Electrolyte strength

A brief list of some common substances classified according to their electrolyte strength is given in Table 4.1. Note that pure water is classified as a nonelectrolyte because it does not dissociate appreciably into H+ and OH ions. We ll explore the dissociation of water in more detail in Chapter 15. 

Note that here using the natural logarithm requires that K be a pure number, while it usually has the units of some power of concentrations. Note also that, to "keep us honest," activities and not concentrations should enter into Eqs. (6.2.14) or (6.2.19). In the pious but understandable desire to keep equations simple when the reagents interact in nonideal fashion, activity coefficients y can be used as premultipliers to [A], [B], etc. (e.g., yA, yB, etc.) to convert concentrations to activities these empirical "fudge factors" are themselves dependent on temperature, on concentration, and on electrolyte strength and hide within them the departure from ideal behavior. 

Since both the feed and the sweep solutions are of similar order of electrolyte strength, it may be assumed that the nonideality effects are about the same on both sides of the membrane, and activities can therefore be replaced by molar concentrations. Equations 34.3 and 34.4 then yield the following ... 

Since both sides have high electrolyte strength, the activity coefficient term can be assumed identical for a given species in the two solutions, in accordance with the correlation based on modified Debye-Huckel theory proposed by Davies [28]. Therefore, the concentration term can replace the activity coefficient term and the condition for Donnan equilibrium becomes... 

To incorporate dilution effect, a separate experiment is carried out with known concentration of NaCl on the sweep side and deionized water on the feed side of the membrane to determine the osmosis of water. Water flux is related to the difference in electrolyte strength of the two solutions and has been validated in previous studies [37]. For the current work, it can be assumed that the water flux. /osmosis (m /m h) is proportional to the difference of ionic concentration in the two solutions. 

More rigorous treatment of comparison of homogeneous and heterogeneous membranes involving conductivity measurements at different electrolyte strengths can help in characterizing the transport properties of these membranes. 

Firstly, ion exchange resins when hydrated generally dissociate to yield equivalent amounts of oppositely charged ions. Secondly, as with conventional aqueous acid or alkali solutions, resins in their acid or base forms may be neutralized to give the appropriate salt form. Finally, the degree of dissociation can be expressed in the form of an apparent equilibrium constant (or pK value) which defines the electrolyte strength of the exchanger and is usually derived from a theoretical treatment of pH titration curves. ... 

Exploration of the Effects of Electrolyte Strength on Solution Ionic Strength. 

The unit for C51 and Cei is meq/mL, and the unit for Cn is water volume fraction in the aqueous phase. The commonly used laboratory units for salinity are wt.% and ppm (mg/L). These units should be converted to meq/mL using Eq. 5.2. In principle, any units could be used, as long as they are used consistently in a study. It is suggested that one unit be used throughout a study. The unit meq/mL is a good scientific unit of salinity because it considers the effects of different ions with different electrolyte strength. In most cases, the unit is chosen based on convenience, not science. Eor example, the salinity is reported... 

Liu (2007) introduced another method called soap extraction to quantify acid number. Because the anionic surfactant can be accurately determined by potentiometric titration (see Appendix A in Liu, 2007) with benzethonium chloride (hyamine 1622), it is reasonable to use this method to find the natural soap amount. Because this potentiometric titration is for the aqueous phase, the soap should be extracted into the aqueous phase as the first step. As an anionic surfactant, the natural soap may stay in the oleic phase and form Winsor type 11 microemulsion when the electrolyte strength is high. To extract the soap into the aqueous phase, NaOH is used to keep the pH high with low electrolyte strength. Also, isopropyl alcohol is added to make the system hydrophilic so that soap will partition into the aqueous phase. 

One can calculate ratio aoi pc dcb/ i, pc-py =1-36 using the data from Tables 9.10 and 9.11 and assess the leveling effect of the solvent propylene carbonate-pyridine on electrolyte strength. The ratio Upc dcb/ pc hac = 3.1 indicates that propylene carbonate-acetic acid has more pronounced leveling effect on electrolyte strength than propylene carbonate-pyridine. 

The same effect can be obtained by means of solvent heating. For instance, LiBr solution in o-dichlorobenzene must be heated to 215 C to reach the same value of association constant as in propylene carbonate at room temperature. For this reason, solvent may be used as an effective means for tailoring electrolyte strength in conditionally universal media as well. 

FIGURE 3.11 ] is shown here with varying electrolyte strengths (k per centimeter). The sphere radii are 1000 A, Ag = 10 ergs, and = 25 mV. Reprinted from Verwey and Overbeek (1948) with permission. 

The above description of acids and bases, in which H (aq) and OH (aq) ions are viewed as responsible for acidic and basic properties, respectively, and different acidic (or electrolytic) strengths are attributed to varying degrees of ionic dissociation, was developed by the Swedish chemist S. Arrhenius between 1880 and 1890. While very useful, this theory has some problems. The first problem has to do with the nature of the positive-charge carrier in aqueous solutions the second problem is that some substances can act as bases, even though they do not release OH (aq) ions. We will now consider both of these problems. 

Acids (and bases) are classified by their strength, the amount of (or OH ) produced per mole of substance dissolved, in other words, by the extent of then-dissociation into ions (see Table 4.2). Because acids and bases are electrolytes, then-strength correlates with electrolyte strength strong electrolytes dissociate completely, and weak electrolytes dissociate slightly. 

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