Weak electrolytes defined

Earlier in this chapter, strong and weak electrolytes were distinguished in terms of the degree to which the dissolved material forms ions. As a particular case, such distinctions can be made in terms of acids, furnishing a quantitative basis for defining the strength of an acid. 

Arrhenius postulated in 1887 that an appreciable fraction of electrolyte in water dissociates to free ions, which are responsible for the electrical conductance of its aqueous solution. Later Kohlrausch plotted the equivalent conductivities of an electrolyte at a constant temperature against the square root of its concentration he found a slow linear increase of A with increasing dilution for so-called strong electrolytes (salts), but a tangential increase for weak electrolytes (weak acids and bases). Hence the equivalent conductivity of an electrolyte reaches a limiting value at infinite dilution, defined as... 

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. 

At the microscopic level, the Arrhenius theory defines acids as substances which, when dissolved in water, yield the hydronium ion (H30+) or H+(aq). Bases are defined as substances which, when dissolved in water, yield the hydroxide ion (OH). Acids and bases may be strong (as in strong electrolytes), dissociating completely in water, or weak (as in weak electrolytes), partially dissociating in water. (We will see the more useful Brpnsted-Lowry definitions of acids and bases in Chapter 15.) Strong acids include ... 

Recently, the Pitzer equation has been applied to model weak electrolyte systems by Beutier and Renon ( ) and Edwards, et al. (10). Beutier and Renon used a simplified Pitzer equation for the ion-ion interaction contribution, applied Debye-McAulay s electrostatic theory (Harned and Owen, (14)) for the ion-molecule interaction contribution, and adoptee) Margules type terms for molecule-molecule interactions between the same molecular solutes. Edwards, et al. applied the Pitzer equation directly, without defining any new terms, for all interactions (ion-ion, ion-molecule, and molecule-molecule) while neglecting all ternary parameters. Bromley s (1) ideas on additivity of interaction parameters of individual ions and correlation between individual ion and partial molar entropy of ions at infinite dilution were adopted in both studies. In addition, they both neglected contributions from interactions among ions of the same sign. 

As it is possible to measure (or closely approximate) the ionic concentrations of a weak electrolyte, it is convenient to define ionic activity coefficients for weak electrolytes in the same way, based on the actual ionic concentrations, or m. Thus,... 

Which electrolytes are called strong, and which are called weak In what form can the law of mass action be applied to strong electrolytes Define the activity, the activity coefficient, and the ionic strength of a solution. 

At infinite dilution a weak electrolyte will be completely dissociated. Thus the conductance A0 at infinite dilution can be associated with complete dissociation and a = 1. Assuming that the decrease of Aeq(Aeq = A/z) with increasing concentration is caused mostly by the decrease of the degree of dissociation, a can be defined as... 

In order to test the reliability of equation (99) it is necessary to know the value of the degree of dissociation at various concentrations of the electrolyte MA in his classical studies of dissociation constants Ostwald, following Arrhenius, assumed that a at a given concentration was equal to the conductance ratio A/Ao, where A is the equivalent conductance of the electrolyte at that concentration and Ao is the value at infinite dilution. As already seen (p. 95), this is approximately true for weak electrolytes, but it is more correct, for electrolytes of all types, to define a as A/A where A is the conductance of 1 equiv. of free ions at the same ionic concentration as in the given solution. It follows therefore, by substituting this value of a in equation (95), that... 

The compounds in the series RSbBr2, R = Et, Pr, Bu are very weak electrolytes in THF as shown by the absence of an oxidation wave for Br" The polarogram of RSbBrj in THF shows several ill-defined waves in the range 0.0-1.8 V vs NHE . Electrolysis at a potential of —1.5 V vs NHE yields a green solution after 2 F mol", which shows a single oxidation wave around —0.3 V vs NHE independent of the substituent R. Reoxidation of the electrolysed solution restores the initial polarogram . After two-electron reduction a yellow product could be isolated which, by MS and NMR spectroscopy, was shown for R = Et to be a mixture of (RSb)4 and (RSb)j and almost exclusively (RSb)5 when R= Pr, Bu these products are identical to the cyclic Sb(I) products obtained by reaction of Mg... 

