Page 251 - Demo

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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.
Chemical Disposal Procedures
249
4
Check the pH of the mixture using pH paper. Neutralize the solution with dilute sodium hydroxide solution or sulfuric acid solution, as needed.
5
The residues from the above procedure must undergo further treatment if they contain chro- mium. The products from the reduction of chromates and dichromates are insoluble chrom- ium hydroxide. These materials can be removed by filtration and require Flinn Disposal Method #27f. Solutions containing chro- mium ions may not be drain disposed.
6
1M 1M NaOH or H2SO4
Flush other solutions down the drain with large quantities of water.
#12b ReducingAgents
FLINN METHOD
NaOCI
pH Paper
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 carbon- ate, 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 reac- tion of sulfite ion with hypochlorite ion produces sulfate and chloride ions as shown below. See Procedure A.
SO3–2 + ClO¯ → SO4–2 + Cl¯
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.
HNO2 + NH4+ → N2 + 2H2O + H+ Procedure A: Sulfites
1a
Perform this procedure in a fume hood. Wear chemical splash goggles, chemical-resistant gloves, and a lab coat or chemical-resistant apron.
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
Sodium carbonate, Na2CO3
Sodium hypochlorite solution (bleach) Ammonium hydroxide, NH4OH (proc. B) Hydrochloric acid, HCl, 3 M (proc. B) Hydrochloric acid, HCl, 1 M
Sodium hydroxide, NaOH, 1 M
Large glass beaker
Glass stirring rod
pH paper
Overview
pH Paper
Reducing agents by definition reduce other substances; that is, 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 consider- able heat is produced.
1M or 1M HCI NaOH
2a
In a large beaker, dissolve an equal amount of sodium carbonate and reduc- ing agent in distilled water.
3a
Slowly add a 25% molar excess of bleach to the mixture, the continuous stirring. Use caution as the reaction may be vigorous and produce heat. Allow the completed mixture to stand for several hours.
4a
Check the pH of the mixture using pH paper and neutral- ize as necessary. Use sodium hydroxide solution if acidic, or hydrochloric acid solution if basic.
5a
Na2CO3
Flush the neutral solution down the drain with a 20-fold excess of water.
FLINN METHOD #12b continued on next page.