Silver cyanide, precipitation

Hydrocyanic acid may be approximately estimated by dissolving 1 gram of oil in 5 c.c. of alcohol, and adding 50 c.c. of water. Then add ammonio-silver nitrate solution and shake well. Acidify slightly with nitric acid, and collect, wash, and dry the silver cyanide precipitated. Ignite and weigh the silver, 4 parts of which correspond to practically 1 of hydrocyanic acid. 

The only difficulty in obtaining a sharp end point lies in the fact that silver cyanide, precipitated by local excess concentration of silver ion somewhat prior to the equivalence point, is very slow to re-dissolve and the titration is time-consuming. In the Deniges modification, iodide ion (usually as KI, ca 0.01 M) is used as the indicator and aqueous ammonia (ca 0.2M) is introduced to dissolve the silver cyanide. 

The value of the solubility product of silver cyanide is A s(AgCN)= 10 -. It is very insoluble. Once the first drop of silver nitrate solution is added, the silver cyanide precipitates according to the reaction... 

REACTIONS AND IDENTIFICATION 325 drop. JNote that the white precipitate of silver salts turns black (showing the presence of silver cyanide). Now add an excess of mercurous nitrate the black precipitate redissolves, but the silver halide remains undissolved. 

Silver Cyanide. Silver cyanide, AgCN, forms as a precipitate when stoichiometric quantities of silver nitrate and a soluble cyanide are mixed. Sdver(I) ion readily forms soluble complexes, ie, Ag(CN) 2 01 Ag(CN) 2> die presence of excess cyanide ion. 

The flake cyanide is dissolved and added to suspensions of finely ground ore under agitation and ia the presence of air. The dissolved gold and silver are precipitated by the addition of 2iac dust and the cmde precipitate is refined to produce bullion. 

A soluble cyanide added to silver nitrate solution precipitates silver cyanide as an ionic compound ... 

When potassium cyanide solution is added to a solution of silver nitrate, a white precipitate of silver cyanide is first formed because the solubility product of silver cyanide ... 

When the above reaction is complete, further addition of silver nitrate solution yields the insoluble silver cyanoargentate (sometimes termed insoluble silver cyanide) the end point of the reaction is therefore indicated by the formation of a permanent precipitate or turbidity. 

Discussion. The theory of the titration of cyanides with silver nitrate solution has been given in Section 10.44. All silver salts except the sulphide are readily soluble in excess of a solution of an alkali cyanide, hence chloride, bromide, and iodide do not interfere. The only difficulty in obtaining a sharp end point lies in the fact that silver cyanide is often precipitated in a curdy form which does not readily re-dissolve, and, moreover, the end point is not easy to detect with accuracy. 

D-Lyxose diacetamide. Ammonia-silver oxide.y Ten grams of pentaaoetyl-n-galactononitrile was dissolved in 30 ml. of ethanol, and a solution of silver oxide (from 5 g. of silver nitrate) in 50 ml. of 30 % ammonia added. After two days at room temperature, the precipitated silver cyanide was removed by filtration and the solution evaporated in vacuo at 40° imtil all ammonia was eliminated. The residue was diluted with water and the soluble silver eliminated by treatment with hydrogen sulfide and filtration. The filtrate was treated with decolorizing carbon, filtered and evaporated to dryness. When the residue crystallized, it was suspended in warm ethanol and filtered yield, 2.5 g. (40%). After recrystallization from 60% ethanol, the product had a melting point of 230-231°. 

In the cyanidation process the ore is crushed and roasted with sodium chloride and then treated with a solution of sodium cyanide. Silver forms a stable silver cyanide complex, [Ag(CN)2]. Adding metallic zinc to this complex solution precipitates sdver. 

Sulphur, as sulphide ion, may be detected by predpitation as black lead sulphide with lead acetate solution and acetic add or by the purple colour produced on addition of disodium pentacyanonitrosoferrate(m). Halogens are detected as the characteristic silver halides by the addition of dilute nitric acid and silver nitrate solution. Cyanide and sulphide ions both interfere with this test for halide by forming silver cyanide and silver sulphide precipitates. If nitrogen or sulphur has been detected, therefore, the interfering ions must be removed by boiling the acidified fusion solution as detailed later, before the silver nitrate solution is added to detect the halogen. 

Silver nitrate Concentrated sulfuric acid, hot Mercury (I) nitrate Copper sulfide White precipitate of silver cyanide Liberation of carbon monoxide (caution highly toxic) Gray precipitate of mercury Formation of colorless tetracyanocuprate (I) ions this test can be done on a section of filter paper... 

Surprisingly, when diphenyl tellurium chloride iodide is stirred with an equimolar amount of silver cyanide in chloroform, silver chloride is precipitated instead of the more insoluble silver iodide and diphenyl tellurium iodide cyanide (m.p. 142°) was isolated in almost quantitative yield1."... 

Dimethylaminophenyl 4-Ethoxypheny] Tellurium Dicyanide7 0.78 g (2 mmol) of 4-dimethylaminophenyl 4-ethoxyphenyl tellurium chloride iodide and 0.6 g (4 mmol) of silver cyanide in 30 ml of chloroform are stirred at 20°. The precipitated silver halides are filtered off and the filtrate is concentrated to give the product yield not given m.p. 192°. 

Silver nitrate solution white precipitate of silver cyanide, AgCN, readily soluble in excess of the cyanide solution forming the complex ion, dicyano-argentate(I) [Ag(CN)2] (cf. Section III.6, reaction 7) ... 

Its heat of formation from the two cyanides is 5-6 Cal.1 Addition of silver chloride to its solution precipitates silver cyanide 2... 

The potential of a silver electrode during the course of the titration of silver nitrate with potassium c an-ide is shown in Fig. 78 the first marked change of potential occurs when one equivalent of cyanide has been added to one of silver, so that the whole of the silver cyanide is precipitated, and the second, when two equivalents of cyanide have been added, corresponds to the complete formation of the Ag(CN)2 ion. It will be seen that the changes of potential occur very sharply in each case this means that the silver cyanide is very slightly soluble and that, t+vo complex ion is very stable. 

Volumetric Methods, (i) Liebig s Method. This depends on the fact that when a neutral or alkaline solution of an alkaline cyanide is treated with silver nitrate solution drop by drop, a white precipitate of silver cyanide forms as each drop enters the solution. This disappears on shaking because the silver cyanide dissolves in the excess of alkali cyanide to form an argento-cyanide ... 

Silver cyanide is a white solid readily formed as a precipitate when solutions of silver are treated with potassium cyanide. It is readily soluble in excess of potassium cyanide forming the double cyanide of silver and potassium. It is also soluble in ammonium hydroxide. 

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