Titrimetric factors acids

Table 11.27 Primary Standards for Aqueous Acid-Base Titrations Table 11.28 Titrimetric (Volumetric) Factors...

Titrimetric (Volumetric) Factors for Acid-Base Titrations... 

Titrimetric (volumetric) factors for acids and bases are given in Table 11.28. Suitable indicators for acid-base titrations may be found in Tables 8.23 and 8.24. 

For the endo isomer, titrimetric rates equalled the polarimetric rates in the three solvents examined acetic acid, aqueous acetone, and aqueous dioxane. However, for the exo isomer the polarimetric rate was greater than the titrimetric rates by factors of 1.40 in 75% aqueous acetone to 3.46 in acetic acid (later increased to 4.6, but without experimental data15 ). If the exo rate is corrected by this factor, the value becomes 1600 times that for the endo isomer. 

Symmetry. Solvolysis of optically active exo-2-norbornyl brosylate (74) yields racemicexo-norbornylderivatives10,513. The rate of racemizationexceeds the titrimetric rate by a factor of 1.40 in 75% acetone, 2.94 in ethanol, and 3.46 in acetic acid10) (later revised to 4.6513 ). This is attributed to recapture of the anion by the racemic cation. Solvolysis of optically active endo-norbornyl brosylate (688) yields exo-norbornyl products with a small amount of retained activity (13% in 75% acetone, 7% in acetic acid, and 3% in formic acid)10,513). Solvent attack with inversion on (688), or the corresponding tight ion pair, must be involved. 

Titrimetric luminescence methods record changes in the indicator emission of radiation during titration. This change is noted visually or by instruments normally used in luminescence analysis. Most luminescence indicators are complex organic compounds which are classified into fluorescent and chemiluminescent, compounds according to the type of emission of radiation. As in titrimetry with adsorption of colored indicators, luminescence titration makes use of acid-base, precipitation, redox, and complexation reactions. Unlike color reactions, luminescence indicators enable the determination of ions in turbid or colored media and permit the detection limit to be lowered by a factor of nearly one thousand. In comparison with direct luminescence determination, the luminescence titrimetric method is more precise. 

In protic solvents, the allylic carbonium ion formed by acid-catalyzed alkyl carbon-oxygen bond fission can recombine either with the carboxylic acid molecule or with a solvent molecule. The electrostatic attraction between the carbonium and carboxylate ions, which is a major factor in isomerization of allylic esters by ion-pair internal return during solvolysis, is absent in the acid-catalyzed reaction. The more numerous, usually more nucleophilic, solvent molecules in the solvation shell of the carbonium ion should compete effectively with the departed carboxylic acid molecule and solvolysis rather than isomerization should be the predominant reaction. For example, in the presence of 0.05 M perchloric acid, solvolyses of cis- and //- //7.s-5-methyl-2-cyclohexenyl p-nitrobenzoates are not only very much faster than in the absence of the acid, but polarimetric and titrimetric rates of solvolysis of optically-active esters were identical within experimental error. For these esters, the acid-catalyzed solvolysis was not accompanied by a detectable amount of isomerization. Braude reported, on the basis of indirect evidence, that isomerization accompanies acid-catalyzed hydrolysis of a-ethynyl-y-methylallyl acetate in aqueous dioxane. It was shown that, under some experimental conditions, the spectrophotometrically determined rate of appearance of the rearranged 1 -yne-3-ene chromophore exceeds the titrimetrically determined rate of hydrolysis,... 

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