Equilibrium with electrolyte solutions

Fic. 136.—Diagram of swollen ionic gel in equilibrium with electrolyte solution. Fixed charges are represented by [T. ... 

Experiments on sufficiently dilute solutions of non-electrolytes yield Henry s laM>, that the vapour pressure of a volatile solute, i.e. its partial pressure in a gas mixture in equilibrium with the solution, is directly proportional to its concentration, expressed in any units (molar concentrations, molality, mole fraction, weight fraction, etc.) because in sufficiently dilute solution these are all proportional to each other. 

The linear energy of the contact line in three-phase equilibrium system could have either positive or negative values. This does not violate the mechanical equilibrium stability condition in such systems. This is proved experimentally by determining k in the case of liquid black films in equilibrium with bulk solutions. The absolute values of k obtained are less than about 10 9 J m 1 (10 4 dyn) they are positive at lower and negative at higher electrolyte (NaCl) concentrations. 

It has already been pointed out that we may take the vapour pressure of a substance as a measure of its active mass, and therefore of its activity In the case of homogeneous reactions in gases it may be recalled that the law of mass action has been fully verified, the terms employed in the expression for K being simply the concentration terms of the various constituents In the case of gases, and therefore in the case of vapours at low concentrations, the concept of activity becomes identical with concentration On this basis the relative activities of the undissociated molecules of an electrolyte will be correctly measured at various concentrations of the solution by determining the values of the partial pressure of the molecules in the vapour in equilibrium with the solutions In the majority of cases the un-dissociated molecules of strong electrolytes are not sufficiently volatile to become measureable in this way There are, however, a few electrolytes in which this is possible, namely, aqueous solutions of HC1, HBr, and HI... 

The elementary theory of the Donnan equilibria has been treated in Part I, Chapter X. Considering a gel of a macromolecular electrolyte in equilibrium with a solution, a situation is met with similar to that of the case of an electrolyte with one ion which cannot diffuse through the membrane. The macromolecules are bound to remain in the gel since they form an intrinsic part of the framework. The concentration of the added electrolyte will always be different in and outside the gel. 

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 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.

It is apparent from this that since the rates of the cathodic and anodic processes at each electrode are equal, there will be no net transfer of charge in fact, with this particular cell, consisting of two identical electrodes in the same electrolyte solution, a similar situation would prevail even if the electrodes were short-circuited, since there is no tendency for a spontaneous reaction to occur, i.e. the system is at equilibrium and AG = 0. 

Soluble corrosion products may increase corrosion rates in two ways. Firstly, they may increase the conductivity of the electrolyte solution and thereby decrease internal resistance of the corrosion cells. Secondly, they may act hygroscopically to form solutions at humidities at and above that in equilibrium with the saturated solution (Table 2.7). The fogging of nickel in SO2-containing atmospheres, due to the formation of hygroscopic nickel sulphate, exemplifies this type of behaviour. However, whether the corrosion products are soluble or insoluble, protective or non-protective, the... 

Rainfall, besides wetting the metal surface, can be beneficial in leaching otherwise deleterious soluble species and this can result in marked decreases in corrosion rate . A recent survey of rainfall analyses for Europe has shown that, with the exception of the UK, the acidity and sulphate content of rainfall markedly increased in the period 1956 to 1966, pH values having fallen by 0 05 to 0-10 units per ann. The exception of the UK may be due to anti-pollution measures introduced in this period. However, even in the UK a pH of 4 is not uncommon for rainfall in industrial areas. The significance of electrolyte solution pH will be discussed in the context of corrosion mechanisms. The remaining cases of electrolyte formation are those in which it exists in equilibrium with air at a relative humidity below 100%. 

In discussing precipitation reactions in Chapter 4, we assumed, in effect, that they went to completion. We must keep in mind, however, that precipitation reactions, like all reactions, reach a position of equilibrium. Putting it another way, even the most "insoluble" electrolyte dissolves to at least a slight extent, thereby establishing equilibrium with its ions in solution. 

Figure 1. Sketch of an electrochemical cell whose equilibrium (open circuit) potential difference is AE. (a) Conventional configuration and (b) short-circuited configuration with an air gap. M and R are the electrodes, S is the solvent (electrolyte solution). Cu indicates the cables connecting the two electrodes to a measuring instrument (or to each other).

Solid Bi2S3 does not appear in the expression for K,p, because it is a pure solid and its activity is 1 (Section 9.2). A solubility product is used in the same way as any other equilibrium constant. However, because ion-ion interactions in even dilute electrolyte solutions can complicate its interpretation, a solubility product is generally meaningful only for sparingly soluble salts. Another complication that arises when dealing with nearly insoluble compounds is that dissociation of the ions is rarely complete, and a saturated solution of Pbl2, for instance, contains substantial... 

Turning now to the case of equilibrium with an infinite external solution containing c/ moles of electrolyte Mp Av per liter, we note that Eq. (B-3) is then equivalent to Eq. (45). As a further condition of equilibrium, it is required that the activity of this electrolyte be the same inside and outside the gel. The activity of the electrolyte being equal to the product of the activities of the individual ions into which it dissociates, this condition may be stated as follows... 

When the metal is in contact with an electrolyte solution not containing its ions, its equilibrium potential theoretically will be shifted strongly in the negative direction. However, before long a certain number of ions will accumulate close to the metal surface as a result of spontaneous dissolution of the metal. We may assume, provisionally, that the equilibrium potential of such an electrode corresponds to a concentration of ions of this metal of about 10 M. In the case of electrodes of the second kind, the solution is practically always saturated with metal ions, and their potential corresponds to the given anion concentration [an equation of the type (3.35)]. When required, a metal s equilibrium potential can be altered by addition of complexing agents to the solution (see Eq. (3.37)]. 

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