Black silver sulfide

SILVER TARNISHES WHEN IT IS EXPOSED TO SULFUR. PLACE A FEW CRYSTALS OF SODIUM THIOSULFATE ("HYPO"] ON A SILVER COIN. HEAT UNTIL HYPO MELTS. WASH. HYPO HAS LEFT STAIN OF BROWN-BLACK SILVER SULFIDE. 

The deposition of silver in tissues is the result of the precipitation of insoluble silver salts, such as silver chloride and silver phosphate. These insoluble silver salts appear to be transformed into soluble silver sulfide albuminates, to bind to or form complexes with amino or carboxyl groups in RNA, DNA, and proteins, or to be reduced to metallic silver by ascorbic acid or catecholamines (Danscher 1981). The blue or gray discoloration of skin exposed to ultraviolet light in humans with argyria may be caused by the photoreduction of silver chloride to metallic silver. The metallic silver is then oxidized by tissue and bound as black silver sulfide (Danscher 1981). Bucklet et al. (1965) identified silver particles deposited in the dermis of a woman with localized argyria as being composed of silver sulfide. 

Metal salts can be used in two ways. First, pretreatment with a trihydroxyben-zene compound such as pyrogallol [87-66-1] is followed by treatment with an ammoniacal silver salt solution. This allows rapid dyeing due to the formation of metallic silver and oxidation products of the trihydroxybenzene derivative. If pre-treatment is performed with thiosulfate instead of a benzene derivative, the process yields unstable silver thiosulfate and finally black silver sulfide. 

The black silver sulfide discoloration of silverware can easily be removed by heating the silver article in a sodium carbonate solution in an aluminum pan. The reaction is... 

Figure 5 Keeping silver bright takes frequent polishing, especially in homes heated by gas. Sulfur compounds found in small concentrations in natural gas react with silver, forming black silver sulfide, Ag2S. 

Why do silver utensils have to be polished, but those made of stainless steel or aluminum don t Silver becomes tarnished through a redox reaction that is a form of corrosion, as rusting is. Tarnish is formed on the surface of a silver object when silver reacts with H2S in air. The product, black silver sulfide, forms the coating of tarnish on the silver. 

In the case of silver turning black , experiments can be used to show reactions of some metals with yellow powder of sulfur to form black sulfides copper sulfide and silver sulfide. One finds the same black silver sulfide, which is caused by the reaction between silver cutlery like knives, spoons and forks with hydrogen sulfide from the air one can remove the black silver sulfide from the surface of pure silver. 

It is possible to recover the silver by heating the black silver sulfide (see E3.3). Similarly, copper can also be recovered from copper oxide through the reaction with hydrogen (see E3.4). However, this leads to the question of how the elements are stored in metal compounds (see Sect. 2.2). This question is difficult and only answered at a level of Dalton s atomic model metal atoms and non-metal atoms remain during these reactions. 

Formation of black silver sulfide is visible on silver wool when 10 mg of silver wool was placed in about 1.5 g of the reference gasoline containing 2.6 ppm elemental sulfur and heated as described above. No visible formation of silver sulfide occurred with silver wool in the reference fuel prior to the addition of elemental sulfur. This is illustrated in Fig. 5, where A represents silver wool heated in the reference fuel versus B, which represents silver wool heated in the reference fuel that contained 2.6 ppm elemental sulfur. Even though the pictures in Fig. 5 were obtained under magnification, the silver corrosion was readily apparent to visual inspection without magnification. The surface of the silver wool became even darker when exposed to higher levels of elemental sulfur as one would expect, and it appeared that the entire surface became coated at about the 10-15 ppm level. Further experimentation was not pursued in this area, as it was demonstrated that the more qualitative silver wool test and quantitative GC analysis appear to be correlated. One could expect to find an optimum ratio between the amount of sample and silver wool to maximize the visible contrast that is observed, however. 

Silver is very ductile and malleable, almost as much as gold. It does not react with water and air at ordinary temperatures. Silver is resistant to aUcaHs and most adds. In contrast to gold, however, it is dissolved by nitric acid. Sulfur and sulfides tarnish silver owing to the formation of black silver sulfide. Contact between silver objects and eggs must be avoided as they contain much sulfur in their protein. 

Sulfites, thiosulfates, and sulfides interfere because they are decomposed by acids the resulting SOg or HgS reduces ferric salts. Black silver sulfide is formed in the case of sulfides. These interferences can be avoided by preliminary oxidation of the neutral or alkaline test solution with several drops of hydrogen peroxide. 

---