Bases in non-aqueous solvents

Abstract Titration of weak bases in non-aqueous solvents can provide valuable information about these weak bases. Some primary amines 1-aminobutane, 1-aminopropane, 2-aminoheptane, aminocyclohexane, 3-amino-l-phenylbutane were titrated with hydrochloric acid in toluene solvent. All the primary amines gave very well-shaped potentiometric titration curves. The same titrations were done with hydrochloric acid in methanol solvent to show the effect of amphiprotic solvent in the titrations with hydrochloric acid. 

Nakazawa and Tanaka described a potentiometric titration of salts of organic bases in non-aqueous solvents with the addition of bismuth nitrate [16], The base halide or hydrohalide (0.7 mequiv.) is dissolved in 40 mL of anhydrous acetic acid, and then 40 mL of 1,4-dioxane and 2.5 mL of 5% Bi(N03)3 solution in acetic acid are added. The Bi(N03)3 prevents interference by the halide. The solution is titrated to a potentiometric endpoint with 0.1 M HC104 (a blank titration is also carried out). Results of purity assays of acetylcholine chloride and other compounds were tabulated, and it was found that the coefficient of variation was 0.18%. 

Abstract Titration of weak bases in non-aqueous solvents can provide valuable information about these weak bases. Some primary amines 1-aminobutane,... 

So ubiquitous is water on this planet that ourview of chemistry, by custom and convenience, is strongly biased in favour of aqueous solutions. What then are we to make of other solvents where the strongest acid may be the least ionised, where a respectable acid like CH3CO2H behaves as a base, where an anion base achieves stabilisation only by bonding to a molecule of its conjugate acid Examination of acids and bases in non-aqueous solvents cannot fail to be enlightening. 

Relative permittivity Acids and bases in non-aqueous solvents... 

SchifT s bases A -Arylimides, Ar-N = CR2, prepared by reaction of aromatic amines with aliphatic or aromatic aldehydes and ketones. They are crystalline, weakly basic compounds which give hydrochlorides in non-aqueous solvents. With dilute aqueous acids the parent amine and carbonyl compounds are regenerated. Reduction with sodium and alcohol gives... 

D. Rosenthal and P. Zuman, Acid-base equilibria, buffers and titrations in water. Chap. 18 in I. M. Kolthoff and P. J. Elving (eds.). Treatise on Analytical Chemistry, 2nd edn., Vol. 2, Part 1, 1979, pp. 157-236. Succeeding chapters (pp. 237-440) deal with acid-base equilibria and titrations in non-aqueous solvents. 

In contrast to the above resins, the chelating resin Amberlite IRC-718 is based upon a macroreticular matrix. It is claimed to exhibit superior physical durability and adsorption kinetics when compared to chelating resins derived from gel polymers and should also be superior for use in non-aqueous solvent systems. 

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. 

Determinations in non-aqueous solvents are of importance for substances which may give poor end points in normal aqueous titrations and for substances which are not soluble in water. They are also of particular value for determining the proportions of individual components in mixtures of either acids or of bases. These differential titrations are carried out in solvents which do not exert a levelling effect. 

As indicated in Section 2.4 the strength of an acid (and of a base) is dependent upon the solvent in which it has been dissolved, and in Sections 10.19-10.21 it has been shown how this modification of strength can be used to carry out titrations in non-aqueous solvents which are impossible to perform in aqueous solution. Potentiometric methods can be used to determine the end point of such non-aqueous titrations, which are mainly of the acid-base type and offer very valuable methods for the determination of many organic compounds. 

As in electroanalysis both ionic and possible electrode aspects are of major interest, both aspects of solutes in non-aqueous solvents have to be considered this can best be done by dividing the theory of the solutions concerned into two parts, viz. (1) the exchange of ionic particles (ionotropy), which leads to acid-base systems, and (2) the exchange of electrons only, which leads to redox systems. 

For the designation of pH in non-aqueous solvents, we use the forms described by Bosch and coworkers6 based on the recommendations of the IUPAC, In Compendium of Analytical Nomenclature. Definitive Rules 1997, 3rd edn, Blackwell, Oxford, UK, 1998. If one calibrates the measuring electrode with aqueous buffers and then measures the pH of an aqueous buffer solution, the term "pH is used if the electrode is calibrated in water and the pH of the neat buffered methanol solution then measured, the term, pH is used, and if the electrode is calibrated in the same solvent and the pH reading is made, then the term pH is used. 

Titrations in non-aqueous solvents have been traditionally an important tool for the accurate determination of various pharmaceuticals, some acids in foods, use of some acids or bases in detergents, cosmetics and textile auxiharies, in the analysis of industrial process streams, the analysis of polymers [1-7]. The determination of the pK or pK values of organic compoimds with acidity or basicity constant less than 10 can only be reahsed in non-aqueous media. Although water has excellent solvent properties, it is not suitable for such organic compotmds since the pH jump at the equivalence point in aqueous solution carmot be evalrrated with reasonable accuracy, with this resrrlt, the end point carmot be found. Moreover, most of this compotmds ate not soluble in water. For these reasons, titration in non-aqueous media has recently acqttired great importance. It is now well known that non-aqueous titrations greatly depend on the solvents used [4, 8-13]. 

As a result, inert and aprotic solvent toluene is suitable for the titration of weak bases in non-aqueous media as solvent, although benzene which is more carcino-genic aromatic hydrocarbon used widely in literature for non-aqueous titrations. The major advantage of toluene is tliat it does not compete for protons with the reactant in the titrations because of its autoprotolysis constant approaching zero. The major disadvantages of solubility can be removed by using small amount of amphiprotic solvents. 

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. 

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). 

Vercellotti et al. (65) studied the rates of / -elimination for O-benzyl ethers of O-glycosyl-N-(2,4-dinitrophenyl)-L-serine and L-threonine methyl esters in non-aqueous solvents using various bases. The corresponding acetyl derivatives of 2-acetamido-2-deoxy-D-galactose were also included. Generally, these substances were highly reactive, but the nature of the solvent and the base type were influencing factors. Thus, no elimination occurred in benzene when trimethylamine was used as base. 

The Bronsted definitions of acids and bases apply to species in non-aqueous solvents as well as in water. For example, when acetic acid is added to liquid ammonia, proton transfer takes place and the following equilibrium is reached ... 

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 mV. The silver/silver chloride electrode also finds application in non-aqueous solutions, although some solvents cause the silver chloride film to become soluble. Some experiments have utilised reference electrodes in non-aqueous solvents that are based on zinc or silver couples. From our own experience, aqueous reference electrodes are as convenient for non-aqueous systems as are any of the prototypes that have been developed to date. When there is a need to exclude water rigorously, double-salt bridges (aqueous/non-aqueous) are a convenient solution. This is true even though they involve a liquid junction between the aqueous electrolyte system and the non-aqueous 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 dramatic 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). 

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