Sunday, May 27, 2012

Benedict's solution

Benedict's solution
Benedict's solution, deep-blue alkaline solution used to test for the presence of the aldehyde functional group, −CHO. The substance to be tested is heated with Benedict's solution; formation of a brick-red precipitate indicates presence of the aldehyde group. Since simple sugars (e.g., glucose) give a positive test, the solution is used to test for the presence of glucose in urine, a symptom of diabetes. One liter of Benedict's solution contains 173 grams sodium citrate, 100 grams sodium carbonate, and 17.3 grams cupric sulfate pentahydrate. It reacts chemically like Fehling's solution; the cupric ion (complexed with citrate ions) is reduced to cuprous ion by the aldehyde group (which is oxidized), and precipitates as cuprous oxide, Cu2O.

Sunday, May 20, 2012

Benedict's reagent

Benedict's reagent

Benedict's reagent (also called Benedict's solution or Benedict's test) is a chemical reagent named after an American chemist, Stanley Rossiter Benedict.[1]

Benedict's reagent is used as a test for the presence of reducing sugars. This includes all monosaccharides and the disaccharides mannose, lactose and maltose. Even more generally, Benedict's test will detect the presence of aldehydes (except aromatic ones), 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.[2].

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) sulfate pentahydrate.[3] It is often used in place of Fehling's solution.

Benedict's reagent contains blue copper(II) ions (Cu2+) which are reduced to copper(I) (Cu+). These are precipitated as red copper(I) oxide which is insoluble in water.

Wednesday, May 9, 2012

Chemical test

Chemical test

During a water bath, which is usually 4-10 minutes, the solution should progress in the colors of blue (with no glucose present), green, yellow, orange, red, and then brick red or brown (with high glucose present).[4] A color change would signify the presence of glucose. The common disacharrides 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 with 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. 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. Once a reducing sugar is detected in urine, further tests have to be undergone in order to ascertain which sugar is present. Only glucose is indicative of diabetes.

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.

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
benedict's test for reducing sugars-how-is-brick-red-coloration-formed

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.
what-is-benedicts-test-for-reducing sugar