Silver concentrations

In both preceding cases, the demands to the electrolysis unit are limited, since there is no need to keep the silver content in the fixer tank constantly low. A steady state silver concentration in the fixer between 3 and 5 g/1 is acceptable, since this causes no substantial loss of fixation speed. 

It is clear that the freight value depends on the type and amount of film and how the production is spread over time. Figure 3 shows 2 simulations of silver concentration in fixer for different processing regimes. In one case, 10 m of film is processed over 3 hours. In another case, 10 m of film is processed over 16 hours. It is clear that the silver level in the fixer not only depends on the amount of film processed, but also on the distribution of the working load. The simulated daily freight values are 44 and 16 mgW respectively. ... 

In case of cascade fixation, an additional fixing step is used to lower the silver concentration in the fixer which is carried over to the rinse section, as is shown in Figure 4. 

Figure 4 Schematic representation of silver concentration and fixer flow for cascade fixation.

The process is designed such that virtually all silver is fixed in the first fixing step. The fixer in the first fixer section has a typical silver concentration of 7 g/1. The silver in the second fixer section originates from the carry-over from the first fixing section, and a typical silver concentration is 0,4 g/1. Hence, the silver carried over to the rinsing section will be substantially lower. 

Quantitative. Classically, silver concentration ia solution has been determined by titration with a standard solution of thiocyanate. Ferric ion is the iadicator. The deep red ferric thiocyanate color appears only when the silver is completely titrated. GravimetricaHy, silver is determined by precipitation with chloride, sulfide, or 1,2,3-benzotriazole. Silver can be precipitated as the metal by electro deposition or chemical reduciag agents. A colored silver diethjldithiocarbamate complex, extractable by organic solvents, is used for the spectrophotometric determination of silver complexes. 

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). 

A silver concentration cell is constructed with the electrolyte at both electrodes being initially 0.10 M AgNO(aq) at 25°C. The electrolyte at one electrode is diluted by a factor of 10 five times and the emf measured each time, (a) Plot the emf of this cell on a graph as a function of In AgT ln)K,e. 

Thus, co-deposition of silver and copper can take place only when the silver concentration in the electrolyte falls to a very low level. This clearly indicates that the electrolytic process can, instead, be used for separating copper from silver. When both silver and copper ions are present, the initial deposition will mainly be of silver and the deposition of copper will take place only when the concentration of silver becomes very low. Another example worth considering here is the co-deposition of copper and zinc. Under normal conditions, the co-deposition of copper and zinc from an electrolyte containing copper and zinc sulfates is not feasible because of the large difference in the electrode potentials. If, however, an excess of alkali cyanides is added to the solution, both the metals form complex cyanides the cuprocyanide complex is much more stable than the zinc cyanide complex and thus the concentration of the free copper ions available for deposition is considerably reduced. As a result of this, the deposition potentials for copper and zinc become very close and their co-deposition can take place to form alloys. 

Silver concentrations (milligrams of silver per kilogram fresh weight [FW], dry weight [DW], or ash weight [AW]) in field collections of selected plants and animals... 

Table 7.4 Silver Concentrations in Representative Nonbiological Materials... 

Silver is usually found in extremely low concentrations in natural waters because of its low crustal abundance and low mobility in water (USEPA 1980). One of the highest silver concentrations recorded in freshwater (38 pg/L) occurred in the Colorado River at Loma, Colorado, downstream of an abandoned gold-copper-silver mine, an oil shale extraction plant, a gasoline and coke refinery, and a uranium processing facility (USEPA 1980). The maximum recorded value of silver in tapwater in the United States was 26 pg/L — significantly higher than finished water from the treatment plant (maximum of 5.0 pg/L) — because of the use of tin-silver solders for joining copper pipes in the home, office, or factory (USEPA 1980). 

In general, silver concentrations in surface waters of the United States decreased between 1970-74 and 1975-79, although concentrations increased in the north Atlantic, Southeast, and lower Mississippi basins (USPHS 1990). About 30 to 70% of the silver in surface waters may be ascribed to suspended particles (Smith and Carson 1977), depending on water hardness or salinity. For example, sediments added to solutions containing 2 pg Ag/L had 74.9 mg Ag/kg DW sediment after 24 h in freshwater, 14.2 mg/kg DW at 1.5% salinity and 6.9 mg/kg DW at 2.3% salinity (Sanders and Abbe 1987). Riverine transport of silver to the ocean is considerable suspended materials in the Susquehanna River, Pennsylvania — that contained as much as 25 mg silver/kg — resulted in an estimated transport of 4.5 metric tons of silver to the ocean each year (USEPA 1980). The most recent measurements of silver in rivers, lakes, and estuaries using clean techniques show levels of about 0.01 pg/L for pristine, nonpolluted areas and 0.01 to 0.1 pg/L in urban and industrialized areas (Ratte 1999). 

Silver is a normal trace constituent of many organisms (Smith and Carson 1977). In terrestrial plants, silver concentrations are usually less than 1.0 mg/kg ash weight (equivalent to less than 0.1 mg/kg DW) and are higher in trees, shrubs, and other plants near regions of silver mining. Seeds, nuts, and fruits usually contain higher silver concentrations than other plant parts (USEPA 1980). Silver accumulations in marine algae (max. 14.1 mg/kg DW) are due mainly to adsorption rather than uptake bioconcentration factors of 13,000 to 66,000 are not uncommon (USPHS 1990 Ratte 1999). 

Season of collection (Fowler and Oregioni 1976 Sanders etal. 1991) and latitude (Anderlini 1974) also influenced silver accumulations. Seasonal variations in silver concentrations of Baltic clams (Macoma balthica) were associated with seasonal variations in soft tissue weight and frequently reflected the silver content in the sediments (Cain and Luoma 1990). Oysters from the Gulf of Mexico vary considerably in whole-body concentrations of silver and other trace metals. Variables that modify silver concentrations in oyster tissues include the age, size, sex, reproductive stage, general health, and metabolism of the animal water temperature, salinity, dissolved oxygen,... 

Among arthropods, pyrophosphate granules isolated from barnacles have the capability to bind and effectively detoxify silver and other metals under natural conditions (Pullen and Rainbow 1991). In a Colorado alpine lake, silver concentrations in caddisflies and chironomid larvae usually reflected silver concentrations in sediments seston, however, showed a high correlation with lake water silver concentrations from 20 days earlier (Freeman 1979). 

Silver concentrations in muscle of Antarctic birds were low (0.01 mg/kg DW) when compared to livers (0.02 to 0.46 mg/kg DW) or feces (0.18 mg/kg DW Szefer et al. 1993). Silver concentrations in avian tissues, especially in livers, were elevated in the vicinity of metals-contaminated areas and in diving ducks from the San Francisco Bay (Table 7.5). Birds with elevated concentrations of silver in tissues — as much as 44 mg/kg DW in liver in the common eider (Somateria mollissima) — seemed outwardly unaffected (Bryan and Langston 1992). 

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