Carboxy group titration

Determination of carboxy groups. Esterification or polyesterification kinetics is usually followed by this titration which is both easy and accurate. Each sample is dissolved in a solvent or a mixture of solvents (CHCI3, QH /EtOH or MeOH, toluene/EtOH or MeOH...) and then titrated with alcoholic KOH. The end point is determined either with an indicator (in most cases phenolphthalein) or with a pH-meter. An accuracy of about 0.1-1% can generally be achieved. 

Determination of hydroxy groups. Hydroxy groups are rarely titrated. Indeed, this titration is more complex and less accurate than carboxy-group determination. However, it is very helpful to know the hydroxy group content, for example in the case of reactions carried out with a great excess of add. In this case, the variations of the add concentration during the esterification are very small and are determined only with poor accuracy. 

Various well established procedures are described in the literature273,274,296. The procedure generally used for polyesters is the phthalylation or acetylation of OH groups with phthalic or acetic anhydrides followed by back titration of excess carboxy groups. Moisture does not interfere but the addity of the sample must be taken into account. The use of pyromellitic dianhydride is also reported320. ... 

During the course of a polyesterification the volume and the weight of the reaction mixture vary because condensation water is released. In most cases, the progress of the reaction is followed by titration of the acid groups at definite intervals the carboxy group concentration is expressed in equivalents per kilogram. Consequently, several authors tried to find out if the weight decrease due to the elimination of water must be taken into account. 

The experimental results may be represented both by the titration curves or property-composition dependences. The extremums or bends on the titration curves indicate the formation of complexes and their composition. Thus, investigating the-possi-bility of complex formation in polyelectrolyte - nonionic polymer systems, one can use the methods of conductometric and potentiometric titration. The formation of interpolymer complexes in these systems, as some authors suggest18,211, is caused by a co-operative formation of hydrogen bonds between carboxy groups of the polyacid and oxygen atoms of nonionic polyvinylpyrrolidone or poly(ethylene glycol) and is therefore accompanied by an increase of pH of the solution. The typical titration curves for the system polyvinylpyrrolidone - copolymer maleic anhydride and acrylic add are shown in Fig. 1. The inflection points of the titration curves indicate the ratio at which the macromolecular components react with each other, i.e. the composition of the formed complexes. 

Like many poly(dimethyl)siloxanes the functionally terminated (carboxy-propyl) polymer dissolves in a number of gases such as carbon dioxide, ethylene, and ethane. Table 9.32 gives the average molecular weight of the parent carboxypropyl-terminated poly(dimethyl)siloxane and the seven fractions derived from the parent by fractionation using supercritical carbon dioxide. The molecular weight analysis was carried out by end-group titration. As... 

Potentiometric titration and other physical data indicated that the native polysaccharide possesses an ordered secondary structure that is stabilized by nonionic interactions, which outweigh the repulsion between adjacent carboxy-groups. An acidic heteropolysaccharide from Xanthomonas SI 9 is composed of residues of D-glucuronic acid, D-glucose, o-galactose, D-mannose, and acetate. Although methylation analysis showed that this polysaccharide possesses many structural features similar to those of other Xanthomonas polysaccharides, it is unusual in that D-mannosyl and D-galactosyl residues occur in the structure. 

In an attempt to clarify the effects of ionic and hydrophobic substituents on the polyelectrolytic behaviour of polysaccharides, potentiometric titrations and activity measurements of the counter-ions have been conducted on carboxy-methyldextrans containing either substituted carboxy-groups or benzyl groups. Dextran glass (10 M, dextran) attached to an alkylamine-bonded phase of glass has been used for the hydrophobic immobilization of enzymes. ... 

What is the isoelectric point of -y-carboxyglutamate Assume that the pK values of the two y-carboxyl groups are identical at 4.1. Draw a titration curve for y-carboxy glutamate. 

Linear peptides will have free amino- and carboxy- terminal groups. Thus they will exhibit titration curves similar to a free amino acid, but with the pKa values shifted closer to simple acid and amine values (there will be no charge stabilization). 

The polymer was found to be insoluble at pH values below 3.5. The titration of the amino groups required the calculated amount of sodium hydroxide, and in comparison with the data for N-carboxy-... 

Amine consumption in reaction II could make it difficult to use the epoxy-carboxy reaction to prepare esters or polyesters. In fact, most of the zwitterion Z, which is formed from E and the amine, is transformed into the ester P and, at least in solution, only a small quantity of side product is formed. Moreover, titrations carried out during the reaction show that epoxy and acid groups are consumed at roughly the same rate, demonstrating that even if the contribution of reaction II to overall kinetics and mechanism is important it does not forbid the use of this method for preparation of polyesters. 

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