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Chemical Disposal Procedures
247
FLINN METHOD
#11 SilverCompounds
Silver and silver compounds are expensive and may be recovered or recycled but often can be reclaimed for future use. Silver compounds are characteristic hazardous wastes. Silver compounds are identified by the EPA as characteristic hazardous wastes due to their toxicity. According to the Resource Conservation and Recovery Act (RCRA), the concentration of silver ions in an extract of a solid suitable for landfill disposal cannot exceed 5 mg/L, based on the Toxicity Characteristic Leaching Procedure (TCLP). Federal guidelines also impose a concentration limit of 5 mg/L on industrial sewer disposal of silver ions in solution. Check with your local sewer authority for the allowable discharge limit in your area. In most cases, dilution with water is not an acceptable means of achieving the concentration limit.
Examples
Silver nitrate, silver chloride, silver oxide
Materials Required
Large glass beaker
Glass stirring rod
Sodium chloride, NaCl, 1 M
Nitric acid, HNO3, 8 M
Sodium hydroxide, NaOH, 6 M Sucrose
Sodium hydroxide, NaOH, 2 M Filtration apparatus
Magnetic stirrer/hot plate with stir bar
Silver Recovery
Silver may be recovered and recycled for future use by dissolving the metal or metal compound in nitric acid, precipitating silver chloride, and reducing the latter to silver metal. Recovery of silver metal from AgCl may be achieved by reduction with sucrose in basic solution or zinc metal in acid solution. The first step must be done in a fume hood due to the possible production of NO2, a toxic brown gas. Note that PbCl2 will co-precipitate with AgCl in the second step if the original silver is contaminated with lead. The third step may be omitted, and the silver reclaimed in the form of the precipitated silver chloride, if the original silver was relatively pure. See Procedure A.
4a
HNO3
The chemical reactions are shown below. Oxidation of silver metal may occur by one of two possible mechanisms:
2Ag(s) + 2HNO3 → 2AgNO3(aq) + H2(g)
or
Ag(s) + 2HNO3(aq) → AgNO3(aq) + NO2(g) + H2O
Ag+(aq) + Cl−(aq) → AgCl(s) AgCl + NaOH(aq) + Sucrose → Ag(s)*
*Sucrose is hydrolyzed to the reducing sugars glucose and fructose in basic solution. These sugars are oxidized to gluconic acids in the process of reducing Ag+ ions to silver metal. The silver metal will be obtained in the form of a brown powder.
Silver(I) compounds such as AgCl are photosensitive and must be stored in dark bottles to prevent light-catalyzed reduction to silver.
Silver or silver ions may also be precipitated in the form of silver chloride to reduce the volume of hazardous waste requiring disposal.
Dissolve the silver metal or silver salts in 8 M nitric acid solution. For about 10 g of silver, use 20–30 mL of 8 M nitric acid.
3a
Add sodium chloride (about 60 g per 100 g of Ag) to precipitate silver chloride. Filter and wash the precipi- tate. (This step is necessary to rid the silver of any copper contaminant.)
NaCl H2O
Suspend the AgCl in 6 M NaOH
(for 100 g of Ag, use about 500 mL
of NaOH), and boil it for about 30
minutes, during which time add
sucrose (about 250 g or 1 cup per
100 g of Ag) in small amounts at
frequent intervals. Stirring is not
necessary, only occasional swirl-
ing. At first there is considerable
frothing, and then the solution
becomes dark brown. Finally a heavy, gray precipitate forms.
5a
Filter, wash and dry this precip- itate. Store in a dark bottle.
Sucrose
See Procedure B.
Ag+(aq) + Cl−(aq) → AgCl(s) Procedure A: Silver Recovery Process
1a
Perform this procedure in a fume hood. Wear chemical splash goggles, chemical-resistant gloves, and a lab coat or chemical-resistant apron.
2a
Silver Waste
FLINN METHOD #11 continued on next page.