Silvering mirrors

Tollens reagent An ammoniacal solution of silver oxide which is used as a lest for aldehydes, which, unlike ketones, cause the deposition of a silver mirror. 

Each newly cleaved mica surface is very clean. Flowever, it is known that mica has a strong tendency to spontaneously adsorb particles [45] or organic contaminants [46], which may affect subsequent measurements. The mica sheets are cut into 10 nun x 10 nun sized samples using a hot platinum wire, then laid down onto a thick and clean 100 nun x 100 nun mica backing sheet for protection. On the backing sheet, the mica samples can be transferred into a vacuum chamber for themial evaporation of typically 50-55 mn thick silver mirrors. 

Partial reflections at the iimer optical interfaces of the interferometer lead to so-called secondary and tertiary fringe patterns as can be seen from figure B 1.20.4. These additional FECO patterns become clearly visible if the reflectivity of the silver mirrors is reduced. Methods for analysis of such secondary and tertiary FECO patterns were developed to extract infonnation about the topography of non-unifonn substrates [54]. 

Give silver mirror with ammoniacal silver nitrate. 

Reduction of ammoniacal silver nitrate. Place about 5 ml. of AgNOj solution in a thoroughly clean test-tube, and add 2-3 drops of dil. NaOH solution. Add dil. ammonia solution, drop by drop, until the precipitated silver oxide is almost redissolved, then add 2 - 3 drops of formaldehyde or acetaldehyde. A silver mirror is formed. 

Reduction of ammoniacal silver nitrate. Add a few drops of a neutral solution of a formate to ammoniacal AgNO (see Test 4, p. 342). A silver mirror or more usually a grey precipitate of metallic sih er is produced on boiling. 

Reduction of ammoniacal silver nitrate. Add i drop of dil. NaOH solution to about 5 ml. of AgNO, solution, and add dil. NH solution drop by drop until the silver oxide is almost redissolved. Add AgNO, solution until a faint but permanent precipitate is obtained (see p.525). Then add 0 5 ml. of a neutral tartrate solution. Place the tube in warm water a silver mirror is formed in a few minutes. 

Reduction of ammoniacal silver nitrate. Place 2 ml. of dilute silver nitrate solution in a clean test-tube. Add 1 drop of NaOH solution and then add dil. ammonia drop by drop until the precipitate formed by the NaOH is just not redissolved. Now add 1-2 ml. of glucose solution and place the test-tube in a water-bath at 50-60° a silver mirror is produced in 1 - 2 minutes. 

Does not reduce ammoniacal silver nitrate or Fehling s solution. If, however, the sucrose solution is warmed for some time with the reagent in question, slight hydrolysis to glucose and fructose does take place and reduction then occurs occasionally samples of sucrose will rapidly give a silver mirror, presumably owing to impurities. 

Oxidation, (a) Ammoniacal silver nitrate. To a few ml. of ammoniacal AgNOj (preparation, p. 525), add a few drops of cold aqueous benzo quinone solution a silver mirror or (more generally) a dark precipitate of metallic silver is formed in the cold. 

Formation of silver mirror or precipitate of silver indicates reducing agent. (This is often a more sensitive test than I (a) above, and some compounds reduce ammoniacal silver nitrate but are without effect on Fehling s solution.) Given by aldehydes and chloral hydrate formates, lactates and tartrates reducing sugars benzoquinone many amines uric acid. 

Dissolve 0-5 g. of the substance in 10 ml. of 50 per cent, alcohol, add 0-5 g. of solid ammonium chloride and about 0 -5 g. of zinc powder. Heat the mixture to boiling, and allow the ensuing chemical reaction to proceed for 5 minutes. Filter from the excess of zinc powder, and teat the filtrate with Tollen s reagent Section 111,70, (i). An immediate black or grey precipitate or a silver mirror indicates the presence of a hydroxyl-amine formed by reduction of the nitro compound. Alternatively, the filtrate may be warmed with Fehling s solution, when cuprous oxide will be precipitated if a hydroxylamine is present. Make certain that the original compound does not aflfect the reagent used. 

