1.3.3 Chemical tests
As well as using the interactions of radiation with matter to help in the identification of materials, forensic scientists make extensive use of chemical tests. Chemical reagents are used to produce a chemical change that results in a change of colour, or some other detectable change. A simple test that can give an indication of the presence of a substance is called a presumptive test. Presumptive tests are often the first kind of test used in looking for a substance but they do not give definite results of the type needed in court.
If you have ever collected wild, field or horse mushrooms you may know a simple colour test. The poisonous mushroom Agaricus xanthodermus can look like a small edible horse mushroom. If in doubt about whether it is an edible variety, mushroom hunters cut the base of the stem or look for the colour when they pull it from the ground. The cut or damaged stem of Agaricus turns a bright chrome-yellow (Figure 9). The yellow stain is formed by a chemical reaction between a substance in the mushroom and the oxygen in the air. Field and horse mushrooms do not show a yellow colour when the stems are cut or damaged.
Box 1 briefly summarises some basic chemical concepts with which you should already be familiar. Read through it quickly and check that you can answer the questions correctly.
Box 1 Some basic chemistry revised
Atoms are the building blocks of all chemical substances. An atom contains three components: negatively charged electrons surrounding a positively charged nucleus containing neutrons and positively charged protons. The different types of atom are called chemical elements and each element has its own symbol.
What are the names of the elements represented by the following symbols: Ag, Al, C, Cl, Cu, Fe, H, N, Na, O, S?
They are, in order: silver, Ag; aluminium, Al; carbon, C; chlorine, Cl; copper, Cu; iron, Fe; hydrogen, H; nitrogen, N; sodium, Na; oxygen, O; and sulfur, S.
Molecules: molecular and structural formulas
Molecules are electrically neutral collections of atoms bound together in specific proportions. Each molecule has a particular geometric arrangement of atoms. One way of describing molecules is to use their molecular formula, such as the familiar H2O for a water molecule. The geometry of molecules can be shown approximately in a structural formula. In water the three atoms are held together by two single bonds (Figure 10). A single bond is composed of two electrons shared between two atoms, holding them together.
Molecules are neutral. Ions are charged. The source of all positive ions is the nucleus of an atom which contains one or more protons, each proton in the nucleus having a single positive charge. The nucleus of a hydrogen atom is a single proton, the hydrogen ion, H+. Negative ions are formed when one or more electrons are added to neutral atoms or molecules. Neutral atoms and molecules have exactly equal numbers of protons and electrons. The electrical charge of an electron is exactly equal in magnitude and is opposite in sign to that of a proton, so in terms of electrical charge an electron neutralises a proton.
A chemical equation describes a chemical change. A chemical equation has on the left-hand side the formulas of the molecules, atoms or ions that are changed during the chemical reaction - the reactants - and has on the right-hand side the formulas of the new molecules, ions or atoms formed by the chemical change - the products. A balanced equation is one in which there are the same number of atoms of each type on each side of the equation. If there are charges involved, the number of charges also needs to be the same on each side.
For the three reactions below, which of the equations are balanced chemical equations?
Equations 1 and 3 are balanced, having equal numbers of C, H and O atoms on each side. Equation 2 with one carbon atom on the left-hand side and two carbon atoms on the right-hand side is not balanced. There are also too many oxygen atoms on the right-hand side of Equation 2. The removal of the 2 in front of the CO2 will balance the equation.
Two types of chemical change are particularly important in diagnostic tests; these are acid-base reactions and redox reactions.
Acid-base reactions involve water, H2O. Although most water does exist in this form, a very small proportion of the H2O molecules break up to form ions, i.e. ionise, according to the following chemical equation:
H+ and OHˉ are called ions as they bear an electrical charge. The hydrogen ion, H+ , is positive. The OHˉ ion, called a hydroxide ion, has a negative charge. An acid is a substance that has a greater concentration of hydrogen ions than water. The concentration of hydrogen ions in water is often measured in pH units. On the pH scale, 7 denotes neutral, 0 is very acidic and 14 is very alkaline. An alkali has a greater concentration of OHˉ ions than water.
The pH of the fluid in a human stomach varies between about 1 and 4. Is the environment in our stomachs acidic or alkaline?
The environment in our stomachs is very acidic. Any pH less than 7 is acidic.
Chemical substances called bases can react with acids to form new compounds by neutralising the hydrogen ions. This is called an acid-base reaction. You are probably familiar with the use of the term 'litmus test' to mean a diagnostic test that gives a quick answer. Litmus is a substance that in the presence of acids is red and in the presence of hydroxide ions or bases is blue. So dipping a piece of blotting paper impregnated with litmus into a liquid and seeing whether it turns red or blue can be used as a simple test for an acid or base, but other tests would be needed to identify exactly how acidic or basic, i.e. to determine the pH.
