Silver nitrate (AgNO3) is a classic technique for developing latent fingerprints. It's a versatile chemical that has been fundamental in forensic investigations for over a century. Here's how it works:
- The Chemistry: Silver nitrate reacts with the chloride ions (primarily from sodium chloride) found in the salt residues within fingerprint sweat. This reaction forms silver chloride (AgCl), a compound that is initially colorless. Upon exposure to light (especially UV radiation), the silver chloride decomposes, forming visible deposits of metallic silver that appear dark gray or black.
Equipment
- Spray Bottle: A clean, chemical-resistant spray bottle is used for larger surfaces.
- Immersion Tray: A tray or container large enough to submerge items is used for smaller objects.
- Tongs: Non-reactive tongs are used to manipulate items during the immersion process.
- Personal Protective Equipment (PPE): Gloves (nitrile), safety glasses, lab coat, and consider a respirator for spray applications.
- Light Source: A powerful UV light or a well-lit area (indirect sunlight can suffice, but may slow the development reaction).
Silver Nitrate Solution (1 Liter)
- Ingredients
- Silver Nitrate: 3 grams
- Distilled Water: 1 Liter
- Important Note: Some sources recommend adding a small amount of ethanol or methanol to the solution. Although this can potentially enhance the reaction, methanol carries health hazards, so a pure distilled water solution is preferred for safety reasons.
Mixing Procedure
- Safety First: Always work in a well-ventilated area with appropriate PPE.
- Distilled Water: Measure out 1 liter of distilled water into a clean, chemically resistant container.
- Adding Silver Nitrate: Slowly add the 3 grams of silver nitrate crystals to the container while gently stirring.
- Thorough Mixing: Continue to stir until all crystals are fully dissolved.
- Filtering: If impurities are visible, filter the solution through filter paper for optimal results.
- Storage: Transfer the solution to a clean, light-proof bottle, ideally an amber glass bottle.
Latent Print Development with Silver Nitrate
- Surface Suitability: Silver nitrate works best on porous surfaces like paper, cardboard, and untreated wood. Avoid using it on surfaces that have been wet, as the fingerprint salts may have dissolved.
- Application:
- Spraying: Mist the item lightly with the solution in a well-ventilated area. Avoid over-saturation.
- Immersion: Submerge the item in the solution for about 5-10 minutes. Use tongs to agitate slightly and prevent items from sticking.
- Air Dry: Remove the items and allow them to dry completely in a dark or low-light environment.
- Light Exposure: Expose the treated surfaces to a strong UV light source or indirect sunlight to develop the silver chloride into visible prints. Development time varies with light intensity.
- Photography: Document any developed latent fingerprints promptly, as the reaction continues, and the background will eventually darken, obscuring detail.
Storage and Shelf Life
- Light Sensitivity: Silver nitrate is highly light-sensitive. Store it in an amber glass bottle, tightly sealed, and keep it in a dark cabinet or cupboard.
- Shelf life: When stored correctly, a silver nitrate solution should remain effective for at least 6 months. However, check for any crystal formation or discoloration before each use.