Solvents nonaqueous

The solution chemistry of nonaqueous solvents is very different from that of water-rich mixed solvents. pH measurement in nonaqueous solvents is difficult or impossible. Salts often show a limited degree of dissociation and limited solubility (see [132] for solubility of salts in organic solvents). Ions that adsorb nonspecifically from water may adsorb specifically from nonaqueous solvents, and vice versa. Therefore, the approach used for water and water-rich mixed solvents is not applicable for nonaqueous solvents, with a few exceptions (heavy water and short-chain alcohols). The potential is practically the only experimentally accessible quantity characterizing surface charging behavior. The physical properties of solvents may be very different from those of water, and have to be taken into account in the interpretation of results. For example, the Smoluchowski equation, which is often valid for aqueous systems, is not recommended for estimation of the potential in a pure nonaqueous solvent. Surface charging and related phenomena in nonaqueous solvents are reviewed in [3120-3127], Low-temperature ionic liquids are very different from other nonaqueous solvents, in that they consist of ions. Surface charging in low-temperature ionic liquids was studied in [3128-3132]. 

Studies of surface charging in nonaqueous solvents can be divided into three categories. Many studies were carried out in allegedly pure solvents without the addition of any solutes. In several studies, a possible correlation between the value of the potential and the parameters characterizing the acid-base properties and the polarity of the solvent was investigated. Detailed discussion of these solvent scales can be found elsewhere [3133-3135], Basically, the solvent scales apply to pure solvents, but the effect of solutes has also been studied [3136]. In fact, the 

In the second category of studies, the effect of a certain solute was investigated that is, the potential was studied at various concentrations of that solute under conditions that were otherwise the same. The potential in such studies is also influenced by impurities, but the levels of the latter are independent of the concentration of the studied solute, and the observed qualitative trend (increase or decrease in the potential and sign reversal) may be correct. Water is among the most often studied solutes, and is also a typical impurity, even in dried solvents. 

In the third category of studies, a similar approach was used as in aqueous system (pH adjusted and measured). Such an approach is limited to relatively polar solvents. 

An area of active research using electrochemical SFG is in nonaqueous solvents systems such as DMSO, acetonitrile, and ionic liquids. An advantage of these systems over water is that the potential window can be much larger and the vibrational spectra are typically less complicated because these nonaqueous solvents do not hydrogen-bond to the same extent as water [111-116]. 

Sum Frequency Ceneration Studies of the Electrified Solid/Liquid Interface 

One solvent studied is DMSO on Pt(lll). As shown in Fig. 5.19, peaks associated with the symmetric (2900 cm ) and antisymmetric (3000 cm ) CH stretches exhibit considerable potential dependence. An analysis of these spectra indicates that at potentials near -i- 200 mV vs Ag (quasi reference electrode, QRE) the DMSO molecule must reorientate so that the preferred orientation of the CH-related dipoles is no longer normal to the electrode surface. 

Another solvent studied with SFG spectroscopy is acetonitrile (CH3CN) [21, 

An interesting application of SFG is to study the structure of room-temperature ionic liquids at the electrode interface [47]. Ionic liquids are composed of 

A problem with 1- and 2-naphthol as substrates is that proton transfer is slower than solvent reorganization and, in any event, not observed in nonaqueous abasic 

Nonlinear behavior is also observed in the wide-range (0.1-2.5 GPa) pressure dependence of the ESPT rate of DCN2 in alcohols [44[. At low pressure, the proto-lytic photodissociation rate slightly increases, reaching the maximum value. With further pressure increase this rate decreases below the initial value at atmospheric pressure (Fig. 13.11). To explain the unique nonexponential dependence of ESPT rate constants on pressure, as well as temperature, Huppert et al. have developed an approximate stepwise two coordinate proton-transfer model that bridges the high-temperature nonadiabatic proton tunneling limit with the rate constant 

Reproduced from Ref [44a] by permission of the American Chemical Society. 

A decrease in the protolytic photodissociation rate of DCN2 with increasing pressure is also observed in supercritical C02/methanol mixtures with constant methanol molarity and molality [45]. This effect is currently under investigation. 