In the same way that litmus changes colour when it comes into contact with acids or bases, other substances can change colour when the pH is changed and this colour change can be used in conjunction with other chemical and biological reagents to give colour tests that are diagnostic for particular biological fluids.
The second kind of chemical reaction that is used extensively in forensic science, and analytical science in general, is the redox reaction. Redox is a contraction of 'reduction and oxidation' and refers to reactions in which one molecule or ion is oxidised and the other reduced. One simple definition of oxidation is the addition of oxygen to a substance. There are lots of examples of an oxidation reaction where oxygen in the atmosphere reacts with something to form a new substance. For example, burning coal, which is carbon, causes it to combine with oxygen to form carbon dioxide (Equation 1) and burning methane gas (CH4) in air produces carbon dioxide and water (Equation 2). In our bodies, glucose (C6H12O6) reacts with the oxygen that we breathe in to produce the carbon dioxide that we breathe out and water (Equation 3).
The definition of oxidation in terms of addition of oxygen is not appropriate for all oxidation reactions, and oxidation is better understood in terms of the loss of electrons. Rusting of iron metal (Fe) is another familiar example of an oxidation reaction. Here, each iron atom loses three electrons to form an iron(III) (ferric) ion as shown in Equation 5, where the symbol eˉ is used to denote an electron and 3eˉ means three electrons. The symbol Fe3+ means that the iron nucleus has three more protons that it has electrons.
If an oxidation is loss of electrons, what do you think a reduction might be?
A reduction is where something gains electrons, such as that shown in Equation 6 where an aluminium ion is reduced to metallic aluminium. (This is the energy-intensive process used to make aluminium metal.)
Another redox reaction, in which the copper atom is oxidised by the silver ion which is correspondingly reduced, is shown in Equation 7.
What this equation shows in chemical language is that if you dip, say, a copper coin into a solution containing silver ions then the copper atoms are oxidised by losing two electrons and the silver ions are simultaneously reduced with each one gaining an electron. The result is that the copper coin becomes silver plated. This is the principle behind Sheffield plating in which copper items such as candlesticks are coated in silver. A similar principle is used in a technique called 'physical developer', where oily or greasy fingerprints may be visualised by the deposition of metallic silver in a redox reaction (Equation 8).
Is the reaction of the silver ion to form silver metal in Equation 8 an oxidation or a reduction?
The addition of an electron to the silver ion is a reduction, because the silver has gained an electron.
Finally, before you read the following extract from Forensic Science about the ways in which latent fingerprints are developed, there are just a few more terms with which you will need to be familiar.
- Autoradiography is a way of photographing materials that contain radioactivity, so that the parts that are radioactive can be seen.
- Polymerisation is the linking together of small molecules to make a long chain of regular repeated segments - a polymer. Polythene, for example, is a polymer of the simple molecule ethane, and polystyrene is a polymer of styrene. Polymers often have very different properties from the simple molecules from which they are composed.
- Fat, grease and oil are different versions of similar substances and the words are used interchangeably in the book.
Please complete Activity 2 as you are working through the above extract from Chapter 4 of Forensic Science by Andrew Jackson and Julie Jackson (2nd edition, 2008).
Activity 2 Summary of fingerprint visualisation techniques
Download and complete the following table as you read about the different techniques used to make latent prints visible. Once you have finished reading the extract, check your completed table with the version below.
Click here to see the completed table.
The flow charts in the Forensic Science extract you have just read show the sequence of processes that forensic scientists go through when visualising latent fingerprints in blood on porous surfaces and latent fingerprints on non-porous surfaces. Charts like this are critically important for people trying to recover fingerprints from crime scenes. As you see, various different routes can be taken, depending mostly on the type of article being tested, and the tests must be used in the correct order until a sufficiently good print has been recorded.
If you were responsible for collecting fingerprint evidence at a major crime scene suggest what methods of visualisation you would choose if one of the likely objects for study was a small fragment of glass, not stained with blood or contaminated with grease.
The protocols that can be followed for a small fragment of glass, such as from a mirror or window, are shown in Figure 4.7 of the extract you've just read. The first step would be to make a visual examination and photograph the fragment (step 1). If the glass is dry, testing could proceed immediately; if not it would be necessary to let it dry at room temperature (step 2). The glass could then be dusted with a fingerprint powder, such as aluminium powder, and photographed or lifted with special adhesive tape or gelatine lifters (step 7 and p. 99 of the extract). An alternative route, which might be taken since this is a major crime scene where the maximum information possible is needed, would be to shine a laser or high-intensity light on the sample and photograph any fluorescence visible (step 3). If this did not give a good record, then the next step would be to subject it to vacuum metal deposition with metallic zinc or gold, or both. The results of this experiment would also be photographed (step 4). Then the fingerprint powder would be used (step 7). If the glass had needed drying, superglue fuming (step 8) would probably not be a success. Similarly small particle reagent (step 9) is unlikely to be successful since it is best used on wet or waxy surfaces.