in Fig. 2.4b. As in the case of CO adsorption from the gas phase, the desorption curve for m/e = 28 exhibits two peaks, one near 450 K for the weakly adsorbed CO and the other at 530 K for the strongly adsorbed CO species. The H2 signal remains at the ground level. A slight increase in C02 concentration compared to the blank is observed, which could be due to a surface reaction with ions of the electrolyte. Small amounts of S02 (m/e = 64) are also observed. 

The processes classified in the third group are of primary importance in elucidating the significance of electric variables in electrosorption and in the double layer structure at solid electrodes. These processes encompass interactions of ionic components of supporting electrolytes with electrode surfaces and adsorption of some organic molecules such as saturated carboxylic acids and their derivatives (except for formic acid). The species that are concerned here are weakly adsorbed on platinum and rhodium electrodes and their heat of adsorption is well below 20 kcal/mole (25). Due to the reversibility and significant mobility of such weakly adsorbed ions or molecules, the application of the i n situ methods for the surface concentration measurements is more appropriate than that of the vacuum... 

Although adsorption of many types of species could be considered, this discussion will focus on surface hydrolysis reactions, that is, adsorption of H+ and OH . Virtually all surface hydrolysis experiments are carried out in the presence of a "background electrolyte," many of which appear to exhibit weak specific chemical interactions (e.g., ion-pair formation) with the surface (10-12). While consideration of these interactions is essential to a complete understanding of the interfacial chemistry, the topic is a subject in itself, and will not be considered in detail here. Treatment of these interactions is readily incorporated within the framework that is presented here. 

In any study of electrosorption of neutral molecules on metallic electrodes, the ions of supporting electrolytes should not be specifically adsorbed. Nevertheless, the interaction of the electrolyte ions with the electrode surface may depend on the interaction of the ions with the solvent. Usually, the stronger the ion-solvent interaction, the weaker the adsorption of the ion. Since the ions are more weakly solvated in nonaqueous solvents than in water, the ions that are not adsorbed from aqueous solutions may still be adsorbed from organic solvents. However, even in the absence of... 

It is instructive to consider the effect of dissociation on the adsorption of amphipathic substances since many of the compounds that behave according to curve 3 are electrolytes. We consider only the case of strong 1 1 electrolytes for weak electrolytes the equilibrium constant for dissociation must be considered. 

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