This article needs additional citations for verification .(May 2015) |
![]() Positive Benedict's test | |
Classification | Colorimetric method |
---|---|
Analytes | Reducing sugars |
Benedict's reagent (often called Benedict's qualitative solution or Benedict's solution) is a chemical reagent and complex mixture of sodium carbonate, sodium citrate, and copper(II) sulfate pentahydrate. [1] It is often used in place of Fehling's solution to detect the presence of reducing sugars. The presence of other reducing substances also gives a positive result. [2] Such tests that use this reagent are called the Benedict's tests. A positive test with Benedict's reagent is shown by a color change from clear blue to brick-red with a precipitate.
Generally, Benedict's test detects the presence of aldehyde groups, alpha-hydroxy-ketones, and hemiacetals, including those that occur in certain ketoses. Thus, although the ketose fructose is not strictly a reducing sugar, it is an alpha-hydroxy-ketone which results to a positive test because the base component of Benedict converts it into aldoses glucose and mannose. Oxidizing the reducing sugar by the cupric (Cu2+) complex of the reagent produces a cuprous (Cu+), which precipitates as insoluble red copper(I) oxide (Cu2O). [3]
The test is named after American chemist Stanley Rossiter Benedict. [4]
Benedict's reagent is a deep-blue aqueous solution. Each litre contains: [4]
Separate solutions of the reagents are made. The sodium carbonate and sodium citrate are mixed first, and then the copper sulfate is added slowly with constant stirring.
Sodium citrate acts as a complexing agent which keeps Cu2+ in solution, since it would otherwise precipitate. Sodium carbonate serves to keep the solution alkaline. In the presence of mild reducing agents, the copper(II) ion is reduced to copper(I), which precipitates in the alkaline conditions as very conspicuous red copper(I) oxide.
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 through the colors of blue (with no reducing sugar present), orange, yellow, green, red, and then brick red precipitate or brown (if a high concentration of reducing sugar is present). A color change would signify the presence of a reducing sugar. [2]
Experiment | Observation | Inference |
---|---|---|
Substance in water + 3 mL Benedict's solution, then boil for few minutes and allow to cool. | Red, green, or yellow precipitate is obtained | Reducing sugar, such as glucose, is present |
Substance in water + 3 mL Benedict's solution, then boil for few minutes and allow to cool. | Solution remains clear or is a little blue | Reducing sugar is not present |
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 undergoing isomerization to an aldehyde, or fructose to alpha-hydroxy-ketone form. Sucrose is thus a non-reducing sugar which does not react with Benedict's reagent. However, sucrose indirectly produces a positive result with Benedict's reagent if heated with dilute hydrochloric acid prior to the test, although it is modified during this treatment as 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 because of lesser number of reducing sugar components that occur at the ends of the carbohydrate chains. Other carbohydrates which produce a negative result include inositol.
Benedict's reagent can also be used to test for the presence of glucose in urine, elevated levels of which is known as glucosuria. Glucosuria can be indicative of diabetes mellitus, but Benedict's test is not recommended or used for diagnosis of the aforementioned condition. This is due to the possibility of a reaction in which the presence of other reducing substances such as ascorbic acid, drugs (levodopa, contrast used in radiological procedures) and homogentisic acid (alkaptonuria) creates a false positive.
As color of the obtained precipitate can be used to infer the quantity of sugar present in the solution, 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% concentration; and red indicates 2 g% or higher concentration.
Benedict's quantitative reagent contains potassium thiocyanate and is used to quantitatively determine the concentration of reducing sugars. [2] This solution forms a copper thiocyanate precipitate which is white and can be used in titration. The titration should be repeated with 1% glucose solution instead of the sample for calibration.
The net reaction between an aldehyde (or an alpha-hydroxy-ketone) and the copper(II) ions in Benedict's solution may be written as:
The hydroxide ions in the equation forms when sodium carbonate dissolves in water. With the citrate included, the reaction becomes:
Other oxidizing reagents
Other reducing reagents
A carbohydrate is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1 and thus with the empirical formula Cm(H2O)n, which does not mean the H has covalent bonds with O. However, not all carbohydrates conform to this precise stoichiometric definition, nor are all chemicals that do conform to this definition automatically classified as carbohydrates.
Glucose is a sugar with the molecular formula C6H12O6. Glucose is overall the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, using energy from sunlight, where it is used to make cellulose in cell walls, the most abundant carbohydrate in the world.
In organic chemistry, an aldehyde is an organic compound containing a functional group with the structure R−CH=O. The functional group itself can be referred to as an aldehyde but can also be classified as a formyl group. Aldehydes are a common motif in many chemicals important in technology and biology.
Fructose, or fruit sugar, is a ketonic simple sugar found in many plants, where it is often bonded to glucose to form the disaccharide sucrose. It is one of the three dietary monosaccharides, along with glucose and galactose, that are absorbed by the gut directly into the blood of the portal vein during digestion. The liver then converts both fructose and galactose into glucose, so that dissolved glucose, known as blood sugar, is the only monosaccharide present in circulating blood.
