Acid-Base Reactions and Chemical Analysis

Equilibria govern diverse phenomena from the folding of proteins to the action of acid rain on minerals to the aqueous reactions used in analytical chemistry. This chapter introduces equilibria for the solubility of ionic compounds, complex formation, and acid-base reactions. Chemical equilibrium provides a foundation not only for chemical analysis, but also for other subjects such as biochemistry, geology, and oceanography. 

The neutralisation of acids with bases provides many valuable volumetric methods of chemical analysis and redox titrations are useful as well. But here we encounter an important difference between acid/base and redox reactions in solution. Acid/base reactions which involve the transfer of protons are very fast indeed they are usually instantaneous for all practical purposes. In protonic solvents, polar H-X bonds are very labile and undergo rapid proton exchange. For example, if B(OH)3 - a very weak acid - is recrystallised from D20, we obtain a fully-deuterated product. Redox reactions, on the other hand, are often very slow under ordinary conditions. To return to the analogy between acid/base and redox titrations, many readers will be familiar with the reaction between permanganate and oxalic acid the reaction is very slow at room temperature and, for titrimetric purposes, should be carried out at about 60 °C. The mechanism whereby a redox reaction takes place tends to be... 

Throughout his career as a chemist Ostwald followed the general approach of applying physical measurements and mathematical reasoning to chemical issues. One of his major research topics was the chemical affinities of acids and bases. To that end, he studied the points of equilibria in reaction systems where two acids in an aqueous solution compete with each other for a reaction with one base and vice versa. Because chemical analysis would have changed the equilibria, he skillfully adapted the measurement of physical properties, such as volume, refractive index, and electrical conductivity, to that problem. From his extensive data he derived for each acid and base a characteristic affinity coefficient independent of the particular acid-base reactions. 

Although X-ray diffraction or analysis of the X-rays emitted by electron microscopy are the preferred methods of identification of minerals, fast and relatively secure identifications can be made by using chemical reactions. Precipitation reactions and acid-base reactions are two of the most common types of reactions used in chemical identifications. 

Techniques responding to the absolute amount of analyte are called total analysis techniques. Historically, most early analytical methods used total analysis techniques, hence they are often referred to as classical techniques. Mass, volume, and charge are the most common signals for total analysis techniques, and the corresponding techniques are gravimetry (Chapter 8), titrimetry (Chapter 9), and coulometry (Chapter 11). With a few exceptions, the signal in a total analysis technique results from one or more chemical reactions involving the analyte. These reactions may involve any combination of precipitation, acid-base, complexation, or redox chemistry. The stoichiometry of each reaction, however, must be known to solve equation 3.1 for the moles of analyte. 

The accuracy of a standardization depends on the quality of the reagents and glassware used to prepare standards. For example, in an acid-base titration, the amount of analyte is related to the absolute amount of titrant used in the analysis by the stoichiometry of the chemical reaction between the analyte and the titrant. The amount of titrant used is the product of the signal (which is the volume of titrant) and the titrant s concentration. Thus, the accuracy of a titrimetric analysis can be no better than the accuracy to which the titrant s concentration is known. 

The last definition has widespread use in the volumetric analysis of solutions. If a fixed amount of reagent is present in a solution, it can be diluted to any desired normality by application of the general dilution formula V,N, = V N. Here, subscripts 1 and 2 refer to the initial solution and the final (diluted) solution, respectively V denotes the solution volume (in milliliters) and N the solution normality. The product VjN, expresses the amount of the reagent in gram-milliequivalents present in a volume V, ml of a solution of normality N,. Numerically, it represents the volume of a one normal (IN) solution chemically equivalent to the original solution of volume V, and of normality N,. The same equation V N, = V N is also applicable in a different context, in problems involving acid-base neutralization, oxidation-reduction, precipitation, or other types of titration reactions. The justification for this formula relies on the fact that substances always react in titrations, in chemically equivalent amounts. 

Seven chemical reactions were identified from the chemistry syllabus. These chemical reactions were selected because they were frequently encountered during the 2-year chemistiy course and based on their importance in understanding concepts associated with three topics, namely, acids, bases and salts, metal reactivity series and inorganic chemistry qualitative analysis. The seven types of chemical reactions were combustion of reactive metals in air, chemical reactions between dilute acids and reactive metals, neutralisation reactions between strong acids and strong alkalis, neutralisation reactions between dilute acids and metal oxides, chemical reactions between dilute acids and metal carbonates, ionic precipitation reactions and metal ion displacement reactions. Although two of the chemical reactions involved oxidation and reduction, it was decided not to include the concept of redox in this study as students had only recently been introduced to ion-electron... 

Neutralization reactions between Lowry-Bronsted acids and bases are frequently employed in chemical analysis. Methods based on them are sometimes termed acidimetric or alkalimetric. 

For most odour nuisance problems, chemical plants, refineries, livestock production, food processing, rendering, water purification plants etc., the compounds responsible for the odour are known. So chemical analysis of the odour can be limited to these odorants, and selective concentrating techniques can be used. Selective concentrating methods are based on specific absorption techniques, using particular chemical reactions of odorant classes. Sometimes several absorption methods have to be used in order to describe the odour problem, thus increasing the labor cost of the analysis. On the other hand absorption methods allow better quantitative results. Selective absorption of odorants from air produces a far less complex mixture. We developed or are developing several of these methods for aldehydes, amines, acids, thiols etc. 

In this article, I shall begin by showing the tremendous scope of Lewis acid-base considerations. Although it is not fully reeilized, it is very difficult to find chemical reactions in which these effects are not operative. This will be followed by a discussion of the kind of data that should be obtained and analyzed in order to learn about the strength of bonding. Since data selection is important, a good deal of space is devoted to complications that can arise from improper design of experiments and improper analysis of experimental results. 

---