Chapter 4: Chemicals for the Home Chemistry Lab 57- ineral acids (hydrochloric, sulfuric, nitric, and so on) M
and most organic acids (formic, acetic, and so on) can
be disposed of safely by neutralizing them with sodium
bicarbonate (baking soda) until the fizzing stops. The
resulting sodium salts (chloride, sulfate, nitrate, acetate,
and so on) can safely be flushed down the drain with lots
of water. If the acid is concentrated, pour it into ten times
its volume of water to dilute it before you neutralize it.
Alternatively, add a few drops of phenolphthalein solution
to the dilute acid and add aqueous sodium hydroxide or
sodium carbonate until the solution turns pink, indicating
that the acid has been neutralized. - Strong bases such as sodium hydroxide and potassium
hydroxide can be disposed of safely by diluting them, if
necessary, and then treating them with a dilute solution of
hydrochloric acid. The resulting chloride salts (for example,
sodium chloride—common table salt—and potassium
chloride) are innocuous and can safely be flushed down
the drain. To neutralize the base, add enough
phenolphthalein solution to the aqueous base solution to
give it a noticeable pink color, and then add dilute acid until
the pink color disappears. - Solutions that contain large amounts of free bromine or
iodine are hazardous to the environment, although the
small amounts that home chemists work with are generally
innocuous. To safely dispose of bromine or iodine solutions,
add sodium thiosulfate solution until the waste solution
turns colorless. This converts the free bromine or iodine to
bromide or iodide ions, which are safe to flush down
the drain. - With the exception of heavy metal ions, described in the
following section, most cations are reasonably benign
and can safely be flushed down the drain. “Safe” cations
include aluminum, ammonium, bismuth, calcium, cerium,
cesium, cobalt, gold, hydrogen, iron, lithium, magnesium,
manganese, potassium, rubidium, sodium, strontium,
and tin. - Most anions are harmless, particularly in small quantities
and low concentrations. “Safe” anions include acetate,
bicarbonate, bisulfite, borate, bromate, bromide, carbonate,
chlorate, chloride, cyanate, iodate, iodide, nitrate, nitrite,
perchlorate, periodate, permanganate, phosphate, silicate,
stannate, sulfate, sulfite, thiocyanate (sulfocyanate),
thiosulfate, titanate, tungstate, and vanadate. Unless
there is another reason not to (such as the presence of a
toxic cation or anion complex), solutions of these anions
can safely be flushed down the drain with copious amounts
of water.
SITEPRvA EWAGE SYSTEMS
It’s safe to dispose of relatively benign chemicals by
flushing them down the drain only if your drain connects
to a sanitary sewer system, which quickly dilutes any
chemicals you flush. If you use a septic tank or other
means of sewage disposal, you might want to think twice
about flushing chemicals into it. It’s difficult to know for
sure whether a particular chemical will “poison” a septic
tank, so the best practice is to avoid flushing any chemical
unless you are absolutely certain that it’s safe to do so.
Instead, neutralize waste chemicals as described in this
section and take them to a waste disposal site or other
location where they can be safely disposed.z
dISALpoS of HEAvy-mETAL ANd oTHER
ToxIC CompoUNdS
Although many common laboratory chemicals can be safely
flushed down the drain, it is unacceptable to flush chemicals
that contain heavy-metal ion species or contain other very toxic
and/or persistent species. Treat any waste that contains organic
compounds as hazardous unless you are certain that it is not, and
treat any waste that contains heavy-metal ion species as toxic.Fortunately, heavy-metal waste can be converted into safer
forms, which can then be disposed of by taking them to a
hazardous waste disposal center. Your goals in pretreating heavy-
metal compounds for disposal are to convert hazardous soluble
heavy-metal ions to much less hazardous insoluble solids, and
to minimize the volume of solid waste that will require proper
disposal. Here are the guidelines I use, although again, you
must verify that these methods are legally acceptable in your
own jurisdiction.Barium
Soluble barium salts are extremely toxic and hazardous for the
environment. Before disposal, treat any solution that contains
soluble barium salts with an excess of a soluble sulfate
salt, such as sodium sulfate. The barium ions precipitate
as insoluble barium sulfate, which can be filtered, dried,
and (including the filter paper) added to a hazardous waste
container for later proper disposal. The filtrate contains few
barium ions, and can safely be flushed down the drain.Chromium
Soluble salts with chromium cations or chromium anion
complexes (chromates, dichromates) are extremely toxic and
hazardous to the environment. Chromium(III) cations can be
precipitated by excess sulfate ions as insoluble chromium(III)