Chapter 22 Laboratory: Forensic Chemistry 399LABORATORY 22.4:
pERfoRm THE mARSH TEST
The Marsh Test provides an incredibly sensitive
test for the presence of arsenic and the similar
element antimony. Because arsenic and
antimony are lethal in small doses and the only
samples available may be body fluids in which
the arsenic or antimony is extremely dilute, a
forensic test for these elements must provide
positive results when arsenic or antimony is
present in the sample at microgram levels.
The Marsh Test has that level of sensitivity.
RIREEqU d EqUIpmENT ANd SUppLIES£ goggles, gloves, and protective clothing£ gas-generating bottle with drawn glass tube nozzle£ small unglazed porcelain plates (one per test)£ pliers, crucible tongs, or other gripping instrument£ grease pencil or other porcelain marker£ matches, lighter, or other source of flame£ arsenic and/or antimony samples (see text)£ hydrochloric acid, concentrated (a few mL per test)£ zinc metal, mossy or granular (a few g per test)£ laundry bleach (a few mL)The fundamental problem in devising a reliable test for these
substances was that the tiny amounts likely to be present in a
sample made it difficult or impossible to use standard chemical
tests that depend on a precipitate forming or a color change
occurring. At the time Marsh devised his test, it was well known
that arsenic and arsenic compounds react with hydrochloric
acid to form arsine gas and that antimony reacts similarly to
form stibine gas. The problem was that the tiny amounts of
arsenic or antimony likely to be present in a sample wouldn’t
form much gas.
Marsh’s stroke of brilliance was to realize that he could “amplify”
the effect of the hydrochloric acid on arsenic or antimony salts
by adding very pure zinc metal to the reaction vessel. Zinc
reacts with hydrochloric acid to form hydrogen gas, which would
serve as a “carrier” for the tiny amounts of arsine or stibine that
were produced by the reaction. Marsh directed the gases from
the reaction to a nozzle, ignited them, and allowed the flame to
impinge on an unglazed porcelain plate. The burning hydrogen
left no mark on the plate, but the burning arsine or stibine
gas left a metallic mirror of arsenic or antimony on the plate.
Although the gas produced by Marsh’s apparatus was typically
99.99% or more hydrogen, the tiny amount of arsine or stibine
present in the gas was sufficient to mark the plate.
In this lab, we’ll reproduce Marsh’s original test, and
discriminate between arsenic and antimony samples. Although
the marks produced by arsenic and antimony have slightly
different appearances, the sure way to discriminate is to treat
the mark with sodium hypochlorite (chlorine bleach). A mark
produced by arsenic disappears in the bleach solution, but a
mark produced by antimony is unaffected.
SBSTITUTIU oNS ANd modIfICATIoNS- You may substitute a large test tube or flask with a
rubber stopper and a drawn glass tube nozzle for the
gas-generating bottle. - You may substitute the unglazed bottoms of porcelain
crucibles for the unglazed porcelain plates. - Antimony trisulfide is used in the heads of ordinary
safety (book) matches. Antimony sulfide is soluble
in hydrochloric acid. You can obtain an antimony
sample by crushing the heads from a book of matches
and soaking the powder in sufficient concentrated
hydrochloric acid to cover it.