Chemical Analysis Techniques: Flame Tests, Fingerprinting, Chromatography, and Titrations
Flame Tests
Different elements emit different colors when electric current is passed through them. This is due to the electron configuration within the atom. Chemists use this theory to analyze materials for the presence of certain elements. By observing the emitted colors, they can identify elements in an unknown substance. Traces of different metals are used in fireworks to produce various colors.
We can demonstrate this effect in the laboratory by heating various metal salts in a Bunsen flame. Wooden splints (soaked in water) are dipped into salts like sodium chloride, lithium chloride, potassium chloride, barium chloride, strontium chloride, and copper (II) chloride and then heated. Damp splints prevent the wood from burning and affecting the results. Different splints are used for each salt to avoid cross-contamination.
- Lithium chloride: Lithium – Crimson (dark red)
- Sodium chloride: Sodium – Yellow
- Potassium chloride: Potassium – Lilac (purple)
- Barium chloride: Barium – Green
- Strontium chloride: Strontium – Red
- Copper (II) chloride: Copper – Blue-green
Fingerprinting
Fingerprinting involves pressing a finger onto an ink pad and then onto paper. The resulting fingerprint can be identified and classified as a loop, arch, or whorl.
Materials: Plain white paper, iodine (I2) crystals, conical flask, Bunsen burner, scissors, tongs, sellotape.
Procedure: Iodine crystals are placed in water in a conical flask. Strips of white paper are attached to the flask using sellotape. The flask is heated over a Bunsen burner. After a while, prints become visible and the paper is removed using tongs.
Dusting for Prints
“Dusting for prints” is a method to identify fingerprints at a crime scene. Items are lightly dusted with a fine white powder, which adheres to the print, making it visible and allowing for classification.
Chromatography
Chromatography, from the Greek words meaning “color writing,” is a technique used to separate different components in a mixture, particularly colored substances. It was invented by Russian botanist Mikhail Tswett in 1903 to separate plant pigments. Today, it is also used in forensic science.
In chromatography, a solvent (like water or ethanol) carries a mixture (like ink or dye) through a material (like filter paper). Components that mix well with the solvent move faster across the paper, leading to separation.
Why use long strips of paper?
It allows the colors to run higher without reaching the bottom of the flask.
Titrations
Acids and bases are at opposite ends of the pH scale. Lower pH values indicate more acidic substances, while higher pH values indicate more basic substances. Hydrochloric acid is very acidic, and caustic soda is very basic; both are dangerous upon skin contact.
Adding the right amount of base to an acid (or vice versa) neutralizes it. Antacids (e.g., Andrews, Alka-Seltzer) neutralize excess stomach acid. A bee sting injects an acidic liquid, neutralized by baking soda (a base). A wasp sting is basic, neutralized by vinegar (an acid). Nettle stings are acidic, neutralized by rubbing a crushed dock leaf (containing a base).
Titration is the method of adding one solution from a burette to another to determine how much of each will react.
A general neutralization reaction is: Acid + Base → Salt + Water
- Purpose of the burette? To precisely control the amount of acid added.
- Purpose of the pipette? To transfer a specific volume of base.
Example: Acid + Base → Sodium Chloride + Water
When reading the volume in a pipette or burette, take the reading from the bottom of the meniscus.
How do we know when the correct amount of acid is added? The solution turns pink due to an indicator (e.g., methyl orange).
- Why use a conical flask? To prevent splashes.
- Why use a white sheet of paper? To easily see color changes.
Example: HCl + NaOH → NaCl + H2O
What is observed in the evaporating basin? Crystals are formed.
Vinegar contains ethanoic acid. To demonstrate this and find the amount of ethanoic acid, replace hydrochloric acid with vinegar and repeat the titration experiment.