Working with Acids and Bases

Strong acids can indeed burn the skin and must be handled with care in the laboratory. However, strong bases can burn skin as well. Chemists must have a more sophisticated understanding of the differences between an acid and a base and their relative strengths than simply their propensity to burn. This chapter focuses on how you can identify acids and bases as well as several ways to determine their strengths. 

SutVetfin Three Complementary Methotls for Defininy Adits and Bases 

As chemists came to understand acids and bases as more than just stuff that burns, their understanding of how to define them evolved as well. It s often said that acids taste sour, while bases taste bitter, but we do not recommend that you go around tasting chemicals in the laboratory to identify them as acids or bases. In the following sections, we explain three much safer methods you can use to tell the difference between the two. 

There s one thing to get out of the way right now, though. As you deal with acids and bases, you see water (H2O) referred to as an acid at certain points and as a base at others. This is totally fine because water is amphoteric, which means it can act as an acid or a base as needed. 

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Use caution when working with acids and bases. 

Use caution when working with acids and bases. Wipe up any water spills to avoid slipping. 

Although water is not an essential participant in all modern acid-base definitions, most laboratory work with acids and bases involves water, as do most environmental, biological, and industrial applications. Recall from our discussion in Chapter 4 that water is a product in all reactions between strong acids and strong bases ... 

When you work with acids and bases, you often need to state the hydronium ion concentration, [H3O+], of a soiution. One simple way is to use the negative logarithm of [H3O+]. This quantity is called pH. For example, pure water has a [H3O+] of 1.00 X 10 M. So, the pH of pure water is... 

The third method of separating optically active substances by combining them with optically active bases or acids had not been employed with any success until E. Fischer took up this question, the study of the optically active amino acids being his first work upon the chemical constitution of the proteins. The non-success of this method was in all probability due to the small affinity which the simple amino acids themselves have for combining with acids and bases even the attempts to separate the monoaminodicarboxylic acids, which are fairly strong acids, were not successful. 

Greenwood, N. N., Eamshaw, A. (1997). Chemistry of the Elements (2nd ed.). Oxford Butterworth-Heinemann. Several chapters in this reference work deal with acids and bases. 

Electrolytes in nonaqueous solvents The most significant work for analytical chemists in this area has been concerned with acids and bases discussion of this topic is reserved for Chapter 4. 

Then, we will consider acids and bases. Hopefully, it will become clear that, if definitions are made broadfy, and in the absence of water to level acid strengths, almost everything can be viewed as either an add or a base, depending on that with which it is reacting While this might suggest that the entire concept of acids and bases is of limited utility, a new viewpoint, that is, that many reactions can be recognized as acid-base processes if the appropriate criteria are applied, is introduced. Eventually, we will find it is not unusual to work with adds (and bases) whose ion-... 

Adolph Baeyer is credited with the first recognition of the general nature of the reaction between phenols and aldehydes in 1872 ([2,5-7] [18], Table 5.1). He reported formation of colorless resins when acidic solutions of pyrogallic acid or resorcinol were mixed with oil of bitter almonds, which consists primarily benzaldehyde. Baeyer also saw resin formation with acidic and basic solutions of phenol and acetaldehyde or chloral. Michael and Comey furthered Baeyer s work with additional studies on the behavior of benzaldehyde and phenols [2,19]. They studied a variety of acidic and basic catalysts and noted that reaction vigor followed the acid or base strength of the catalyst. Michael et al. also reported rapid oxidation and darkening of phenolic resins when catalyzed by alkaline materials. 

You need to know the strong acids and bases to work with acid-base reactions. 

The problem with the Arrhenius definitions is that they are specific to one particular solvent, water. When chemists studied nonaqueous solvents, such as liquid ammonia, they found that a number of substances showed the same pattern of acid-base behavior, but plainly the Arrhenius definitions could not be used. A major advance in our understanding of what it means to be an acid or a base came in 1923, when two chemists working independently, Thomas Lowry in England and Johannes Bronsted in Denmark, came up with the same idea. Their insight was to realize that the key process responsible for the properties of acids and bases was the transfer of a proton (a hydrogen ion) from one substance to another. The Bronsted-Lowry definition of acids and bases is as follows ... 

The first four steps of the seven-step strategy are identical to the ones in Example. In this example, addition of a strong acid or base modifies the concentrations that go into the buffer equation. We need to determine the new concentrations (Step 5) and then apply the buffer equation (Step 6). In dealing with changes in amounts of acid and base, it is often convenient to work with moles rather than molarities. The units cancel in the concentration term of the buffer equation, so the ratio of concentrations can be... 

In 1923, the same year that Bransted and Lowry came up with their idea of what acids and bases were, an American chemist named Gilbert Newton Lewis began to work on his own acid-base theory. Lewis defined acid as any substance that accepted an electron pair. A base, on the other hand, is any substance that donates an electron pair. 

Another method that may work is to change the pH. This works with weak organic acids and bases. In this case the material is adsorbed at some optimum pH. To desorb it the pH is changed and the adsorbed material is removed in a more... 

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. 

Compound 51 was found to be unstable and difficult to purify, as described in the literature [93—95]. Therefore, 51 was not isolated, but was instead converted to the stable pinacol 1-acetamido-l-hexylboronate derivative 52. However, the acylated derivative 52 could not be purified by column chromatography as it was destroyed on silica gel and partially decomposed on alumina. Fortunately, we found that it dissolves in basic aqueous solution (pH > 11) and can then be extracted into diethyl ether when the pH of the aqueous layer is 5—6. Finally, pure 52 was obtained by repeated washing with weak acids and bases. It should be mentioned here that exposure to a strongly acidic solution, which also dissolves compound 51, results in its decomposition. Compared with other routes, the present two-step method involves mild reaction conditions (THF, ambient temperature) and a simple work-up procedure. It should prove very useful in providing an alternative access to a-aminoboronic esters, an important class of inhibitors of serine proteases. 

As mentioned earlier, in the Ruhrchemie-Rhone Poulenc process for propene hydroformylation the pH of the aqueous phase is kept between 5 and 6. This seems to be an optimum in order to avoid acid- and base-catalyzed side reactions of aldehydes and degradation of TPPTS. Nevertheless, it has been observed in this [93] and in many other cases [38,94-96,104,128,131] that the [RhH(CO)(P)3] (P = water-soluble phosphine) catalysts work more actively at higher pH. This is unusual for a reaction in which (seemingly) no charged species are involved. For example, in 1-octene hydroformylation with [ RhCl(COD) 2] + TPPTS catalyst in a biphasic medium the rates increased by two- to five-fold when the pH was changed from 7 to 10 [93,96]. In the same detailed kinetic studies [93,96] it was also established that the rate of 1-octene hydroformylation was a significantly different function of reaction parameters such as catalyst concentration, CO and hydrogen pressure at pH 7 than at pH 10. 

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