Fehling's solution
Overview
 
Fehling's solution is a chemical test
Chemical test
In chemistry, a chemical test is a qualitative or quantitative procedure designed to prove the existence of, or to quantify, a chemical compound or chemical group with the aid of a specific reagent...

 used to differentiate between water-soluble 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....

 and ketone
Ketone
In organic chemistry, a ketone is an organic compound with the structure RCR', where R and R' can be a variety of atoms and groups of atoms. It features a carbonyl group bonded to two other carbon atoms. Many ketones are known and many are of great importance in industry and in biology...

 functional group
Functional group
In organic chemistry, functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. The same functional group will undergo the same or similar chemical reaction regardless of the size of the molecule it is a part of...

s, and as a test for monosaccharides. The test was developed by German chemist Hermann von Fehling
Hermann von Fehling
Hermann von Fehling was a German chemist, famous as the developer of Fehling's solution used for estimation of sugar.-Biography:...

 in 1849.
Fehling's solution is always prepared fresh in the laboratory. It is made initially as two separate solutions, known as Fehling's A and Fehling's B. Fehling's A is a blue aqueous solution of copper(II) sulfate, while Fehling's B is a clear solution of aqueous potassium sodium tartrate
Potassium sodium tartrate
Potassium sodium tartrate is a double salt first prepared by an apothecary, Pierre Seignette, of La Rochelle, France. As a result the salt was known as Seignette's salt or Rochelle salt....

 (also known as Rochelle salt) and a strong alkali (commonly sodium hydroxide).

Equal volumes of the two mixtures are mixed together to get the final Fehling's solution, which is a deep blue colour.
Encyclopedia
Fehling's solution is a chemical test
Chemical test
In chemistry, a chemical test is a qualitative or quantitative procedure designed to prove the existence of, or to quantify, a chemical compound or chemical group with the aid of a specific reagent...

 used to differentiate between water-soluble 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....

 and ketone
Ketone
In organic chemistry, a ketone is an organic compound with the structure RCR', where R and R' can be a variety of atoms and groups of atoms. It features a carbonyl group bonded to two other carbon atoms. Many ketones are known and many are of great importance in industry and in biology...

 functional group
Functional group
In organic chemistry, functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. The same functional group will undergo the same or similar chemical reaction regardless of the size of the molecule it is a part of...

s, and as a test for monosaccharides. The test was developed by German chemist Hermann von Fehling
Hermann von Fehling
Hermann von Fehling was a German chemist, famous as the developer of Fehling's solution used for estimation of sugar.-Biography:...

 in 1849.

Laboratory preparation

Fehling's solution is always prepared fresh in the laboratory. It is made initially as two separate solutions, known as Fehling's A and Fehling's B. Fehling's A is a blue aqueous solution of copper(II) sulfate, while Fehling's B is a clear solution of aqueous potassium sodium tartrate
Potassium sodium tartrate
Potassium sodium tartrate is a double salt first prepared by an apothecary, Pierre Seignette, of La Rochelle, France. As a result the salt was known as Seignette's salt or Rochelle salt....

 (also known as Rochelle salt) and a strong alkali (commonly sodium hydroxide).

Equal volumes of the two mixtures are mixed together to get the final Fehling's solution, which is a deep blue colour. In this final mixture, aqueous tartrate ions from the dissolved Rochelle salt chelate
Chelation
Chelation is the formation or presence of two or more separate coordinate bonds between apolydentate ligand and a single central atom....

 to Cu2+ (aq) ions from the dissolved copper(II) sulfate, as bidentate
Denticity
Denticity refers to the number of atoms in a single ligand that bind to a central atom in a coordination complex. In many cases, only one atom in the ligand binds to the metal, so the denticity equals one, and the ligand is said to be monodentate...

 ligands giving the bistartratocuprate(II)4- complex as shown below. The tartarate ions, by complexing copper prevent the formation of Cu(OH)2 from the reaction of CuSO4.2H2O and NaOH present in the solution.
Fehling's can be used to determine whether a carbonyl-containing
Carbonyl
In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups....

 compound is an aldehyde or a ketone. The bistartratocuprate(II) complex in Fehling's solution is an oxidizing agent
Oxidizing agent
An oxidizing agent can be defined as a substance that removes electrons from another reactant in a redox chemical reaction...

 and the active reagent in the test. The compound to be tested is added to the Fehling's solution and the mixture is heated. Aldehydes are oxidized, giving a positive result, but ketones do not react, unless they are alpha-hydroxy-ketones. The bistartratocuprate(II) complex oxidizes the aldehyde to a carboxylate anion, and in the process the copper(II) ions of the complex are reduced to copper(I) ions. Red copper(I) oxide then precipitates out of the reaction mixture, which indicates a positive result i.e. that redox
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....

 has taken place (this is the same positive result as with Benedict's solution. A negative result is the absence of the red precipitate; it is important to note that Fehling's will not work with aromatic aldehydes; in this case Tollens' reagent
Tollens' reagent
Tollens' 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...

 should be used.

Fehling's test can be used as a generic test for monosaccharides. It will give a positive result for aldose
Aldose
An aldose is a monosaccharide that contains only one aldehyde group per molecule. The chemical formula takes the form Cnn. The simplest possible aldose is the diose glycolaldehyde, which only contains two carbon atoms....

 monosaccharides (due to the oxidisable aldehyde group) but also for ketose
Ketose
A ketose is a sugar containing one ketone group per molecule.With 3 carbon atoms, dihydroxyacetone is the simplest of all ketoses and is the only one having no optical activity. Ketoses can isomerize into an aldose when the carbonyl group is located at the end of the molecule...

 monosaccharides, as they are converted to aldoses by the base in the reagent, and then give a positive result. For this reason, Fehling's reagent is sometimes referred to as a general test for monosaccharides.

Fehling's can be used to screen for 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...

, thus detecting diabetes. Another use is in conversion / breakdown of starch to glucose syrup and maltodextrin
Maltodextrin
Maltodextrin is a polysaccharide that is used as a food additive. It is produced from starch by partial hydrolysis and is usually found as a creamy-white hygroscopic spraydried powder. Maltodextrin is easily digestible, being absorbed as rapidly as glucose, and might be either moderately sweet or...

s, to measure the amount 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 and calculating the dextrose equivalent
Dextrose equivalent
Dextrose 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...

 (DE) of the starch sugar
Glucose syrup
Glucose syrup is a food syrup, made from the hydrolysis of starch. Maize is commonly used as the source of the starch in the USA, in which case the syrup is called "corn syrup", but glucose syrup is also made from other starch crops, including potatoes, wheat, barley, rice and cassavap.21...

.

Formic acid
Formic acid
Formic acid is the simplest carboxylic acid. Its chemical formula is HCOOH or HCO2H. It is an important intermediate in chemical synthesis and occurs naturally, most notably in the venom of bee and ant stings. In fact, its name comes from the Latin word for ant, formica, referring to its early...

 (HCOOH - methanoic acid) also gives a positive Fehling's test result, as it does with Tollens'
Tollens' reagent
Tollens' 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...

 test and Benedict's
Benedict's reagent
Benedict's reagent is a chemical reagent named after an American chemist, Stanley Rossiter Benedict....

 test also. This is because it is readily oxidizable to carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

 and water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

.'

Safety

Sodium hydroxide is corrosive at high concentrations and precautions should be taken as such not to come into direct contact with it. Copper(II) sulfate is also toxic if ingested.
The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
x
OK