Tollens’ test definition

Chemical analysis known as the Tollens’ test is used to distinguish between reducing and non-reducing carbohydrates. The results from this test are sometimes referred to as the “silver mirror test.”

Objectives of Tollens’ test

• to differentiate between reducing and non-reducing sugars.
• to recognize and separate ketone-containing carbohydrates from aldehyde-containing carbohydrates.

Principle of Tollens’ test

• An alkaline combination of liquid ammonia (NH3) and silver nitrate (AgNO3), results in the formation of a complex.
• In the silver aqua complex made using the aqueous solution of silver nitrate, water acts as a ligand.
• The subsequent action of hydroxide ions on the aqua complexes results in silver oxide (Ag2O) formation.
• The [Ag(NH3)2]+ combination is created when water ammonia dissolves the brown precipitate that silver oxide creates.
• This compound is the main ingredient in Tollen’s reagent and a potent oxidizer.
• The complex produces a carboxylic acid by oxidizing the aldehyde group in certain carbohydrates.
• The reagent’s silver ions are converted to metallic silver simultaneously.
• Reducing silver ions into metallic silver produces a silver mirror on the bottom and sides of the test tube.
• An -hydroxy ketone, however, results in a positive Tollen test because the Tollen’s reagent oxidizes it into an aldehyde.

Reaction

2AgNO3 + 2NaOH    →    Ag2O (brown ppt) + 2NaNO3 + H2O

Ag2O (brown ppt) + 4NH3 + 2NaNO3 + H2O    →    2[Ag(NH3)2]NO3 + 2NaOH

Glucose + 2[Ag(NH3)2]NO3 + H2O    →    2 Ag(silver mirror) + 4 NH3 + Gluconic acid + 2 H+

Requirements

Reagent

• To make the Tollen’s reagent, combine 25 ml of 0.8 M KOH with 50 ml of 0.1 M AgNO3 in a beaker. Add enough aqueous ammonia at this point to completely dissolve the dark precipitate.
• Test sample

Materials required

• Test tubes
• Test tube stand
• Pipette

Equipment

• Water bath

The procedure of Tollens’ test

• Use two clean, dry test tubes and fill one with the test sample and the other with distilled water to serve as a blank.
• Tollen’s reagent in 2 ml should be added to each test tube.
• Keep both test tubes submerged in water for one minute.
• Take notice of the color formation as you watch it happen.

Result and Interpretation of Tollens’ test

• While a dark grey precipitate or silver mirror appears on the test tube’s bottom and sides, it indicates that the sample presented contains reducing sugars or aldoses.
• A negative result shows that there are no reducing sugars, aldoses, or -hydroxy ketoses in the test sample. This precipitate’s absence demonstrates this.

Uses of Tollens’ test

• Tollen’s test is frequently used in chemical laboratories to discriminate between aldehydes and ketones during qualitative organic analysis.
• Additionally, reducing sugars and non-reducing sugars are distinguished using this test.

Limitations of Tollens’ test

• Tollen’s test may yield a positive result for certain carbohydrates that do not include an aldehyde group because these sugars areomerize in an alkaline environment.

 References and Sources

• Tiwari A. (2015). Practical Biochemistry. LAP Lambert Academic Publishing.
• 3% – https://projects.ncsu.edu/project/chemistrydemos/Organic/TollensTest.pdf
• 2% – https://byjus.com/chemistry/tollens-test/
• 2% – https://byjus.com/chemistry/tests-of-carbohydrates/
• 1% – https://science.blurtit.com/658440/an-aqueous-solution-of-silver-nitrate-is-added-to-an-aqueous-solution-of-ironiichloride
• 1% – https://memberfiles.freewebs.com/31/91/47149131/documents/bio%20chem%20prelims%20hand%20outs.docx
• 1% – https://chemdemos.uoregon.edu/demos/Fehling-Test
• 1% – http://www.chem.boun.edu.tr/wp-content/uploads/2014/04/Chem-415-Experiment-1.pdf