Titrimetric factors

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. 

The use of a colour change to indicate the end point is common to a wide variety of titrimetric methods. Visual detection of end points is a major factor in maintaining the simplicity of titrimetry, hence the capability of the human eye to detect colour change plays an important role in these techniques. 

In addition to assay features already mentioned, other factors may influence the choice of assay by the user. In terms of sensitivity of the assay, the threshold of detection of lipase activity, using the procedures as described in this unit, is on the order of 10 2 U for titrimetry, 10H U for colorimetry, and 10 4 U for spectrophotometry (where U is the amount of enzyme required to yield 1 imol product per minute). The smallest amounts (volumes) of materials, including enzyme, are required for the spectrophotometric method, and progressively more material is required for the colorimetric and titrimetric methods. Unless a flow cell adapter is available, the spectrophotometric method is not suitable for analysis of particulate (immobilized) enzyme preparations, whereas the other assay procedures are. 

Starch in helical conformation is indicated by the blue color developed with iodine. In a typical test, a tissue or extract is submerged in a KIS solution. DP heterogeneity is a factor the higher the DP, the higher is the I2 absorption and the more intense is the violet-blue color. The starch-iodine reaction is sensitive enough for starch to be a titrimetric indicator of I2, mindful of the nonstoichiometry and nonspecificity of the reaction. 

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. 

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