Disposal procedures for a large number of compounds are listed in section 3.4 Waste Disposal of the Chemistry Department Safety Manual. A shorter list of disposal instructions can also be found in Padias p. 4.
This page describes some principles of waste disposal and suggestions for disposal by category of chemical compound.
Principles of waste disposal
If you think about it, there are very few realistic options for disposing of unwanted chemicals. You might be able to sell them to someone else. More likely, you will have to choose between sending them into the natural environment unchanged (destinations include water, earth, and air), insulating them from the environment by storing them in special containers, or chemically changing them into chemically innocuous forms (e.g., many organics can be burnt to produce carbon dioxide and water).
In fact, only a few "safe" chemicals can be sent into the natural environment directly, i.e., by way of the lab sink or garbage can. Disposal by evaporation should be avoided.
The chemicals that may be flushed down the sink must be: water-soluble, nonflammable, noncorrosive, nonreactive, and nontoxic (as defined by the EPA). If a chemical fails to satisfy even one of these criteria, it cannot be disposed of in this way. It is also illegal to change the properties of a chemical by simply diluting it with water prior to sending it down the sink.
Solid nonhazardous compounds may be placed in a garbage can, but the list of "safe" solids is also quite short (see the Safety Manual). All solids must be collected in an appropriate container and labeled (compound name + "nonhazardous waste") before putting them in the trash. Unwanted liquids can never be disposed of in garbage cans.
"Unsafe" chemicals - the vast majority that you will encounter - may not be sent into the natural environment directly. We collect them in waste containers and send them to disposal facilities off-campus. Depending on the nature of the chemicals, they will either be burnt in an incinerator or buried at a hazardous waste disposal site.
Organic compounds (liquids and solids)
As a rule, organic compounds are flammable and should be placed in the organic waste container (fume hood). Some exceptions to this rule include: salicylic acid (place in special container), ethyl acetate and/or hexane (place in special container).
Smelly compounds (aka stench compounds), like acetic acid and banana oil (isopentyl acetate), should be placed in the organic waste container too. Remember to cap the container after you dispose of your material (the container should always be capped to minimize evaporation).
Glassware that is coated with stench compounds should be rinsed with acetone (send the rinses directly into the organic waste container). Solids (like silica gel, filter paper, etc.) that are coated with stench compounds should be placed in a fume hood so that the stench compounds can evaporate (evaporation of stench vapors through a fume hood, although undesirable, is far better than evaporation into the lab atmosphere).
Organic compounds that are mixed with a substantial quantity of water should not be placed in the organic waste container because this makes the container's contents less flammable. Consult your instructor for instructions.
Inorganic compounds (liquids and solids)
Many dry solid inorganic compounds are innocuous and can be placed in a garbage can in a suitably labeled container. Since it is impractical for each student to prepare a separate waste container, we provide labeled jars for unwanted compounds. Innocuous solids include: sodium bicarbonate, calcium chloride, sodium chloride, potassium chloride, sodium sulfate, and magnesium sulfate.
Aqueous solutions of the inorganic compounds listed above are innocuous and may be sent into a sink. Aqueous solutions of sodium hydroxide, potassium hydroxide, hydrochloric acid, nitric acid, and sulfuric acid may also be sent into a sink, provided that you test the solution with pH paper first and establish that the solution's pH is between 5-11. If the solution's pH falls outside this range, you must partially neutralize the solution before disposal (do not simply dilute the solution with water unless a completely trivial amount of solution is involved).
Inorganic compounds that have adsorbed organics should not be sent down the sink and should not be placed in the garbage can. In many cases, they can be left in a fume hood until the organics have evaporated (undesirable, but evaporation in the fume hood is better than evaporation in the lab or garbage) and then placed in the garbage.
Spent silica gel should be returned to a special waste container (fume hood).
Disposable equipment includes items like filter paper, rubber tubing, rubber gloves, paper towels, pipets, pipet bulbs, pH paper, and so on.
Glass items (pipets, melting point capillary tubes, etc.) should be placed in the special “sharps” cardboard containers.
Rubber and paper items should be placed in a garbage can, but you should check them for chemical contamination first. If necessary, use acetone to rinse organic chemicals into an organic waste container, or use water to rinse water-soluble innocuous compounds into a sink. If a stench compound has soaked into the item, place the item in a fume hood to let the compound evaporate, then move the item to a garbage can.
TLC plates should be returned to a special waste collection bag (center of lab).