Forensic Chemistry

1.         Detecting Arsenic: The Use of a Reducing Agent in Criminal Investigation

70 to 180 mg of arsenic is enough to kill an adult. Although arsenic targets different enzymes, like cyanide, it interferes with energy-related processes of cell mitochondria. Arsenic has long been used as a poison, but historically people could not prove its presence in a corpse.

The Marsh Test treats the sample suspected of containing As with a combination of As-free Zn and sulfuric acid, H2SO4. If there are even minute amounts of arsenic present, the Zn reduces the As+3 *. Here are the two half reactions:

Oxidation:         Zn → Zn+2 + 2e

Reduction:        As2O3 + 12e- + 6H+1   → 2As-3 + 3 H2O

(the trivalent ion in the oxide is not only the most common form of arsenic but it’s the one that’s most toxic)

Overall the reaction is:

As2O3 + 6 Zn + 6H+1(aq)→ 2As-3 (aq)+ 6 Zn+2 (aq)+ 3 H2O

But in an acidic medium, As-3 actually forms arsine (AsH3), so adding 6 H2SO4 to each side of the equation we obtain:

As2O3 + 6 Zn + 6 H+1 (aq)+ 6 H2SO4(aq) → 2 As-3 (aq)+ 6 H2SO4(aq) +6 Zn+2 (aq)+ 3 H2O,or as the As-3  combines with the H+ from sulfuric acid to form arsine:

As2O3 + 6 Zn + 6 H+1 (aq)+ 6 H2SO4(aq) → 2AsH3 +6 ZnSO4 + 3 H2O  + 6 H+1(aq)  ,or eliminating the common ions:

As2O3 + 6 Zn + 6 H2SO4(aq) → 2 AsH3 + 6 ZnSO4 + 3 H2O

The Zn, acid and sample are placed in a flask. It is connected to a drying tube, through which the water vapour and arsine pass. The emerging arsine is heated, and upon decomposing, it forms arsenic. The elemental powder is black and shiny and often referred to as an arsenic mirror. The length of the powdery line is directly proportional to the concentration of arsenic present in the original sample, and by running standards of known concentrations of arsenic, the result of the Marsh test can become quantitative.

Antimony, Sb, an element in the same family as arsenic, can give a false positive test by also forming a black shiny product. But antimony will not react with NaOCl (sodium hypochlorite), whereas arsenic will.

History Behind Marsh Test

  • 1775: Carl Scheele discovered that arsine, a garlic-smelling gas could be released from this reaction: As2O3 + 6 Zn + 12 HNO3 à 2 AsH3 + 6 Zn(NO3)2 + 3 H2O. Note how similar the reaction is to the Marsh Test.
  • later in the same century: Johann Metzger discovered that when arsenious oxide was heated in the presence of charcoal, a black shiny powder (arsenic mirror) would form over it. This was a reduction of As2O3 by carbon:

2 As2O+ 3 C → 3 CO+ 4 As

This is a very similar reaction to the one demonstrated in the physical science lab in grade 10:

            2 CuO + C → CO2 + 2 Cu

  • 1806: Dr. Valentine Rose took the stomach of a suspected poison victim and treated it with K2CO3, CaO and HNO3.The resulting white compound ( arsenious oxide) was then subjected to Metzger’s test.
  • 1832: A man was accused of poisoning his grandfather with arsenic-laced coffee. James Marsh was not able to convince a jury with his explanation of his lab analysis that was based on Metzger’s technique.
  • 1836: Realizing that Metzger’s procedure was not accurate enough, Marsh went on to develop the technique that bears his name, which is a refinement of Scheele’s test but sensitive enough to detect as little as 0.02 mg of arsenic.


Bodies of Evidence. Brian Hinnes. Amber. 2000

Beyond the Crime Lab. Jon Zonderman, Wiley & Sons. 1990

Casarett and Doull’s Toxicology.  Edited by Amdur and al. McGraw-Hill. 1993

Columbia Encyclopedia. Edited by Chernow and al. 1993

  1. Revealing Fingerprints: The Use of an Oxidizing Agent in Criminal Investigation

Investigators cannot always rely on dusting for revealing fingerprints. Materials such as paper and wood are porous, so the powder used will not only cling to the ridges of the fingerprint, but it will fill in little pockets of the uneven surface of such materials.

Iodine is a halogen, and like all members of this family, it is a good oxidizing agent, meaning that it is eager to steal electrons. The oils from skin, which allow the fingerprint to be created in the first place, are the victims of oxidation.

When iodine is heated gently, it sublimates—it passes directly from a solid to a gas. A few crystals of solid iodine are placed in a fuming pipe. A match is used to sublimate the solid, and the gas is blown over the surface where the fingerprint is expected. The gas oxidizes the skin’s oils to yellowish brown compounds. This reveals the fingerprints, which have to be quickly photographed because the yellow colour fades as the gas resolidifies.

