Tollens' reagent (chemical formula Ag(NH3)2OH) is a chemical reagent used to distinguish between

aldehyde In organic chemistry, an aldehyde () is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred to as an aldehyde but can also be classified as a formyl grou ...

s and ketones along with some alpha-hydroxy ketones which can tautomerize into aldehydes. The reagent consists of a solution of

silver nitrate Silver nitrate is an inorganic compound with chemical formula . It is a versatile precursor to many other silver compounds, such as those used in photography. It is far less sensitive to light than the halides. It was once called ''lunar causti ...

ammonia Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous wa ...

and some sodium hydroxide (to maintain a basic pH of the reagent solution). It was named after its discoverer, the German chemist Bernhard Tollens. A positive test with Tollens' reagent is indicated by the precipitation of elemental silver, often producing a characteristic "silver mirror" on the inner surface of the reaction vessel.

Laboratory preparation

This reagent is not commercially available due to its short

shelf life Shelf life is the length of time that a commodity may be stored without becoming unfit for use, consumption, or sale. In other words, it might refer to whether a commodity should no longer be on a pantry shelf (unfit for use), or no longer on a ...

, 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

. The OH- ions convert the silver aquo complex form into

silver(I) oxide Silver oxide is the chemical compound with the formula Ag2O. It is a fine black or dark brown powder that is used to prepare other silver compounds. Preparation Silver oxide can be prepared by combining aqueous solutions of silver nitrate and a ...

, Ag2O, which precipitates from the solution as a brown solid: :2AgNO3 + 2NaOH -> Ag2O(s) + 2NaNO3 + H2O In the next step, sufficient aqueous

is added to dissolve the brown silver(I) oxide. The resulting solution contains the g(NH₃)₂sup>+ complexes in the mixture, which is the main component of Tollens' reagent. Sodium hydroxide is reformed: :Ag2O(s) + 4NH3 + 2NaOH + H2O -> 2 g(NH3)2H + 2NaOH Alternatively,

aqueous ammonia Ammonia solution, also known as ammonia water, ammonium hydroxide, ammoniacal liquor, ammonia liquor, aqua ammonia, aqueous ammonia, or (inaccurately) ammonia, is a solution of ammonia in water. It can be denoted by the symbols NH3(aq). Although ...

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 A coordination complex consists of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of bound molecules or ions, that are in turn known as '' ligands'' or complexing agents. ...

, g(NH3)2. Filtering the reagent before use helps to prevent false-positive results.

Uses

Qualitative organic analysis

Once the presence of a

carbonyl group In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups. A compound containi ...

has been identified using

2,4-dinitrophenylhydrazine 2,4-Dinitrophenylhydrazine (2,4-DNPH or DNPH) is the organic compound C6H3(NO2)2NHNH2. DNPH is a red to orange solid. It is a substituted hydrazine. The solid is relatively sensitive to shock and friction. For this reason DNPH is usually handle ...

(also known as

Brady's reagent 2,4-Dinitrophenylhydrazine (2,4-DNPH or DNPH) is the organic compound C6H3(NO2)2NHNH2. DNPH is a red to orange solid. It is a substituted hydrazine. The solid is relatively sensitive to shock and friction. For this reason DNPH is usually handled a ...

or 2,4-DNPH or 2,4-DNP), Tollens' reagent can be used to distinguish ketone vs

. 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 Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a ...

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 An oxidizing agent (also known as an oxidant, oxidizer, electron recipient, or electron acceptor) is a substance in a redox chemical reaction that gains or " accepts"/"receives" an electron from a (called the , , or ). In other words, an oxi ...

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 In organic chemistry, a carboxylate is the conjugate base of a carboxylic acid, (or ). It is an ion with negative charge. Carboxylate salts are salts that have the general formula , where M is a metal and ''n'' is 1, 2,...; ''carboxylat ...

