Nonaqueous solvents titrations

In order to examine the influence of these interactions, the charge properties of protein molecules have to be investigated in nonaqueous solvents. Titration, electrophoretic, and conductometric experiments could yield important information in this connection, but few studies in nonaqueous solvents have been performed. 

The majority of titrations involving basic analytes, whether conducted in aqueous or nonaqueous solvents, use HCl, HCIO4, or H2SO4 as the titrant. Solutions of these titrants are usually prepared by diluting a commercially available concentrated stock solution and are stable for extended periods of time. Since the concentrations of concentrated acids are known only approximately,the titrant s concentration is determined by standardizing against one of the primary standard weak bases listed in Table 9.7. 

The NH3 is removed by distillation and titrated with HCl. Alternatively, N03 can be titrated as a weak base in an acidic nonaqueous solvent such as anhydrous acetic acid, using HCIO4 as a titrant. 

Another important example of a redox titration for inorganic analytes, which is important in industrial labs, is the determination of water in nonaqueous solvents. The titrant for this analysis is known as the Karl Fischer reagent and consists of a mixture of iodine, sulfur dioxide, pyridine, and methanol. The concentration of pyridine is sufficiently large so that b and SO2 are complexed with the pyridine (py) as py b and py SO2. When added to a sample containing water, b is reduced to U, and SO2 is oxidized to SO3. 

Fritz, J. S. Acid-Base Titrations in Nonaqueous Solvents. Allyn and Bacon Boston, 1973. 

Potcntiomctric Titrations In Chapter 9 we noted that one method for determining the equivalence point of an acid-base titration is to follow the change in pH with a pH electrode. The potentiometric determination of equivalence points is feasible for acid-base, complexation, redox, and precipitation titrations, as well as for titrations in aqueous and nonaqueous solvents. Acid-base, complexation, and precipitation potentiometric titrations are usually monitored with an ion-selective electrode that is selective for the analyte, although an electrode that is selective for the titrant or a reaction product also can be used. A redox electrode, such as a Pt wire, and a reference electrode are used for potentiometric redox titrations. More details about potentiometric titrations are found in Chapter 9. 

Amides can be titrated direcdy by perchloric acid ia a nonaqueous solvent (60,61) and by potentiometric titration (62), which gives the sum of amide and amine salts. Infrared spectroscopy has been used to characterize fatty acid amides (63). Mass spectroscopy has been able to iadicate the position of the unsaturation ia unsaturated fatty amides (64). Typical specifications of some primary fatty acid amides and properties of bisamides are shown ia Tables 5 and 6. 

The secondary amine function of dobutamine hydrochloride may be determined by potentiometric titration with perchloric acid using glacial acetic acid as a nonaqueous solvent. Mercuric acetate is used to tie up the chloride ion. 

Fritz, J.S., Acid-Base Titration in Nonaqueous Solvents Allyn and Bacon Boston, 1973 Chapter 2. 

Popov, A. I., Caruso, H. Acid-base equilibria and titrations in nonaqueous solvents. B. Amphiprotic solvents, in Ref. 1, pp. 303-348. 

J. S. Fritz, Acid-Base Titrations in Nonaqueous Solvents (Boston Allyn Bacon, 1973) J. Kucharsky and L. Safarik, Titrations in Non-Aqueous Solvents (New York Elsevier, 1963) W. Huber, Titrations in Nonaqueous Solvents (New York Academic Press, 1967) I. Gyenes, Titration in Non-Aqueous Media (Princeton, NJ Van Nostrand, 1967). 

One advantage of the amperometric titration is its ease of automation. A titrator can be signaled to shut off when a specified current level is reached. The advantage of the two-working-electrode variation is the elimination of a reference electrode, which can be troublesome in nonaqueous solvents. 

Some of the reasons for considering coulometric titrations in nonaqueous solvents are that many organic compounds are not soluble in water, metals can exist in oxidation states that are not found in water, and advantage can be taken of the acidity or basicity of the solvent to improve the basic or acidic strength of a base or acid, respectively. 

Table 5.21 Commonly used pH indicators for titrations in nonaqueous solvents...

For most potentiometric measurements either the saturated calomel reference electrode or the silver/silver chloride reference electrode are used. These electrodes can be made compact, are easily produced, and provide reference potentials that do not vary more than a few millivolts. The discussion in Chapter 5 outlines their characteristics, preparation, and temperature coefficients. The silver/silver chloride electrode also finds application in nonaqueous titrations, although some solvents cause the silver chloride film to become soluble. Some have utilized reference electrodes in nonaqueous solvents that are based on zinc or silver couples. From our own experience, aqueous reference electrodes are as convenient for nonaqueous systems as are any of the prototypes that have been developed to date. When there is a need to rigorously exclude water, double-salt bridges (aqueous/nonaqueous) are a convenient solution. This is true even though they involve a liquid junction between the aqueous electrolyte system and the nonaqueous solvent system of the sample solution. The use of conventional reference electrodes does cause some difficulties if the electrolyte of the reference electrode is insoluble in the sample solution. Hence the use of a calomel electrode saturated with potassium chloride in conjunction with a sample solution that contains perchlorate ion can cause erratic measurements due to the precipitation of potassium perchlorate at the junction. Such difficulties normally can be eliminated by using a double junction that inserts another inert electrolyte solution between the reference electrode and the sample solution (e.g., a sodium chloride solution). 

The complexes are useful as indicators for titrating weak bases in nonaqueous solvents and also for oxidation-reduction and aromatic diazotization titrations.4 8... 

The classical method for the determination of low water contents in organic solvents is the nonaqueous iodometric titration introduced by Karl Fischer in 1935, using a solution of sulfur dioxide, iodine, and pyridine in a benzene/methanol mixture [139, 140], By replacing some of the toxic ingredients (pyridine, benzene, methanol), this titration method has more recently been developed into a simple and environmentally safe standard procedure [141]. 

Some liquid covalent halides can act as nonaqueous solvents " based on Lewis acid-base behavior, according to the donor-acceptor definition. The self-dissociated ions consist of a cation formed by subtraction of a hahde ion from the neutral compound, while the anion is formed by its addition (equation 24). Salts derived from such covalent halides can be considered as titration products of the parent acidic and basic compounds (equations 25 and 26). In such cases, both the cation and the anion usually possess a stable coordination number with a high geometrical symmetry. 

Water is highly unusual in the extent of its interactions with solutes, but even minimal solvent-solute interactions can play a major role in the nature of chemical reactions. To calculate pH during acid-base titrations in a nonaqueous solvent, we must consider not only the equilibria discussed in Chapter 3 but also reactions discussed in Sections 4-2, 4-3, and 4-4. 

From the practical point of view, nonaqueous solvents can be described broadly as acidic, basic, or neutral. For carrying out titrations, properties such as dielectric constant, melting point, boiling point, and (for amphiprotic solvents. Section 4-3) autoprotolysis constant are important (Table 6-1). 

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