Tollens Test - Formula, Preparation, Uses, FAQs

Edited By Team Careers360 | Updated on Jul 02, 2025 04:46 PM IST

Tollens Test or Tollens Reagent Test
The Tollens reagent test was discovered by Bernhard Christian Gottfried Tollens. Tollens test is used to distinguish between the compounds as they are aldehyde or ketones. The silver mirror test is the other name of the tollens test. In the Tollens test one of the main components is tollens reagent.

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Tollens Reagent
Tollens’ Reagent formula
Tollens Reagent Preparation
Tollens Test :-
Its Uses:-

Tollens Reagent

Tollens reagent is used to determine the aldehyde functional group and an aromatic aldehyde functional group and alpha hydroxy ketone group in any given substance.

Tollens reagent is named after Bernhard Tollens, who coined the term Tollen’s reagent. Bernhard Tollens is a famous Chemist from Germany.

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Tollens’ Reagent formula

Tollen reagent formula is Ag(NH3)2OH

Q – What other term can we use in place of the tollens test?

The Tollens test is also known as silver mirror test
Q- What is the composition of Tollens reagent?
The tollens reagent is a mixture above-mentioned silver nitrate and ammonia.
Silver nitrate = AgNO3
Ammonia = Nh3

Q – Tollens test mechanism?

As we know tollens reagent is an alkaline solution of ammoniacal silver nitrate, tollens reagent i..e.. ammoniacal silver nitrate is used for the test of aldehyde or alpha diketones. Silver ions play an important role in this as among the first silver nitrate oxidises aldehydes group to carboxylic group and the silver ions being reduced to metallic silver and due to this conversion, we say this compound is true for the over mirror test or not true for silformirror test.
Tollens reaction of Tollens reagent reaction
2Ag⁺ (aq) + 2OH^- (aq) Aga 2 O(s) + H2O
Ag₂O (s) + the 4 NH₃ (aq)a + H₂O(l) ? 2 [Ag(NH₃)₂]+ (aq) are2 OH^- (aq)

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Tollens Reagent Preparation

Since Tollens reagent has a short life span, it has to be prepared in the laboratory with precautions:

So to form tollens reagent we have to follow these given steps : -
First of all we have to take an aqueous silver nitrate solution and then we have to introduce a few drops of dilute NaOH ( we can not use concentrated NaOH in place of dilute NaOH ) in the aqueous solution of silver nitrate . Then the reaction occurs in which water ( from aqueous silver nitrate ) will act as ligand and the aqueous solution of silver nitrate also contains silver aqua complexes. And then the hydroxide ions ( from water ) convert silver aqua complexes to become silver oxide. And due to formation of this silver oxide precipitation will occur in the solution i..e... silver oxide precipitate in the form of brown solid from their solutions.
Reaction of above discussed event will be 2AgNO₃ + 2NaOH → Ag₂O ( brown ppt ) + 2NaNO₃ + H₂O
Silver oxide = Ag2O ( molecular symbol )
Then we have to dissolve the precipitated brown solid of silver oxide in aqueous ammonia. Then the new solution will be formed after reaction containing [Ag(NH₃)₂]⁺ complex. The complex i..e.. formed is the primary component of tollens reagents.
And the reaction of above mentioned step i..e.. formation of primary component of tollens reagent [Ag(NH₃)₂]⁺ complex will be:-
2 Ag⁺ (aq) + 2 OH^- (aq) Ag₂O(s) + H₂O(l)
Ag₂O ( brown ppt ) + 4NH₃ + 2NaNO₃ + H₂O → 2[Ag(NH₃)₂]NO₃ + 2NaOH

Tollens Test :-

Q - What happens when we mix aldehyde with the tollens reagent we recently prepared in the laboratory ?

The first thing that will happen is the oxidation of aldehyde i..e.. aldehyde is oxidized by tollens reagent to form a carboxylic acid.
In this process silver ions that are present in tollens reagent will be reduced and form metallic silver.
Basically any test i..e.. any laboratory test is done in a glass test tube . And due to the presence of glass test tubes when silver ions are reduced to metallic silver then formation of silver mirror will happen on the test tube.
And then we can easily find out why tollens test is also known as silver mirror test i..e.. due to the formation of silver mirror on the test tube ( because of reduction of silver ions to metallic silver ) .

Q - what tollens reagent? describe one usefulness of this reagent ?

Tollens reagent is the solution of silver nitrate and aqueous ammonia .

Tollens reagent complex is [Ag(NH₃)₂]⁺

Its Uses:-

We use tollens reagent to determine either of these functional groups i.e.. aldehyde , aromatic aldehyde or alpha hydroxy ketone is present.

Benzaldehyde tollens test :-

Benzaldehyde i.e... benzene + aldehyde will give positive tollens test due to presence of aldehyde group to it as its aldehyde portion will be reduced by silver aqua complex and forms mirror i..e.. silver mirror on the test tube and hence proves it positive test.

