Tollens' reagent (chemical formula Ag(NH3)2OH) is a chemical

reagent In chemistry, a reagent ( ) or analytical reagent is a substance or compound added to a system to cause a chemical reaction, or test if one occurs. The terms ''reactant'' and ''reagent'' are often used interchangeably, but reactant specifies a ...

used to distinguish between

aldehyde In organic chemistry, an aldehyde () (lat. ''al''cohol ''dehyd''rogenatum, dehydrogenated alcohol) is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred ...

s and

ketone In organic chemistry, a ketone is an organic compound with the structure , where R and R' can be a variety of carbon-containing substituents. Ketones contain a carbonyl group (a carbon-oxygen double bond C=O). The simplest ketone is acetone ( ...

s along with some alpha-hydroxy ketones which can

tautomerize In chemistry, tautomers () are structural isomers (constitutional isomers) of chemical compounds that readily interconvert. The chemical reaction interconverting the two is called tautomerization. This conversion commonly results from the reloca ...

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 ...

ammonium hydroxide 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 ...

and some

sodium hydroxide Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula . It is a white solid ionic compound consisting of sodium cations and hydroxide anions . Sodium hydroxide is a highly corrosive base (chemistry), ...

(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 A mirror, also known as a looking glass, is an object that Reflection (physics), reflects an image. Light that bounces off a mirror forms an image of whatever is in front of it, which is then focused through the lens of the eye or a camera ...

" 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 s ...

, so it must be freshly prepared in the laboratory. One common preparation involves two steps. First a few drops of dilute

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 Ag2 O. 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 ...

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

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 ...

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 + 2NaNO3 + H2O -> 2 g(NH3)2O3 + 2NaOH Alternatively,

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 is a chemical compound consisting of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of chemical bond, bound molecules or ions, that are in turn known as ' ...

, 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 with the formula , composed of a carbon atom double-bonded to an oxygen atom, and it is divalent at the C atom. It is common to several classes of organic compounds (such as aldehydes ...

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 handled ...

(also known as Brady's reagent or 2,4-DNPH or 2,4-DNP), Tollens' reagent can be used to distinguish

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

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 ''electron donor''). In ot ...

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 anion, an ion with negative charge. Carboxylate salts are salts that have the general formula , where M is a metal and ''n'' is 1, 2,... ...

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 In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group () attached to an Substituent, R-group. The general formula of a carboxylic acid is often written as or , sometimes as with R referring to an organyl ...

. The carboxylic acid is not directly formed in the first place as the reaction takes place under

alkaline In chemistry, an alkali (; from the Arabic word , ) is a basic salt of an alkali metal or an alkaline earth metal. An alkali can also be defined as a base that dissolves in water. A solution of a soluble base has a pH greater than 7.0. The ...

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 group is derived from a cy ...

group. :2 g(NH3)2 + R-CHO + H2O -> 2Ag(s) + 4NH3 + R-COOH + 2H+ Tollens' reagent can also be used to test for terminal

alkyne \ce \ce Acetylene \ce \ce \ce Propyne \ce \ce \ce \ce 1-Butyne In organic chemistry, an alkyne is an unsaturated hydrocarbon containing at least one carbon—carbon triple bond. The simplest acyclic alkynes with only one triple bond and n ...

s (R-C2H). A white precipitate of the

acetylide In chemistry, an acetylide is a compound that can be viewed as the result of replacing one or both hydrogen atoms of acetylene (ethyne) by metallic or other cations. Calcium carbide is an important industrial compound, which has long been used ...

(AgC_2-R) is formed in this case. Another test relies on reaction of the

furfural Furfural is an organic compound with the formula C4H3OCHO. It is a colorless liquid, although commercial samples are often brown. It has an aldehyde group attached to the 2-position of furan. It is a product of the dehydration of sugars, as occu ...

with

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

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

hydrazine Hydrazine is an inorganic compound with the chemical formula . It is a simple pnictogen hydride, and is a colourless flammable liquid with an ammonia-like odour. Hydrazine is highly hazardous unless handled in solution as, for example, hydraz ...

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 aldehydes. ...

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' rea ...

give positive results with

formic acid Formic acid (), systematically named methanoic acid, is the simplest carboxylic acid. It has the chemical formula HCOOH and structure . This acid is an important intermediate in chemical synthesis and occurs naturally, most notably in some an ...

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 macroscopic, microscopic, biochemical, immunologic and molecular examination ...

, ammonical silver nitrate is used in the Fontana–Masson stain, which is a silver stain technique used to detect

melanin Melanin (; ) is a family of biomolecules organized as oligomers or polymers, which among other functions provide the pigments of many organisms. Melanin pigments are produced in a specialized group of cells known as melanocytes. There are ...

, argentaffin, and

lipofuscin Lipofuscin is the name given to fine yellow-brown pigment Granule (cell biology), granules composed of lipid-containing residues of Lysosome, lysosomal digestion. It is considered to be one of the aging or "wear-and-tear" pigments, found in the l ...

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 sugar with the Chemical formula#Molecular formula, molecular formula , which is often abbreviated as Glc. It is overall the most abundant monosaccharide, a subcategory of carbohydrates. It is mainly made by plants and most algae d ...

(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 ac ...

stabilised in

hydrochloric acid Hydrochloric acid, also known as muriatic acid or spirits of salt, is an aqueous solution of hydrogen chloride (HCl). It is a colorless solution with a distinctive pungency, pungent smell. It is classified as a acid strength, strong acid. It is ...

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 tamarinds, bananas, avocados, and citrus. Its salt (chemistry), salt, potassium bitartrate, commonly known as cream of ta ...

and

ethanol Ethanol (also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound with the chemical formula . It is an Alcohol (chemistry), alcohol, with its formula also written as , or EtOH, where Et is the ps ...

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 fr:Réaction de Tollens pl: Próba Tollensa Silver compounds Oxidizing agents Coordination complexes Chemical tests Analytical reagents Ammine complexes