The technique works with recent fingerprints only, so it is useful to the investigator who needs an indication of how fresh the prints are.

You can roughly simulate the technique with this simple lab:

  1. Pick up a clean filter paper with forceps.
  2. Rub your finger along the external side of your nose. (That part of the face is an excellent source of skin oils.)
  3. Add three crystals of iodine to a 200 mL bottle.
  4. Press your finger on the paper.
  5. With forceps, place it in the bottle and seal it.
  6. Wait a few scenonds for the print to appear, and if possible photograph it with a digital camera.

Note the instability of the print: the bottom picture was taken about 4.5 hours after it was developed in an iodine atmosphere, and the print has started to fade. Compare it to the fresh print in the second picture.


Bodies of Evidence. Brian Hinnes. Amber. 2000

Beyond the Crime Lab. Jon Zonderman, Wiley & Sons. 1990

  1. Revealing Fingerprints: The Use of a Precipitation Reaction in Criminal Investigation

A classic precipitation reaction in analytical chemistry involves aqueous solutions of silver nitrate and sodium chloride which react to produce soluble sodium nitrate and a precipitate of silver chloride:

AgNO3(aq) + NaCl(aq)  → NaNO3(aq)  + AgCl(s)

This is the reaction behind another useful technique for revealing fingerprints on porous materials. Rather than attacking the oils with iodine, the investigator uses silver nitrate to go after the salt naturally released through the fingers’ pores. Either with an aspirator, brush or swab, the technician applies a solution of AgNO3 on the suspected surface. After about 5 minutes,the area dries, and in uv or bright light the precipitate appears on the plateau-ridges formed by fingers—in other words—the fingerprint appears. As with the iodine-method, the prints are not chemically stable, so the investigator has to be photograph them not to end up with transient evidence.


Bodies of Evidence. Brian Hinnes. Amber. 2000

Beyond the Crime Lab. Jon Zonderman, Wiley & Sons. 1990

  1. The Role of Activation Energy in Explosives

Explosives are practical when it comes to blasting through rock for purposes of constructing roads, dams and bridges. Unfortunately when they end up in the hands of angry people, both hands and lives are lost because unleashed potential energy has no discriminating mind of its own. Quickly expanding gases wreck anything in their path.

Before the World Trade Center was demolished by terrorists on September 11, 2001, another attempt to wreak havoc was made in its underground parking lot in 1993. The ’93 terrorists had an “instructive” videotape with a bomb-recipe that exploited chemicals’ ability to set off chain-reactions. These are not to be confused with nuclear chain reactions. In a chemical chain reaction, reactions B and C would not occur on their own. But reaction A is deployed so that it supplies the energy that reaction B needs to happen. When B releases its energy, then C finally explodes. In other words each initial reaction provides the activation energy needed by the subsequent reaction.

The destructive quartet used a gel of urea nitrate for their reaction “C”. Investigators realized this because when they analyzed the debris in the parking lot, they found high levels of urea and nitric acid. Urea nitrate will release its energy only when subjected to both heat and friction. The activation energy was supplied by homemade nitroglycerin (part B). By attempting to make nitroglycerin in a Jersey City apartment, the terrorists were lucky enough not to have killed themselves in the process. (Upon mixing the necessary acids with glycerin, the reaction generates enough heat to explode the synthesized nitroglycerin.)

Once nitroglycerin cools it is more stable, so, in a van, they attached a fuse to some gunpowder to detonate ( provide the activation energy for) the nitroglycerin. ( reaction “A”).

The explosion was powerful enough to cause the Twin Towers to sway. Five people were killed. The bombers were each sentenced to 240 years in prison.


Merck Index. Thirteenth Edition. 1996.

Crime Science. Nickell and Fischer. University Press of Kentucky. 1999.

5.         Detecting Illicit Drugs: The Use of Characteristic Properties in Criminal Investigation

Scientists working in the crime lab often make use of characteristic properties when looking for the presence of illegal drugs. A group characteristic property of heroin and similar opiates(morphine, opium) is their peculiar reaction with Mecke’s (or Lafon’s) reagent. The reagent consists of selenious acid(H2SeO3)in concentrated sulfuric acid( H2SO4). If heroin and the reagent react, the following colours appear: green, quickly greenish blue, changing to blue,slowly to bluish green with a yellow-brown edge, then finally olivaceous green.

This is one of a variety of color tests that are used. Most, however, are presumptive tests, meaning that they suggest the presence of a drug but don’t absolutely prove it. Final confirmation is obtained through instrumentation, either gas chromatography-mass-spectrometry or by Fourier Transform Infrared Spectroscopy.


American Academy of Forensic Sciences Educational Conference. UTA Oct. 2003(Max Courtney. Forensic Consultant Services. Fort Worth, Texas)