Standardization of acids and bases

Solutions of strong base attack glass and therefore should be stored in polyethylene containets. Polyethylene is slightly permeable to carbon dioxide (permeability constant about 1 X 10 ml-cm/sec-cmS-cm Hg), but no significant carbonate contamination occurs within a reasonable time (see Problem 6-6). 

When carbonate contamination is undesirable, protection from the carbon dioxide of the atmosphere should be provided. The usual method is to use a storage bottle with a protective tube containing either sodium hydroxide and asbestos (ascarite) or sodium and calcium hydroxides (soda lime). A siphon is used to deliver the clear supernatant liquid to the buret without exposure to carbon dioxide of the air. 

Under the auspices of the International Union of Pure and Applied Chemistry, an Analytical Methods Committee carried out a critical examination of some primary standards suggested for use in acid-base titrations, with a view to making recommendations concerning practical applications. Some of the findings of this committee were as follows (1) Benzoic acid as a primary standard is not recommended because 

Potassium hydrogen phthalate as a primary standard for bases Potassium hydrogen phthalate, C6H4COOKCOOH, is used almost exclusively as the primary 

When a solution of standard base is used only for titration of strong acids, a small amount of carbonate is not a serious source of error provided the end point is taken with an indicator that changes color at a pH of about 4 or 5. For standardizing such carbonate-containing solutions, potassium hydrogen phthalate is an unsuitable primary standard. An alternative is pure potassium chloride, which is passed through a cation-exchange column, converted to hydrochloric acid, and titrated with the sodium hydroxide. 

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The purpose of this chapter is to consider the preparation and standardization of acids and bases and to review some of the important applications of acid-base titrations in aqueous and nonaqueous systems. For end points to be detected most precisely, the pH in the vicinity of the equivalence point should change sharply. For this reason a solution of strong acid or base is chosen as titrant whenever possible. 

Although many quantitative applications of acid-base titrimetry have been replaced by other analytical methods, there are several important applications that continue to be listed as standard methods. In this section we review the general application of acid-base titrimetry to the analysis of inorganic and organic compounds, with an emphasis on selected applications in environmental and clinical analysis. First, however, we discuss the selection and standardization of acidic and basic titrants. 

Most chemicals used in the procedure will appear in the index. Thus, there will generally be entries for all starting materials, reagents, intermediates, important by-products, and final products. Most products shown in the Tables in the discus.sion sections of this volume are included unless the compounds are quite similar in which case a general descriptive name was entered. Chemicals generally nut indexed included coiimion solvents, standard inorganic acids and bases, reactants shown in the Tables, and compounds cited in the discussion section in connection with other methods of preparation. 

The benzoic acid was quantitatively coupled within 5 min via its cesium salt by using a dedicated multi-mode batch reactor, carried out in standard glassware under atmospheric reflux conditions. In a more extended study, various substituted carboxylic acids (Fig. 7.7) were coupled to chlorinated Wang resin, employing an identical reaction protocol. In a majority of cases, the microwave-mediated conversion reached at least 85% after 3-15 min. These microwave conditions represented a significant rate enhancement, in contrast to the conventional protocol, which took 24-48 h. The microwave protocol has additional benefits in comparison to the conventional method, as the amounts of acid and base equivalents can be reduced and potassium iodide as an additive can be eliminated from the reaction mixture27. 

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. 

A very considerable portion of the text has been wholly rewritten, and the entire text has been subjected to a revision and rearrangement. Specific new exercises and discussions which have been introduced include such topics as the determinations of vapor density and molecular weight, the standardization of acids and the titration of acids and bases, Faraday s law, and the use of the pH scale of hydrogen-ion concentration. Several new preparar tions have been introduced, and a few of the old ones have been discontinued. A complete list of apparatus and chemicals required in the course has been added to the Appendix. 

The standard molar Gibbs energy change for reaction (4-16), AG° = —RT In K, is then a measure of the relative acidity of HA and BH (or of the relative basicity of B and A ). Series of acids and bases have been studied to establish a scale of relative acidities in the same manner as p.K a values are determined in solution. 

Reactions involving the transfer of a proton are known as protolytic reactions. The equilibrium constants of protolyses allow a comparison to be made of acid and base strengths. The conjugate pair, H3O+, H2O, is used as a standard,... 

We can titrate an acid solution of unknown concentration hy adding a standardized solution of sodium hydroxide dropwise from a buret (see Figure 11-1). A common huret is graduated in large intervals of 1 mL and in smaller intervals of 0.1 mL so that it is possible to estimate the volume of a solution dispensed to within at least 0.02 mL. (Experienced individuals can often read a buret to 0.01 mL.) The analyst tries to choose an indicator that changes color clearly at the point at which stoichiometrically equivalent amounts of acid and base have reacted, the equivalence point. The point at which the indicator changes color and the titration is stopped is called the end point. Ideally, the end point should coincide with the equivalence point. Phenolphthalein is colorless in acidic solution and reddish violet in basic solution. In a titration in which a base is added to an acid, phenolphthalein is often used as an indicator. The end point is signaled by the first appearance of a faint pink coloration that persists for at least 15 seconds as the solution is swirled. 

Let us now describe the use of a few primary standards for acids and bases. One primary standard for solutions of acids is sodium carbonate, Na2C03, a solid compound. 

You know the concentration of the NaOH solution, so it is your standard solution. You can use the reaction, the volumes of acid and base used, plus the molarity of the base to determine the molarity of the im-labeled HCl. Follow the first part of this process in Figure 15.16. 

The lower the value of this constant, the larger the deferences in acidity indices (pH) between the standard solutions of strong acids and bases, that results in a wider acid-base range for the solvent. This refers not only to the acid-base equilibria in aqueous solutions but also applies to any donor-acceptor interaction in molecular solvents which are prone to heterolytic dissociation with the formation of acidic and basic particles, as provided by an appropriate definition of acids and bases. It follows from equations (1.1.3) and (1.1.4) that the Arrhenius definition can only be used for the description of acid-base interactions in aqueous solutions, since the reaction between the acid of solvent and the base of solvent can result in the formation only of the solvent molecules. In the case considered, this solvent is water. 

Vapors of acids and bases are usually collected by liquid phases in washing bottles, so-called impingers. Figure 6.65 b shows an example of a liquid absorber, model B 70 according to the German BGIA [6-100]. For a liquid, a standard diluted aqueous add or base can be used. 

Acid-Base with Indicator. An indicator snch as bromophenol blue (blue to yellow. Figure 4-13b) or phenolphthalein (pink to colorless) is added to the bulk liquid. Small additions of acid and base can be used to color the entire mixing vessel and then strip ont the color. A small excess of the acid-base used for the color change is typically nsed. The last point to remain colored is easily observed and selected as a suitable probe position. For standard geometries, the... 

We have referred somewhat vaguely above to the relative strengths of acids and bases. It is now necessary to give quantitative significance to such terms. The strengths of acids for a particular solvent are measured and expressed with respect to a chosen standard/or that solvent. Thus, for water, the standard is the acid-base pair H3OVH2O. The strength of some other acid. A, may then be defined with respect to the reaction... 

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