These include monosaccharides like glucose and fructose and disaccharides like lactose and maltose []. It is a bright blue solution prepared by mixing copper sulfate pentahydrate CuSO 4. The presence of the alkaline sodium carbonate converts the sugar into a strong reducing agent called enediols. During the reduction reaction, the mixture will change its color from blue to brick-red precipitate due to the formation of cuprous oxide Cu 2 O.
The red-colored cuprous oxide is insoluble in water and hence, separated. If the concentration of the sugar is high, then the color becomes more reddish, and the volume of the precipitate increases [5]. The color of the mixture will change accordingly resulting in precipitates, which indicates positive results.
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Search for: 'Benedict's test' in Oxford Reference ». All Rights Reserved. Under the terms of the licence agreement, an individual user may print out a PDF of a single entry from a reference work in OR for personal use for details see Privacy Policy and Legal Notice. Oxford Reference. Class Benedict's Reagent Results Aside from our controls, we tested three solutions for glucose: starch, acid-treated starch, and amylase-treated starch.
As starch is a polysaccharide, it is unsurprising that the starch solution tested negative for simple sugars. We mixed HCl an acid into starch and re-tested for simple sugars.
First, we had to adjust the pH of the solutions back to neutral before adding the Benedict's reagent. We used a pH indicator and NaOH a base for this. We then added the Benedict's reagent. We got moderately positive results orangish color. This is because HCl breaks starch back down into its component monosaccharides glucose, in this case.
Amylase is an enzyme that removes glucose molecules from starch. Both plants and animals use amylase when digesting starch. Unfortunately, amylase cannot break the beta-bonds which hold the glucose molecules together in cellulose. If it could, we'd be able to eat hay. Based on this information, can you figure out what our results should be if we tested amylase-treated starch and amylase-treated cellulose solutions for reducing sugars?
Some sugars such as glucose are called reducing sugars because they are capable of transferring hydrogens electrons to other compounds, a process called reduction. When reducing sugars are mixed with Benedicts reagent and heated, a reduction reaction causes the Benedicts reagent to change color. The color varies from green to dark red brick or rusty-brown, depending on the amount of and type of sugar.
This solution forms a copper thiocyanate precipitate which is white and can be used in a titration. This reaction is caused by the reducing property of simple carbohydrates. The red copper I oxide formed is insoluble in water and is precipitated out of solution. This accounts for the precipitate formed. As the concentration of reducing sugar increases, the nearer the final colour is to brick-red and the greater the precipitate formed.
Sometimes a brick red solid, copper oxide, precipitates out of the solution and collects at the bottom of the test tube. Sodium carbonate provides the alkaline conditions which are required for the redox reaction.
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