Editing Tollens' reagent (section)

==Uses==

===Qualitative organic analysis===
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 [[acyloin|alpha-hydroxy ketones]] being one exception.

The test rests on the premise that aldehydes are more readily [[oxidation and reduction|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.<ref name="chemguide">{{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}}</ref>

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

Tollens' reagent can also be used to test for terminal [[alkyne]]s (<chem>R-C2H</chem>). A white precipitate of the [[metal acetylide|acetylide]] (<chem>AgC_2-R</chem>) is formed in this case.  Another test relies on reaction of the [[furfural]] with [[phloroglucinol]] to produce a colored compound with high molar absorptivity.<ref>{{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 }}</ref> It also gives a positive test with [[hydrazine]]s, [[hydrazone]]s, [[acyloin|α-hydroxy ketones]] and [[dicarbonyl|1,2-dicarbonyls]].

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

===Staining===
In [[anatomic pathology]], ammonical silver nitrate is used in the [[Histological techniques|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.<ref name = webpath>Webpath website http://library.med.utah.edu/WebPath/HISTHTML/MANUALS/FONTANA.PDF. Retrieved 4 February 2009</ref>

===In silver mirroring===
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.<ref>{{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}}</ref>

For applications requiring the highest optical quality, such as in [[reflecting telescope|telescope mirrors]], the use of [[tin(II) chloride]] is problematic, since it creates nanoscale roughness and reduces the reflectivity.<ref>
{{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 }}</ref><ref name="silvering_mirrors"/>
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]].<ref name="silvering_mirrors">{{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}}</ref>