Benedict's reagent
Encyclopedia
Benedict's reagent is a chemical reagent
named after an American chemist, Stanley Rossiter Benedict
.
Benedict's reagent is used as a test for the presence of reducing sugar
s. This includes all monosaccharide
s and the disaccharide
s, lactose
and maltose
. Even more generally, Benedict's test will detect the presence of aldehyde
s (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.
The copper sulphate in Benedict's solution reacts with reducing sugars.
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. 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) ions (Cu+). These are precipitated as red copper(I) oxide
which is insoluble in water.
, which is usually 4–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.
Starch
es 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 found to be present in urine is an indication of diabetes mellitus
. 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.
reagent is used to determine how much reducing sugar is present. This solution forms as white precipitate rather than a red one and so can be used in a titration
. The titration should be repeated with 1% glucose solution instead of the sample for calibration
Other reducing reagents (opposite)
Reagent
A reagent is a "substance or compound that is added to a system in order to bring about a chemical reaction, or added to see if a reaction occurs." Although the terms reactant and reagent are often used interchangeably, a reactant is less specifically a "substance that is consumed in the course of...
named after an American chemist, Stanley Rossiter Benedict
Stanley Rossiter Benedict
Stanley Rossiter Benedict is an American chemist best known for discovering Benedict's reagent, a solution that detects certain sugars....
.
Benedict's reagent is used as a test for the presence of reducing sugar
Reducing sugar
A reducing sugar is any sugar that either has an aldehyde group or is capable of forming one in solution through isomerisation. This functional group allows the sugar to act as a reducing agent, for example in the Tollens' test or Benedict's test.-Chemistry:...
s. This includes all monosaccharide
Monosaccharide
Monosaccharides are the most basic units of biologically important carbohydrates. They are the simplest form of sugar and are usually colorless, water-soluble, crystalline solids. Some monosaccharides have a sweet taste. Examples of monosaccharides include glucose , fructose , galactose, xylose...
s and the disaccharide
Disaccharide
A disaccharide or biose is the carbohydrate formed when two monosaccharides undergo a condensation reaction which involves the elimination of a small molecule, such as water, from the functional groups only. Like monosaccharides, disaccharides form an aqueous solution when dissolved in water...
s, lactose
Lactose
Lactose is a disaccharide sugar that is found most notably in milk and is formed from galactose and glucose. Lactose makes up around 2~8% of milk , although the amount varies among species and individuals. It is extracted from sweet or sour whey. The name comes from or , the Latin word for milk,...
and maltose
Maltose
Maltose , or malt sugar, is a disaccharide formed from two units of glucose joined with an αbond, formed from a condensation reaction. The isomer "isomaltose" has two glucose molecules linked through an α bond. Maltose is the second member of an important biochemical series of glucose chains....
. Even more generally, Benedict's test will detect the presence of aldehyde
Aldehyde
An aldehyde is an organic compound containing a formyl group. This functional group, with the structure R-CHO, consists of a carbonyl center bonded to hydrogen and an R group....
s (except aromatic ones), and alpha-hydroxy-ketones
Hydroxy ketone
A hydroxy ketone in organic chemistry is a functional group consisting of a ketone flanked by a hydroxyl group. In the two main classes the hydroxyl group can be placed in the alpha position or in the beta position .* An alpha-hydroxy ketone is called an acyloin* Prominent beta-hydroxy ketones...
, including those that occur in certain ketoses. Thus, although the ketose fructose
Fructose
Fructose, or fruit sugar, is a simple monosaccharide found in many plants. It is one of the three dietary monosaccharides, along with glucose and galactose, that are absorbed directly into the bloodstream during digestion. Fructose was discovered by French chemist Augustin-Pierre Dubrunfaut in 1847...
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
Glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...
and mannose
Mannose
Mannose is a sugar monomer of the aldohexose series of carbohydrates. Mannose is a C-2 epimer of glucose. It is not part of human metabolism, but is a component of microbial cell walls, and is therefore a target of the immune system and also of antibiotics....
by the base in the reagent.
The copper sulphate in Benedict's solution reacts with reducing sugars.
One litre of Benedict's reagent can be prepared from 100 g of anhydrous sodium carbonate
Sodium carbonate
Sodium carbonate , Na2CO3 is a sodium salt of carbonic acid. It most commonly occurs as a crystalline heptahydrate, which readily effloresces to form a white powder, the monohydrate. Sodium carbonate is domestically well-known for its everyday use as a water softener. It can be extracted from the...
, 173 g of sodium citrate
Sodium citrate
Trisodium citrate has the chemical formula of Na3C6H5O7. It is sometimes referred to simply as sodium citrate, though sodium citrate can refer to any of the three sodium salts of citric acid. It possesses a saline, mildly tart flavor. For this reason, citrates of certain alkaline and alkaline earth...
and 17.3 g of copper(II) sulfate pentahydrate. It is often used in place of Fehling's solution
Fehling's solution
Fehling's solution is a chemical test used to differentiate between water-soluble aldehyde and ketone functional groups, and as a test for monosaccharides. The test was developed by German chemist Hermann von Fehling in 1849.-Laboratory preparation:...
.
Benedict's reagent contains blue copper(II) ions (Cu2+) which are reduced to copper(I) ions (Cu+). These are precipitated as red copper(I) oxide
Copper(I) oxide
Copper oxide or cuprous oxide is the inorganic compound with the formula Cu2O. It is one of the principal oxides of copper. This red-coloured solid is a component of some antifouling paints. The compound can appear either yellow or red, depending on the size of the particles, but both forms...
which is insoluble in water.
