Sink-Float Test
Definition
Qualitative density screening in which a solid fragment is immersed in a liquid of adjustable density until neutral buoyancy is achieved; the liquid density at that point equals the fragment density.
- Method
- Immerse fragment in liquid, adjust density to neutral buoyancy
- Endpoint
- Fragment neither sinks nor floats
- Output
- Liquid density at that point equals fragment density
- Typical use
- Screening plastics, glass and fibre fragments
Common questions
Why is the sink-float test only a screening step and not a final identification?+
Many different materials share overlapping density ranges, so a matching density narrows the field of possible materials or sources but cannot alone distinguish between, say, two different polymer formulations; it is normally followed by spectroscopic or chromatographic confirmation.
How is the density gradient adjusted during the test?+
The examiner mixes two liquids of known differing densities in varying proportions, or uses a pre-made density gradient column, adding or diluting until the fragment stops sinking or floating and sits suspended, marking the matching density.
Related terms
- ASTM E2365
- The American Society for Testing and Materials standard for forensic comparison of glass by density; specifies temperature control, calibration, and precision requirements...
- Bromoform-Bromobenzene System
- The reference liquid mixture used in forensic density-gradient columns and sink-float tests; bromoform (2.890 g/cm³) and bromobenzene (1.495 g/cm³) mix in variable...
- Density
- Mass per unit volume (ρ = m/V), expressed in g/cm³ or kg/m³; a physical property used to compare and discriminate glass, mineral,...
- Density-Gradient Column
- Glass column filled with stepwise layers of liquids of decreasing density (commonly bromoform 2.89 g/cm3 and bromobenzene 1.50 g/cm3 mixed in known...
- DFSS
- Directorate of Forensic Science Services, India; issues the Forensic Laboratory Manual that governs glass and soil examination procedures, including the bromoform-based density...
- ENFSI ENG3-2013
- The ENFSI Glass Expert Working Group guideline for glass comparison casework; specifies the examination sequence, within-source variation requirements, statistical comparison framework, and...
- Hydrostatic Weighing
- Density measurement exploiting Archimedes' principle; the apparent weight loss of a solid submerged in a liquid of known density equals the weight...
- Neutral Buoyancy
- The condition in which a solid object's density exactly matches the density of the surrounding liquid; the object neither sinks nor rises...
- Oscillating-U-Tube Densitometer
- An instrument that measures liquid density from the resonant frequency of a liquid-filled U-shaped tube; precision of ±0.0001 g/cm³, widely used for...
- Pycnometry
- Absolute density measurement using a calibrated-volume flask; the density of a solid is calculated from the mass of liquid displaced when the...
- Stokes Settling Formula
- The equation v = 2r²g(ρₛ - ρ_f) / 9η relating the settling velocity of a spherical particle to its radius, the density...