Fehling's solution
Fehling's sample for the detection of reducing agents, such as aldehydes and reducing sugar.
History
The 1848 published by Hermann Fehling detection reaction allowed the quantitative determination of sugar in urine by titration. This was for the diagnosis of diabetes (diabetes ) is important. Previously this was only qualitatively by simple taste test or fermentation possible, later also quantitatively by polarimetry. Today the Fehling's test is part of school chemistry.
Fehling's solution
For carrying out the Fehling's test using two solutions as detection reagents, which are referred to by Hermann Fehling as " Fehling I" and " II Fehling ".
- The light blue Fehling's solution I is a dilute copper (II ) sulfate solution.
- The colorless Fehling's solution II is an alkaline sodium potassium tartrate solution.
After merging the two equal volumes of Fehling's solutions, the Fehling reagent possesses due to the complex formation of Cu ( II ) ions with tartrate ions have a characteristic dark blue color. The tartrate is in this case, a complexing agent: The high complex stability, the solubility of the copper ( II) hydroxide will not be achieved. If the copper ( II) ions were available not complexed, the OH would - ions react with the copper ( II) ions to the sparingly soluble blue copper ( II) hydroxide Cu (OH) 2, and the desired detection reaction could not be take place more.
The addition of glycerol before filling with water extends the shelf life of a self- prepared solution.
After addition of the test substance, the solution is heated. Thus, the detection reaction is accelerated according to the RGT rule. The monosaccharides are shown in their open-chain form, since the oxidability of the aldehyde group is used, which is bound in the ring shapes as the hemiacetal. The open-chain form is connected to the various ring shapes in a chemical equilibrium. Such are, for example, when glucose in aqueous solution is less than 0.1% of the glucose molecules in open-chain form.
There is then a reduction of the copper ( II) ions to only yellow copper (I ) hydroxide ( CuOH ) and then dehydration to copper (I ) oxide ( Cu2O ). Aldehydes are oxidized by addition of Fehling's reagent to carboxylic acids, while the copper sulfate ( CuSO4 ) (I ) oxide ( Cu2O ) is reduced to copper and precipitates as a red-brown precipitate.
Not least because of the emergence of a solid product, the equilibrium of this reaction almost entirely on the side of the carboxylic acid. Characterized additional sugar molecules are converted to the open chain form, until the reaction is almost completely expired:
For prolonged heating or in simpler aldehydes such as formaldehyde or acetaldehyde may also arise elemental copper.
Redox reaction
Since the oxidation of the test substance is carried out by reduction of the copper (II) ion, the overall reaction can be separated as in all redox reactions in an oxidation and reduction reaction. Here, in the example below is not considered for simplicity that the copper ions actually present in a complex with tartrate ions ( copper tartrate ):
Oxidation:
Reduction:
Redox reaction:
Confines
Be mentioned at this point that ketones are not oxidized by Fehling's solution. Consequently, the detection allows a distinction between a ketone and aldehyde. However ketones with the carbonyl group adjacent OH group (alpha- ketols ) respond to the Fehling reagent (eg by keto -enol tautomerism ); Reducing agent is the resulting in alkaline solution Endiolation here.
The Fehling's reaction with reducing sugars generally follows not the simple stoichiometry shown above, since in this case oxidation products are formed, the self-reducing effect ( keto, hydroxy diketones as well as products of retro - aldol reactions ).
Further detection reactions of aldehydes
- Angeli - Rimini reaction
- Schiff test with Schiff's reagent
- Tollensprobe ( silver mirror test ) with the Tollens reagent
- Benedict 's reagent
- Nylanders reagent