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Benedict's reagent (often sold as ''Benedict's Qualitative Solution'' () or ''Benedict's Solution'') is a chemical reagent named after an American chemist, Stanley Rossiter Benedict.〔 〕 Benedict's reagent is a chemical reagent commonly used to detect presence of reducing sugar(s), however other reducing substances also give a positive reaction.〔Collins Edexcel International GCSE Biology, Student Book (ISBN-13 978 0 00 745000 8) p.42-43〕 This includes all monosaccharides and many disaccharides, including lactose and maltose. Generally, Benedict's test will detect the presence of aldehydes, and alpha-hydroxy-ketones, including those that occur in certain ketoses. Thus, although the ketose fructose is not strictly a reducing sugar, it is an alpha-hydroxy-ketone, and gives a positive test because it is converted to the aldoses glucose and mannose by the base in the reagent. A positive test with Benedict's reagent is shown by a colour change from clear blue to a brick-red precipitate. One litre of Benedict's reagent can be prepared from 100 g of anhydrous sodium carbonate, 173 g of sodium citrate and 17.3 g of copper(II) sulphate pentahydrate. It is often used in place of Fehling's solution. The principle of Benedict's test is that when reducing sugars are heated in the presence of an alkali, they get converted to powerful reducing compounds known as enediols. Enediols reduce the cupric ions (Cu2+) present in the Benedict's reagent to cuprous ions (Cu+) which get precipitated as insoluble red copper(I) oxide(Cu2O). The color of the obtained precipitate gives an idea about the quantity of sugar present in the solution, hence the test is semi-quantitative. A greenish precipitate indicates about 0.5 g% concentration; yellow precipitate indicates 1 g% concentration; orange indicates 1.5 g% and red indicates 2 g% or higher concentration. ==Chemical test== To test for the presence of monosaccharides and reducing disaccharide sugars in food, the food sample is dissolved in water, and a small amount of Benedict's reagent is added. During a water bath, which is usually 4–10 minutes, the solution should progress in the colors of blue (with no reducing sugar present), green, yellow, orange, red, and then brick red or brown (with high reducing sugar present). A colour change would signify the presence of a reducing sugar.〔 The common disaccharides lactose and maltose are directly detected by Benedict's reagent, because each contains a glucose with a free reducing aldehyde moiety, after isomerization. Sucrose (table sugar) contains two sugars (fructose and glucose) joined by their glycosidic bond in such a way as to prevent the glucose isomerizing to aldehyde, or the fructose to alpha-hydroxy-ketone form. Sucrose is thus a non-reducing sugar which does not react with Benedict's reagent. Sucrose indirectly produces a positive result with Benedict's reagent if heated with dilute hydrochloric acid prior to the test, although after this treatment it is no longer sucrose. The acidic conditions and heat break the glycosidic bond in sucrose through hydrolysis. The products of sucrose decomposition are glucose and fructose, both of which can be detected by Benedict's reagent, as described above. Starches do not react or react very poorly with Benedict's reagent, due to the relatively small number of reducing sugar moieties, which occur only at the ends of carbohydrate chains. Inositol (myo-inositol) is another carbohydrate which produces a negative test. Benedict's reagent can be used to test for the presence of glucose in urine. Glucose in urine is called glucosuria and can be indicative of diabetes mellitus, but the test is not recommended or used for the diagnosis of diabetes mellitus. False positive reaction can be due to the presence of other reducing substances in urine such as ascorbic acid (during intake of vitamin supplements), drugs (levodopa, contrast used in radiological procedures) and homogentisic acid (alkaptonuria). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Benedict's reagent」の詳細全文を読む スポンサード リンク
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