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BENEDICT'S TEST FOR REDUCING SUGAR

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Sunday, October 11, 2009

What chemical solution is the best to use for testing sugar in food plant?

What chemical solution is the best to use for testing sugar in food plant?

The standard chemical test for sugar is Benedict's test. The reagent, a mixture of (mainly) copper sulfate and sodium hydroxide, is called Benedict's reagent. It can be purchased from many drug stores because it was once the standard test for sugar in the urine of diabetics.

Some of the solution to be tested (this could be the juice or extract of the fruit or vegetable in question) is mixed with Benedict's test reagent (by volume, usually about 4 solution to 1 reagent) and heated almost to boiling. A color change from the blue of the reagent to almost any other color -- green, yellow, orange, red, brown -- is an indication of the presence of what are called "reducing" sugars. For practical purposes, reducing sugars are monosaccharides (simple sugars) like glucose and fructose. It is important to note the most disaccharides (like sucrose, which is the sugar most often used at the table) will not give a positive Benedict's reaction.

The color sequence given -- green, yellow, orange, red, brown -- is in order of increasing concentration of Benedict's test for reducing sugar in the test solution.
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The reaction between Benedict's solution and reducing sugars?













The reaction between Benedict's solution and reducing sugars?
Benedict's Test For Reducing Sugars
Fehling's solution versus Benedict's. They are chemically very similar both being alkaline solutions of copper(II) sulphate solution. They are made by having copper(II) sulphate solution, sodium hydroxide solution and a complexing agent (a ligand). The latter is tartrate ions (2,3-dihydroxybutanedioate ions) in Fehling's but there is no information what it is for Benedict's. It is different but acts the same way. It will be the anion of a carboxylic acid and will act as a ligand for copper(II) ions like tartrate. The ligand forms a complex with the copper(II) ions and stops the copper(II) ion from precipitating with the hydroxide ions as copper(II) hydroxide. This would not be a very good oxidising agent. The trick is that the copper(II) ions have to be alkaline to be oxidising but that they must be in solution not in a precipitate. The tartrate ions complex to the copper(II) ions and protect it from precipitating with the hydroxide ions.

Now the redox reaction can be carried out. The reaction works as aldehydes and reducing sugars are easily oxidised. i.e they like to lose electrons. Aldehydes are oxidised to carboxylic acids. Fehling's and Benedict's are weak oxidising agents and can do the job. The copper(II) ion gains an electron to form copper(I) in the form of copper(I) oxide, Cu2O.

2Cu2+ + H2O + 2e- = Cu2O + 2H+

The hydroxide ions remove the H+ ions and help the forward reaction to go better and become more electron accepting.

The reducing sugar e.g glucose, gets oxidised because it exists in aqueous solution in three different forms (two cyclic and one non-cyclic). The non-cyclic is the least common but it is present in small amounts. This form has an aldehydic group at the end of the chain and is the group that is oxidised. As the aldehyde form gets oxidised, more of the non-cyclic form is produced due to Le Chatelier's principle.
Benedict's Test for Reducing Sugar
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Benedict's test for the presence of a reducing sugar

Benedict's test for the presence of a reducing sugar

Benedict's reagent and Fehling's solution are used to test for the presence of a reducing sugar. The reducing sugar reduces copper(II) ions in these test solutions to copper(I), which then forms a brick red copper(I) oxide precipitate. 3,5-Dinitrosalicylic acid is another test reagent that allows quantitative spectrophotometric measurement of the amount of reducing sugar present.

Sugars having acetal or ketal linkages are not reducing sugars, as they do not form free aldehyde chains. They therefore do not react with any of the reducing-sugar test solutions. However, a non-reducing sugar can be hydrolysed using dilute hydrochloric acid to convert the acetal or ketal into a hemiacetal or hemiketal. After hydrolysis and neutralization of the acid, the product may be a reducing sugar that gives normal reactions with the test solutions.

All carbohydrates respond positively to Molisch's reagent.
Benedict's test for reducing sugars
Benedicts test for reducing sugars-how-is-brick-red-coloration-formed

How is the brick-red coloration formed by Benedict's reagent in the prescence of reducing sugars?

How is the brick-red coloration formed by Benedict's reagent in the prescence of reducing sugars?
One of the components of Benedict's reagent is copper(II) oxide.when it is placed in solution with a reducing sugar, the copper(II) oxide is reduced to copper(I) oxide ions. this has a brick red colour. if there a small or large amount of the reducing sugar present, the color would range from green to brick red respectively.
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Benedict's Solution, a Reagent for Measuring Reducing Sugars: the Clinical Chemistry of Stanley R. Benedict

Benedict's Solution, a Reagent for Measuring Reducing Sugars: the Clinical Chemistry of Stanley R. Benedict
Stanley Rossiter Benedict was born in Cincinnati in 1884. While a student at the University of Cincinnati he worked with J. F. Snell, and together they published nine papers describing new analytical methods in inorganic chemistry. This research experience as a college student provided the intellectual foundation for his career. After a mistaken year in medical school at Cincinnati, he went to Yale, to the Department of Physiological Chemistry, to study with Russell Chittenden and Lafayette Mendel where he received training in metabolism and physiology. He received his Ph.D. in 1908, 2 years after entering graduate school. (Current students take note.) In 1910, he became Professor of Chemistry at Cornell University Medical College, the position he held until his death in 1936 at the age of 52 (1).