Sucrose, a disaccharide, is a sugar composed of glucose and fructose subunits. It is produced naturally in plants and is the main constituent of white sugar. It has the molecular formula C
12H
22O
11.
In organic chemistry, Fehling's solution is a chemical reagent used to differentiate between water-soluble carbohydrate and ketone functional groups, and as a test for reducing sugars and non-reducing sugars, supplementary to the Tollens' reagent test. The test was developed by German chemist Hermann von Fehling in 1849.
In organic chemistry, a ketose is a monosaccharide containing one ketone group per molecule. The simplest ketose is dihydroxyacetone, which has only three carbon atoms. It is the only ketose with no optical activity. All monosaccharide ketoses are reducing sugars, because they can tautomerize into aldoses via an enediol intermediate, and the resulting aldehyde group can be oxidised, for example in the Tollens' test or Benedict's test. Ketoses that are bound into glycosides, for example in the case of the fructose moiety of sucrose, are nonreducing sugars.
A reducing sugar is any sugar that is capable of acting as a reducing agent. In an alkaline solution, a reducing sugar forms some aldehyde or ketone, which allows it to act as a reducing agent, for example in Benedict's reagent. In such a reaction, the sugar becomes a carboxylic acid.
Barfoed's test is a chemical test used for detecting the presence of monosaccharides. It is based on the reduction of copper(II) acetate to copper(I) oxide (Cu2O), which forms a brick-red precipitate.
Classical qualitative inorganic analysis is a method of analytical chemistry which seeks to find the elemental composition of inorganic compounds. It is mainly focused on detecting ions in an aqueous solution, therefore materials in other forms may need to be brought to this state before using standard methods. The solution is then treated with various reagents to test for reactions characteristic of certain ions, which may cause color change, precipitation and other visible changes.
Sucrases are digestive enzymes that catalyze the hydrolysis of sucrose to its component monosaccharides, fructose and glucose. One form, sucrase-isomaltase, is secreted in the small intestine on the brush border. The enzyme invertase, which occurs more commonly in plants, fungi and bacteria, also hydrolyzes sucrose but by a different mechanism: it is a fructosidase, whereas sucrase is a glucosidase.
Tollens' reagent is a chemical reagent used to distinguish between aldehydes and ketones along with some alpha-hydroxy ketones which can tautomerize into aldehydes. The reagent consists of a solution of silver nitrate, ammonium hydroxide and some sodium hydroxide. It was named after its discoverer, the German chemist Bernhard Tollens. A positive test with Tollens' reagent is indicated by the precipitation of elemental silver, often producing a characteristic "silver mirror" on the inner surface of the reaction vessel.
In chemistry, a chemical test is a qualitative or quantitative procedure designed to identify, quantify, or characterise a chemical compound or chemical group.
The Cannizzaro reaction, named after its discoverer Stanislao Cannizzaro, is a chemical reaction which involves the base-induced disproportionation of two molecules of a non-enolizable aldehyde to give a primary alcohol and a carboxylic acid.
The Kiliani–Fischer synthesis, named for German chemists Heinrich Kiliani and Emil Fischer, is a method for synthesizing monosaccharides. It proceeds via synthesis and hydrolysis of a cyanohydrin, followed by reduction of the intermediate acid to the aldehyde, thus elongating the carbon chain of an aldose by one carbon atom while preserving stereochemistry on all the previously present chiral carbons. The new chiral carbon is produced with both stereochemistries, so the product of a Kiliani–Fischer synthesis is a mixture of two diastereomeric sugars, called epimers. For example, D-arabinose is converted to a mixture of D-glucose and D-mannose.
Seliwanoff’s test is a chemical test which distinguishes between aldose and ketose sugars. If the sugar contains a ketone group, it is a ketose. If a sugar contains an aldehyde group, it is an aldose. This test relies on the principle that, when heated, ketoses are more rapidly dehydrated than aldoses. It is named after Theodor Seliwanoff, the chemist who devised the test. When added to a solution containing ketoses, a red color is formed rapidly indicating a positive test. When added to a solution containing aldoses, a slower forming light pink is observed instead.
Glucose-galactose malabsorption is a rare condition in which the cells lining the intestine cannot take in the sugars glucose and galactose, which prevents proper digestion of these molecules and larger molecules made from them.
The Walden reductor is a reduction column filled with metallic silver which can be used to reduce a metal ion in aqueous solution to a lower oxidation state. It can be used e.g. to reduce UO22+ to U4+. The method is named after George H. Walden, who developed it jointly with a Ph.D. student, Sylvan M. Edmonds, at Columbia University.
Copper forms a rich variety of compounds, usually with oxidation states +1 and +2, which are often called cuprous and cupric, respectively. Copper compounds, whether organic complexes or organometallics, promote or catalyse numerous chemical and biological processes.