Tollens' reagent
Encyclopedia
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Aldehyde
An aldehyde is an organic compound containing a formyl group. This functional group, with the structure R-CHO, consists of a carbonyl center bonded to hydrogen and an R group....
or a ketone
Ketone
In organic chemistry, a ketone is an organic compound with the structure RCR', where R and R' can be a variety of atoms and groups of atoms. It features a carbonyl group bonded to two other carbon atoms. Many ketones are known and many are of great importance in industry and in biology...
. It is usually ammoniacal silver nitrate, but can also be other mixtures, as long as aqueous
Aqueous solution
An aqueous solution is a solution in which the solvent is water. It is usually shown in chemical equations by appending aq to the relevant formula, such as NaCl. The word aqueous means pertaining to, related to, similar to, or dissolved in water...
diamminesilver(I) complex
Complex (chemistry)
In chemistry, a coordination complex or metal complex, is an atom or ion , bonded to a surrounding array of molecules or anions, that are in turn known as ligands or complexing agents...
is present. It was named after its discoverer, Bernhard Tollens
Bernhard Tollens
Bernhard Christian Gottfried Tollens was a German chemist.- Life and work :Tollens attended school at the Gelehrtenschule des Johanneums in Hamburg where he was influenced by his science teacher, Karl Möbius. After graduating in 1857, Tollens started an apprenticeship in pharmacy...
.
A positive test with Tollens' reagent results in elemental silver precipitating out of solution, occasionally onto the inner surface of the reaction vessel, producing a characteristic and memorable "silver mirror" on the inner vessel surface.
Laboratory preparation
This reagent is not commercially available due to its short shelf life; it must be prepared freshly in the laboratory. One common preparation involves two steps. First a few drops of dilute sodium hydroxide is added to some aqueous silver nitrateSilver nitrate
Silver nitrate is an inorganic compound with chemical formula . This compound 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...
. In this solution, the Ag+ ions from the aqueous silver nitrate exist in a hydrated form as [Ag(H2O)2]+ complexes, i.e. diaquasilver(I) ion. The OH- ions from the sodium hydroxide deprotonate the water ligand
Ligand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...
s that are attached to the Ag+ ions, to give [Ag(OH)2]- ions. These dihydroxyargentate(I) complexes are not at all stable, and immediately they are dehydrated further to give silver oxide, Ag2O. This is insoluble, and precipitates out of the solution as a brown solid. Aqueous sodium nitrate
Sodium nitrate
Sodium nitrate is the chemical compound with the formula NaNO3. This salt, also known as Chile saltpeter or Peru saltpeter to distinguish it from ordinary saltpeter, potassium nitrate, is a white solid which is very soluble in water...
is also produced in the mixture as a by-product.
- 2 AgNO3 (aq) + 2 NaOH (aq) → Ag2O (s) + 2 NaNO3 (aq) + H2O (l)
In the next step, aqueous ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
is added until all of the brown silver(I) oxide is dissolved. At this point the mixture will be clear, and there are now aqueous silver ions existing as [Ag(NH3)2]+ complexes in the mixture, which is the main component of Tollens' reagent. Sodium hydroxide is reformed at the end of the preparation.
- Ag2O (s) + 4 NH3 (aq) + 2 NaNO3 (aq) + H2O (l) → 2 Ag(NH3)2NO3 (aq) + 2 NaOH (aq)
Alternatively, aqueous ammonia can be added in a continuous fashion directly to silver nitrate solution. At first, silver oxide will be formed and precipitate out, but as more ammonia solution is added the precipitate dissolves and the solution becomes clear as diamminesilver(I) is formed. At this point the addition of the ammonia should be stopped. This may be a preferable method as less reagents are involved. Filtering the reagent before use helps to prevent false-positive results.
Analytical Uses
Once it has been ascertained that there is a carbonyl group on an organic molecule using 2,4-dinitrophenylhydrazine2,4-Dinitrophenylhydrazine
2,4-Dinitrophenylhydrazine is the chemical compound C6H32NHNH2. Dinitrophenylhydrazine is relatively sensitive to shock and friction; it is a shock explosive so care must be taken with its use. It is a red to orange solid, usually supplied wet to reduce its explosive hazard...
(also known as Brady's reagent or 2,4-DNPH), Tollens' reagent can be used to determine whether the compound is a ketone
Ketone
In organic chemistry, a ketone is an organic compound with the structure RCR', where R and R' can be a variety of atoms and groups of atoms. It features a carbonyl group bonded to two other carbon atoms. Many ketones are known and many are of great importance in industry and in biology...
or an aldehyde
Aldehyde
An aldehyde is an organic compound containing a formyl group. This functional group, with the structure R-CHO, consists of a carbonyl center bonded to hydrogen and an R group....
. Importantly, there is a special case in which Tollens' reagent will give a positive for a ketone; if the ketone is an alpha-hydroxy ketone, then the Tollens' reagent will react.