Although reactions in gases and solids are by no means rare, it is the enormous number of reactions carried out in solutions that is the subject of this chapter. However, there is no question but that the vast majority of reactions are carried out in solutions where water is the solvent. It is important to note that most nonaqueous solvents present some difficulties when their use is compared to that of water as a solvent. Some of the more important nonaqueous solvents are NH3, HF, S02, SOCl2, 

CH3COOH, POCI3, and H2S04. Some of these compounds (NH3, HF, and S02) are gases at ambient temperature and pressure. Some of them are also highly toxic. Some of the compounds are both gaseous and toxic. It is almost never as convenient to use a nonaqueous solvent as it is to use water. 

In view of the difficulties that accompany the use of a nonaqueous solvent, one may certainly ask why such use is necessary. The answer includes several of the important principles of nonaqueous solvent chemistry that will be elaborated on in this chapter. First, solubilities are different. In some cases, classes of compounds are more soluble in some nonaqueous solvents than they are in water. Second, the strongest acid that can be used in an aqueous solution is H30+. As was illustrated in Chapter 9, any acid that is stronger than H30+ will react with water to produce H30+. In some other solvents, it is possible to routinely work with acids that are stronger than H30+. Third, the strongest base that can exist in aqueous solutions is OH-. Any stronger base will react with water to produce OH-. In some nonaqueous solvents, a base stronger than OH - can exist, so it is possible to carry out certain reactions in such a solvent that cannot be carried out in aqueous solutions. These differences permit synthetic procedures to be carried out in nonaqueous solvents that would be impossible when water is the solvent. As a result, chemistry in nonaqueous solvents is an important area of inorganic chemistry, and this chapter is devoted to the presentation of a brief overview of this area. 

When one considers the incredible number of chemical reactions that are possible, it becomes apparent why a scheme that systemizes a large number of reactions is so important and useful. Indeed, classification of reaction types is important in all areas of chemistry, and a great deal of inorganic chemistry can be systematized or classified by the broad types of compounds known as acids and bases. Many properties and reactions of substances are understandable, and predictions can often be made about their reactions in terms of acid-base theories. In this chapter, we will describe the most useful acid-base theories and show their applications to inorganic chemistry. However, water is not the only solvent that is important in inorganic chemistry, and a great deal of chemistry has been carried out in other solvents. In fact, the chemistry of nonaqueous solvents is currently a field of a substantial amount of research in inorganic chemistry, so some of the fundamental nonaqueous solvent chemistry will be described in this chapter. 

Permittivity (dielectric constant) Specific conductance Viscosity 

Water will be discussed only briefly here but a summary of ils physical properties is given in Table 10.1 for comparison with the nonaqueous solvents to follow. One notable property is the very high permittivity which makes it a good solvent for ionic and polar compounds. The solvating properties of water and some of the related effects have been discussed in Chapter 8. Electrochemical reactions in water are discussed on pages 378-381. 

Allhough many nonaqueous solvent systems have been studied, the discussion here will be limited to a few representative solvents ammonia, a basic solvent sulfuric acid, an acidic solvent and bromine trifluoride, an aprotic solvent. In addition a short discussion of the chemistry taking place in solutions of molten salts is included. 

Precipitation reactions take place in ammonia just as they do in water. Because of the differences in solubility between the two solvents, the results may be considerably different. As an example, consider the precipitation of silver chloride in aqueous solution  

In ammonia solution the direction of the reaction is reversed so that  

FIGURE 10.6 An illustration of a nonpolar covalent compound in contact with water. There is no interaction between the two and, in addition, the water molecules are held together by the hydrogen bond, so there is no mixing and no dissolving. 

FIGURE 10.7 An illustration of a polar solvent, other than water, dissolving another polar compound such as sugar. The solvent dissolves the compound, but since it is less polar that water, the solubility will likely be lower. 

The old adage like dissolves like appears to cover all these cases. Polar (or ionic) substances dissolve in other polar substances, polar (or ionic) substances do not dissolve in nonpolar substances, and nonpolar substances dissolve in other nonpolar substances. 

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Table 8.11 pK, Values for Proton-Transfer Reactions in Nonaqueous Solvents 8.81... 

FIGURE 8.1 Approximate potential ranges in nonaqueous solvents. 

Solubility can often be decreased by using a nonaqueous solvent. A precipitate s solubility is generally greater in aqueous solutions because of the ability of water molecules to stabilize ions through solvation. The poorer solvating ability of nonaqueous solvents, even those that are polar, leads to a smaller solubility product. For example, PbS04 has a Ks of 1.6 X 10 in H2O, whereas in a 50 50 mixture of H20/ethanol the Ks at 2.6 X 10 is four orders of magnitude smaller. 

