Density Gradient Tube
Definition
A vertical glass tube filled with a bromoform-bromobenzene mixture in stepped concentrations, producing a stable density gradient. Soil particles settle at depths matching their specific gravities, generating a comparison pattern.
- Medium
- Bromoform-bromobenzene mixture
- Structure
- Stepped concentration gradient in a vertical tube
- Application
- Soil particle comparison by specific gravity
- Result
- Layered settling pattern unique to a soil sample
Common questions
How is a soil comparison made using the tube?+
A small quantity of the questioned soil and a reference soil are each dropped into separate, identically prepared gradient tubes. The particles settle at heights matching their specific gravities, producing a banding pattern; the two patterns are compared visually or photographically for matching layer positions and spacing.
Why has this method declined in routine casework?+
The reagents are toxic and require careful handling and disposal, the gradient must be freshly prepared and calibrated, and the comparison is subjective. Many labs now prefer instrumental methods such as X-ray diffraction or elemental analysis for soil evidence, keeping the gradient tube as a legacy or screening technique.
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