The test rests on the premise that aldehydes are more readily oxidised compared with ketones; this is due to the carbonyl-containing carbon in aldehydes having an attached hydrogen. The diamminesilver(I) complex in the mixture is an oxidizing agent
Oxidizing agent
An oxidizing agent can be defined as a substance that removes electrons from another reactant in a redox chemical reaction...
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 diamminesilver(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.
- [Ag(NH3)2]+ (aq) + e- → Ag (s) + 2 NH3 (aq)
- RCHO (aq) + 3 OH- → RCOO- + 2 H2O + 2 e-
The negative result for the test is no precipitate of silver formed when the carbonyl to be tested is added. A ketone will give a negative result because it cannot be oxidized easily. A ketone has no available hydrogen atom attached to the carbonyl carbon, meaning it can't be so easily oxidized - unlike an aldehyde, which has this hydrogen atom.
Tollens' reagent is also a test for alkyne
Alkyne
Alkynes are hydrocarbons that have a triple bond between two carbon atoms, with the formula CnH2n-2. Alkynes are traditionally known as acetylenes, although the name acetylene also refers specifically to C2H2, known formally as ethyne using IUPAC nomenclature...
s with a triple bond in the 1-position. A yellow precipitate of the metal acetylide
Metal acetylide
Acetylide, ethynide, dicarbide, or percarbide is the divalent anion with formula C22− or 2−. It may be regarded as the result of removing two protons from acetylene C2H2 or H-C≡C-H, the prototypical alkyne — that behaves as a weak acid.These terms are also used for any monovalent anion...
is formed in this case.
Both Tollens' reagent and Fehling's reagent also give positive results with formic acid
Formic acid
Formic acid is the simplest carboxylic acid. Its chemical formula is HCOOH or HCO2H. It is an important intermediate in chemical synthesis and occurs naturally, most notably in the venom of bee and ant stings. In fact, its name comes from the Latin word for ant, formica, referring to its early...
(methanoic acid - HCOOH), which is fully oxidised to water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
and carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
.
In anatomic pathology, ammoniacal silver nitrate is used in the Fontana-Masson Stain, which is a silver stain
Silver stain
Silver staining is the use of silver to selectively alter the appearance of the target.-Use in medicine:It is used to stain histologic sections. This kind of staining is important especially to show proteins and DNA. It is used to show both substances inside and outside cells...
technique used to detect melanin
Melanin
Melanin is a pigment that is ubiquitous in nature, being found in most organisms . In animals melanin pigments are derivatives of the amino acid tyrosine. The most common form of biological melanin is eumelanin, a brown-black polymer of dihydroxyindole carboxylic acids, and their reduced forms...
, argentaffin
Argentaffin
Argentaffin refers to cells which take up silver stain. Enteroendocrine cells are sometimes called "argentaffins", because they take up this stain....
and lipofuscin
Lipofuscin
Lipofuscin is the name given to finely granular yellow-brown pigment granules composed of lipid-containing residues of lysosomal digestion. It is considered one of the aging or "wear-and-tear" pigments, found in the liver, kidney, heart muscle, adrenals, nerve cells, and ganglion cells...
in tissue sections. Melanin and the other chromaffins reduce the silver nitrate to metallic silver.
In Silver Mirroring
Tollens' reagent is also used to apply a silver mirror to glassware; for example the inside of an insulated vacuum flask. Around 500 mL of solution is prepared, much more than would be made for analytical use. This is then introduced to the clean glass surface which is to be mirrored and the solution reduced using a glucose solution. For a high quality finish the glass surface is cleaned using an oxidizing acid to remove all traces of organic compounds and the glass surface is pretreated with aqueous tin(II) chlorideTin(II) chloride
Tin chloride is a white crystalline solid with the formula 2. It forms a stable dihydrate, but aqueous solutions tend to undergo hydrolysis, particularly if hot. SnCl2 is widely used as a reducing agent , and in electrolytic baths for tin-plating...
.
Safety
The reagent should be freshly prepared and stored refrigerated in a dark glass container. It has an approximate shelf-life of 24 hours when stored in this way. After the test has been performed, the resulting mixture should be acidified with dilute acid before disposal. These precautions are to prevent the formation of the highly explosive silver nitrideSilver 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 a silver-amide or imide complex which subsequently breaks down to Ag3N...
.
See also
- Benedict's reagentBenedict's reagentBenedict's reagent is a chemical reagent named after an American chemist, Stanley Rossiter Benedict....
- Fehling's solutionFehling's solutionFehling's solution is a chemical test used to differentiate between water-soluble aldehyde and ketone functional groups, and as a test for monosaccharides. The test was developed by German chemist Hermann von Fehling in 1849.-Laboratory preparation:...
- Walden reductorWalden reductorThe Walden reductor is a reduction column filled with metallic silverwhich can be used to reduce a metal ion in aqueous solution to a lower oxidation state. It can be used e.g. to reduce UO22+ in U4+. The method is named after George H. Walden who developed together with a Ph.D. student Sylvan M...
(opposite use involving metallic silver)
