Pentosans determination

The researches have dealt with each phase of the pentosan determination. Many modifications have been introduced in the distillation procedure, in attempts to obtain ma.ximal yields of furfural. For e.xample, some workers prefer to distil with 23 % hydrobromic acid, rather than with the conventional 12% hydrochloric acid. Others distil in the presence of added sodium chloride, to avoid changes in acid concentration. Steam distillation has been used by a number of workers, who claim theoretical yields of furfural from pentoses, but Launer and Wilson found no advantage either in salts or in steam in the analysis of pulps and papers. Interfering substances are of two types materials other than pentosans which form furfural in the pentosan analysis, and substances which yield products which may be determined as furfural. TJronic acids and polyuronides yield furfural, although not quantitatively, and, in the case of materials containing appreciable quantities of these substances, it is usual to make a correction. The value of the correction to be applied has been determined experimentally by several workers, with somewhat differing results. 

Norris and Resch determined furfural yields from uronic acids, under the conditions normally used for pentosan determination, and obtained about 37 to 44% of the theoretical yield of furfural. In later work, the effect of various furfural-yielding substances on one another was ascertained, and a standardized procedure was developed for analysis of the types of mixtures commonly met with in cell-wall chemistry. Apparently, the yield of furfural from uronic acids and polyuronides varies with the proportion of these substances present, and with their ratio to other sugars. From oxycellulose, TJnruh and Kenyon obtained furfural yields of 18 to 20 % of the theoretical on the basis of carboxyl content. Wise and Ratliff, however, in summative analysis of wood, used a correction based on the assumption that the uronic acids present evolve 35% of the theoretical amount of furfural. 

N. Durust and M.E. Meyerhoff, Determination of pentosan polysulfate and its binding to polycationic species using polyion-sensitive membrane electrodes. Arm Chim. Acta 432, 253-260 (2001). 

The determination of the structure of a plant gum involves the establishment of its homogeneity, its equivalent weight, and rotation and its uronic acid and pentosan content by the well-known methods. The nature of the constituent sugars and the uronic acid are then determined, after hydrolysis of the gums, by ciystallization or by conversion into characteristic crystalline derivatives. As an example the hexose mannose may be identified as the free sugar or in the form of its anilide, methyl glycoside or phenylhydrazone. Autohydrolysis of the acid... 

Pentosans. This determination is made by Tollens and Kriiger s method, which is based on the transformation of pentosans into furfural by distillation with hydrochloric acid and on subsequent precipitation of the furfural with phloroglucinoL The analysis comprises, therefore, two distinct operations ... 

Pentosan was determined by the method of Dische and Borenfreund (28) on grains grown at Swift Current, 1981... 

The only pentosans which have been studied to any great extent are the xylans. These can be obtained from all lignified cell membranes by extraction with 5% sodium hydroxide solution after lipids, pectic materials etc. have been removed. The conditions necessary to avoid degradation during isolation have been outlined.2 Early structural determinations indicated that arabinose was present as a non-reducing end-group in a branched molecule which had a chain-length of 18-20 D-xylo-furanose residues.27 It has been shown recently, however, that the xylan from esparto holocellulose can be purified free of arabinose after several re-precipitations of the copper-complex.28... 

Volumetric methods for determination of furfural from pentosans in-... 

A number of reagents give, with furfural, colored compounds which are useful for quantitative colorimetry. The reaction with aniline has been studied by Stillings and Browning, who were able to determine furfural at low concentrations even in the presence of appreciable amounts of methylfurfural and hydroxymethylfurfural. For further details on the determination of pentosans, the reader should consult the recent re dew by Browning. ... 

When an acid solution of phloroglucinol (Pt. 11) is added to furfural a black precipitate, known as a phloroglucid, is formed. From the weight of this phloroglucid we. may calculate, by empirical methods, the amount of furfural and, from this, the amount of pentose sugar with which we started, or also, the amount of pentosan.. This is the basis of the analytical method for the determination of pentosan compounds in plant materials. As the pentosans possess considerable nutritive... 

Pentosan Reagent.—With furfural (p. 853) it forms an insoluble greenish black phloroglucid compound. As pentosans (p. 338) yield furfural on boiling with hydrochloric acid, phloroglucinol is used as a reagent for the determination of the furfural obtained and from this the amount of pentosan is calculated empirically. This is the official method for determination of pentosans in food stuffs. 

The analytical determinations most often made on a polyuronide before it is hydrolyzed are those for uronic acid, pentosan, methyl-pentosan, methoxyl and ash. The percentage of hexosan is usually estimated by difference. When the polyuronide has been obtained as a free acid it can in many cases be titrated with a base. Such metals as sodium, potassium, magnesium, calcium and iron often occur and especially so in plant gums and mucilages. 

Pentosan and methylpentosan determinations are described in standard works. When these determinations are made on polyuronides, they are inaccurate and of restricted use in calculating the relative number of pentosan and methyl pentosan units per uronic acid unit. This is especially true when pentoses, methylpentoses and hexoses occur together in a polyuronide. 

Tappi standard T 223 outlines the procedure for pentosan analysis. Briefly, wood meal is boiled in 3.85 N HCl with some NaCl added. Furfural is generated and distilled into a collection flask. The furfural is determined colorimetrically with orcinol-iron(III) chloride reagent. Another method also generates furfural, and the furfural is determined gravimetrically by precipitation with 1,3,5-benzenetriol. These and other methods of pentosan analysis are described and discussed in Browning s book (36). 

With the sealed ampoule process used for their kinetic studies, Root, Saeman, Harris, and Neill [20] achieved fiirfiiral yields well in excess of 70 % at temperatures above 220 °C, whereas industrial furfural processes, operating at lower temperatures and featuring a continuous removal of the furfural by steam stripping, have typical yields below 60 %. By contrast, in analytical chemistry, at a proven yield of 100 % [21], the formation of furfural from xylose or pentosan is routinely used for the quantitative determination of these substances. It is of great importance to elaborate the reasons for this yield paradox . 

The pentosan content is measured by converting the pentosan to furfural, and by then determining the furfural, usually by precipitation with barbituric acid. The procedure is described... 

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