Titration inert solvents

A1C13, or S02 in an inert solvent cause colour changes in indicators similar to those produced by hydrochloric acid, and these changes are reversed by bases so that titrations can be carried out. Compounds of the type of BF3 are usually described as Lewis acids or electron acceptors. The Lewis bases (e.g. ammonia, pyridine) are virtually identical with the Bransted-Lowry bases. The great disadvantage of the Lewis definition of acids is that, unlike proton-transfer reactions, it is incapable of general quantitative treatment. 

As would be expected, the larger titration potential ranges offer much more scope for mutually distinguishing between individual acids or bases in amphiprotic solvents, as a consequence of self-dissociation, the potential ranges are rather limited, whereas in the aprotic protophilic solvents and "aprotic inert solvents these ranges are considerably more extensive. 

In Fig. 4.6, the titration of p-hydroxybenzoic acid in pyridine shows that the COOH and OH groups can be clearly determined. However, in acetonitrile there is half-way the titration of the COOH group an additional potential jump this can be explained by a phenomenon which was already known for acetic acid23, viz., in the inert solvent acetonitrile intermolecular hydrogen-bridge... 

From eqns. 4.50 and 4.51 it can be seen that complex 1 is converted into complex 2 by a stronger base 2 or into complex 3 by a stronger acid 2 in other words, complexes 2 and 3 are much more stable than complex 1. Whereas reactions 4.48 and 4.49 are addition reactions, reactions 4.50 and 4.51 are exchange reactions often Lewis titrations must be carried out in completely inert solvents such as alkanes or benzene because of instability of the titrants and titrands in other media. Examples of potentiometric Lewis titration curves are given in Fig. 4.9 for CS2 and C0220, where one of their resonance structures can react as a Lewis acid with OH as a Lewis base ... 

In order to increase our knowledge of the complex electrochemistry in non-aqueous media, we shall consider the phenomena that occur during titrations in solvents with very low e in this connection, it is useful to treat these titrations separately for protogenic and the aprotic solvents, with the latter being subdivided into protophilic and inert solvents. 

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

In the above titrations of Lewis acids or with the use of the highly reactive titrant LiAlH4, one needs inert solvents of Bronsted class 8, having a low dielectric constant (e.g., at 20° C = 2.0 for cyclohexane or 4.3 for diethyl... 

Steigman, J. Acid-base equilibria and titrations in nonaqueous solvents. D. Inert solvents, in Ref. 1, pp. 385-423. 

After the acid strength of a catalyst surface has been bracketed by means of colors of adsorbed indicators, the next logical step in the determination of surface acidity is the measurement of the number of acidic groups. This is generally done by titrating a suspension of the catalyst with a solution of a suitable amine in an inert solvent the previously described indicators are used to determine endpoints. 

The bromination of cinnamic acid dissolved in carbon tetrachloride or other inert solvent.offers a convenient system for study. The dibromocinnamic acid produced remains in the carbon tetrachloride solution. The thermal reaction is so slow that it can barely be measured at room temperature and it is entirely negligible in comparison with the photochemical reaction at ordinary intensities. The quantum yield is so large that considerable reaction occurs even if the intensity of light is much reduced by the monochromator or other device for confining the light to a narrow range of frequencies. Furthermore, the reaction is easily and accurately followed by titration with sodium thiosulfate. Potassium iodide is added and the iodine liberated is a measure of the remaining bromine. 

Heat and titration with acid can accomplish the required H bond ruptures. Cavalieri and Rosenberg (358) have described this process, and they illustrate how it relates to the Watson and Crick structure for nucleic acids. They show further that the temperature required in an H bonding solvent is lower than in inert solvent, consequently these two factors can work in the same direction. Denaturation may also occur by ionization of amino groups (23), and the effect of radiation on DNA has been imputed to breaking of H bonds (454, 2147a). Mechanical stress may be able to break H bonds and denature proteins (1105). 

Many sulfa drugs such as sulfanilamide, sulfathio-zole, and sulfathalidine can be titrated in ethylene-amine with tetrabutylammonium hydroxide. The ability of dimethylformamide to dissolve salts, polymers, and many organic compounds accounts for its wide use as a titration solvent. Inert solvents such as acetone can be used for titration of acids, acetonitrile for both acids and bases, and ethyl acetate for amines. A suitable non-aqueous medium such as methyl... 

Although boron trifluoride, sulphur trioxide and the like may be titrated against bases in inert solvents, there is no unequivocal instance of a Lewis acid causing catalysis in the absence of protons. Thus the concept is... 