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 In organic chemistry, an alkyl group is an alkane missing one hydrogen. The term ''alkyl'' is intentionally unspecific to include many possible substitutions. An acyclic alkyl has the general formula of . A cycloalkyl is derived from a cycloa ...

group. :2 g(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 In organometallic chemistry, acetylide refers to chemical compounds with the chemical formulas and , where M is a metal. The term is used loosely and can refer to substituted acetylides having the general structure (where R is an organic side c ...

(AgC-R) is formed in this case. Another test relies on reaction of the furfural with

phloroglucinol Phloroglucinol is an organic compound with the formula C6H3(OH)3. It is a colorless solid. It is used in the synthesis of pharmaceuticals and explosives. Phloroglucinol is one of three isomeric benzenetriols. The other two isomers are hydroxyqu ...

to produce a colored compound with high molar absorptivity. It also gives a positive test with hydrazines,

hydrazone Hydrazones are a class of organic compounds with the structure . They are related to ketones and aldehydes by the replacement of the oxygen =O with the = functional group. They are formed usually by the action of hydrazine on ketones or aldehyde ...

s, α-hydroxy ketones and 1,2-dicarbonyls. Both Tollens' reagent and

Fehling's reagent In organic chemistry, Fehling's solution is a chemical reagent used to differentiate between water-soluble carbohydrate and ketone () functional groups, and as a test for reducing sugars and non-reducing sugars, supplementary to the Tollens' reag ...

give positive results with formic acid.

Staining

anatomic pathology Anatomical pathology (''Commonwealth'') or Anatomic pathology (''U.S.'') is a medical specialty that is concerned with the diagnosis of disease based on the gross examination, macroscopic, Histopathology, microscopic, biochemical, immu ...

, ammonical silver nitrate is used in the Fontana–Masson Stain, which is a

silver stain In pathology, silver staining is the use of silver to selectively alter the appearance of a target in microscopy of histological sections; in temperature gradient gel electrophoresis; and in polyacrylamide gels. In traditional stained glass, silv ...

technique used to detect

melanin Melanin (; from el, μέλας, melas, black, dark) is a broad term for a group of natural pigments found in most organisms. Eumelanin is produced through a multistage chemical process known as melanogenesis, where the oxidation of the amino ...

, argentaffin and

lipofuscin Lipofuscin is the name given to fine yellow-brown pigment granules composed of lipid-containing residues of lysosomal digestion. It is considered to be one of the aging or "wear-and-tear" pigments, found in the liver, kidney, heart muscle, reti ...

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

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 Glucose is a simple sugar with the molecular formula . Glucose is overall the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, u ...

(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 Tin(II) chloride, also known as stannous chloride, is a white crystalline solid with the formula . It forms a stable dihydrate, but aqueous solutions tend to undergo hydrolysis, particularly if hot. SnCl2 is widely used as a reducing agent (in aci ...

stabilised in

hydrochloric acid Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid Acid strength is the tendency of an acid, symbol ...

solution. For applications requiring the highest optical quality, such as in telescope mirrors, the use of

is problematic, since it creates nanoscale roughness and reduces the reflectivity. Methods to produce telescope mirrors include additional additives to increase adhesion and film resilience, such as in Martin's method, which includes

tartaric acid Tartaric acid is a white, crystalline organic acid that occurs naturally in many fruits, most notably in grapes, but also in bananas, tamarinds, and citrus. Its salt, potassium bitartrate, commonly known as cream of tartar, develops naturally ...

and

ethanol Ethanol (abbr. EtOH; also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound. It is an alcohol with the chemical formula . Its formula can be also written as or (an ethyl group linked to a ...

Safety

Aged reagent can be destroyed with dilute acid to prevent the formation of the highly explosive

silver nitride Silver nitride is an explosive chemical compound with symbol Ag3N. It is a black, metallic-looking solid which is formed when silver oxide or silver nitrate, is dissolved in concentrated solutions of ammonia, causing formation of the diammine silve ...

References

External links

Video of experimental process involving Tollens' reagent

Univ. of Minnesota Organic Chemistry Class DemoResult
{{Analytical reagents Silver compounds Oxidizing agents Coordination complexes Chemical tests Analytical reagents Ammine complexes