The Salt Effect

The dissolution of electrolytes in water has a strong effect on the internal pressure of the solvent, a phenomenon known as the salt effect. Almost all electrolytes (perchloric acid is the exception) increase the internal pressure of water by elec-trostriction, a term used to describe the polarization and attraction of water molecules. The effect of this internal pressure is to squeeze out the organic 

2 THE BENEFITS AND PROBLEMS ASSOCIATED WITH USING WATER IN CHEMICAL SYNTHESIS 

Water is abundant and has many properties that make it a desirable solvent (perhaps only superseded by not using a solvent at all). These desirable properties include the following  

1 Polar and therefore relatively easy to separate from apolar solvents. 

4 Odourless and colourless-contamination is therefore easy to recognize. 

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The indicator method is especially convenient when the pH of a weU-buffered colorless solution must be measured at room temperature with an accuracy no greater than 0.5 pH unit. Under optimum conditions an accuracy of 0.2 pH unit is obtainable. A Hst of representative acid—base indicators is given in Table 2 with the corresponding transformation ranges. A more complete listing, including the theory of the indicator color change and of the salt effect, is also available (1). 

An important cautionary note must be inserted here. It may seem that the study of the salt effect on the reaction rate might provide a means for distinguishing between two kinetically equivalent rate terms such as k[HA][B] and k [A ][BH ], for, according to the preceding development, the slope of log k vs. V7 should be 0, whereas that of log k vs. V7 should be — 1. This is completely illusory. These two rate terms are kinetically equivalent, which means that no kinetic experiment can distinguish between them. To show this, we write the rate equation in the two equivalent forms, making use of Eq. (8-26) ... 

It is important to note that the solubility product relation applies with sufficient accuracy for purposes of quantitative analysis only to saturated solutions of slightly soluble electrolytes and with small additions of other salts. In the presence of moderate concentrations of salts, the ionic concentration, and therefore the ionic strength of the solution, will increase. This will, in general, lower the activity coefficients of both ions, and consequently the ionic concentrations (and therefore the solubility) must increase in order to maintain the solubility product constant. This effect, which is most marked when the added electrolyte does not possess an ion in common with the sparingly soluble salt, is termed the salt effect. 

Fig. 6.3.6 Effects of salt concentration (left panel) and pH (right panel) on the initial light intensity emitted from the homogenate of the Symplectoteuthis oualaniensis light organ. The salt effect was tested in 50 mM Tris-HCl, pH 7.2, and the pH effect in the various buffers containing 0.5MKC1 or NaCl. From Tsuji and Leisman, 1981.

The St content at which the line broadening starts to occur coincides with the one marking the sharp decrease in the salt-effect on reduced viscosity (see Fig. 6). 

The salt effect is very strong in polyconjugated polyelectrolytes. Figure 15 is a graph of the proton dissociation energy vs. the dissociation degree of PPA of different structures. Also, the graphs for poly(methacrylic acid) and a copolymer... 

However, the addition of even small volumes of alkali leads to the screening of these groups, with a subsequent decrease of the proton dissociation energy at low dissociation degrees. This complies with the salt effect (Fig. 15). 

The special salt effect is a constant feature of the activation of substrates in cages subsequent to ET from electron-reservoir complexes. In the present case, the salt effect inhibits the C-H activation process [59], but in other cases, the result of the special effect can be favorable. For instance, when the reduction of a substrate is expected, one wishes to avoid the cage reaction with the sandwich. An example is the reduction of alkynes and of aldehydes or ketones [60], These reductions follow a pathway which is comparable to the one observed in the reaction with 02. In the absence of Na + PFg, coupling of the substrate with the sandwich is observed. Thus one equiv. Na+PFg is used to avoid this cage coupling and, in the presence of ethanol as a proton donor, hydrogenation is obtained (Scheme VII). 

We shall demonstrate that the magnitude of the salt effect is an attribute of the rate law, not of the reaction mechanism. To do so, let us consider two mechanisms by which the second term of the rate law in Eq. (9-49) might proceed. It is easy to envisage that... 

Equivalent mechanisms. Consider two schemes for the first term in Eq. (9-47). One involves a prior equilibrium to form FeOH2+, the other SnOH+. Calculate the salt effects at each step and for the overall composite that gives ki. 

One factor that complicates the kinetic picture is the salt effect. An increase in ionic strength of the solution usually increases the rate of an SnI reaction (p. 451). But when the reaction is of charge type II, where both Y and RX are neutral, so that X is negatively charged (and most solvolyses are of this charge type), the ionic strength increases as the reaction proceeds and this increases the rate. This effect must be taken into account in studying the kinetics. Incidentally, the fact that the addition of outside ions increases the rate of most SnI reactions makes especially impressive the decrease in rate caused by the common ion. 

Kinetic studies also provide other evidence for the SnI mechanism. One technique used F NMR to follow the solvolysis of trifluoroacetyl esters. If this mechanism operates essentially as shown on page 393, the rate should be the same for a given substrate under a given set of conditions, regardless of the identity of the nucleophile or its concentration. In one experiment that demonstrates this, benzhy-dryl chloride (Ph2CHCl) was treated in SO2 with the nucleophiles fluoride ion, pyridine, and triethylamine at several concentrations of each nucleophile. In each case, the initial rate of the reaction was approximately the same when corrections were made for the salt effect. The same type of behavior has been shown in a number of other cases, even when the reagents are as different in their nucleophilicities (see p. 438) as H2O and OH . 

