Chapter 22 Laboratory: Forensic Chemistry 401PII: RTA TSTHEE T SAmpLES
- If you have not already done so, put on your splash
goggles, gloves, and protective clothing. - Prepare the sample to be tested. Ideally, you want the
sample to be as finely divided as possible, in the form of
dust or small granules. A sample weighing two or three
grams is sufficient. An even smaller sample is likely to
provide positive results, but there’s no reason to make
things harder than necessary.
CUTIOA nS
Be careful not to inhale any of the dust or expose your
bare skin to it. Although the amount of arsenic contained
in one of the sources described in this lab will be extremely
small, you must develop habits that minimize your
exposure to such chemicals. So even when handling trace
amounts, take the same precautions you would for larger
samples. Wear an inhaler or face mask when you are
processing the samples.z
3.ut the sample in the gas-generating bottle and add P
about 10 mL of concentrated hydrochloric acid. Swirl
the bottle to mix the contents thoroughly, and allow the
acid to react with the sample for a few seconds. Once the
sample is thoroughly mixed, add about 3 g of mossy or
granular zinc, recap the bottle, and repeat steps 3 and 4
from Part I.- Repeat the procedure until you have tested all of your
samples. After each test run, discard the spent solution
(flush it down the drain with copious water) and rinse the
bottle thoroughly. Use a grease pencil or similar marker to
label each test plate after it has cooled.
FIGURE 22-6: A positive Marsh Test for arsenic
PIII: RTA dISCRImINATE BETwEEN ARSENIC
ANd ANTImoNy
1. If you have not already done so, put on your splash
goggles, gloves, and protective clothing.- Set aside one of the porcelain plates that shows a
positive Marsh Test for arsenic and one that shows
a positive test for antimony. Put one or two drops of
undiluted chlorine laundry bleach on each sample.
Observe and record the results.
dISpoSAL:
So much of the arsenic or antimony present in any
sample will have been converted to arsine or stibine gas
that it leaves only a trace quantity orders of magnitude
smaller than the sample you started with. Even so, it
would be poor practice to flush samples that contain
arsenic or antimony into the public sewer system, so
follow whatever hazardous waste disposal guidelines are
in effect in your community.