A separatory funnel is a fundamental piece of laboratory glassware used to separate immiscible liquid phases and perform liquid-liquid extractions. Choosing the correct funnel involves more than simply selecting its capacity. Body shape, glass type, stopcock material, stopper, joint size and chemical compatibility can all affect performance.
How Does a Separatory Funnel Work?
Two immiscible liquids form separate layers according to their physical properties, particularly density.
The basic process is:
Mixing → Settling → Phase separation → Controlled drainage of the lower phase
The lower layer is drained through the stopcock.
Choosing the Right Capacity
Common capacities include 50, 100, 250, 500, 1000 and 2000 mL.
A separatory funnel should not normally be filled completely. Adequate headspace is required for effective mixing and safer handling.
Choose a funnel that provides sufficient space for both the liquids and the mixing process.
Pear-Shaped vs. Squibb-Type
Pear-shaped funnels taper toward the stopcock and are widely used for routine liquid-liquid extraction.
Squibb-type funnels have a more conical body geometry.
The preferred shape depends on the procedure, sample volume and laboratory preference.
Why Borosilicate 3.3?
Borosilicate 3.3 glass provides:
- low thermal expansion,
- high hydrolytic resistance,
- good resistance to many chemicals,
- excellent transparency.
Transparency is particularly important because the user must clearly observe the phase boundary and any emulsion formation.
Glass vs. PTFE Stopcocks
Glass Stopcock
Glass stopcocks offer good chemical compatibility and traditional all-glass construction. Depending on the design, suitable grease may be required.
PTFE Stopcock
PTFE stopcocks provide:
- broad chemical resistance,
- low friction,
- generally grease-free operation,
- convenient handling.
Specific chemical compatibility should always be verified.
Stopper and Joint Size
Separatory funnels may use glass or polymer stoppers.
With ground-glass systems, joint sizes such as NS 19/26, NS 24/29 and NS 29/32 may be encountered.
Checking the joint size is important for stopper replacement and compatibility.
Why Must a Separatory Funnel Be Vented?
Pressure can develop during mixing due to volatile solvents, gas-generating reactions or temperature changes.
The funnel should therefore be vented according to the procedure, with the outlet directed away from people.
Pressure may need to be released several times during extraction.
Which Layer Is the Organic Phase?
The organic layer is not always the upper layer.
Some organic solvents, particularly certain halogenated solvents, are denser than water and may form the lower phase.
Always identify layers based on the actual solvent system.
Emulsion Formation
An emulsion is a persistent cloudy layer between the two liquid phases.
It may be promoted by:
- excessive shaking,
- surfactants,
- fine particles,
- complex sample matrices.
Controlled mixing can help reduce unnecessary emulsion formation.
Selection Guide
| Feature | What to Consider |
|---|---|
| Capacity | Liquids plus sufficient mixing space |
| Glass | Suitable laboratory borosilicate glass |
| Shape | Pear-shaped or Squibb-type |
| Stopcock | Glass or PTFE |
| Stopper | Glass or compatible polymer |
| Joint | Correct standard size |
| Compatibility | Solvents and chemicals used |
| Transparency | Clear phase observation |
| Flow | Controlled drainage |
| Support | Suitable ring or clamp |
Conclusion
Selecting the right separatory funnel requires consideration of:
capacity + body shape + glass type + stopcock + stopper + joint size + chemical compatibility.
Borosilicate glass with a suitable PTFE stopcock can be a practical option for many general laboratory applications, but the final choice should always match the chemicals and extraction procedure being used.
