Forensic Chemistry: What it is and what it does

By Kate Fraser on May 25, 2015

Perkins School for the Blind Secondary school student Robert Evans shares his science fair project.

 

Forensic chemistry is the analytic study of substances that could easily be misidentified or confused, and finding their identity.  Forensic chemists analyze the physical and chemical properties of different materials in order to identify them correctly where one or more materials may have similar properties.  Forensic science is crucial to law enforcement because it has improved the efficiency of crime solving drastically.  Most often the strength of a prosecution relies on the physical evidence collected.  Using that evidence, forensic chemists can help connect the evidence to the perpetrator of the crime and to the victim.  Any physical evidence collected is sealed in special containers to prevent contamination and degradation, and is then catalogued carefully.  A well-trained forensic chemist can determine the composition and nature of materials, as well as predict their sources, and match substances sample against sample.
 

Brief History

Poisons were first used by early Egyptians, and the ancient Greeks and Romans.  Democritus was most likely the first chemist to study poisons, and he communicated many of his findings to Hippocrates.  Poisons were used both for murder and execution; Greek philosopher Socrates was condemned to death by drinking hemlock, which was a popular poison.  Another common poison during that time period was arsenic.  Before the development of systematic, scientific criminal investigation, guilt was determined by mostly circumstantial evidence and hearsay, because it was the only information that they had.  The Blandy trial of 1752 was the first ever chemical test for poison, and the Marsh Test in 1836 was the first reliable analysis that determined whether or not arsenic was present in the body of a victim.
 

robert with his experiment

What does a Forensic Chemist do?

Forensic chemistry encompasses organic and inorganic analysis, arson investigation, toxicology, serology, and other different skill sets and techniques.  Each type of analysis uses different techniques and instrumentation.
 

How Does this Pertain to the Experiment at Hand?

Picture this scenario:  A woman is sent to the ER with complaints of severe abdominal pain, lethargy, and Tinitus.  She started feeling poorly while baking for her grandchildren.  Suspecting a drug interaction, doctors question the woman and find out that she takes no medications aside for Aspirin for her arthritis.  Because she cannot swallow pills well she buys a powdered form and keeps it in the same cabinet as her baking products, which are table salt, table sugar, and cornstarch.  She also admits to testing her baking.  Knowing this information, doctors think they know the cause and pump her stomach.
 
In this situation, the role of a forensic chemist would be to identify which substance is which by examining various physical and chemical properties: solubility, pH, density, reactivity, etc.
 
The reason I chose this experiment is because I've always been fascinated by how modern detectives use forensics to solve crimes.  Being a very analytical person I wanted to see what it was like to be in their shoes.
 

Preparation:

Question:

You have four powder substances: table sugar, salt, cornstarch, and powdered aspirin.  How can you properly identify the aspirin among of the other substances?
 

Hypothesis:

If aspirin has a lower pH than the other three substances, then this substance is the one that poisoned that poisoned the grandmother baking for her grandchildren.
 
1.  Gather all necessary materials to perform the experiment.
 
2.  Grind the aspirin using a Mortar and Pestle.
 

Materials

  • 1experiment beakers50mL canister table sugar
  • 150mL canister table salt
  • 150mL canister corn starch
  • 150mL canister powdered aspirin
  • 1 Mortar and Pestle
  •  4 stirring rods
  • 1 water source
  • 12 100mL beakers
  • 1 box of gloves
  • 1 lab quest plus 1 pH sensor

Procedure

  1. Measure 15mmL out of each 150mL canister for all four of the substances into clean beakers.
  2. Test the solubility of each substance by adding it to 100mL of water.
  3. Using a Lab Quest with an attached pH sensor, test the pH of each solution.
  4. Use buffer soution to clean the snensor between each use.
 

robert using lab questData:

  • all four substances titrated to  58g.
  • cornstarch = 6.48 pH units
  • table salt = 6.10 pH units
  • sucrose = 8.99 pH units
  • aspirin = 3.47 pH units
  • tested aspirin again, titrated to  58G.
  • second aspirin = 3.23
 

Observation and Analysis:

  • cornstarch is the least soluble substance; it sticks to the bottom of the beaker and when sitting for an extended period of time it settles back to the bottom of the beaker.
  • salt was completely soluble
  • sucrose was completely soluble
  • some of the aspirin hardened at the bottom of the beaker, so it is not fully soluble.
 

Conclusion: 

The hypothesis was supported because the aspirin indeed had a lower pH than the sucrose, cornstarch, and table salt.  The acidity of the aspirin would be due to the fact that salicylic acid being one of the main ingredients.    

Variations

Variables that could have possibly had an impact on the results of the experiment are the temperature of the room, the ratio of each substance to the amount of water, the acidity or basicity of the water source, and how effectively each mixture is stirred.

NGSS Standards:

HSPS1-3
Plan and conduct an investigation to gather evidence to compare the structure of substances at the bulk scale to infer the strength of electrical forces between particles.

forensic chemistry collage

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