Difference between revisions of "Reagent recycling"

Revision as of 03:33, 25 August 2016

Recycling and reusing reagents in the laboratory is an easy way to effectively cut costs by not having to buy reagents from a supplier again or disposing of waste at a facility. While many lab materials react irreversibly and thus cannot be recovered, some of the more expensive ones may be easily regenerated.

Solvents

Many reactions, especially those in organic chemistry, require the use of one or more liquid reagents, either to contain the reaction or to participate directly as a reactant. For example, the synthesis of many esters by Fischer esterifications require a primary alcohol as a reactant, but the reaction also benefits from having a large excess of the alcohol, meaning little of the alcohol is actually consumed in the process. Instead of boiling the excess solvent off to claim the product, it is much more resourceful to recollect it by distillation. Likewise, many solid reaction products are often cleaned or washed with a solvent that is intended to dissolve impurities, consuming quite a lot of liquid. Distilling the dirty solvent to recover it afterward makes it much more economical to use them for this purpose. If one has access to a rotary evaporator, it is often worth while to install a condenser to recover the solvent.

Metals

Some metals can be plated out of solution. Copper and zinc are two of most commonly discussed. Simply passing a low-voltage current through a solution containing the metal will plate the metal out of solution as a powder. This process can also be used decoratively or functionally for plating metal surfaces, though the amperage will need to be reduced to allow for more even coating.

Another common method of plating metals is to displace them with a more reactive metal. Copper can be regenerated from waste solutions containing copper salts by the simple addition of a cheaper, more reactive metal such as aluminium or zinc. Likewise, copper can be used to remove silver from solution, which, given the high price of silver, is very highly recommended if not vital.

Metals which may be plated out of solution are those which have a standard reduction potential greater than -0.76 V (the standard reduction potential of zinc). However, not all metals which have a low reduction potential are easy or feasible to plate out of solution, and the substrate on which plating occurs matters a great deal as well.

Rare metals

Certain hard to find metals, like lithium, rubidium, cesium, strontium, scandium are difficult to recycle due to their high reactivity. Best way is to recover their salts via recrystallization, precipitation, and storing them, until they are needed. They can be converted back to their metallic form via electrolysis from their molten salts or thermite-like reactions.

Metal oxides

Some metal oxides, such as manganese dioxide left over from reactions with potassium permanganate can be easily recovered and converted back into potassium permanganate if necessary. Manganese dioxide itself may be used for a variety of reactions.

Aluminium oxide can be made by heating aluminium hydroxide, which in turn results from the hydrolysis of aluminium halides.

Excess oxidizers

Waste chromate solutions are useful for cleaning glassware if they are acidified with sulfuric acid.

Wet desiccants

Thermally stable desiccants, such as calcium chloride, magnesium sulfate can be converted back to their anhydrous form, via heating. Hydroxides, such as calcium hydroxide, require temperatures too high to be properly dehydrated, but they can be used as a neutralization agent. Some however, may require drying in a stream of halogen or acid gas.

References

Relevant Sciencemadness threads

• Lab-scale waste recycling