Sputtered silver mirrors are used for solar energy (qv) collectors and astronomical telescope mirrors. Approximately 3 t/yr of silver are used in the United States for low emissivity windows. 

Readions. — i. Adda few chops of the acid solution or solution of the calcium salt to a few c.c. of ammonia-silver nitrate and warm in hot water. A silver mirror is deposited. 

Apart from its structural interest, SnCl2 is important as a widely used mild reducing agent in acid solution. The dihydrate is commercially available for use in electrolytic tin-plating baths, as a sensitizer in silvering mirrors and in the plating of plastics, and as a perfume stabilizer in toilet soaps. The anhydrous material can be obtained either by dehydration using acetic... 

Dichromate oxidation of secondary alcohols produces ketones in good yield, with little additional oxidation. For example, CH,CH2CH(OH)CH3 can be oxidized to CH CH2COCH3. The difference between the ease of oxidation of aldehydes and that of ketones is used to distinguish them. Aldehydes can reduce silver ions to form a silver mirror—a coating of silver on test-tube walls—with Tollens reagent, a solution of Ag1" ions in aqueous ammonia (Fig. 19.3) ... 

FIGURE 19.3 An aldehyde (left) produces a silver mirror with Tollens reagent, but a ketone (right) does not. 

Front-silvered mirrors can be pohshed with optical rouge on a pad of chamois leather over cotton wool. Care is necessary because it is easy to polish completely through the silver. Back-silvered mirrors can be protected by first varnishing the silver and then painting the varnish. Front-silvered mirrors can be protected with a thin coat of lacquer, such as a dilute solution of cellulose nitrate in amyl acetate. The lacquer should not be too thin, or interference colours may be produced. 

In the presence of a saturated tetraaza macrocycle such as cyclam, disproportionation of Ag(i) occurs to produce a silver mirror and a stable Ag(n) complex of the macrocycle (Kestner Allred, 1972 Barefield Mocella, 1973). In some cases the Ag(n) complexes so formed may then be oxidized further to Ag(m) species either electrochemically or chemically [using nitrosyl (NO+) salts]. 

Figure 3.1 Schematic diagram of an AAS spectrometer. A is the light source (hollow cathode lamp), B is the beam chopper (see Fig. 3.2), C is the burner, D the monochromator, E the photomultiplier detector, and F the computer for data analysis. In the single beam instrument, the beam from the lamp is modulated by the beam chopper (to reduce noise) and passes directly through the flame (solid light path). In a double beam instrument the beam chopper is angled and the rear surface reflective, so that part of the beam is passed along the reference beam path (dashed line), and is then recombined with the sample beam by a half-silvered mirror.

Lithium containing films were unusual in that the film after curing was damp on the surface with what appeared to be the solvent, DMAC. No other films exhibited this property. The AgNO containing film had the appearance of a silver mirror but the film was exceedingly brittle and "flaky-like". Only two truly flexible films were produced from BTDA + m,m -DABP. These contained Al(acac)3 and NiCl2 6H2O respectively. 

The "silver mirror test" is used to distinguish an aldehyde from a ketone. Tollen s reagent, Ag(NH3)20H, acts as an oxidizing agent. When it is mixed with an aldehyde, the aldehyde oxidizes to the salt of a carboxylic acid. The silver ions in Tollen s reagent are reduced to silver atoms, and coat the glass of the reaction container with solid silver metal. 

Uses Manufacture of acetic acid, acetic anhydride, aldol, aniline dyes, 1-butanol, 1,3-butylene glycol, cellulose acetate, chloral, 2-ethylhexanol, paraldehyde, pentaerythritol, peracetic acid, pyridine derivatives, terephthalic acid, trimethylolpropane, flavors, perfumes, plastics, synthetic rubbers, disinfectants, drugs, explosives, antioxidants, yeast silvering mirrors hardening gelatin fibers. 

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