The majority of titrations involving basic analytes, whether conducted in aqueous or nonaqueous solvents, use HCl, HCIO4, or H2SO4 as the titrant. Solutions of these titrants are usually prepared by diluting a commercially available concentrated stock solution and are stable for extended periods of time. Since the concentrations of concentrated acids are known only approximately,the titrant s concentration is determined by standardizing against one of the primary standard weak bases listed in Table 9.7. 

The NH3 is removed by distillation and titrated with HCl. Alternatively, N03 can be titrated as a weak base in an acidic nonaqueous solvent such as anhydrous acetic acid, using HCIO4 as a titrant. 

Another important example of a redox titration for inorganic analytes, which is important in industrial labs, is the determination of water in nonaqueous solvents. The titrant for this analysis is known as the Karl Fischer reagent and consists of a mixture of iodine, sulfur dioxide, pyridine, and methanol. The concentration of pyridine is sufficiently large so that b and SO2 are complexed with the pyridine (py) as py b and py SO2. When added to a sample containing water, b is reduced to U, and SO2 is oxidized to SO3. 

Fritz, J. S. Acid-Base Titrations in Nonaqueous Solvents. Allyn and Bacon Boston, 1973. 

The concentration of anionic surfactants at the sub-ppm level in natural waters and industrial waters are determined spectrophotometrically. The anionic surfactants are extracted into a nonaqueous solvent following the formation of an ion association complex with a suitable cation. 

Potcntiomctric Titrations In Chapter 9 we noted that one method for determining the equivalence point of an acid-base titration is to follow the change in pH with a pH electrode. The potentiometric determination of equivalence points is feasible for acid-base, complexation, redox, and precipitation titrations, as well as for titrations in aqueous and nonaqueous solvents. Acid-base, complexation, and precipitation potentiometric titrations are usually monitored with an ion-selective electrode that is selective for the analyte, although an electrode that is selective for the titrant or a reaction product also can be used. A redox electrode, such as a Pt wire, and a reference electrode are used for potentiometric redox titrations. More details about potentiometric titrations are found in Chapter 9. 

A. Popoff, "Anhydrous Acetic Acid as a Nonaqueous Solvent", iu J. J. Lagowski, ed., Chemisty oJAonaqueous Solvents Vol. 3, Academic Press, 1970. 

Metal organic decomposition (MOD) is a synthesis technique in which metal-containing organic chemicals react with water in a nonaqueous solvent to produce a metal hydroxide or hydrous oxide, or in special cases, an anhydrous metal oxide (7). MOD techniques can also be used to prepare nonoxide powders (8,9). Powders may require calcination to obtain the desired phase. A major advantage of the MOD method is the control over purity and stoichiometry that can be achieved. Two limitations are atmosphere control (if required) and expense of the chemicals. However, the cost of metal organic chemicals is decreasing with greater use of MOD techniques. 

The activity of the hydrogen ion is affected by the properties of the solvent in which it is measured. Scales of pH only apply to the medium, ie, the solvent or mixed solvents, eg, water—alcohol, for which the scales are developed. The comparison of the pH values of a buffer in aqueous solution to one in a nonaqueous solvent has neither direct quantitative nor thermodynamic significance. Consequently, operational pH scales must be developed for the individual solvent systems. In certain cases, correlation to the aqueous pH scale can be made, but in others, pH values are used only as relative indicators of the hydrogen-ion activity. 

Other difficulties of measuring pH in nonaqueous solvents are the complications that result from dehydration of the glass pH membrane, increased sample resistance, and large Hquid-junction potentials. These effects are complex and highly dependent on the type of solvent or mixture used (1,5). 

The solubihty of potassium permanganate in aqueous potassium hydroxide (108) is shown in Figure 7. Permanganates are soluble in certain nonaqueous solvents such as hquid NH, but not in hquid SO2. Organic solvents such as glacial acetic acid, acetone, acetonitrile, tert-huty alcohol. 

Some inorganic nonaqueous solvents can be used in systems operable at near room temperature, eg, thionyl chloride others, however, require special handling, eg, Hquid ammonia, which must be used below its boiling point of —33° C in a thermally insulated container and in an inert atmosphere. 

