:Tollens' reagent

This reagent is not commercially available due to its short shelf life, so it must be freshly prepared in the laboratory. One common preparation involves two steps. First a few drops of dilute sodium hydroxide are added to some aqueous 0.1 M silver nitrate. The OH- ions convert the silver aquo complex form into silver(I) oxide, Ag2O, which precipitates from the solution as a brown solid:

:2AgNO3 + 2NaOH -> Ag2O(s) + 2NaNO3 + H2O

In the next step, sufficient aqueous ammonia is added to dissolve the brown silver(I) oxide. The resulting solution contains the [Ag(NH₃)₂]⁺ complexes in the mixture, which is the main component of Tollens' reagent. Sodium hydroxide is reformed:

:Ag2O(s) + 4NH3 + 2NaNO3 + H2O -> 2[Ag(NH3)2]NO3 + 2NaOH

Alternatively, aqueous ammonia can be added directly to silver nitrate solution. At first, ammonia will induce formation of solid silver oxide, but with additional ammonia, this solid precipitate dissolves to give a clear solution of diamminesilver(I) coordination complex, [Ag(NH3)2]+. Filtering the reagent before use helps to prevent false-positive results.

Once the presence of a carbonyl group has been identified using 2,4-dinitrophenylhydrazine (also known as Brady's reagent or 2,4-DNPH or 2,4-DNP), Tollens' reagent can be used to distinguish ketone vs aldehyde. Tollens' reagent gives a negative test for most ketones, with alpha-hydroxy ketones being one exception.

The test rests on the premise that aldehydes are more readily oxidized compared with ketones; this is due to the carbonyl-containing carbon in aldehydes having attached hydrogen. The diamine silver(I) complex in the mixture is an oxidizing agent and is the essential reactant in Tollens' reagent. The test is generally carried out in a test tube in a warm water bath.

In a positive test, the diamine silver(I) complex oxidizes the aldehyde to a carboxylate ion and in the process is reduced to elemental silver and aqueous ammonia. The elemental silver precipitates out of solution, occasionally onto the inner surface of the reaction vessel, giving a characteristic "silver mirror". The carboxylate ion on acidification will give its corresponding carboxylic acid. The carboxylic acid is not directly formed in the first place as the reaction takes place under alkaline conditions. The ionic equations for the overall reaction are shown below; R refers to an alkyl group.{{cite web |url=http://www.chemguide.co.uk/organicprops/carbonyls/oxidation.html |title=Oxidation of aldehydes and ketones |website=chemguide.co.uk |access-date=31 January 2010}}

:2[Ag(NH3)2]+ + R-CHO + H2O -> 2Ag(s) + 4NH3 + R-COOH + 2H+

Tollens' reagent can also be used to test for terminal alkynes (R-C2H). A white precipitate of the acetylide (AgC_2-R) is formed in this case. Another test relies on reaction of the furfural with phloroglucinol to produce a colored compound with high molar absorptivity.{{cite journal|last1=Oshitna|first1=K.|last2=Tollens|first2=B.|title=Ueber Spectral-reactionen des Methylfurfurols|journal=Ber. Dtsch. Chem. Ges.|volume=34|page=1425|date=1901|issue=2 |doi=10.1002/cber.19010340212 |url=https://zenodo.org/record/1735046 }} It also gives a positive test with hydrazines, hydrazones, α-hydroxy ketones and 1,2-dicarbonyls.

Both Tollens' reagent and Fehling's reagent give positive results with formic acid.{{citation needed|date=December 2020}}

In anatomic pathology, ammonical silver nitrate is used in the Fontana–Masson stain, which is a silver stain technique used to detect melanin, argentaffin, and lipofuscin in tissue sections. Melanin and the other chromaffins reduce the silver nitrate to metallic silver.Webpath website http://library.med.utah.edu/WebPath/HISTHTML/MANUALS/FONTANA.PDF. Retrieved 4 February 2009

Tollens' reagent is also used to apply a silver mirror to glassware; for example the inside of an insulated vacuum flask. The underlying chemical process is called silver mirror reaction. The reducing agent is glucose (an aldehyde) for such applications. Clean glassware is required for a high quality mirror. To increase the speed of deposition, the glass surface may be pre-treated with tin(II) chloride stabilised in hydrochloric acid solution.{{cite book|last=Hart|first=M.|title=Manual of scientific glassblowing|year=1992|publisher=British Society of Scientific Glassblowers|location=St. Helens, Merseyside [England]|isbn=0-9518216-0-1}}

For applications requiring the highest optical quality, such as in telescope mirrors, the use of tin(II) chloride is problematic, since it creates nanoscale roughness and reduces the reflectivity.

{{cite journal

| last1 = N. Chitvoranund1

| last2 = S. Jiemsirilers

| last3 = D.P. Kashima

| title = Effects of surface treatments on adhesion of silver film on glass substrate fabricated by electroless plating

| journal = Journal of the Australian Ceramic Society

| volume = 49

| pages = 62–69

| date = 2013 }}

Methods to produce telescope mirrors include additional additives to increase adhesion and film resilience, such as in Martin's method, which includes tartaric acid and ethanol.{{cite journal|last1 = C. Heber D.|title = Methods of Silvering Mirrors|journal = Publications of the Astronomical Society of the Pacific|volume = 23|issue = 135|pages = 15–19|date = Feb 1911|doi=10.1086/122040|bibcode=1911PASP...23...13C| url = https://zenodo.org/record/1431273|doi-access = free|hdl = 2027/mdp.39015018047608|hdl-access = free}}

Aged reagent can be destroyed with dilute acid to prevent the formation of the highly explosive silver nitride.{{cite book

|author1=Svehla, G. |author2=Vogel, Arthur Anton |title=Vogel's Qualitative Inorganic Analysis

|publisher=Longman

|location=New York

|year=1996

|isbn=0-582-21866-7

}}

• Benedict's reagent
• Walden reductor (opposite use involving metallic silver)

{{Reflist}}

• [https://web.archive.org/web/20050329000040/http://www.chemistry.wustl.edu/~courses/genchem/Labs/RedoxIdentity/silver.htm Video of experimental process involving Tollens' reagent]
• [http://www.wiu.edu/users/mftkv/Chemistry102/oxidationaldehydes.html Tollens' reagent on www.wiu.edu]
• [http://jialigao.org/teaching/silv2.jpg Univ. of Minnesota Organic Chemistry Class Demo] {{Webarchive|url=https://web.archive.org/web/20160303195839/http://jialigao.org/teaching/silv2.jpg |date=3 March 2016 }} [http://jialigao.org/teaching/silv3.jpg Result] {{Webarchive|url=https://web.archive.org/web/20160303190235/http://jialigao.org/teaching/silv3.jpg |date=3 March 2016 }}

{{silver compounds}}

{{Analytical reagents}}

fr:Réaction de Tollens

pl: Próba Tollensa

Category:Silver compounds

Category:Oxidizing agents

Category:Coordination complexes

Category:Chemical tests

Category:Analytical reagents

Category:Ammine complexes