Download Determination of an Unknown Chemical Substance

Determination of an
Unknown Chemical
Substance
Rachel Zammit
One type of investigation that a chemist
performs is chemical analysis.
The goal of chemical analysis is to answer
the questions: "What is this sample
composed of, and how much of
each constituent is present?"
The analysis can be qualitative or
quantitative.
Qualitative analysis
The result of the analysis is a list of
ingredients ("this sample contains
chemicals X and Y")
Quantitative analysis
The result is a list of ingredients and the
concentration of each ingredient ("this
sample contains 3.2 percent of chemical X
and 96.8 percent of chemical Y”)
If a chemist is asked to determine if a
sample of soil has mercury in it, at any
level, is that a qualitative or a quantitative
question?
Question
Forensic scientists often need to identify unknown
powders that may be illegal drugs.
There are two categories of forensic tests used to
analyse drugs and other unknown substances:
Presumptive tests (such as colour tests) give
only an indication of which type of substance is
present but they can't specifically identify the
substance.
Confirmatory tests (such as
chromatography/mass spectrometry) are more
specific and can determine the precise identity of
the substance.
Physical Properties
 Appearance
 Colour
 Melting Point
Chemical Properties
 Solubility in water
 Chemicals it reacts with
Techniques used to identify an
unknown substance
Colour Test
Expose an unknown drug to a chemical or
mixture of chemicals. What colour the test
substance turns can help determine the
type of substance that's present.
Techniques used to identify an
unknown substance
Chromatography
Thin-layer chromatography incorporates a solid
stationary phase and a moving
liquid phase to cause a separation of the
constituents of a mixture.
The distance a component has travelled up a plate
can be assigned a numerical value known as the Rf
value.
Rf = Distance travelled by the component
Distance travelled by the solvent
Techniques used to identify an
unknown substance
Determine the Rf values for the
following substances
Ultraviolet spectrophotometry
Analyses the way the substance reacts to
ultraviolet (UV) and infrared (IR) light. A
spectrophotometry machine emits UV and
IR rays, and then measures how the
sample reflects or absorbs these rays to
give a general idea of what type of
substance is present.
Techniques used to identify an
unknown substance
It is possible that a forensic scientist might find
a white powder at the scene of a crime.
Without testing the substance, the scientist
cannot know if the substance was legal or
illegal.
Many commonly found white powders have
been previously tested so that the results of
the tests can be compared to the test results
of the powder taken from the crime scene.
This allows the scientist to rule out legal
substances.
Setting the Scene
Can you identify these substances?
Salt
Corn Starch
Sugar
A police officer makes a routine traffic stop
in an area where there has been illegal
drug activity. The officer notices white
powder on the seat of the car and requests
a sample to be taken.
The powder might just be where the driver
had groceries and a container of sugar
opened and spilled… Or the powder might
be an illegal substance.
Scenario 1
Several kids have become sick after eating
at the school cafeteria, each complaining
about stomach ache and nausea. After
questioning the students, it became clear
that they all had one dish in common – the
chilli.
Upon further investigation, it turns out that
a bottle containing aspirin in the form of
crystals is missing from a chemistry
classroom. Did the aspirin somehow end up
in the chilli?
Scenario 2
An elderly woman is rushed to the hospital complaining of
severe abdominal pain. The doctor learns that the woman
takes no medicines except aspirin for her arthritis.
Since she cannot swallow pills well she takes a powdered
form of aspirin which she buys in bulk and keeps in a plastic
container in her kitchen next to her baking goods.
She had just finished a day of baking sweets and admits she
samples while she bakes and thought that she had just had
too many sweets. Immediately the doctor suspects that she
has mixed up the aspirin and another ingredient and is
suffering from aspirin poisoning.
The doctor quickly pumps her stomach. Soon the woman is
feeling well again, but the doctor needs to confirm the
diagnosis before releasing her. If the doctor is wrong, the
woman could be suffering from something far more severe. If
the doctor is right, the poisoning could easily occur again
unless the aspirin is located and properly labelled
Scenario 3
http://chemistry.about.com/od/growingcrystals/ig/Crys
tal-Photo-Gallery/Sugar-Crystals---Sucrose.htm
http://biozz.deviantart.com/art/Table-Salt-179603487
http://www.starwest-botanicals.com/product/1cornstarch-powder.html
http://www.sciencebuddies.org/science-fairprojects/project_ideas/Chem_p093.shtml#summary
http://resources.mhs.vic.edu.au/science/elective/resou
rces/chromatography.htm
References