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Q – What is the name of tollens reagent?

Basically tollens reagent is ammoniacal silver nitrate as to be more accurate we would define tollens reagent as an alkaline solution of ammoniacal silver nitrate i..e.. used in distinguishing functional groups like aldehyde and ketones.
As We all know that main working principle for tollens test is that aldehyde is oxidised to carboxylic acid and tollens reagent is reduced ( silver ions is reduced to metallic silver ) and thus gives us symbol in the form of silver mirror about the presence of aldehyde group in our given compound or the compound we uses in tollens test.

Q – what is an ammoniacal silver nitrate formula ?

As we know that ammoniacal silver nitrate is tollens reagent means the reagent used in tollens reagent is ammoniacal silver nitrate itself.
And formula of ammoniacal silver nitrate is Ag(NH₃)₂OH)
In ammoniacal silver nitrate formula the ratio of Ag : N : H : O is 1 : 2 : 7 : 1
Nature Ammoniacal silver nitrate is alkaline .

Questions like; which of the following reduces tollen's reagent , we have to note one thing that the compound which will reduce tollens reagent will also give positive tollens test or silver mirror test.

Q – which compounds will reduce tollens reagent?

Answer will be that all aldehyde and terminal alpha hydroxy ketone will reduce tollens reagent i..e.. ammoniacal silver nitrate and also these compounds ( all aldehyde and terminal alpha hydroxy ketone ) will give positive silver mirror test and tollens test.

Q – Do ketone reduce tollens reagent ?

And the answer will be no as ketones do not reduce tollens reagent and give tollens test or silver mirror test .
But there is an exception in case of alpha diketones i...e... compound consists of two ketone functional groups which are situated at adjacent positions to each other.
In general ketones neither reduce tollens reagent nor reduce fehling's reagent that means ketone does not give tollens and fehlings tests.

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NCERT Chemistry Notes:

Frequently Asked Questions (FAQs)

1. Can the Tollens' test distinguish between different aldehydes?

No, the Tollens' test can only confirm the presence of an aldehyde. It cannot distinguish between different types of aldehydes as they all produce the same silver mirror effect.

2. Can the Tollens' test detect all aldehydes?

The Tollens' test can detect most aldehydes, but some exceptions exist. For example, aromatic aldehydes like benzaldehyde may not react as readily due to resonance stabilization.

3. Can the Tollens' test be used to quantify the amount of aldehyde present?

While primarily qualitative, the Tollens' test can be adapted for quantitative analysis by measuring the amount of silver deposited. However, other methods like HPLC are generally preferred for quantification.

4. How does the Tollens' test relate to oxidation and reduction?

In the Tollens' test, the aldehyde is oxidized to a carboxylic acid (or its salt), while the silver ions are reduced to metallic silver. It's a classic example of a redox reaction in organic chemistry.

5. Why is glucose positive in the Tollens' test despite not being an aldehyde?

Glucose exists in solution as an equilibrium mixture of its cyclic and open-chain forms. The open-chain form contains an aldehyde group, which reacts with Tollens' reagent, giving a positive result.

6. Why is the Tollens' test considered a "mild" oxidation method?

The Tollens' test is considered mild because it selectively oxidizes aldehydes without affecting other oxidizable groups that might be present in the molecule. This selectivity makes it useful for identifying aldehydes specifically.

7. How does the Tollens' test relate to the reducing properties of aldehydes?

The Tollens' test demonstrates the reducing properties of aldehydes. Aldehydes act as reducing agents, donating electrons to the silver ions and reducing them to metallic silver, while being oxidized themselves.

8. Can the Tollens' test be used to distinguish between an aldehyde and a hemiacetal?

Generally, no. Hemiacetals can exist in equilibrium with their open-chain aldehyde form in solution, which would react with Tollens' reagent. This is why glucose, a cyclic hemiacetal, gives a positive Tollens' test.

9. What is the role of the ammonia in Tollens' reagent?

Ammonia forms a complex with silver ions, [Ag(NH3)2]+, which is more stable in solution than silver oxide. This complex is the active species that reacts with aldehydes in the test.

10. How does the Tollens' test relate to the concept of functional groups in organic chemistry?

The Tollens' test is a perfect example of how different functional groups react differently. It specifically targets the aldehyde functional group, demonstrating how the presence of specific functional groups determines a molecule's chemical behavior.

11. How is Tollens' reagent prepared?

Tollens' reagent is prepared by adding sodium hydroxide to silver nitrate solution, forming a brown precipitate of silver oxide. Aqueous ammonia is then added until the precipitate dissolves, forming the diamminesilver(I) complex.

