Titrimetry acid-base titrations

StreuU, C.A. Titrimetry acid-base titrations in non-aqueous solvents. In Treatise on Analytical Chemistry, Part / Kolthoff, I.M., Elving, P.J., Eds. Wiley-Interscience New York, 1975 Vol. II. Vydra, F. Stulik, K. Julakova, E. Electrochemical Stripping Analysis, Ellis Norwood, 1976. Wang, J. Stripping Analysis, VCH Publishers, Inc. Deerfield Beach, FL, 1985. 

The utility of acid-base titrimetry improved when NaOH was first introduced as a strong base titrant in 1846. In addition, progress in synthesizing organic dyes led to the development of many new indicators. Phenolphthalein was first synthesized by Bayer in 1871 and used as a visual indicator for acid-base titrations in 1877. Other indicators, such as methyl orange, soon followed. Despite the increasing availability of indicators, the absence of a theory of acid-base reactivity made selecting a proper indicator difficult. 

In practice, however, any improvement in the sensitivity of an acid-base titration due to an increase in k is offset by a decrease in the precision of the equivalence point volume when the buret needs to be refilled. Consequently, standard analytical procedures for acid-base titrimetry are usually written to ensure that titrations require 60-100% of the buret s volume. 

Luminescence titrimetry has been developed chiefly for acid-base titrations. Therefore, fluorescence pH-indicators are now widely used. Their application is based on changes of fluorescence spectrum upon the addition of a proton or its loss. At present, over 200 fluorescence pH-indicators are available the structural formulae of the most the widely applied indicators are given in Table 8. Some of them (No. 2, 8, 9, 12, 16, 17, 23, 25 and 29) and also, primuline, tripaflavine, and rhodamine 6G are widely used as adsorption fluorescence indicators. The titration end point can be detected in this case because of the differences in of the indicator in the adsorbed state and in solution. Redox fluorescence indicators including rhodamines B and 6 G, 3,6-dihydroxy-phthalic acids, complexes of Ru(II) with 2,2 -dipyridyl or 1,10-phenanthroline and other... 

An acid-base titration is a quick and convenient method for the quantitative analysis of substances with acidic or basic properties. Many inorganic and organic acids and bases can be titrated in aqueous media, but others, mainly organic, are insoluble in water. Fortunately, most of them are soluble in organic solvents hence they are conveniently determined by nonaqueous acid-base titrimetry. Although acid-base titrations can usually be followed potentio-metrically, visual endpoint detection is quicker and can be very precise and accurate if the appropriate indicator is chosen. 

Thus far we have assumed that the acid and base are in an aqueous solution. Indeed, water is the most common solvent in acid-base titrimetry. When considering the utility of a titration, however, the solvent s influence cannot be ignored. 

Equilibrium Constants Another application of acid-base titrimetry is the determination of equilibrium constants. Consider, for example, the titration of a weak acid, HA, with a strong base. The dissociation constant for the weak acid is... 

As with acid-base and complexation titrations, redox titrations are not frequently used in modern analytical laboratories. Nevertheless, several important applications continue to find favor in environmental, pharmaceutical, and industrial laboratories. In this section we review the general application of redox titrimetry. We begin, however, with a brief discussion of selecting and characterizing redox titrants, and methods for controlling the analyte s oxidation state. 

It is clear that reactions suitable for use in titrimetric procedures must be stoichiometric and must be fast if a titration is to be carried out smoothly and quickly. Generally speaking, ionic reactions do proceed rapidly and present few problems. On the other hand, reactions involving covalent bond formation or rupture are frequently much slower and a variety of practical procedures are used to overcome this difficulty. The most obvious ways of driving a reaction to completion quickly are to heat the solution, to use a catalyst, or to add an excess of the reagent. In the last case, a hack titration of the excess reagent will be used to locate the stoichiometric point for the primary reaction. Reactions employed in titrimetry may be classified as acid-base oxidation-reduction complexation substitution precipitation. 

The precision and accuracy with which the end point can be detected is a vital factor in all titrations. Because of its simplicity and versatility, chemical indication is quite common, especially in acid-base titrimetry. 

This chapter provides in troductory material that applies to all types of titrimetric methods of analysis, using precipitation titrimetry to illustrate the various theoretical aspects of the titration process. Chapters 14, 15, and 16 are devoted to the various types of neutralization titrations, in which the analyte and titrants undergo acid/base reactions. Chapter 17 provides information about titrations in which the analytical reactions involve complex formation. These methods are of particular importance for the determination of a variety of cations. Finally, Chapters 18 and 19 are devoted to volumetric methods, in which the analytical reactions involve electron transfer. These methods are often called redox titrations. Some additional titration methods are explored in later chapters. These methods include ampero-metric titration, in Section 23B-4, and spectrophotometric titrations, in Section 26A-4. 

PART III Classical Methods of Analysis 311 Chapter 12 Gravimetric Methods of Analysis 314 Chapter 13 Titrimetric Methods Precipitation Titrimetry 337 Chapter 14 Principles of Neutralization Titrations 368 Chapter 15 Titration Curves for Complex Acid/Base Systems 395 Chapter 16 Applications of Neutralization Titrations 428 Chapter 17 Complexation Reactions and Titrations 449... 

Titrimetric luminescence methods record changes in the indicator emission of radiation during titration. This change is noted visually or by instruments normally used in luminescence analysis. Most luminescence indicators are complex organic compounds which are classified into fluorescent and chemiluminescent, compounds according to the type of emission of radiation. As in titrimetry with adsorption of colored indicators, luminescence titration makes use of acid-base, precipitation, redox, and complexation reactions. Unlike color reactions, luminescence indicators enable the determination of ions in turbid or colored media and permit the detection limit to be lowered by a factor of nearly one thousand. In comparison with direct luminescence determination, the luminescence titrimetric method is more precise. 

Ultraviolet-visible spectrophotometry has also been applied to titrimetry. In this case the variation in the absorbance of the analyte with addition of titrant is used to obtain a spectrophotometric profile from which titration end points and/or equilibrium constants, etc., can be determined. This has been applied to the whole range of titrations in which a chromophore is generated. These include acid-base, redox, and complexometric titrations. 

Contaminant by-products depend upon process routes to the product, so maximum impurity specifications may vary, eg, for CHA produced by aniline hydrogenation versus that made by cyclohexanol amination. Capillary column chromatography has improved resolution and quantitation of contaminants beyond the more fliUy described packed column methods (61) used historically to define specification standards. Wet chemical titrimetry for water by Kad Eisher or amine number by acid titration have changed Httle except for thein automation. Colorimetric methods remain based on APHA standards. 

From the viewpoint of titrimetry in glacial acetic acid, the behavior of indicators during the titration of weak bases with perchloric acid is most important. It at first seems that Equation (4-108) could be used in a straightforward way nevertheless, this is not an experimentally useful form because the concentration of molecular perchloric acid, [HCIO4], is unknown. 

Compounds that may be determined by nonaqueous titrimetry include amines, amino acids, phenols, and Schiff s bases. Carbonyl compounds (by oxidation and titration of the released H ) can also be determined. Such titrations are especially useful in the pharmaceutical industry. 

Nonaqueous titrations are frequently desirable or required because of the increased sensitivity, improved selectivity, or greater solubility achieved with nonaqueous solvents. A far greater number of acids and bases can be determined in nonaqueous solvents than in aqueous media. This is primarily true because of the numerous organic acids and bases that require organic solvents. Properties such as dissolving or solvating, diffusion or equilibrium constants, acidity or basicity, and dielectric constant or polarity extend the capability of titrimetry to a far wider range when nonaqueous solvents are used. 

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