Analyzing Acids and Bases

The model that utilized regression analysis was one that built upon previous work by the same authors [36,39]. In this case, the dataset was expanded to 125-129 drugs and the number of assessed descriptors increased to 210. Models for acidic and basic compounds were developed separately as well as a model using all compounds, and the advantages of analyzing acids and bases separately were minimal. Mean-fold errors were generally around 1.8. Descriptors that dominated the models included lipophilicity, fraction anionic or cationic, surface electrostatic potential, and parameters specific to aliphatic carbons and fluorine. 

Skill 10.1 Analyzing acids and bases according to acid-base theories (i.e., Arrhenius, Bronsted-Lowry, Lewis)... 

Write reaction equations that illustrate various ways to prepare salts, and do calculations using the concept of an equivalent of salt. (Section 9.8) Demonstrate an understanding of the words weak and strong as applied to acids and bases. (Section 9.9) Demonstrate an understanding of the titration technique used to analyze acids and bases. (Section 9.10)... 

Demonstrate an understanding of the titration technique used to analyze acids and bases. 

A common procedure often used to analyze acids and bases is called titration (> Figure 9.4). Suppose the total acidity of an unknown acid solution needs to be determined. A known volume of the acidic solution is first measured out by drawing it up... 

The neutrahzation reaction of acids and bases is used in a process called titration to analyze acids and bases. During a typical titration, a base solution of known concentration is added slowly to an acid solution of unknown concentration. The titration is stopped at the endpoint when a color change occurs in an indicator. The volumes of acid and base required are used to calculate the acid concentration. 

In this experiment the effect of a mixed aqueous-organic solvent on the color transition range of common indicators is investigated. One goal of the experiment is to design an appropriate titrimetric method for analyzing sparingly soluble acids and bases. 

Only the last factor is a little tricky it is also different with and without additives. With no additive, polycaprolactam can be represented A BABAB. . . ABAB, where the A and B are acid and base groups, respectively, and those marked with the asterisk are those analyzed. Thus every molecule has one of each. In this case, then, we use the average of 12.0 and 11.6 as the end group concentration, and unity as the number of ends of each kind to obtain... 

The product yield of a thermodynamically controlled reaction depends on pH when acids and bases participate in the reaction. This pH-dependence can be analyzed using known values of p AT -values of the acidic and basic groups of the reactants and the products. For thermodynamically controlled processes the apparent eqnilibrium constant for the product yield in condensation reactions, K, mnst be determined. This equilibrium constant is defined by the following equation ... 

Separations and Analyses. After removal of trace amounts of acids and bases from the <200° C distillates by extraction methods (16), a chromatographic separation with silica gel provided a check for the presence of olefins. No olefins were detected thus, the acid- and base-free distillates were analyzed by ASTM D2789-71, Standard Method of Test for Hydrocarbon Types in Low Olefinic Gasoline by Mass Spectrometry. The separation and analysis scheme for this distillate is shown in Figure 2. 

Exercise 1.2. In the discussion of Lewis acids and bases in the Appendix, the compound [13] is analyzed as an adduct of the base I- with the acid I2. It probably is not clear how I2, a diatomic that perfectly fits the two-center-two-electron bond model and the eight electron rule, can act as a Lewis acid. Show how a HOMO-LUMO analysis of the acid-base interaction rationalizes the interaction and predicts a linear structure. 

In Figure 8-18, a mixture of acids and bases was analyzed on three types of columns phenyl, polar embedded, and C18 column. Significant differences in selectivity were obtained. The separation could be further optimized by modifying the gradient slope and employing off-line method development tools such as Drylab for further optimization and resolution of the critical pairs. 

In this chapter we have encountered many different situations involving aqueous solutions of acids and bases, and in the next chapter we will encounter still more. In solving for the equilibrium concentrations in these aqueous solutions, you may be tempted to create a pigeonhole for each possible situation and to memorize the procedures necessary to deal with each particular situation. This approach is just not practical and usually leads to frustration Too many pigeonholes are required, because there seems to be an infinite number of cases. But you can handle any case successfully by taking a systematic, patient, and thoughtful approach. When analyzing an acid-base equilibrium problem, do not ask yourself how a memorized solution can be used to solve the problem. Instead, ask yourself this question What are the major species in the solution, and how does each behave chemically ... 

In addition to a wide range of polar and nonpolar hydrocarbons that can be analyzed by RP-HPLC, it is also possible to separate ionic species. Because water is used as part of almost all mobile phases, those species which are acids and bases can be neutralized by control of pH. In cases where neutralization is not possible, then the addition of a counterion into the mobile phase so that the analyte will form a neutral complex can be used to enhance RP retention. The same principle can be applied to inorganic species by forming a neutral complex that results in reversed-phase retention. 

Colonic Absorption. In our earlier paper (1 ) we analyzed the absorption of acids and bases from the rat colon and compared these results with alternative analyses. We reported eq 4 for the bases (Table II), which compares favorably with eq 5 (5). (The absolute t-value is shown below each parameter.)... 

Intermolecular proton exchange between 3,6-di-f-butyl-2-hydroxyphenoxyl radical and a variety of organic acids and bases was studied under various conditions . Line broadening was analyzed in terms of a mechanism shown in equation 30 for... 

Solubility theory based on pH-solubility profiles for weak acids and bases is well established (Grant and Higuchi, 1990, Butler, 1998). From a knowledge of the intrinsic solubility of the unionized form, the dissociation constant (plC. ) and the solubility of a salt, one should be able to construct the pH-solubility profile. If multiple solubility data are available, data can be analyzed through the use of a non-linear regression model to calculate plC, . If the solubilities of the various salts are also determined, the complete pH-solubility profile can be constructed. 

Approximately 10% of this material was not analyzed. 1.50% Heteroatomic compounds determined in addition to acids and bases extracted. 

A detailed analytical scheme is described elsewhere (9). Briefly, organic compounds from raw and drinking water were collected with GAC or XAD-2. A soxhlet extraction with dichloromethane was nsed to disadsorb the organics, evaporated and redissolved in ether. The ether solnble componnds were fractionated into acids and base+neutrals and analyzed by GC/MS and FAB. The ether-insolnble fraction was fractionated by HPLC-UV diode array detection usually in four fractions, and analyzed by FAB mass spectrometry. FAB spectra of surfactants were obtained using triethanolamine or thioglycerol+NaCl for anionic and cationic compounds, whereas the last was the most suitable for non ionic snrfactants and real samples. 

In this chapter we have applied the methods of chapter 4 to ionic equilibria other than those between acids and bases. Of course, complexation, extraction, solubility, precipitation, and redox equilibria may also involve acid-base equilibria, which is why we treated acid-base equilibria first. The examples given here illustrate that the combination of exact theory with the computational power of a spreadsheet allows us to solve many problems that occur in quantitative chemical analysis, and to analyze experimental data accordingly. Even quite complicated titrations, such as the multi-component precipitation titrations, the von Liebig titration, and redox titrations involving many species and complicated stoichiometries, can be handled with ease. 

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