12. Why is it important to use freshly prepared Tollens' reagent?

Freshly prepared Tollens' reagent is crucial because it can form explosive silver compounds if left standing for too long, especially if exposed to light or heat.

13. What is the role of sodium hydroxide in the Tollens' reagent?

Sodium hydroxide serves two purposes: it initially forms silver oxide with silver nitrate, and it provides the basic conditions necessary for the redox reaction between the aldehyde and the silver complex.

14. What happens if you use too much ammonia in preparing Tollens' reagent?

Excess ammonia can form a stable silver-ammonia complex that won't react with aldehydes, leading to false negative results. It's important to add ammonia only until the silver oxide precipitate just dissolves.

15. What is the chemical formula for Tollens' reagent?

Tollens' reagent doesn't have a single chemical formula. It's a mixture of silver nitrate (AgNO3), sodium hydroxide (NaOH), and aqueous ammonia (NH3). The active species is the diamminesilver(I) complex, [Ag(NH3)2]+.

16. What is the Tollens' test?

The Tollens' test is a chemical reaction used to distinguish between aldehydes and ketones. It involves the reduction of silver ions to metallic silver, which forms a silver mirror on the inside of the test tube when an aldehyde is present.

17. Why doesn't the Tollens' test work for ketones?

Ketones do not react with Tollens' reagent because they lack a hydrogen atom attached to the carbonyl group. This hydrogen is necessary for the oxidation reaction that reduces silver ions to metallic silver.

18. What is the balanced equation for the Tollens' test with an aldehyde?

The general equation is:

19. What safety precautions should be taken when performing the Tollens' test?

Wear safety goggles and gloves. Work in a well-ventilated area. Handle ammonia carefully as it's corrosive. Dispose of silver-containing waste properly. Never heat the mixture, as it can form explosive silver compounds.

20. Why is the Tollens' test also called the "silver mirror test"?

It's called the "silver mirror test" because when an aldehyde reduces the silver ions, metallic silver is deposited on the inside of the test tube, forming a reflective silver surface that looks like a mirror.

21. How does the Tollens' test compare to other carbonyl tests like Fehling's or Benedict's?

All these tests detect reducing sugars and aldehydes, but Tollens' is often considered more sensitive. Fehling's and Benedict's use copper ions instead of silver and produce a red-orange precipitate rather than a mirror.

22. Can the Tollens' test distinguish between aldehydes and reducing sugars?

No, the Tollens' test cannot distinguish between aldehydes and reducing sugars because both contain aldehyde groups (or can form them in solution) that react similarly with the reagent.

23. What is the chemical name of the silver-containing product in a positive Tollens' test?

The product is elemental silver (Ag). It's not a compound but pure metallic silver in a finely divided state, which gives the characteristic mirror-like appearance.

24. How does temperature affect the Tollens' test?

Increasing temperature generally speeds up the reaction, making the silver mirror form more quickly. However, excessive heating should be avoided due to safety concerns with silver compounds.

25. Why is it important to clean glassware thoroughly before performing the Tollens' test?

Any organic residue on the glassware could reduce the silver ions, leading to false positive results. Thorough cleaning ensures that only the test sample can cause the silver mirror formation.

26. Can the Tollens' test be used to detect formaldehyde in the air?

While the Tollens' test can detect formaldehyde in solution, it's not typically used for air testing. Other methods like colorimetric badges or HPLC analysis of air samples are more practical for detecting airborne formaldehyde.

27. What happens if you mix Tollens' reagent with a ketone?

Generally, nothing visible happens. Ketones don't react with Tollens' reagent, so there's no silver mirror formation or other observable change. The solution remains clear.

28. How does the structure of an aldehyde affect its reactivity in the Tollens' test?

The reactivity of aldehydes in the Tollens' test can vary. Generally, aliphatic aldehydes react more readily than aromatic ones. Steric hindrance around the carbonyl group can also slow down the reaction.

29. Can the Tollens' test detect aldehydes in mixture with ketones?

Yes, the Tollens' test can detect aldehydes even in a mixture with ketones. The aldehyde will react to form the silver mirror, while the ketone remains unreacted.

30. What is the shelf life of prepared Tollens' reagent?

Tollens' reagent should be prepared fresh for each use and discarded after the experiment. It doesn't have a long shelf life due to the potential formation of explosive silver compounds over time.

31. How does the concentration of the aldehyde affect the Tollens' test results?

Higher concentrations of aldehyde generally result in faster and more pronounced silver mirror formation. Very dilute solutions may take longer to show a positive result or may produce a faint mirror.

32. Can the Tollens' test be used to detect aldehydes in natural products?

Yes, the Tollens' test can be used to detect aldehydes in natural products, such as vanillin in vanilla extract. However, other compounds in the mixture might interfere, so additional tests may be necessary for confirmation.

