Titration method, methyl ester determination

Methyl Esters In Pectin. A titration method has been reported twT in which methyl ester levels are calculated from the number of equivalents of standard sodium hydroxide required to neuturalize the pectin sample before and after saponification. The copper titration procedure described earlier for determination of galac-turonic acid residues in pectin (15), is also used to determine methyl ester levels from the increase in copper-binding after hydrolysis of the esters. An accurate and sensitive colorimetric method (M) is rather time-consuming, but several samples can be run in parallel. Samples are saponified, the released methanol oxidized to formaldehyde, and the formaldehyde determined by spectrophometric assay (4l2nm) of its condensation product with pentane-2,4-dione. 

The total concentration of free fatty acids is usually determined by extrac-tion/titration methods or spectrophotometrically as Cu soaps. Early attempts to quantify the concentration of individual short-chain fatty acids involved steam distillation and adsorption chromatography. Complete separation and quantitation of free fatty acids can be achieved by GC, usually as their methyl esters, for which several preparative techniques have been published. Free fatty acids are major contributors to the flavor of some varieties, e.g., Romano, Feta, and Blue in the latter, up to 25% of the total fatty acids may be in the free form. Short chain fatty acids are important contributors to cheese aroma, while longer chain acids contribute to taste. Excessive concentrations of either cause off-flavors (rancidity) and the critical concentration is quite low in many varieties, e.g., Cheddar and Gouda. 

The instrumentation became increasingly sophisticated with the addition of provision to heat the column, an automatic titrating device and then the first successful sensitive and universal detector, the gas-density balance, all of which required to be handcrafted [597]. In a companion paper to the description of the detector, the technique was demonstrated with the first separations of methyl ester derivatives of fatty acids [433]. It appears that the initial contact with the problem of the resolution of fatty acids must have inspired James to continue with the study of lipids, and in 1957 he independently published a paper on the determination of the structures of longer-chain fatty acids using gas chromatography and microchemical methods [434]. He subsequently went on to a distinguished career in lipid biochemistry. 

Methyl salicylate, if free from interfering substances, may be determined by the general method for esters by direct saponification with 0-5N ethanolic potash, and back-titration with 0-5N acid, using phenolphthalein as indicator. 1 ml 0-5N == 0-07608 g. 

The total of a-sulfo fatty acid methyl ester and a-sulfo fatty acid can be determined by the standard two-phase titration method (Chapter 16) with benzethonium chloride titrant and either methylene blue or mixed indicator. The titration is carried out at acid pH so that the carboxylic acid groups of unesterified material and free fatty acid are not measured (120,121). 

This reaction forms the basis of a method for the quantitative determination of methyl chloroformate which consists essentially in treating the substance to be tested with aniline solution and titrating the hydrochloric acid formed. In this case it is not possible to determine the carbamic ester formed gravimetrically (as in the phosgene estimation), for it is very soluble in water. ... 

Acetyl and Feruloyl Esters in Pectin. A colorimetric method for determining degrees of acetylation in pectins from various sources ( ), has been shown to be rapid and quite sensitive. Hydroxy-lamine is reactive toward both the methyl and acetyl esters in pectin, and ferric ion complexes with the resulting hydroxamic acids are red. The pectin complex is insoluble and removed by filtration the intensity at 520nm in the soluble fraction, consisting of the ferric complex with acetohydroxamic acid, is a measure of acetyl content. The accuracy of the method was demonstrated in determinations of 0-acetyl levels in standard per-ace-tylated polysaccharides. Another method ( ) involves alkaline hydrolysis of the acetyl groups from pectin, followed by distillation of acetic acid and its titration with standard base. 

A suitable method for determining the anhydride group is titration with aqueous potassium hydroxide in pyridine after previous esterification of the carboxyl group with diazomethane. This esterification is carried out in diethyl ether methanol (9 + 1). After methylation, which takes about 10 minutes for 0.5 g of sample, solvents are removed by evaporation and a portion of the derivatised polymer is dissolved in pyridine and titrated. In the IR spectra of the resin before and after methylation, the absorption band of the acid group at 1710 cm (5.84 pm) disappears and a carbonyl band of the ester at 7104 cm (5.74 pm) is formed. The acid content of the sample is found from the difference in titres of an unmethylated and a methylated product. 

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