Volumetric analysis, acid-base titrations

Acid-base titrations are an example of volumetric analysis, a technique in which one solution is used to analyze another. The solution used to carry out the analysis is called the titrant and is delivered from a device called a buret, which measures the volume accurately. The point in the titration at which enough titrant has been added to react exactly with the substance being determined is called the equivalence point, or the stoichiometric point. This point is often marked by the change in color of a chemical called an indicator. The titration procedure is illustrated in Fig. 4.18. 

The diammoniate can be identified by means of its x-ray powder diffraction pattern and by chemical analysis. For analysis a sample of the substance is hydrolyzed with 6 N hydrochloric acid for several hours at 90° in a previously evacuated and sealed tube, which is equipped with a breakable side arm allowing gas transfer into the vacuum system. Hydridic hydrogen is measured volumetrically nitrogen and boron are determined by standard acid-base titrations. Anal. Calcd. for [BH2(NH3)2][BH4] hydridic H, 9.80 B, 35.05 N, 45.35. Found hydridic H, 9.74 B, 34.5 N, 45.7. 

The normality of a solution of an acid or base is the number of equivalents of acid or base per liter a 1 solution contains 1 equivalent per liter of solution. By determining, with use of an indicator, such as litmus, the relative volumes of acidic and alkaline solutions which are equivalent the normality of one solution can be calculated from the known value of the other. This process of acid-base titration (the determination of the titer or strength of an unknown solution), with use of special apparatus such as graduated burets and pipets, is an important method of volumetric quantitative analysis. 

Many titration procedures in volumetric analysis use an indicator that changes color to signal the endpoint of the titration. For example, acid-base titrations are often performed... 

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. 

Attention is finally focused on the advantages of conductometric titrations, which include (i) colored solutions where no indicator is found to function satisfactorily can be successfully titrated by this method (ii) the method is useful for titrating weak acids against weak bases, which does not produce a sharp change in color with indications in ordinary volumetric analysis and (iii) more accurate results are obtained because of the graphical determination of the end-point. 

The peroxide value (PV) of an oil or fat is defined as the quantity of peroxide oxygen present in the sample. This classical iodometric method is a volumetric analysis based on the titration of iodine released from potassium iodide by peroxides in a biphasic system using a standardized thiosulfate solution as the titrant and a starch solution as the indicator (see Background Information, discussion of peroxide value). This method will detect all substances that oxidize potassium iodide under the acidic conditions of the test, therefore the purity of the reagents is critical. 

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... 

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. 

DIE may be used for the same applications as discussed for thermometric titrations, for example, for the volumetric analysis of materials, such as boric acid, which are virtually impossible to titrate using endpoint indicators or pH indicators. DIE can also be used in biological studies where the reaction rates may be slow. Eor example, proteins have been titrated with acid or base, antibodies have been titrated with antigen, and enzyme-coenzyme systems have been studied. DIE is used to determine kinetic parameters for slow reactions. The use of a large excess of one reactant (the titrant) favors the forward reaction (according to Le Chatelier s principle) even if the equilibrium constant is small, so equilibria may be studied using DIE that cannot be studied using other titrimetric methods. 

Among them, volumetric methods are presumably the most widely used for water analysis. They are titrimetric techniques which involve a chemical reaction between a precise concentration of a reagent or titrant and an accurately known volume of sample. The most common types of reactions as used within this method are acid-base neutralization, oxidation-reduction, precipitation, and complexation. The use of an indicator which identifies the equivalence point is required to develop this kind of method. The modem laboratories usually employ automated endpoint titrators, which largely improve the efficiency and reliability of the determination. Moreover, spectrophotometric, potentiometric, or amperometric methods to determine the endpoint of the reaction can... 

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