In a biographical review of Benedict's career, E. V. McCollum wrote,“ It is not possible to give an accurate account of the scientific work of Stanley Benedict without at the same time discussing the parallel researches of Otto Folin... they succeeded, through many years of intensive investigations, in devising and refining analytical procedures for determination of minute amounts of the principal non-protein constituents of blood and urine so that, for the first time, chemical analysis became a highly useful technic (sic) for the discovery of the chemical processes in the normal functioning of the body” (1).

Of Benedict's relationship with Folin, Shaffer wrote, “Both excelled in designing very clever analytical methods of the widest usefulness, and in using these tools with rare success for the discovery of new facts about metabolism. In spite of seventeen years difference in their age (Folin was the older), of the rivalry and controversy sometimes evident in their papers, there early developed between them a warm friendship which reveals the fine character of both. They were kindred spirits” (2). (We will present a classic paper by Otto Folin in a subsequent installment of JBC Classics, stay tuned.)

As McCollum and Shaffer described, Benedict's major contributions to biochemistry were in devising analytical methods. Although he published many papers in the Journal of Biological Chemistry (JBC), the paper reprinted here seemed appropriate to characterize a distinguished career. It had been known for many years that the common sugars had carbonyl groups and were therefore, “reducing sugars.” That is, they were oxidized by a variety of metal ions, Ag+, Fe3+, and Cu2+. Treatment with hot alkali fragments the sugars, and the resulting products reduce Cu2+ to Cu+ with the formation of a precipitate of Cu2O. As noted in the paper, Benedict's goal was to improve this general method to make the reagent less corrosive and more stable. He accomplished this by substituting carbonate for hydroxide as the alkali component, to reduce the corrosiveness, and by substituting citrate for tartrate as the agent to chelate the Cu2+, to make the reagent more stable.

Benedict's Solution, or one of the many variants that evolved over the years, was used as the reagent of choice for measuring sugar content for more than 50 years. It was the most common test for diabetes and was the standard procedure for virtually all clinical laboratories. Saul Roseman remembers that all inductees into the army during World War II had their urine tested for sugar with Benedict's Solution.1 Although Benedict's assay was the method of choice for more than 50 years, it suffered from lack of sugar specificity and was eventually supplanted by the use of enzymatic methods such as glucose oxidase.
Benedict's test for reducing sugars....
what-is-benedicts-test-for-reducing sugar ?

What is the Benedict's test for reducing sugars?

What is the Benedict's test for reducing sugars?

A commonly used test to detect carbohydrates is the Benedict's test for reducing sugars such as glucose and fructose. Benedict's reagent, containing sodium bicarbonate, sodium citrate, and copper sulfate, is added to a solution and heated.
The Benedict's test identifies reducing sugars based on their ability to reduce the cupric ions to cuprous oxide at high pH values (basic solutions). Cuprous oxide is green to reddish orange. A green solution indicates a small amount of reducing sugars, while a reddish-orange solution indicates an abundance of reducing sugars. If the solution contains sucrose, a nonreducing sugar, there is no change in color in the solution, and it remains blue.
Benedict's test for reducing sugars....
what-chemical-solution-is-best-to-use

BENEDICT'S TEST FOR REDUCING SUGAR

BENEDICT'S TEST FOR REDUCING SUGAR

Voiceover
Benedict's reagent is a solution of copper sulfate, sodium hydroxide, and tartaric acid. Aqueous glucose is mixed with Benedict's reagent and heated. The reaction reduces the blue copper (II) ion to form a brick red precipitate of copper (I) oxide. Because of this, glucose is classified as a reducing sugar.

Discussion
Aqueous glucose is mixed with Benedict's reagent, a solution of copper sulfate, sodium hydroxide, and tartaric acid. The mixture is heated. Carbohydrates which react with Benedict's reagent to reduce the blue copper (II) ion to form a brick red precipitate of copper (I) oxide are classified as reducing sugars.
benedict's test for reducing sugar
what-is-benedicts-test-for-reducing sugars ?

BENEDICT'S TEST FOR REDUCING SUGAR

BENEDICT'S TEST FOR REDUCING SUGAR

Voiceover
Benedict's reagent is a solution of copper sulfate, sodium hydroxide, and tartaric acid. Aqueous glucose is mixed with Benedict's reagent and heated. The reaction reduces the blue copper (II) ion to form a brick red precipitate of copper (I) oxide. Because of this, glucose is classified as a reducing sugar.

Discussion
Aqueous glucose is mixed with Benedict's reagent, a solution of copper sulfate, sodium hydroxide, and tartaric acid. The mixture is heated. Carbohydrates which react with Benedict's reagent to reduce the blue copper (II) ion to form a brick red precipitate of copper (I) oxide are classified as reducing sugars.
benedict's test for reducing sugar