Hydroxyl group titration

End-Group Analysis. In end-group analysis the concentration of an end group is measured by a suitable technique, and then, from the known structure of the polymer, the value for Mn may be calculated (10-15). In condensation polymers one end of the polymer should have a specific group that can be titrated by appropriate means. Polyesters (13), polyamides (13), and polyurethanes (16) have been subjected to this analysis. Polyethers have been analyzed by hydroxyl-group titrations similar to those used for polyesters or by reaction with excess phenyl isocyanate followed by reaction with excess di-n-butylamine the latter was back-titrated with perchloric acid (17). Nonaqueous titrations (18) have been used to... 

The excess of unchanged acetic anhydride is then hydrolysed by the addition of water, and the total free acetic acid estimated by titration with standard NaOH solution. Simultaneously a control experiment is performed identical with the above except that the alcohol is omitted. The difference in the volumes of NaOH solution required in the two experiments is equivalent to the difference in the amount of acetic add formed, i.e., to the acetic acid used in the actual acetylation. If the molecular weight of the alcohol is known, the number of hydroxyl groups can then be calculated. 

The other analytical methods necessary to control the typical specification given in Table 5 are, for the most part, common quality-control procedures. When a chemical analysis for purity is desired, acetylation or phthalation procedures are commonly employed. In these cases, the alcohol reacts with a measured volume of either acetic or phthalic anhydride in pyridine solution. The loss in titratable acidity in the anhydride solution is a direct measure of the hydroxyl groups reacting in the sample. These procedures are generally free from interference by other functional groups, but both are affected adversely by the presence of excessive water, as this depletes the anhydride reagent strength to a level below that necessary to ensure complete reaction with the alcohol. Both procedures can be adapted to a semimicro- or even microscale deterrnination. 

In order that the reaction may proceed rapidly it is important to shake the mixture thoroughly after adding the iodine solution. When this is done the iodine compound is formed completely within one minute. With thymol it affords thymol di-iodide. In order to make sure that any iodine wnich may have entered into the hydroxyl-group is again liberated, care should be taken that a little hydriodic acid is always present hence the addition of the potassium iodide solution before the exc ess of iodine is titrated back with thiosulphate. Titration can only be regarded as complete when the blue coloration does not return in 10 minutes. 

Discussion. Hydroxyl groups present in carbohydrates can be readily acetylated by acetic (ethanoic) anhydride in ethyl acetate containing some perchloric acid. This reaction can be used as a basis for determining the number of hydroxyl groups in the carbohydrate molecule by carrying out the reaction with excess acetic anhydride followed by titration of the excess using sodium hydroxide in methyl cellosolve. 

Carry out a blank determination on the acetic anhydride/ethyl acetate solution following the above procedure without adding the carbohydrate. Use the difference between the blank titration, Vb, and the sample titration, Vs, to calculate the number of hydroxyl groups in the sugar (Note 2). 

Sodium hydroxyalkanesulfonates may be determined in the presence of an unsaturated hydrocarbon, including sodium alkenesulfonate. The sulfonates are converted to the free sulfonic acids using a slight excess of 2,4-dinitrobenzene-sulfonic acid. The hydroxyl group of the sulfonic acid liberated is acetylated in ethyl acetate solution by a known excess of acetic anhydride. The unconsumed anhydride is hydrolyzed by a pyridine-water mixture and the acids titrated potentiometrically with standard sodium hydroxide solution. The hydroxy-alkanesulfonate content is calculated after correction for any traces of acidity or alkalinity in the original sample. 

Three pKa values have been reported for leucovorin (free acid) they are 3.1, 4.8, and 10.4, as determined by electrometric titration.6 The first two values are attributed to the glutamyl carboxyls, and 10.4 is assigned to the hydroxyl group at the 4 position,8 by comparison to model compounds. 

Another key contribution of the Schwarz group was the recognition of the dramatic influence of oxide surfaces on bulk solution pH. In a landmark 1989 paper, Noh and Schwarz [7] demonstrated the method of mass titration, in which successive additions of oxide cause stepwise shifts in solution pH. This procedure is illustrated in Figure 6.7 [7], As indicated in Figure 6.1, the protonation-deprotonation chemistry of the surface hydroxyl groups is coupled to the liquid-phase pH. In mass titration, as the mass (or more appropriately, the surface area) of oxide in solution increases, the solution pH is brought to the PZC of the oxide, at which point no driving force for proton transfer exists... 

The carbon blacks used by Puri and Bansal (59) seemed to be different from the carbons used by Boehm et al. (35). Titration curves published by Puri and Bansal (59) show distinct breaks at pH 10-11. Other authors (26, 44) did not find this inflection. Possibly, no phenolic groups were present in Puri s samples. As will be shown below, the group neutralized by NaOH, but not by Na COa, was identified as phenolic hydroxyl group. 

A determination of the surface hydroxyl groups was made by Noll et al. (183). The total water content was determined by heating to 1100°. The content ofmolecular water was titrated by the Karl Fischer method. Silanol groups react with this reagent only very slowly. Good agreement was observed with silanol contents determined independently by other methods. With silica gel, 5.2 silanol groups were found per 100 A. ... 

Chemical analysis of the unreacted monomer functional groups as a function of time is useful for step polymerizations. For example, polyesterification can be followed accurately by titration of the carboxyl group concentration with standard base or analysis of hydroxyl groups by reaction with acetic anhydride. The rate of chain polymerization of vinyl monomers can be followed by titration of the unreacted double bonds with bromine. 

Adsorption and desorption reactions of protons on iron oxides have been measured by the pressure jump relaxation method using conductimetric titration and found to be fast (Tab. 10.3). The desorption rate constant appears to be related to the acidity of the surface hydroxyl groups (Astumian et al., 1981). Proton adsorption on iron oxides is exothermic potentiometric calorimetric titration measurements indicated that the enthalpy of proton adsorption is -25 to -38 kj mol (Tab. 10.3). For hematite, the enthalpy of proton adsorption is -36.6 kJ mol and the free energy of adsorption, -48.8 kJ mol (Lyklema, 1987). 

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