Weak electrolytes in aqueous solutions

Many of the undesirable substances present in gaseous or liquid streams form volatile weak electrolytes in aqueous solution. These compounds include ammonia, hydrogen sulfide, carbon dioxide and sulfur dioxide. The design and analysis of separation processes involving aqueous solutions of these materials require accurate representation of the phase equilibria between the solution and the vapor phase. Relatively few studies of these types of systems have been published concerning solutions of weak electrolytes. This paper will review the methods that have been used for such solutions and, as an example, consider the alkanolamine solutions used for the removal of the acid gases (H2S and C02) from gas streams. 

On the Solubility of Volatile Weak Electrolytes in Aqueous Solutions... 

The solubility of gaseous weak electrolytes in aqueous solutions is encountered in many chemical and petrochemical processes. In comparison to vapory-liquid equilibria in non reacting systems the solubility of gaseous weak electrolytes like ammonia, carbondioxide, hydrogen sulfide and sulfur dioxide in water results not only from physical (vapor-liquid) equilibrium but also from chemical equilibrium in the liquid phase. 

Maurer G. On the solubility of volatile weak electrolytes in aqueous solutions. ACS Symp Ser 1980 133 139-172. 

J. Kendall, Electrical conductivity and ionization constants of weak electrolytes in aqueous solution, in International Critical Tables (ed. E. Washburn), 1st edn., McGraw-HiU, New York, 1929, pp. 259-304. 

Conductance H2O i =25 c = 0.03-0.001 Kendall J, Electrical conductivity and ionizaticm constants of weak electrolytes in aqueous solution, in Washburn EW, Editor-in-Chief, International Critical Tables, Vol. 6, McGraw-Hill, NY, 9-304... 

From G. Maurer, "On the Solubility of Volatile Weak Electrolytes In Aqueous Solutions", Thermodynamics of Aqueous Syatema with Industrial... 

Maurer, G., "On the Solubility of Volatile Weak Electrolytes in Aqueous Solutions", Thermodynamics of Aqueous Systems with Industrial Applications. S.A. Newman, ed., ACS Symposium Series 133, Washington D.C.. ppl39-172 (1980)... 

The species appearing as strong electrolytes in aqueous solutions lose this property in low-permittivity solvents. The ion-pair formation converts them to a sort of weak electrolyte. In solvents of very low-permittivity (dioxan, benzene) even ion triplets and quadruplets are formed. 

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. ... 

Ampholytes (amphoteric electrolytes) can function as either weak acids or weak bases in aqueous solution and have plC values corresponding to the ionisation of each group. They may be conveniently divided into two categories - ordinary ampholytes and zwitterionic ampholytes - depending on the relative acidity of the two ionisable groups. 

Acids are classified as strong or weak, depending on the extent to which they are ionized in solution. In a weak acid the transfer of hydrogen ions to water does not proceed to completion. A weak acid such as acetic acid is thus also a weak electrolyte its aqueous solutions do not conduct electricity as well as a strong acid of the same concentration because fewer ions are present. A weak acid shows smaller values for colligative properties than a strong acid (recall the effect of dissolved acetic acid on the freezing point of water in Fig. 11.13). 

SOLUBILITY OF A WEAK ELECTROLYTE IN SALT SOLUTIONS. Calculation of the solubility of a volatile strong electrolyte, such as HCl, in aqueous salt solutions is straightforward. However, solubilities of weak electrolytes are more difficult to model accurately, since the dissolved speciation must frequently be determined in addition to the activity of the component of interest. Thus, in the case of NH3, the relevant ionic interactions involving NH4 and OH" must be known in addition to parameters for the interaction of dissolved salts with the neutral NH3 molecule. See, for example, the work of Maeda et al. (47) on the dissociation of NH3 in LiCl solutions. 

Electrolytes in aqueous solution In Chapter 8, you read that ionic compounds are called electrolytes because they dissociate in water to form a solution that conducts electric current, as shown in Figure 14.19. Some molecular compounds ionize in water and are also electrolytes. Electrolytes that produce many ions in a solution are called strong electrolytes those that produce only a few ions in a solution are called weak electrolytes. 

Another variation of crystallization out of a solution, is separation by adding a salt (chemical precipitation). Nonelectrolytes or weak electrolytes are replaced by strong electrolytes in aqueous solutions [7.1, 7.18]. 

Perchlorate salts have been widely used in laboratory studies to adjust the total electrolyte concentration because the anion was thought to be noncoordinating. Thus, addition of perchlorates in almost any amount could be made without affecting the identity of complex ions also present in the solution. This assumption is now known to be incorrect, and coordinated perchlorate has been identified in a number of cases involving both main group and transition metal ions. The donor ability of perchlorate is quite weak, however, and the use of this ion as an inert electrolyte in aqueous solution is still widespread. 

Salts and acids and bases are electrolytes. In aqueous solutions, electrolytes dissolve and dissociate into ions. Molecules that dissociate completely are called strong electrolytes, and those that dissociate only partly are called weak electrolytes. The degree of dissociation can be represented by the equilibrium dissociation constant K,... 

In aqueous electrolyte solutions the molar conductivities of the electrolyte. A, and of individual ions, Xj, always increase with decreasing solute concentration [cf. Eq. (7.11) for solutions of weak electrolytes, and Eq. (7.14) for solutions of strong electrolytes]. In nonaqueous solutions even this rule fails, and in some cases maxima and minima appear in the plots of A vs. c (Eig. 8.1). This tendency becomes stronger in solvents with low permittivity. This anomalons behavior of the nonaqueous solutions can be explained in terms of the various equilibria for ionic association (ion pairs or triplets) and complex formation. It is for the same reason that concentration changes often cause a drastic change in transport numbers of individual ions, which in some cases even assume values less than zero or more than unity. 

Electrolytes are defined as substances whose aqueous solutions conduct electricity due to the presence of ions in solution. Acids, soluble bases and soluble salts are electrolytes. Measuring the extent to which a substance s aqueous solution conducts electricity is how chemists determine whether it is a strong or weak electrolyte. If the solution conducts electricity well, the solute is a strong electrolyte, like the strong acid, HC1 if it conducts electricity poorly, the solute is a weak electrolyte, like the weak acid, HF. 

This time a solid sample of a weak base is being added to a solution of its conjugate acid. We let represent the concentration of acetate ion from the added sodium acetate. Notice that sodium acetate is a strong electrolyte, completely dissociated in aqueous solution. 

Many of the reactions that you will study occur in aqueous solution. Water readily dissolves many ionic compounds as well as some covalent compounds. Ionic compounds that dissolve in water (dissociate) form electrolyte solutions— solutions that conduct electrical current due to the presence of ions. We may classify electrolytes as either strong or weak. Strong electrolytes dissociate (break apart or ionize) completely in solution, while weak electrolytes only partially dissociate. Even though many ionic compounds dissolve in water, many do not. If the attraction of the oppositely charged ions in the solid is greater than the attraction of the water molecules to the ions, then the salt will not dissolve to an appreciable amount. 

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