A second type of very useful behavior occurs in aprotic (or sometimes inert ) solvents, which usually exhibit very weak acid properties. Examples are dimethylformamide, dimethylsulfoxide, dioxane, ether, various nitriles, methyl isobutyl ketone, hydrocarbons, carbon tetrachloride. These solvents often permit differentiation (or stepwise titration) of a series of acidic or basic species which, in water, either titrate together or not at all. For example, perchloric, hydrochloric, salicylic, and acetic acids and phenol can be titrated stepwise in methyl isobutyl ketone solvent to obtain discernible endpoints for each compound, using tetrabutyl ammonium hydroxide in isopropyl alcohol as titrant. 

The species variability, unknown molecular weights, and poor solubility in most solvents of the melanins present difficulties in the choice and preparation of the material for titration. In the conductimetric study cited melanin was prepared according to Piatelli and Nicolaus substituting frozen Japanese squid (cuttlefish) for the Mediteranean sepia species used by the Italian authors. A sample of human melanin from Japanese hair was made available by Bolt, who used a new and mild procedure for extraction and purification. The natural melanins varied in their solubility in suitable solvents. Melanin derived from human liver melanoma appeared to be the least polymeric and most readily soluble. For squid melanin DM SO was used as the solvent, although some insoluble residue was left. DMSO, however, is not a truly inert solvent, but interacts with melanin, particularly at high temperatures. To minimize this reaction all tests were carried out in cold saturated solutions of melanin in DMSO, or suitable dilutions thereof. Triethanolamine completely dissolved human hair melanin and was employed in a series of tests. This solvent also... 

The potentiometric titration experiments have provided information on the electron affinities in inert solvents, such as 1,2-dimethoxyethane or tetrahydrofuran (2S). 

I would now like to consider the titration of acidic compounds in nonaqueous solutions. If you wish to titrate an acid in nonaqueous solution, you should choose a solvent that is not acidic and a titrant that is as strong a base as possible. The paper that really aroused people s imagination and created a lot of interest was the one published by Moss, Elliot, and Hall in 1948, in which they introduced ethylenediamine as a solvent. This compound certainly doesn t have any acidic properties and these authors showed that you can titrate phenol, which is normally too weak to titrate as an acid. In recent years, however, the trend has been away from the use of strongly basic solvents because they have a leveling effect on many bases and they are somewhat unpleasant to handle. Solvents now in use are pyridine, which is an inert solvent and a very weak base, acetonitrile, and acetone. Acetone and certain other ketones are surprisingly good. Recently we have done some work with tertiary butyl alcohol, an excellent solvent for certain cases. Sodium or potassium hydroxide can be used as tltrants, but these are not particularly... 

The experiinental procedure used depends on the nature of the end-group present. The niol.wl. of polymers containitJg-COOH groups is determined by titrating a known weight of the polymer dissolved in an inert solvent against a standard. solution of KOH using phenolphthalein indicator. 

Titanium, D. of as oxide, via tannic acid and phenazone complexes, (g) 470 by hydrogen peroxide, (s) 696 Titan yellow 692 Titrand 257 Titrant 257 Titration 257 classification of, 258 in an inert atmosphere, 376, 629 in non-aqueous solvents, 281 aniline (and ethanolamine), D. of, 307 indicators for, 283 solvents for, 283... 

Van der Heijde24 mentioned the frequent occurrence of similar irregularities, especially for titrations of acids in inert or weakly basic solvents, and we shall return to this later. 

The formation of a weakly basic tertiary amine in reaction 4.102 does not alter the titrant normality, but in the titration of an acid it may suppress the height of the titration curve on the basic side. In an extensive study of twelve quaternary ammonium titrants in non-aqueous media (mainly isopropyl alcohol), Harlow73 observed large differences in stability the presence of water had a profound stabilizing action but at the sacrifice of basic strength inert and basic solvents increased the rate of decomposition (see Fig. 4.18). 

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. 

The characteristics of redox reactions in non-aqueous solutions were discussed in Chapter 4. Potentiometry is a powerful tool for studying redox reactions, although polarography and voltammetry are more popular. The indicator electrode is a platinum wire or other inert electrode. We can accurately determine the standard potential of a redox couple by measuring the electrode potential in the solution containing both the reduced and the oxidized forms of known concentrations. Poten-tiometric redox titrations are also useful to elucidate redox reaction mechanisms and to obtain standard redox potentials. In some solvents, the measurable potential range is much wider than in aqueous solutions and various redox reactions that are impossible in aqueous solutions are possible. 

---