インドフェニン反応を利用したベンゼン中のチオフェンの改良定量法
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概要
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An improved spectrophotometric method was described for determination of small amounts of thiophene in benzene using the indophenine reaction.<BR>The spectrophotometric measurement of a blue indophenine coloration produced by a reaction of thiophene with isatin in concentrated sulfuric acid solution in the presence of the oxidizing agents such as iron(III)ions or concentrated nitric acid has been widely employed as a rapid and convenient method for determination of thiophene in benzene, although it is less reliable than the gravimetric methods. It was found that the temperature and the time of this reaction should be strictly controlled for good reproducibility to be obtained. The isatin-sulfuric acid solution used in this study, which was heated once at about 100 °C for half an hour when prepared afresh, could be safely used for a period of about 3 weeks.<BR>A part of thiophene seems to react with concentrated sulfuric acid to give a by-product, thiophenesulfonic acid.<BR>An improved procedure is as follows. A stoppered pear-type flask containing 10 m<I>l</I> of isatin-sulfuric acid solution {prepared by adding 0.1 g of isatin and 0.25 g of iron alum to 1000 m<I>l</I> of 92% (by weight) sulfuric acid and then being heated at 100°C for 0.5 h} was fixed to a shaking apparatus in a bath thermostated at (30.6±0.2) °C and allowed to stand until the temperature of the solution became constant. Then 1 m<I>l</I> of benzene sample solution containing thiophene was added from pipet, and after just 30 seconds had passed from the beginning of adding the benzene solution, shaking of the reaction vessel was started and continued for 10 minutes. The blue colored isatin-sulfuric acid solution was transferred to a 10-mm cell, and after 5 minutes had passed the absorbance at 540 nm was, measured against the reagent blank. <BR>The relation between the absorbance and concentration of thiophene was linear within a relative error of less than 1.5% fbr a concentration range of 4.864 μ<I>l</I>/<I>l</I>.
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