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FLINN METHOD
#11 Silver Compounds
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
Nitric acid, HNO3, 8 M
Sodium chloride, NaCl, 1 M
Sodium hydroxide, NaOH, 2 M
Sodium hydroxide, NaOH, 6 M
Sucrose
Filtration apparatus
Glass stirring rod
Large glass beaker
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).
The chemical reactions are as follows. Oxidation of silver metal may occur by one
of two possible mechanisms:
2Ag(s) + 2HNO3 → 2AgNO3(aq) + H2(g)
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 (see Procedure B).
Ag+(aq) + Cl−(aq) → AgCl(s)
Procedure A: Silver Recovery Process
or
1a
Perform this procedure in a fume hood. Wear chemical splash goggles, chemicalresistant
gloves and a lab coat or chemical-resistant apron.
FLINN METHOD #11 continued on next page.
2a
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.
HNO3
Silver
Waste
3a
Add sodium chloride (about 60 g per
100 g of Ag) to precipitate silver chloride.
Filter and wash the precipitate.
(This step is necessary to rid the silver
of any copper contaminant.)
NaCl H2O
4a
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 swirling. At
first there is considerable frothing, and
then the solution becomes dark brown.
Finally a heavy, gray precipitate forms. Sucrose
5a
Filter, wash and dry this precipitate.
Store in a dark bottle.