Principles of Neutralization Titrations

Standard solutions of strong acids and strong bases are used e.xtensively for determining analytes that are themselves acids or bases or that can be converted to such species by chemical treatment. This chapter introduces neutralization titrations, deals with the principles of titration, and discusses the common indicators that are used. In addition, titration curves that are plots of pH versus volume of titrant are explored, and several examples of pH calculations are presented. Titration curves for strong and weak acids and bases are described. 

Like all titrations, neutralization titrations depend on a chemical reaction between the analyte and a standard reagent. The point of chemical equivalence is indicated by a chemical indicator or an instrumental method. The discussion here focuses on the types of standard solutions and the chemical indicators that are used for neutralization titrations. 

Standard solutions of bases are ordinarily prepared from solid sodium, potassium, and occasionally barium hydroxides. Again, eye protection should always be used when handling dilute solutions of these reagents. 

Many substances, both naturally occurring and synthetic, display colors that depend on the pH of the solutions in which they are dissolved. Some of these substances, which have been used for centuries to indicate the acidity or alkalinity of water, are still employed as acid/base indicators. 

An acid/base indicator is a weak organic acid or a weak organic base whose undissociated form differs in color from its conjugate base or its conjugate acid form. For example, the behavior of an acid-type indicator, HIn, is described by the equilibrium 

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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... 

Principle. By means of potentiometric titration (in nonaqueous media) of a blend of sulfonic and sulfuric acids, it is possible to split the neutralization points corresponding to the first proton of sulfuric acid plus that of sulfonic acid, and to the second proton of sulfuric acid. The first derivate of the titration curve allows identification of the second points the corresponding difference in the volume of titrating agent is used as a starting point in the calculation method (Fig. 4). 

Acid-Base Titrations We can apply the principles of acid-base neutralization and stoichiometry to a common laboratory procedure called a titration. In a titration, a substance in a solution of known concentration is reacted with another substance in a solution of unknown concentration. For example, consider the following acid-base reaction ... 

Another satisfactory process for the determination of total hardness, based on a somewhat similar principle, is due to Blacher.4 The water is first titrated with decmormal hydrochloric acid until it is neutral to methyl orange, as in the method described above for temporary hardness. After the removal of the carbon dioxide by a current of air, the methyl orange is bleached by the addition of a drop of bromine water a little phenolphthalein and a few drops of alcoholic potassium hydroxide are added, the liquid is just decolorised with dccinormal hydrochloric acid and is then titrated with an alcoholic decmormal solution of potassium palmitate until a decided red colour is produced. The quantity of the potassium palmitate solution required is proportional to the total hardness. 

Other methods of titration of amino-acids have been employed that are based on principles already described as with other very weak bases the neutralization by acid may be carried out in glacial acetic acid solution (cf. p. 398), or use may be made of the fact that charged cation acids and anion bases give sharper pH inflexions when neutralized in media of low dielectric constant, e.g., alcohol or acetone (cf. p. 397). ... 

Protonation State. At optimal pH for enzymatic activity ( 5-6) [101, 102, 105], the Asp dyad can in principle exist in three protonation states, a deprotonated, a mono-protonated or a doubly protonated form. Because hydrogen atoms cffe invisible in the X-ray structure, evidence for a specific protonation state must be inferred indirectly by spectroscopic or titration measurements. Up to now, the existence of the doubly protonated, neutral form hfree enzyme. The existence of the deprotonated, doubly negative form is supported by a recent NMR study [102] at pH 6. However, this study has been subjected to criticism [106] and it is not conclusive. Our ab initio simulations of this form show that the Asp dyad is unstable even in the ps timesccde because of the strong Asp-Asp repulsion, which turns out to be -t-30 kcal/mol as estimated with a simple electrostatic model [100]). Thus, our calculations do not support the existence of this form. 

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