If, in the first instance, the plasma membrane is considered to be a strip of lipoidal material, homogeneous in nature and with a defined thickness, one must assume that only lipid-soluble agents will pass across this barrier. As most dmgs are weak electrolytes it is to be expected that the unionised form (U) of either acids or bases, the lipid-soluble species, will diffuse across the membrane, while the ionised forms (1) will be rejected. This is the basis of the pH-partition hypothesis in which the pH dependence of drug absorption and solute transport across membranes is considered. The equations of Chapters 3 and 5 are relevant here. 

Briefly describe or define and give an example of (a) a weak electrolyte. 

Define and distinguish among (a) strong electrolytes, (b) weak electrolytes, and (c) nonelectrolytes. 

Define and illustrate the following terms clearly and concisely. Give an example of each, (a) strong electrolyte (b) weak electrolyte (c) nonelectrolyte (d) strong acid (e) strong base (f) weak acid (g) weak base (h) insoluble base. 

Derivation of the thermodynamic equations for an electrolyte system with ion pairing follows the same procedure given for a weak electrolyte. However, in the following the ion pairing equilibrium is defined in terms of an association process. For a 1-1 electrolyte, ion pairing is described as... 

First, the concept of mobility must be defined for weak electrolytes. The initial Tiselius perceptions serves as the basis and it can be expressed as follows the substance, present in the solution in more forms, whose molar fractions are x , Xj,... x , mobilities Uj>, Uj,... u and individual forms are in a rapid dynamic equilibrium with one another, migrates through the electric field as the only substance with a certain effective mobility, u, defined by the relationship... 

Conduction of electric current in conductors can be electronic or ionic, depending on the type of charges involved. Electronic conduction is found in all metals and also in certain other nonmetals. Ionic conductors are also known as electrolytes. Substances that ordinarily are not conducting can produce ionic conduction after being dissolved in water or another solvent (e.g., electrolyte solution and weak electrolyte ). The relative amount of substance present in a solution or a mixture is known as its concentration. The different concentration units used mostly are molarity, molality, normality, and mole fraction. The acidity or basicity of a solution is measured by a relative measurement called the pH of solution. It is defined as the cologarithm of the activity of dissolved hydrogen ions (H" "). Pure water is said to be neutral. The pH for pure water at 25 °C (77 °F) is close to 7.0. Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are said to be basic or alkaline. 

Indeed, PAAc cryogels coupled with a bromate oscillator oscillated between swollen and collapsed states [31]. The reactions of bromate, sulfite, and ferrocyanide ions were conducted in an open continuously stirred tank reactor. Four feed solutions (potassium bromate, sodium sulfite, potassium ferrocyanide, and sulfuric acid) were supplied continuously to the reactor, during which the pH of the reaction solution was monitored as a function of time. The flow rate of the feed solutions is an important parameter in determining the extent of pH oscillations. In Fig. 21, pH versus time plots are shown for four different reduced flow rates k, defined as the flow rate of the feed solutions divided by the reaction volume. It is seen that the pH of the solution oscillates between 6.2-6.9 and 3.2-3.8. The dissociation degree a of a weak electrolyte relates to pH by ... 

Two especially important categories of aqueous solutions are acids and bases. Examples of them are easy to find in our everyday lives as well as in the chemical industry. For our present purposes, we will define an acid as any substance that dissolves in water to produce H (or H3O+) ions and a base as any substance that dissolves in water to produce OH ions. Table 3.2 lists some common acids and bases. Like other solutes, acids and bases can be either strong or weak electrolytes. 