Among the experiments that have been cited for the viewpoint that borderline behavior results from simultaneous SnI and Sn2 mechanisms is the behavior of 4-methoxybenzyl chloride in 70% aqueous acetone. In this solvent, hydrolysis (i.e., conversion to 4-methoxybenzyl alcohol) occurs by an SnI mechanism. When azide ions are added, the alcohol is still a product, but now 4-methoxybenzyl azide is another product. Addition of azide ions increases the rate of ionization (by the salt effect) but decreases the rate of hydrolysis. If more carbocations are produced but fewer go to the alcohol, then some azide must he formed by reaction with carbocations—an SnI process. However, the rate of ionization is always less than the total rate of reaction, so some azide must also form by an Sn2 mechanism. Thus, the conclusion is that SnI and Sn2 mechanisms operate simultaneously. ... 

We have seen how the polarity of the solvent influences the rates of Sn 1 and Sn2 reactions. The ionic strength of the medium has similar effects. In general, the addition of an external salt affects the rates of SnI and Sn2 reactions in the same way as an increase in solvent polarity, though this is not quantitative different salts have different effects. However, there are exceptions though the rates of SnI reactions are usually increased by the addition of salts (this is called the salt effect), addition of the leaving-group ion often decreases the rate (the common-ion effect, p. 395). 

Winstein Robinson (1958) used this concept to account for the kinetics of the salt effects on solvolysis reactions. They considered that carbonium ions (cations) and carbanions could exist as contact ion-pairs, solvated ion-pairs and as free ions and that all these forms participated in the reactions and were in equilibrium with each other. These equilibria can be represented, thus ... 

Allain L., Canada T., Xue Z. Optical sensors and the salt effect a dual-transducer approach to acidity determination in a salt-containing concentrated strong acids, Anal. Chem. 2001 73 4592-4598. 

The solubility of any solid can be either increased or decreased by the addition of an electrolyte to the solvent, a phenomenon known as the salt effect. Salting-out describes the situation in which the solubility of the solid is decreased by the salt effect, whereas salting-in is the term used when the solubility is increased. Salting-out takes place when the added electrolyte sufficiently modifies the water structure so that the amount of water available for solute dissolution is effectively reduced, and it is a procedure convenient for the isolation of highly soluble substances. 

The common-ion effect describes the lowering of the solubility of a compound in a solution due to the presence of one of the ions of that compound in the solution. The salt effect describes the enhancement of the solubility of a compound in a solution due to the presence of a different type of ion in solution. 

Re (ii). The "salt effect" is more intriguing. At low lithium concentrations (lithium is the most effective cation) the reaction is first order in the salt concentration and zero order in rhodium, methyl iodide, and carbon monoxide. The rate steeply increases with the lithium concentrations. At high lithium concentrations the rate dependencies equal the Monsanto process, i.e. first order in rhodium and methyl iodide, and zero order in CO. The metal salts are involved in two reactions ... 

Assuming the argument is valid, it would then be possible to contact fused NaCl (or, presumably NaOH, Na2S, or smelts with these constituents) with water and to state that the resulting explosion stemmed from a homogeneous nucleation of a solution of salt in water. Their hypothesis therefore explains qualitatively the effect of variations in smelt composition on explosivity. It also clarifies the result that green liquor normally explodes more violently than pure water since, in the former, there are dissolved salts (of the NaCl type) to enhance the salt effect at the interface. 

Therefore, the presence of the electrolyte solute, LiPFe, added complexity to the thermal decomposition of the LiMn204-based cathode. Contrary to the salt effect found with the LiCo02 cathode, the onset temperature of exothermic activity as represented by SHR > 0.02 °C min decreased as the concentration of LiPFe increased. Apparently, LiCo02 and LiMn204 have fundamental differences in the way they react with solvent in the presence of salt. It seemed that the salt mainly contributes to an initial thermal instability, which increases with LiPFe-concentr ation. ... 

The nature of salt effects in monomolecular heterolysis has been reviewed. The experimental work of the same group on salt effects has continued with a study of the negative salt effect of lithium perchlorate on the heterolysis of 1-iodoadamantane in y-butyrolactone. It is assumed that the salt effect of lithium perchlorate is caused by the salt action on the solvent-separated ion pair of the substrate. 

The salt effects just considered are counterion effects. Sometimes, however, an added salt can induce electron transfer from a donor to an acceptor. Here are several examples. 

The decomposition of the salt [(cp)Fe(CgHg) Na+] displaces the equilibrium with the participation of this salt to the right. Hence, the difference in stability of the (20e) anion (cp)Fe(CgHg) depending on the cation nature is the major factor responsible for the salt effect. The NaPFg salt can induce electron transfer between neutral organometallic species very efficiently. 

Many salt minerals have water of crystallization in their crystal structnre. Such water of hydration can provide information on the isotope compositions and/or temperatures of brines from which the minerals were deposited. To interpret snch isotope data, it is necessary to know the fractionation factors between the hydration water and the solntion from which they are deposited. Several experimental studies have been made to determine these fractionation factors (Matsno et al. 1972 Mat-subaya and Sakai 1973 Stewart 1974 Horita 1989). Becanse most saline minerals equilibrate only with highly saline solutions, the isotopic activity and isotopic concentration ratio of water in the solntion are not the same (Sofer and Gat 1972). Most studies determined the isotopic concentration ratios of the sonrce solntion and as Horita (1989) demonstrated, these fractionation factors have to be corrected using the salt effect coefficients when applied to natural settings (Table 3.2). 

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