Thiosulfate complexes

Silver Thiosulfate. Silver thiosulfate [23149-52-2], Ag 2 y is an insoluble precipitate formed when a soluble thiosulfate reacts with an excess of silver nitrate. In order to minimize the formation of silver sulfide, the silver ion can be complexed by haUdes before the addition of the thiosulfate solution. In the presence of excess thiosulfate, the very soluble Ag2(S203) 3 and Ag2(S203) 3 complexes form. These soluble thiosulfate complexes, which are very stable, are the basis of photographic fixers. Silver thiosulfate complexes are oxidized to form silver sulfide, sulfate, and elemental sulfur (see Thiosulfates). 

Silver compounds having anions that are inherently toxic, eg, silver arsenate and silver cyanide, can cause adverse health effects. The reported rat oral LD values for silver nitrate, silver arsenate [13510-44-6] and silver cyanide are 500—800 (29), 200—400 (29), and 123 mg/kg (30), respectively. Silver compounds or complexes ia which the silver ion is not biologically available, eg, silver sulfide and silver thiosulfate complexes, are considered to be without adverse health effects and essentially nontoxic. 

The impact that a silver compound has in water is a function of the free or weaMy complexed silver ion concentration generated by that compound, not the total silver concentration (3—5,27,40—42). In a standardized, acute aquatic bioassay, fathead minnows were exposed to various concentrations of silver compounds for a 96-h period and the concentration of total silver lethal to half of the exposed population (96-h LC q) deterrnined. For silver nitrate, the value obtained was 16 )-lg/L. For silver sulfide and silver thiosulfate complexes, the values were >240 and >280 mg/L, respectively, the highest concentrations tested (27). 

The chronic aquatic effects which relate silver speciation to adverse environmental effects were studied on rainbow trout eggs and fry. The maximum acceptable toxicant concentration (MATC) for silver nitrate, as total silver, was reported to be 90—170 ng/L (43). Using fathead minnow eggs and fry, the MATC, as total silver, for silver thiosulfate complexes was reported as 21—44 mg/L, and for silver sulfide as 11 mg/L, the maximum concentration tested (27). 

In secondary wastewater treatment plants receiving silver thiosulfate complexes, microorganisms convert this complex predominately to silver sulfide and some metallic silver (see Wastes, INDUSTRIAL). These silver species are substantially removed from the treatment plant effluent at the settling step (47,48). Any silver entering municipal secondary treatment plants tends to bind quickly to sulfide ions present in the system and precipitate into the treatment plant sludge (49). Thus, silver discharged to secondary wastewater treatment plants or into natural waters is not present as the free silver ion but rather as a complexed or insoluble species. 

Other Uses. Photochromic glass contains silver chloride (80) and silver molybdate [13765-74-7] (81) (see Chromogenic materials). An apparatus coated with silver nitrate has been described for the detection of rain or snow (82). Treatment with silver-thiosulfate complex has been reported as dramatically increasing the post-harvest life of cut carnations (83). Silver sulfate has been used in the electrolytic coloring of aluminum (84). Silver sulfate also imparts a yellowish red color to glass bulbs (85). 

Stmcture (1) explains the formation of sulfur and sulfite in the presence of acid stmcture (2) is consistent with the formation of sulfide and sulfate in the presence of heavy metals. The bonding in thiosulfate complexes and the chemistry of thiosulfates are normally explained on the basis of (2) (see also... 

Gold thiosulfate complexes of the form Na2[Au(S202)2] 2H20 [19153-98-1] are prepared by addition of gold trichloride to concentrated sodium thiosulfate solution (89). The gold is completely reduced and some thiosulfate is oxidized to tetrathionate. This complex has been used in the treatment of rheumatoid arthritis. 

Uptake of small organic metal complexes over transport systems of organic metabolites may be possible, for example, of small organic acids like citrate or amino acids. However, only few examples of such processes have been studied so far. Increased uptake of cadmium by an alga has been observed in the presence of citrate and has been attributed to accidental transport of the metal-citrate complex over a citrate transporter [212]. Transport systems of inorganic anions may also play a role in metal transport. Silver uptake by algae was enhanced in the presence of thiosulfate. In this case, the silver thiosulfate complex was transported over a sulfate uptake system [213]. It remains to be demonstrated how widespread these processes may be for metal uptake in the aquatic environment [12]. 

Initial stages of Ag electrocrystallization from silver thiosulfate complexes have been studied by Simons et al. [368] and Gonnissen et al. [369] The formation of subcritical 2D silver clusters, preceding the formation of supercritical, stable 3D nuclei and their growth, has been suggested. 

The complex Na2Hg(S03)2-H20 has been prepared. It contains discrete Hg(S03)2 anions with S-bonded S03 units (Hg—S = 240.2,241.1 pm).393,394 Mercury(II) thiosulfate complexes obtained in solution also appear to be S bonded.395 Mercury(II) forms numerous compounds with different sulfur donor ligands. Many of them have been reviewed by McAuliffe7 and by Constable.5... 

Golddll) chloride or tetrachloroaurates(lll) also form thiosulfate complexes. especially in the presence of Nal. of the form Na dAutS Oibl. in which the gold is monovalenL. 

This reaction prevails with the deposition of metallic Cd (Eq. (6.5)), as expected from its more positive equilibrium potentia- Howevei, this thiosulfate complex is chemically unstable, providing another route for the formation of CdS. 

Koslowsky in 1936 observed that certain gold salts, such as aurous thiocyanate and aurous thiosulfate complexes, are sensitizers, but this discovery remained a trade secret for several years (157-159). Gold salts used alone are relatively poor sensitizers, but in combination with sulfur sensitizers they can increase the sensitivity beyond that obtainable by S-sensitiza-tion alone (160). 

Scow et al. (1981) reported that the median silver concentrations in sewage treatment plant influent and effluent were 0.008 mg/L and 0.002 mg/L, respectively. Treated effluents from a large photographic processing plant contained an average of 0.07 mg/L silver (range <0.02-0.30 mg/L) in the form of silver thiosulfate complexes, silver bromide, and silver sulfide (Bard et al. 1976). 

When the film paper is exposed to light, silver ions are reduced to silver atoms. Next excess silver ions are removed by combining them with thiosulfate ions to form a silver thiosulfate complex ion. This happen when the exposed film paper is placed in a hypo solution. To completely stop the developing process, the film paper is next placed in a fixer such as acetic acid to neutralize the basic hypo solution. 

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