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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. It is often used in place of Fehling's solution to detect the presence of
reducing sugar 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 react ...
s and other reducing substances.Collins Edexcel International GCSEBiology, Student Book () p.42-43 Tests that use this reagent are called Benedict's tests. A positive result of Benedict's test is indicated 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
ketose 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 keto ...
s. In example, although the ketose
fructose Fructose (), or fruit sugar, is a Ketose, ketonic monosaccharide, 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 gal ...
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 Glucose is a sugar with the Chemical formula#Molecular formula, molecular formula , which is often abbreviated as Glc. It is overall the most abundant monosaccharide, a subcategory of carbohydrates. It is mainly made by plants and most algae d ...
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). The test is named after American chemist Stanley Rossiter Benedict.


Composition and preparation

Benedict's reagent is a deep-blue aqueous solution. Each litre contains: * 17.3 g copper sulfate * 173 g sodium citrate * 100 g anhydrous sodium carbonate or, equivalently, 270 g sodium carbonate decahydrate 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.


Organic analysis

To test for the presence of
monosaccharide Monosaccharides (from Greek '' monos'': single, '' sacchar'': sugar), also called simple sugars, are the simplest forms of sugar and the most basic units (monomers) from which all carbohydrates are built. Chemically, monosaccharides are polyhy ...
s and reducing
disaccharide A disaccharide (also called a double sugar or ''biose'') is the sugar formed when two monosaccharides are joined by glycosidic linkage. Like monosaccharides, disaccharides are simple sugars soluble in water. Three common examples are sucrose, ...
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. 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 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 . For human consumption, sucrose is extracted and refined ...
(table
sugar Sugar is the generic name for sweet-tasting, soluble carbohydrates, many of which are used in food. Simple sugars, also called monosaccharides, include glucose Glucose is a sugar with the Chemical formula#Molecular formula, molecul ...
) 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 Hydrochloric acid, also known as muriatic acid or spirits of salt, is an aqueous solution of hydrogen chloride (HCl). It is a colorless solution with a distinctive pungency, pungent smell. It is classified as a acid strength, strong acid. It is ...
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 Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution reaction, substitution, elimination reaction, elimination, and solvation reactions in which water ...
. The products of sucrose decomposition are glucose and fructose, both of which can be detected by Benedict's reagent as described above.
Starch Starch or amylum is a polymeric carbohydrate consisting of numerous glucose units joined by glycosidic bonds. This polysaccharide is produced by most green plants for energy storage. Worldwide, it is the most common carbohydrate in human diet ...
es 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 A carbohydrate () is a biomolecule composed of carbon (C), hydrogen (H), and oxygen (O) atoms. The typical hydrogen-to-oxygen atomic ratio is 2:1, analogous to that of water, and is represented by the empirical formula (where ''m'' and ''n'' ...
chains. Other carbohydrates which produce a negative result include inositol. Benedict's reagent can also be used to test for the presence of
glucose Glucose is a sugar with the Chemical formula#Molecular formula, molecular formula , which is often abbreviated as Glc. It is overall the most abundant monosaccharide, a subcategory of carbohydrates. It is mainly made by plants and most algae d ...
in
urine Urine is a liquid by-product of metabolism in humans and many other animals. In placental mammals, urine flows from the Kidney (vertebrates), kidneys through the ureters to the urinary bladder and exits the urethra through the penile meatus (mal ...
, elevated levels of which is known as glucosuria. Glucosuria can be indicative of
diabetes mellitus Diabetes mellitus, commonly known as diabetes, is a group of common endocrine diseases characterized by sustained hyperglycemia, high blood sugar levels. Diabetes is due to either the pancreas not producing enough of the hormone insulin, or th ...
, 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 Ascorbic acid is an organic compound with formula , originally called hexuronic acid. It is a white solid, but impure samples can appear yellowish. It dissolves freely in water to give mildly acidic solutions. It is a mild reducing agent. Asco ...
, drugs ( levodopa, contrast used in radiological procedures) and homogentisic acid (
alkaptonuria Alkaptonuria is a rare inherited genetic disease which is caused by a mutation in the ''HGD'' gene for the enzyme homogentisate 1,2-dioxygenase (); if a person inherits an abnormal copy from both parents (it is a dominance relationship, recessive c ...
) 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.


Quantitative reagent

Benedict's quantitative reagent contains potassium thiocyanate and is used to quantitatively determine the concentration of reducing sugars. This solution forms a copper thiocyanate precipitate which is white and can be used in
titration Titration (also known as titrimetry and volumetric analysis) is a common laboratory method of Quantitative research, quantitative Analytical chemistry, chemical analysis to determine the concentration of an identified analyte (a substance to be ...
. The titration should be repeated with 1% glucose solution instead of the sample for
calibration In measurement technology and metrology, calibration is the comparison of measurement values delivered by a device under test with those of a calibration standard of known accuracy. Such a standard could be another measurement device of known ...
.


Net reaction

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: :.


See also

* Dextrose equivalent Other oxidizing reagents * Fehling's solution * Tollens' reagent Other reducing reagents * Jones reductor * Walden reductor


References

{{Organic reactions Analytical reagents Biochemistry detection methods Carbohydrate methods Chemical tests Coordination complexes Copper compounds Oxidizing agents