Acidic solvents, titrations

All other things being equal, the strength of a weak acid increases if it is placed in a solvent that is more basic than water, whereas the strength of a weak base increases if it is placed in a solvent that is more acidic than water. In some cases, however, the opposite effect is observed. For example, the pKb for ammonia is 4.76 in water and 6.40 in the more acidic glacial acetic acid. In contradiction to our expectations, ammonia is a weaker base in the more acidic solvent. A full description of the solvent s effect on a weak acid s piQ or on the pKb of a weak base is beyond the scope of this text. You should be aware, however, that titrations that are not feasible in water may be feasible in a different solvent. 

The concentration of o-phthalic acid in an organic solvent, such as n-butanol, may be determined by an acid-base titration using aqueous NaOH as the titrant. As the titrant is added, the o-phthalic acid is extracted into the aqueous... 

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. 

QuaHty control in the production of organic solvent finish removers may be done by gas—Hquid chromatography, which allows the manufacturer to determine the actual ratio of volatile solvent present in the finished product. If the product does not meet specifications, solvents can be added to bring the product to an acceptable composition. A less expensive approach is to use a hydrometer to determine the specific gravity of the product. The specific gravity indicates if the proper blend has been reached. Nonaqueous acid—base titration may be used to determine the amount of acid or alkaline activator present in a remover. 

In the discussion of the relative acidity of carboxylic acids in Chapter 1, the thermodynamic acidity, expressed as the acid dissociation constant, was taken as the measure of acidity. It is straightforward to determine dissociation constants of such adds in aqueous solution by measurement of the titration curve with a pH-sensitive electrode (pH meter). Determination of the acidity of carbon acids is more difficult. Because most are very weak acids, very strong bases are required to cause deprotonation. Water and alcohols are far more acidic than most hydrocarbons and are unsuitable solvents for generation of hydrocarbon anions. Any strong base will deprotonate the solvent rather than the hydrocarbon. For synthetic purposes, aprotic solvents such as ether, tetrahydrofuran (THF), and dimethoxyethane (DME) are used, but for equilibrium measurements solvents that promote dissociation of ion pairs and ion clusters are preferred. Weakly acidic solvents such as DMSO and cyclohexylamine are used in the preparation of strongly basic carbanions. The high polarity and cation-solvating ability of DMSO facilitate dissociation... 

The Bronsted-Lowry theory of acids and bases referred to in Section 10.7 can be applied equally well to reactions occurring during acid-base titrations in non-aqueous solvents. This is because their approach considers an acid as any substance which will tend to donate a proton, and a base as a substance which will accept a proton. Substances which give poor end points due to being weak acids or bases in aqueous solution will frequently give far more satisfactory end points when titrations are carried out in non-aqueous media. An additional advantage is that many substances which are insoluble in water are sufficiently soluble in organic solvents to permit their titration in these non-aqueous media. 

Table 15.6 indicates common reagents and solvents and the appropriate electrode combination for a variety of acid-base titrations. 

An analytical solution for molecules with alkaline functionality is acid/base titration. In this technique, the polymer is dissolved, but not precipitated prior to analysis. In this way, the additive, even if polymer-bound, is still in solution and titratable. This principle has also been applied for the determination of 0.01 % stearic acid and sodium stearate in SBR solutions. The polymer was diluted with toluene/absolute ethanol mixed solvent and stearic acid was determined by titration with 0.1 M ethanolic NaOH solution to the m-cresol purple endpoint similarly, sodium stearate was titrated with 0.05 M ethanolic HC1 solution [83]. Also long-chain acid lubricants (e.g. stearic acid) in acrylic polyesters were quantitatively determined by titration of the extract. 

This example shows also that the proton theory, in addition to being valid for aprotic solvents, also works for amphiprotic solvents, and so represents a more general theory. How in an acid-base titration the theory works out can be followed from the titration of a certain amount of HC1 gas introduced into pyridine as an aprotic solvent ... 

Fig. 4.3. Potentiometric titration of phenol and salicylic acid. Solvent pyridine-dimethylfuran-diethylamine (4 7 9, v/v) titrant (C4H9)3CH3NOH, 0.1 Nin IPA glass-calomel electrode (aq.).

Exploratory acid-base titrations in solvents with very low dielectric constant (e)... 

Acid-base titrations in protogenic solvents with low e... 

Szabolcs determined the active principle in preparations based on miconazole and clotrimazole [15]. The two drugs were determined in ointments by extraction with chloroform, evaporation of the solvent, dissolution of the residue in acetic acid, and titration with 0.1 N perchloric acid in the presence of Gentian Violet. 

In glacial acetic acid (an acidic solvent) and in dioxane (a neutral solvent), the perchloric acid (HC104) behaves as more acidic (i.e., less protophyllic) than HC1 and, therefore, many base-hydrochlorides (i.e., chlorides) may be titrated with standard HC104, just as carbonates may be titrated in aqueous solution with standard HC1. 

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

Acid-base reactions in non-aqueous solvents have been extensively studied. Many books and reviews are available concerning acid-base equilibria and acid-base titrations in non-aqueous solvents. References [1-3] are particularly useful. 

B. Chemical Reactions in Solvents and Melts, Pergamon Press, Oxford, 1969 Gyenes, 1. Titrationen in Nichtwdsserigen Medien, F. Enke, Stuttgart, 1970 Fritz, J.S. Acid-Base Titrations in Nonaqueous Media, Allyn Bacon, Needham Heights, MA, 1973 Kratochvil, B. 

There are many books and. review articles review articles covering redox titrations in nondealing with acid-base titrations in non-aque- aqueous solvents, of which Ref. [9] seems to be... 

The pH window is very wide in solvents that are weak both in acidity and basicity. The widths of the pH window are well over 30 in such solvents, compared to about 14 in water (Table 6.6). The usefulness of these expanded pH regions is discussed in Section 3.2.2. In particular, potentiometric acid-base titrations in such solvents are highly useful in practical chemical analyses as well as physicochemical studies [22]. Acid-base titrations in lion-aqueous solvents were popular until the 1980s, but now most have been replaced by chromatographic methods. However, the pH-ISFETs are promising to realize simple, rapid and miniature-scale acid-base titrations in lion-aqueous solvents. For example, by use of an Si3N4-type pH-ISFET, we can get an almost complete titration curve in less than 20 s in a solution containing several different acids [17d]. 

Vapor phase chromatography was used to analyze allylic oxidation products. The column, inch X 35 feet, was packed with 20% UCON 5100 on Celite (50-70). NMR analyses were performed using a Varian A-60 instrument. Molecular weights were determined by cryoscopic methods (benzene and acetic acid solvents). Organoselenium compounds were analyzed for selenium by the method of Gould (2), which involves wet digestion of the sample, followed by iodometric titration of the selenium dioxide produced. 

Peroxide Determinations. Bawn and Williamson report two iodometric procedures for determining peracetic acid (Methods I and III below) and one method for determining total peroxide (4) (Method II). Bawn and Jolly report another method for total peroxide (5) (Method IV below). The difference between total peroxide and peracetic acid is assumed to be acetaldehyde monoperacetate (AMP). Each method was tested in our preliminary studies. Method III is preferred for peracetic acid because the results are more reproducible. In Method I a large blank titration was always observed, while in Method III the blank titration was very small. Method IV is preferred for total peroxide because it seems to be more sensitive to total peroxide and less sensitive to water content of the acetic acid solvent. 

A base too weak to be titrated by H30 in water might be titrated by HCI04 in acetic acid solvent. 

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). 

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