400 DIY Science: Illustrated Guide to Home Chemistry Experiments
RAGENTE pURITy
When forensic chemists run a Marsh Test, they use
extremely pure hydrochloric acid and zinc, because less
pure grades of hydrochloric acid and zinc may contain
enough arsenic as an impurity to yield a positive result
even if the sample contains no arsenic. (Yes, the Marsh
Test is really that sensitive.) I used ordinary muriatic acid
from the hardware store and lab-grade mossy zinc, at
first reacting them without an arsenic sample present to
determine whether they were contaminated with arsenic.
As it turned out, my samples were not contaminated (or at
least not with arsenic), but that’s no guarantee that yours
won’t be. If you want to perform a definitive Marsh Test,
either test your reagents first without a sample, or use
reagent-grade hydrochloric acid and zinc. Or both.ASENICR ANd oLd wood
Arsenic salts were formerly commonplace, found in nearly
every household. Nowadays, though, arsenic and arsenic
salts are very difficult to come by, unavailable even from
many specialty chemical suppliers. The best way to obtain
an arsenic sample for this lab is probably to find a piece
of pressure-treated wood that was treated with CCA
(chromated copper arsenate), which can be recognized by
its green tint. Although CCA is still widely used worldwide,
including the United States, recent restrictions on its use
have made it harder to find. Any landscape timber more
than a few years old was probably treated with CCA, and a
small chip suffices for the purposes of this lab. (If possible,
use a freshly sawn sample from the interior of the timber.)
Antimony and its salts are readily available from specialty
chemical suppliers. You can also get a sample by crushing
the heads of safety matches (from a paper matchbook).
Match heads contain antimony(III) sulfide.CUTIOA nS
This experiment involves burning hydrogen gas. Keep
the quantities of reactants small, be careful with the flame,
and have a fire extinguisher readily available. Arsenic and
antimony compounds are toxic, as are the arsine and
stibine gas produced during the experiment. Run the
experiment outdoors or in a well-ventilated area. The odor
of arsine gas is detectable at less than 1 ppm, and has
a garlic smell. stibine gas is similar, with an undefinable
but unpleasant odor. Hydrochloric acid is corrosive and
toxic, and emits noxious fumes. Laundry bleach is toxic
and corrosive. Wear splash goggles, gloves, and protective
clothing.z
CUTIOA nS
The hydrogen flame is extremely hot and may be invisible.
Verify that the hydrogen is burning by placing the match
or a wood splint near the nozzle to reveal the flame.
When you heat a porcelain plate in the hydrogen flame, it
may become extremely hot. (A hot porcelain plate looks
exactly like a cold porcelain plate, as I can tell you from
experience.) If you put down the porcelain plate, make sure
to place it on a heat-proof surface, and ensure that it has
cooled completely before you touch it with your fingers.z
POCEDURER
This laboratory is divided into three parts. In Part I, we establish
the purity of our reagents. In Part II, we run the Marsh Test
on our samples. In Part III, we treat the porcelain plates with
laundry bleach to observe the effects of bleach on the arsenic
and antimony mirrors that the Marsh Test produces on the
porcelain plates.
PI: RTA ETABLISHS REAGENT pURITy
If you have not already done so, put on your splash
goggles, gloves, and protective clothing.
Set up your gas-generating bottle. Pour about 10 mL
of concentrated hydrochloric acid into the bottle, add
about 3 g of mossy or granular zinc metal, and replace
the stopper.
The zinc metal begins reacting with the hydrochloric acid,
producing hydrogen gas. Allow a few seconds for the air
to be displaced from the bottle, and then apply a lighted
match or lighter to the glass nozzle to ignite the hydrogen.
4. Using pliers or another gripper to hold a porcelain plate,
position the porcelain plate so that the hydrogen flame
impinges directly on it. Allow the flame to touch the plate
for 30 seconds to a minute, and examine the plate for any
evidence of a deposit.
If your hydrochloric acid or zinc is contaminated with arsenic or
antimony, there will be a visible mark on the plate. If so, you will
need purer reagents for the actual test.