Weak electrolytes chemical potential

The methods for obtaining expressions for the chemical potential of a component that is a weak electrolyte in solution are the same as those used for strong electrolutes. For illustration we choose a binary system whose components are a weak electrolyte represented by the formula M2A and the solvent. We assume that the species are M +, MA , A2-, and M2A. We further assume that the species are in equilibrium with each other according to... 

An expression can now be written for the chemical potential of the weak electrolyte system. On the basis of the composition of the solution... 

This result shows that the chemical potential of the weak electrolyte system may be expressed in terms of the activities of the ions only, without explicitly including the activity of the undissociated molecule. Equation (3.6.38) is no different in form from those for a strong electrolyte (equations (3.6.1) and (3.6.2)). Of course, the activities of the ions are much less for the weak electrolyte than those for the strong electrolyte for a given molality. Thus, on the basis of the present analysis for a weak electrolyte... 

This result shows that the chemical potential of the electrolyte can be expressed in terms of the activities of the two ions without considering that of the ion pair. It was obtained earlier for weak electrolytes as equations (3.6.38) and (3.6.40). 

At equilibrium, during each step of the potentiometric titration of a weakly acidic (or weakly basic) gel (HA)V, in the presence of a simple electrolyte MX, the chemical potential fi of each diffusible component (HX, MX, and H20) is equal in both phases for example,... 

In this chapter we discuss some of the properties of electrolyte solutions. In Sec. 12-1, the chemical potential and activity coefficient of an electrolyte are expressed in terms of the chemical potentials and activity coefficients of its constituent ions. In addition, the zeroth-order approximation to the form of the chemical potential is discussed and the solubility product rule is derived. In Sec. 12-2, deviations from ideality in strong-electrolyte solutions are discussed and the results of the Debye-Hiickel theory are presented. In Sec. 12-3, the thermodynamic treatment of weak-electrolyte solutions is given and use of strong-electrolyte and nonelectrolyte conventions is discussed. 

Application of the expression for the chemical potential of a strong electrolyte given by Eq. (12-10) to the system consisting of weak-electrolyte solute HA in a solvent yields the result... 

Application of the conventions used for nonelectrolytes to the weak electrolyte HA yields an expression for the chemical potential of HA in the form... 

Neither the strong-electrolyte nor the nonelectrolyte formulation is applicable to weak electrolytes throughout the entire range of concentration. At high molality a weak electrolyte behaves like a nonelectrolyte and at low molality it behaves like a strong electrolyte. In order to develop expressions for chemical potentials of weak electrolytes which may be used over the entire composition range it is customary to resort to a nonoperational treatment based on certain structural considerations which involve nonmeasurable quantities. 

Thus, at high concentration we can formally associate 1 — a with an activity coefficient. Equation (12-57) is an expression for the chemical potential of weak electrolyte HA using the nonelectrolyte convention of Eq. (12-36). An alternative expression for //ha in zero approximation can be obtained from Eq. (12-39) in the form... 

The main objective of this chapter is to introduce the reader to physical chemistry of electrolyte solutions. An electrolyte solution consists of charged species (ions), and it makes these solutions very useful for electrochanical science and engineering. Concentration, activity, activity coefQdent, and chemical potential of both solute and solvent are described in detail. The concentration of species in weak electrolytes and pH of aqueous solutions are discussed, and physical chemistry of buffer solutions is explained. 

Thus, the transport of hydrated ions and chemical debonding processes can be studied by means of the SKP. Fig. 31.6 shows the potential distribution measured with the SKP when a thin electrolyte layer enters the interface between an adhesive and an iron surface covered by a thin (about 6 nm) nonconducting SiOx layer precipitated by a plasma-polymerization process [51, 52]. The SiO layer inhibits the electron-transfer reaction. Consequently, no corrosive degradation of the interface takes place (see Section 31.3.2.1). However, as the adhesion of the epoxy adhesive to the siUca-Uke layer is weak, the polymer is replaced by... 

The employment of suitable organic solvents, such as acetonitrile and acetic acid, with oxidation-resistant supporting electrolytes permits the anodic formation of reactive radical cations from many organic materials. Most aromatic compounds and olefins, as well as those alkanes which have particularly weak C—H bonds, are oxidised in acetonitrile containing fluoroborate or hexafluorophosphate electro-lytes. °" 2 Some aromatic radical cations can be further oxidised to dications within the available potential range. Radical cations in general either deprotonate or attack nucleophiles present in the medium reactions with pyridine, methanol, water, cyanide ion, acetate ion or acetonitrile itself produce addition or substitution products. The complete reactions involve a second electron transfer and coupled chemical... 

For some systems, there is no resonance Raman or SERS effect to be utilized, and the sensitivity becomes the main problem. In this case, a potential difference method will be of great help [11], Here, a spectrum is acquired at potentials where there is no or only a weak surface signal which is subtracted from that at the potential of interest. In addition, a change in the composition of the electrolyte or an isotopic labeling experiment may be considered to identify the surface species and verify its orientation and structure. For temporally resolved studies, electrochemical transient techniques are helpful to understand the surface dynamics and the reconstruction processes of surfaces. For nonuniform surfaces, spatially resolved measurements provide more reliable and complete information on the surface. This is also useful for electrode surfaces that change either chemically or topographically in a microzone upon variation of potential. 

Micellar Effects on Chemical Equflibria.—A few studies have been made of acid-base equilibria in micelles. Hydronium ion activity in anionic micelles has been measured conductimetrically using hydrophilic indicators, it being found that a plot of mn+ versus [H ]-t-[Na ] is linear with a slope of 0.82. The quantity mH+ is defined as the number of micellized hydrogen ions per surfactant head group, namely mH = [H ]tot—[H ]w/ [D]tot c.m.c., where [DJtot is the total catalyst concentration. The use of fluorescent indicators (21a) and (21b) in anionic, neutral, and cationic surfactantspermitted the evaluation of the electrical potential at the micellar surface as a function of added electrolytes. Indicator pK values for mixed micelles and pK values of weak... 

---