33. What is the minimum concentration of aldehyde that the Tollens' test can detect?

The sensitivity of the Tollens' test can vary, but it can typically detect aldehydes at concentrations as low as 0.1-1 mM. However, the exact limit of detection depends on various factors including reaction conditions and observation time.

34. How does the Tollens' test relate to the oxidation number of carbon?

In the Tollens' test, the carbon atom of the aldehyde group is oxidized, increasing its oxidation number from +1 to +3. This change corresponds to the conversion of the aldehyde (R-CHO) to a carboxylate ion (R-COO-).

35. Can the silver mirror formed in a positive Tollens' test be removed from the test tube?

The silver mirror can be difficult to remove once formed. It often requires treatment with strong acids or oxidizing agents. For this reason, disposable test tubes are often used for the Tollens' test in educational settings.

36. Can the Tollens' test be used to detect aldehydes in cosmetic or food products?

Yes, the Tollens' test can be used to detect aldehydes in cosmetic or food products. However, in practice, more sophisticated analytical methods like gas chromatography are often preferred for regulatory testing due to their higher specificity and quantitative capabilities.

37. How does the Tollens' test compare to the Schiff's test for aldehydes?

Both tests detect aldehydes, but they work differently. The Tollens' test produces a silver mirror, while the Schiff's test produces a pink-purple color. The Schiff's test can also detect some ketones, making Tollens' more specific for aldehydes.

38. What happens if an alcohol is subjected to the Tollens' test?

Alcohols do not react with Tollens' reagent. The test will be negative, with no silver mirror formation. This is because alcohols cannot be oxidized under the mild conditions of the Tollens' test.

39. How does the Tollens' test relate to the concept of oxidation levels in organic compounds?

The Tollens' test demonstrates the progression of oxidation levels in organic compounds. Aldehydes, at a lower oxidation level, are oxidized to carboxylic acids (or their salts), which are at a higher oxidation level.

40. Can the Tollens' test be used to distinguish between an aldehyde and an α,β-unsaturated ketone?

Yes, the Tollens' test can distinguish between these. Aldehydes will give a positive result (silver mirror), while α,β-unsaturated ketones, despite being more reactive than simple ketones, still won't reduce the silver ions and thus give a negative result.

41. How does the presence of other functional groups in a molecule affect its behavior in the Tollens' test?

Other functional groups generally don't interfere with the Tollens' test unless they can be oxidized more easily than the aldehyde group. However, some groups might affect the reaction rate or the appearance of the silver mirror.

42. Can the Tollens' test be used to detect aldehydes in biological samples like blood or urine?

While theoretically possible, the Tollens' test is not typically used for biological samples due to potential interferences and the availability of more specific and sensitive methods. In medical contexts, enzymatic assays or chromatographic methods are preferred.

43. How does the Tollens' test relate to the concept of chirality in organic molecules?

The Tollens' test itself doesn't distinguish between chiral molecules. However, it's relevant to chirality because the oxidation of an aldehyde to a carboxylic acid can create a new chiral center if the aldehyde carbon is attached to two different groups.

44. Can the Tollens' test be used to detect aldehyde groups in polymers?

Yes, the Tollens' test can detect aldehyde groups in polymers, such as in oxidized cellulose. However, the test may be less effective if the aldehyde groups are not easily accessible due to the polymer's structure.

45. How does the electronic structure of the carbonyl group relate to its reactivity in the Tollens' test?

The carbonyl group's reactivity in the Tollens' test is due to its polarized nature. The partially positive carbon is susceptible to nucleophilic attack, while the oxygen can form hydrogen bonds with water, facilitating the reaction.

46. Can the Tollens' test be adapted for use in organic solvents instead of water?

The standard Tollens' test is performed in aqueous solution. Adapting it to organic solvents is challenging because the silver-ammonia complex is water-soluble. Some modified versions for non-aqueous media exist, but they're not commonly used.

47. How does the Tollens' test relate to the concept of Lewis acids and bases?

In the Tollens' test, the aldehyde acts as a Lewis base, donating electrons to the silver ion (Lewis acid). This electron transfer is key to the reduction of silver ions to metallic silver.

48. Can the Tollens' test detect aldehydes in gaseous form?

The standard Tollens' test is designed for liquid samples. To test gaseous aldehydes, they would need to be dissolved in a suitable solvent first. There are other methods more suitable for detecting gaseous aldehydes directly.

49. How does the Tollens' test demonstrate the principle of green chemistry?

The Tollens' test aligns with some green chemistry principles: it uses relatively safe reagents, operates at room temperature, and doesn't require complex equipment. However, the silver waste produced is a concern and needs proper disposal.

50. Can the silver produced in the Tollens' test be used for any practical applications?

While the primary purpose of the Tollens' test is qualitative analysis, the silver produced can be used to create silver mirrors or silver-coated glassware. However, the small scale and potential impurities usually make this impractical for commercial applications.