Solvent Gleaning. Solvent cleaning employs the natural solubilizing properties of various nonaqueous solvents or blends of solvents. Either 100% solvents or aqueous emulsions of solvents can be used. AppHcation is typically by immersion, hand appHcation, or via a vapor degreaser machine. 

Chemical Grafting. Polymer chains which are soluble in the suspending Hquid may be grafted to the particle surface to provide steric stabilization. The most common technique is the reaction of an organic silyl chloride or an organic titanate with surface hydroxyl groups in a nonaqueous solvent. For typical interparticle potentials and a particle diameter of 10 p.m, steric stabilization can be provided by a soluble polymer layer having a thickness of - 10 nm. This can be provided by a polymer tail with a molar mass of 10 kg/mol (25) (see Dispersants). 

Amides can be titrated direcdy by perchloric acid ia a nonaqueous solvent (60,61) and by potentiometric titration (62), which gives the sum of amide and amine salts. Infrared spectroscopy has been used to characterize fatty acid amides (63). Mass spectroscopy has been able to iadicate the position of the unsaturation ia unsaturated fatty amides (64). Typical specifications of some primary fatty acid amides and properties of bisamides are shown ia Tables 5 and 6. 

Activated tertiary amines such as triethanolamine (TEA) and methyl diethanolamine (MDEA) have gained wide acceptance for CO2 removal. These materials require very low regeneration energy because of weak CO2 amine adduct formation, and do not form carbamates or other corrosive compounds (53). Hybrid CO2 removal systems, such as MDEA —sulfolane—water and DIPA—sulfolane—water, where DIPA is diisopropylamine, are aqueous alkaline solutions in a nonaqueous solvent, and are normally used in tandem with other systems for residual clean-up. Extensive data on the solubiUty of acid gases in amine solutions are available (55,56). 

The original hot carbonate process developed by the U.S. Bureau of Mines was found to be corrosive to carbon steel (55). Various additives have been used in order to improve the mass transfer rate as well as to inhibit corrosion. Vetrocoke, Carsol, Catacarb, Benfteld, and Lurgi processes are all activated carbonate processes. Improvements in additives and optimization of operation have made activated carbonate processes competitive with activated MDEA and nonaqueous solvent based systems. Typical energy requirements are given in Table 9. 

Table 3. Solubilities of the Hydrox ybenzoic Acids in Nonaqueous Solvents, Wt % ...

Table 1. Solubility of Sodium Nitrite in Nonaqueous Solvents ...

In industrial production of acid-modified starches, a 40% slurry of normal com starch or waxy maize starch is acidified with hydrochloric or sulfuric acid at 25—55°C. Reaction time is controlled by measuring loss of viscosity and may vary from 6 to 24 hs. For product reproducibiUty, it is necessary to strictly control the type of starch, its concentration, the type of acid and its concentration, the temperature, and time of reaction. Viscosity is plotted versus time, and when the desired amount of thinning is attained the mixture is neutralized with soda ash or dilute sodium hydroxide. The acid-modified starch is then filtered and dried. If the starch is washed with a nonaqueous solvent (89), gelling time is reduced, but such drying is seldom used. Acid treatment may be used in conjunction with preparation of starch ethers (90), cationic starches, or cross-linked starches. Acid treatment of 34 different rice starches has been reported (91), as well as acidic hydrolysis of wheat and com starches followed by hydroxypropylation for the purpose of preparing thin-hoiling and nongelling adhesives (92). 

Micellar properties are affected by changes in the environment, eg, temperature, solvents, electrolytes, and solubilized components. These changes include compHcated phase changes, viscosity effects, gel formation, and Hquefication of Hquid crystals. Of the simpler changes, high concentrations of water-soluble alcohols in aqueous solution often dissolve micelles and in nonaqueous solvents addition of water frequendy causes a sharp increase in micellar size. 

Antimony trichloride is used as a catalyst or as a component of catalysts to effect polymerisation of hydrocarbons and to chlorinate olefins. It is also used in hydrocracking of coal (qv) and heavy hydrocarbons (qv), as an analytic reagent for chloral, aromatic hydrocarbons, and vitamin A, and in the microscopic identification of dmgs. Liquid SbCl is used as a nonaqueous solvent. 

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