Titration method, methyl ester

Extraction and Identification. Identification of PE as the clearing enzyme in citrus juices progressed rapidly after MacDonnell et al. (17) reported on cation requirement for extraction and solubilization of the enzyme from various portions of the fruit. PE was assayed by the method introduced by Kertesz (18) and modified by Lineweaver and Ballou (19). The method involved measuring the rate at which the methyl ester groups in the pectin molecule are hydrolyzed by titrating the free carboxyl groups with 0.1N NaOH as they are formed. One unit of PE was defined as the amount of enzyme which will hydrolyze 1 meq carboxyl groups per min from a 0.5% solution of pectin in 0.15M NaCl at pH 7.5, 30°C (86°F). McDonnell et al. (17) showed that the enzyme... 

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. 

Non-aqueous titration of methyl salicylate using lithium methoxide with quinaldine red as indicator or with tetrabutylammonium hydroxide and a potentiometric finish (see p. 794) has been applied by Allen. This method is applicable to the ester itself and to the liniment, but with ointments the end-point tends to be somewhat sluggish although probably adequate for routine laboratory control. 

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). 

A manual titrimetric method using methyl orange as indicator and sodium tetraphenylboron as titrant has been described [273]. Dodecyl sulfate at concentrations of approximately 10"5 M have been analyzed by a visual two-phase titration in the presence of dichloroethane and tetrabromophenolphthalein ethyl ester [274]. 

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. ... 

Roberts and Whiting developed a method for effecting the anhydrous hydrolysis of esters and nitriles in DMSO. A solution of sodium methylsulfinylmethide is prepared from sodium hydride and DMSO and titrated with a solution of an appropriate amount of water in DMSO, using triphenylmethane as indicator. This produces a fine suspension of sodium hydroxide which hydrolyzes ethyl benzoate very rapidly at room temperature. As compared with reactions in hydroxylic solvents, rates are enhanced by a factor of 10 -10 . Benzonitrile is hydrolyzed to benzamide. Methyl and ethyl mesitoates are hydrolyzed readily at 25°. 

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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