SILVERING SOLUTIONS

Do not heat the silver solution or allow it to stand even for a few hours, since explosive silver fulminate may be formed. The ammoniacal solution of silver nitrate is prepared by treating 3 ml. of 0-lN silver nitrate solution with very dilute ammonia solution dropwise until the precipitate which is first formed just redissolves. 

The procedure may be illustrated by the following simple experiment, which is a modification of the Gay Lussac-Stas method. The sodium chloride solution is added to the silver solution in the presence of free nitric acid and a small quantity of pure barium nitrate (the latter to assist coagulation of the precipitate). 

A silvering solution exploded when disturbed. This is a particularly dangerous mixture, because both silver nitride and silver fulminate could be formed. 

During preparation of an oxidising agent on a larger scale than described [1], addition of warm sodium hydroxide solution to warm ammoniacal silver nitrate with stirring caused immediate precipitation of black silver nitride which exploded [2], Similar incidents had been reported previously [3], including one where explosion appeared to be initiated by addition of Devarda s alloy (Al—Cu—Zn) [4], The explosive species separates at pH values above 12.9, only produced when alkali is added to ammoniacal silver solutions, or when silver oxide is dissolved with ammonia [5], The Sommer Market reagent mixture used to identify cellulose derivatives led to a severe explosion [6],... 

See Silver nitride, also Ammonia Silver compounds See also silvering solutions, tollens reagent... 

See entries SILVER-CONTAINING EXPLOSIVES, SILVERING SOLUTIONS... 

Li Y.S., Cheng J.C., Coons L.B., A silver solution for surface-enhanced Raman scattering, Spectrochimica Acta Part A 1999 55 1197-1207. 

Experiments.—Being a primary hydrazide (of carbamic acid), semicarbazide reduces ammoniacal silver solutions and Fehling s solution. It reacts readily with aldehydes and ketones with the elimination of water and formation of semicarbazones, which, since they are more easily hydrolysed than are phenylhydrazones and oximes, are to be preferred to the latter for purposes of separation and purification of carbonyl compounds. Shake an aqueous solution of the hydrochloride (prepared as described above) with a few drops of benzaldehyde, isolate the semicarbazone and purify it by recrystallisation from alcohol. Melting point 214° decomp. Benzaldehyde semicarbazone is decomposed into its constituents by gentle warming with concentrated hydrochloric acid. 

The aldehydes are readily oxidised and therefore behave towards ammoniacal silver solution and towards Fehling s solution as reducing agents. 

Experiment 1.—Dilute a few drops of formaldehyde or acetaldehyde with a few c.c. of water, add a small amount of ammoniacal silver solution, and divide the mixture between two test tubes. Into one test tube run a few drops of sodium hydroxide solution an immediate separation of metallic silver takes place. From the other solution after standing for some time in the cold, or more quickly on warming, the silver separates. Thus the oxidising action of ammoniacal silver solution is very considerably increased by sodium hydroxide (Tollens). Also test the reducing action of the aldehydes on Fehling s solution. 

Test the behaviour of phenylhydrazine towards Fehling s solution and towards ammoniacal silver solution. 

Silver solutions used in photography can become explosive under a variety of conditions. Ammoniacal silver nitrate solutions, on storage, heating or evaporation eventually deposit silver nitride ( fulminating silver ). Silver nitrate and ethanol may give silver fulminate, and in contact with azides or hydrazine, silver azide. These are all dangerously sensitive explosives and detonators [1], Addition of ammonia solution to silver containing solutions does not directly produce explosive precipitates, but these are formed at pH values above 12.9, produced by addition of alkali, or by dissolution of silver oxide in ammonia [2]. 

Brashear s silvering solution (alkaline ammoniacal silver oxide containing glucose) or residues therefrom should not be kept for more than 2 hours after preparation, since an explosive precipitate forms on standing [1], The danger of explosion may... 

Figure 6.12 shows the IR spectra of Ag/Si02 composites prepared by different methods calcined at 550°C. Because it appears no difference in chemical bond indicates physical structures of the three composite systems are similar. The IR spectra of Ag/Si02 composites prepared by the two-step method are shown in Fig. 6.13. The mixture ofthe silver solution and Si02 solution exhibits two peaks at 1382cm and 949 cm which can be attributed respectively to the vibration of and the vibration...

A simple non-aqueous reference electrode is made up to contain 1x10 mol of Ag in 1.0 cm of MeCN, i.e. at a concentration of 1 x 10 mol dm . In contact with the silver solution is a silver wire. During a coulometric experiment, 5x10 faradays of charge (i.e. moles of electrons) are passed oxidatively through the solution. 

Under the usual conditions of commercial practice, the development reaction does not occur entirely at the silver/silver halide interface. Some reduction of silver ions from solution takes place. Such reduction presumably can occur at any point on the silver/solution interface, and the mechanism should be the same as that for post-fixation physical development. The relative extent of the physical development in comparison with that at the silver/silver halide interface will depend upon the silver halide solvent action of the developing solution and upon the rate of the direct development. 

The liquid is now neutralised with baryta, and barium nitrate is added, so long as a precipitate of barium sulphate is formed this is filtered off and washed. The filtrate is concentrated to 300 c.c. and treated with silver nitrate, as before, till a test drop gives a yellow colour with baryta when this occurs it is exactly neutralised with baryta, and from a burette small quantities of baryta are added till the silver salt of histidine is completely precipitated this is determined by taking out a drop when the precipitate has settled and testing with ammoniacal silver solution if a precipitate easily soluble in excess of ammonia be formed, when the two liquids come together, histidine is still present and more baryta water must be added, until it is completely thrown out, when it is filtered off, stirred up with water, again filtered off and washed out. 

Silver chloride is used in silver plating and to obtain pure silver. The salt also finds applications in photography and optics in photochromic glass and in electrodes and batteries. It is used to make antiseptic silver solution. It occurs as the mineral cerargyrite. 

It will be noted that in the derivation of the transverse potential difference the product ijv should be constant for the same system under uniform conditions. A change in tj can be produced most conveniently by alteration of the temperature. Burton (Physical Properties of Colloidal Solutions, p. 145) gives the following data for colloidal silver solutions in support of the validity of the equation. 

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