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.
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+
- 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
- Test tubes
- Test tube stand
- 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