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Many reducing agents may be safely reacted with an oxidizing agent that will
destroy their reducing power prior to disposal. Sulfides are commonly oxidized
using bleach or sodium hypochlorite. This works best in a weakly basic solution.
Never mix bleach with acid! Dissolve the reducing agent in water, then make
it basic with sodium carbonate and finally react it with the hypochlorite ion. After
the material is oxidized, the pH is adjusted to neutral and the resulting mixture
containing innocuous ions may be flushed down the drain. The reaction of sulfite
ion with hypochlorite ion produces sulfate and chloride ions as shown. (see
Procedure A).
Nitrites are a unique class of compounds in that the nitrogen is in an intermediate
oxidation state (+3). It can be either oxidized to the +5 state (NO3¯) or reduced to a
lower state (NO or N2). Nitrites are easily destroyed by adding 50% excess ammonia
and acidifying to pH 1. The resulting product is nitrogen gas. (see Procedure B).
Procedure A: Sulfites
SO3–2 + ClO¯ → SO4–2 + Cl¯
HNO2 + NH4+ → N2 + 2H2O + H+
1a
Perform this procedure in a fume hood. Wear chemical splash goggles, chemicalresistant
gloves and a lab coat or chemical-resistant apron.
Flinn Method #12a, continued
3
Allow the mixture to stand for about one hour for the redox reaction to proceed to
completion. There may be a temperature rise during the reaction.
6
Flush other solutions down the drain with large quantities of water.
FLINN METHOD
#12b Reducing Agents
Strong reducing agents will react vigorously with oxidizing agents to produce heat
and possibly fire. Some reducing agents may cause a fire when in contact with
moist combustible materials. A simple oxidation reaction will render most reducing
agents safe for disposal.
Examples
Potassium nitrite, sodium sulfite, sodium thiosulfate
Materials Required
Ammonium hydroxide, NH4OH (proc. B)
Hydrochloric acid, HCl, 1 M
Hydrochloric acid, HCl, 3 M (proc. B)
Sodium carbonate, Na2CO3
Sodium hydroxide, NaOH, 1 M
Sodium hypochlorite solution (bleach)
Glass stirring rod
Large glass beaker
pH paper
Overview
Reducing agents by definition reduce other substances (i.e., they readily change
the oxidation state of a substance from a high value to a lower value). They are the
opposite of oxidizing agents. In the example involving carbon reacting with oxygen,
the oxygen was the oxidizing agent because it oxidized the carbon. On the other
hand, in this same reaction, carbon is the reducing agent because it changed the
oxidation state of oxygen from zero to negative 2 (–2). In the process, considerable
heat is produced.
5a
Flush the neutral solution down the drain with a 20-fold excess of water.
FLINN METHOD #12b continued on next page.
4
Check the pH of the mixture using pH
paper. Neutralize the solution with
dilute so dium hydroxide solution or
sulfuric acid solution, as needed.
pH Paper
1 M
NaOH or
1 M
H2SO4
5
The residues from this procedure
must undergo further treatment if
they contain chromium. The products
from the reduction of chromates and
dichromates are insoluble chromium
hydroxide. These materials can
be removed by filtration and require
Flinn Disposal Method #27f. Solutions
containing chromium ions may not be
drain disposed.
2a
In a large beaker, dissolve an equal
amount of sodium carbonate and
reducing agent in distilled water.
Na2CO3
3a
Slowly add a 25% molar excess of
bleach to the mixture with continuous
stirring. Use caution as the reaction
may be vigorous and produce heat.
Allow the completed mixture to stand
for several hours.
NaOCI
4a
Check the pH of the mixture using
pH paper and neutralize as necessary.
Use sodium hydroxide solution
if acidic or hydrochloric acid solution
if basic.
pH Paper
1 M
HCI or 1 M
NaOH