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 bathHeated bath
A heated bath is used in the laboratory to allow a chemical reaction to occur at an elevated temperature.The heated bath is a fluid placed in an open pot. Water and silicone oil are the most commonly used fluids. A water bath is used for temperatures up to 100 °C...
, which is usually 4–moiety, after isomerization.
Sucrose
Sucrose
Sucrose is the organic compound commonly known as table sugar and sometimes called saccharose. A white, odorless, crystalline powder with a sweet taste, it is best known for its role in human nutrition. The molecule is a disaccharide composed of glucose and fructose with the molecular formula...
(table sugar
Sugar
Sugar is a class of edible crystalline carbohydrates, mainly sucrose, lactose, and fructose, characterized by a sweet flavor.Sucrose in its refined form primarily comes from sugar cane and sugar beet...
) contains two sugars (fructose and glucose) joined by their glycosidic bond
Glycosidic bond
In chemistry, a glycosidic bond is a type of covalent bond that joins a carbohydrate molecule to another group, which may or may not be another carbohydrate....
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
Glycosidic bond
In chemistry, a glycosidic bond is a type of covalent bond that joins a carbohydrate molecule to another group, which may or may not be another carbohydrate....
in sucrose through hydrolysis
Hydrolysis
Hydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...
. The products of sucrose decomposition are glucose and fructose, both of which can be detected by Benedict's reagent, as described above.
Starch
Starch
Starch or amylum is a carbohydrate consisting of a large number of glucose units joined together by glycosidic bonds. This polysaccharide is produced by all green plants as an energy store...
es 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
Carbohydrate
A carbohydrate is an organic compound with the empirical formula ; that is, consists only of carbon, hydrogen, and oxygen, with a hydrogen:oxygen atom ratio of 2:1 . However, there are exceptions to this. One common example would be deoxyribose, a component of DNA, which has the empirical...
chains. Inositol
Inositol
Inositol or cyclohexane-1,2,3,4,5,6-hexol is a chemical compound with formula 6126 or 6, a sixfold alcohol of cyclohexane. It exists in nine possible stereoisomers, of which the most prominent form, widely occurring in nature, is cis-1,2,3,5-trans-4,6-cyclohexanehexol, or myo-inositol...
(myo-inositol) is another carbohydrate which produces a negative test.
Benedict's reagent can be used to test for the presence of glucose
Glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...
in urine
Urine
Urine is a typically sterile liquid by-product of the body that is secreted by the kidneys through a process called urination and excreted through the urethra. Cellular metabolism generates numerous by-products, many rich in nitrogen, that require elimination from the bloodstream...
. Glucose found to be present in urine is an indication of diabetes mellitus
Diabetes mellitus
Diabetes mellitus, often simply referred to as diabetes, is a group of metabolic diseases in which a person has high blood sugar, either because the body does not produce enough insulin, or because cells do not respond to the insulin that is produced...
. 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.
| Experiment | Observation | Inference |
|---|---|---|
| Substance in water + 3ml Benedict's solution, then boil for few minutes and allow to cool. | Red ppt or green ppt or yellow ppt obtained | Reducing sugar e.g. Glucose is present |
| Substance in water + 3ml Benedict's solution, then boil for few minutes and allow to cool. | Solution remains clear | Reducing sugar is not present |
Quantitative reagent
Benedict's quantitativeQuantitative analysis (chemistry)
In chemistry, quantitative analysis is the determination of the absolute or relative abundance of one, several or all particular substance present in a sample....
reagent is used to determine how much reducing sugar is present. This solution forms as white precipitate rather than a red one and so can be used in a titration
Titration
Titration, also known as titrimetry, is a common laboratory method of quantitative chemical analysis that is used to determine the unknown concentration of an identified analyte. Because volume measurements play a key role in titration, it is also known as volumetric analysis. A reagent, called the...
. The titration should be repeated with 1% glucose solution instead of the sample for calibration
Calibration
Calibration is a comparison between measurements – one of known magnitude or correctness made or set with one device and another measurement made in as similar a way as possible with a second device....
See also
Other oxidizing reagents- Dextrose equivalentDextrose equivalentDextrose equivalent is a measure of the amount of reducing sugars present in a sugar product, relative to glucose, expressed as a percentage on a dry basis. For example, a maltodextrin with a DE of 10 would have 10% of the reducing power of dextrose , while sucrose, with a DE of 120, would have...
- Tollens' reagentTollens' reagentTollens' reagent is a chemical reagent most commonly used to determine whether a known carbonyl-containing compound is an aldehyde or a ketone. It is usually ammoniacal silver nitrate, but can also be other mixtures, as long as aqueous diamminesilver complex is present...
- Fehling's solutionFehling's solutionFehling's solution is a chemical test used to differentiate between water-soluble aldehyde and ketone functional groups, and as a test for monosaccharides. The test was developed by German chemist Hermann von Fehling in 1849.-Laboratory preparation:...
Other reducing reagents (opposite)
- Jones reductorJones reductorA Jones reductor is a device which can be used to reduce a metal ion in aqueous solution to a very low oxidation state. The active component is a zinc/mercury amalgam...
- Walden reductorWalden reductorThe Walden reductor is a reduction column filled with metallic silverwhich 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+ in U4+. The method is named after George H. Walden who developed together with a Ph.D. student Sylvan M...