As we have seen in several examples in this chapter, HCN acts as an acid in aqueous solutions. We introduced a few fundamental concepts of acids and bases in Chapter 3, but the context of equilibrium allows us to explore them further. Recall that we distinguished between strong acids (or bases), which dissociate completely in solution, and weak acids (or bases), which dissociate only partially. At this point in our study of chemistry, we should realize that this partial dissociation of weak electrolytes was an example of a system reaching equilibrium. So we can use equilibrium constants to characterize the relative strengths of weak acids or bases. One common way to do this is to use the pH scale, which we will define in this section. 

The selectivity of the glass electrode for NH3 and CO2 may be improved over that of pH measurement by inclusion of a gas permeable membrane between the enzyme layer and the pH electrode. At constant solution, pH a defined relation exists between the potential of the glass electrode and the concentration of the gas-forming ions HC03 or NH4 which are formed in the enzyme reaction. Maximum sensitivity of the electrode is reached when the H+ concentration in the solution is sufficient to ensure maximum conversion of the weak electrolyte into its undissociated form, that is CO2 or NH3. With NH3 this occurs at pH > 10 and with CO2 at pH > 5. Generally, these pH values differ substantially from the pH optima of deaminase and decarboxylase enzymes therefore, for the respective enzyme electrodes a compromise pH has to be found. To obtain optimal conditions for both steps, the enzyme reaction is often separated from the potentiometric indication and a pH change is included between these stages. This setup is termed a reactor electrode. 

In our consideration we have not yet taken into account that liquid crystals are weak electrolytes possessing the corresponding specific properties [2, 3, 97]. Ions could be created by the action of the external electric field, which favors the dissociation of neutral molecules (chemical degradation) or as a result of electrochemical reactions near the electrode boundaries. The latter process is mainly defined by the injection of the additional charge carriers from the electrodes. In some cases, the appearance of electrohydrodynamic instabilities in the nematic phase does not correlate with such crystalline properties as dielectric Ae or conductivity Aa anisotropy. The only physical reason for these instabilities is nonuniform ion distribution along the direction z parallel to the electric field and perpendicular to the cell substrate. 

Define the terms strong electrolyte and weak electrolyte. Give an example of each. 

Electrolytic conduction is due to the presence of ions thus (T is a function of the concentration of equivalents of ions, Strong electrolytes like NaCl, HNOg, etc., are fully dissociated while weak electrolytes like HCN are not. In the former case C is the same as the normality of the electrolyte C in the latter it is not. As a consequence the conductivity of a strong electrolyte is roughly proportional to C while that of a weak one is not. It is convenient to define the equivalent conductance which accounts for the concentration dependence... 

The strength of an electrolyte, first discussed in Section 4.5, depends on the extent of its dissociation into its component ions in solution. A strong electrolyte completely dissociates into ions in solution, whereas a weak electrolyte only partially dissociates. We define strong and weak acids accordingly. A strong acid completely ionizes in solution, whereas a weak acid only partially ionizes. In other words, the strength of an acid depends on the equilibrium ... 

PK. A measurement of the complete ness of an incomplete chemical reaction. It is defined as the negative logarithm ito the base 101 of the equilibrium constant K for the reaction in question. The pA is most frequently used to express the extent of dissociation or the strength of weak acids, particularly fatty adds, amino adds, and also complex ions, or similar substances. The weaker an electrolyte, the larger its pA. Thus, at 25°C for sulfuric add (strong acid), pK is about -3,0 acetic acid (weak acid), pK = 4.76 bone acid (very weak acid), pA = 9.24. In a solution of a weak acid, if the concentration of undissociated acid is equal to the concentration of the anion of the acid, the pAr will be equal to the pH. 

We consider simple polyelectrolytes, which we define as homopolymers In which each monomer unit carries an ionizable group. Such a group may be a strong acid or base, in which case its charge is virtually independent of pH (strong poly electrolyte]. On the other hand, weak polyelectrolytes carry weakly acidic (for example carboxyl) or weakly basic (for example amino) groups their solution behaviour depends on pH. 

---