You aspirate ethanol, lift the pipette, and suddenly a drop appears at the tip—even though your thumb has not moved.
Is the pipette leaking? Not necessarily. A pipette that handles water without dripping can behave differently with absolute (anhydrous) ethanol. The reason starts with the liquid itself, not automatically with a faulty seal.
Quick answer: Ethanol is volatile. In an air-displacement pipette, its vapor changes the pressure of the air cushion above the liquid and can push liquid out of the tip. Ethanol also has lower surface tension than water, which influences how droplets form and wet the tip. Pre-wetting, reverse pipetting, and reducing delays may help, but none of these guarantees accurate delivery. For demanding solvent transfers, a positive-displacement pipette may be a better choice.
Why Does Ethanol Leak from an Air-Displacement Pipette?
1. Ethanol vapor changes the pressure inside the tip
In a standard air-displacement micropipette, the piston does not touch the liquid. A small air cushion separates the piston from the sample in the disposable tip.
When ethanol is aspirated, some of it evaporates into that air space. Its relatively high vapor pressure can make the air cushion expand, pushing the liquid column downward. If the pressure change is large enough, a drop can leave the tip without any movement of the plunger. This vapor-pressure effect is a recognized reason ethanol and other volatile solvents can drip from air-displacement tips.
A leaking droplet therefore does not, by itself, prove that the pipette seal is damaged. It also does not prove that the intended volume has been delivered accurately.
2. Ethanol has much lower surface tension than water
Surface tension influences the way liquid forms a droplet at a narrow opening and wets surrounding surfaces. At approximately 20°C, common reference values are:
| Liquid | Surface tension at 20°C (approx.) |
|---|---|
| Water | 72.8 mN/m |
| Ethanol | 22.3 mN/m |
In other words, ethanol's surface tension is about one-third of water's. This is a useful explanation for the difference in droplet behavior, although vapor-pressure effects are the more direct explanation of spontaneous dripping in an air-displacement pipette.
What Is Reverse Pipetting—and Why Does It Help with Ethanol?
The practical trick demonstrated in the source video is a form of reverse pipetting: aspirate the target volume plus an extra reserve, but dispense only the intended volume. The reserve remains in the tip.
For a typical two-stop manual air-displacement pipette, the general sequence is:
- Set the target volume. Fit a compatible tip securely, and press the plunger smoothly to the second stop before aspiration.
- Aspirate the extra volume. Immerse the tip as specified by the pipette manufacturer, then release the plunger smoothly to the rest position. This draws in more than the nominal target volume.
- Dispense only to the first stop. Move to the receiving vessel promptly. Press down to the first stop, not the blowout/second stop, to deliver the target portion.
- Keep the excess out of the sample. A small reserve stays inside the tip. Dispose of that remainder appropriately rather than blowing it into the destination vessel.
Why it can help: If a small amount of liquid is lost during handling, having an extra reserve may reduce its effect on the intended dispense. Reverse pipetting is one technique to evaluate for volatile and low-surface-tension liquids.
What it cannot guarantee: If the tip keeps dripping, if leakage exceeds the reserve, or if the technique is inconsistent, the dispensed volume can still be wrong. The extra liquid may also increase solvent consumption. Reverse pipetting is a method to evaluate and validate, not an automatic correction for every ethanol transfer.
Four Practical Ways to Improve Ethanol Pipetting
Pre-wet the pipette tip with the same liquid
Before the measured transfer, aspirate and dispense ethanol two or three times using the same tip and liquid. This conditions the air space inside the tip with ethanol vapor and can reduce the pressure changes that cause dripping.
Pre-wetting is a useful first step for routine work, but it does not independently establish a method's accuracy.
Minimize the time between aspiration and dispensing
Do not leave ethanol suspended in the tip longer than necessary. Use a consistent, controlled movement and dispense promptly into the correct vessel. Avoid abrupt plunger movement and prevent droplets from contaminating neighboring samples.
Consider a positive-displacement pipette when accuracy matters
Unlike an air-displacement pipette, a positive-displacement system has a piston in direct contact with the liquid inside a dedicated capillary/piston tip. With no air cushion to expand, it is less sensitive to changes in vapor pressure and other liquid properties.
For repeated or accuracy-critical ethanol transfers, this may be more appropriate than trying to compensate with technique alone. Confirm solvent compatibility and the device-specific instructions.
Check fit, range, and the actual delivered volume
Make sure the tip fits the pipette, including its sealing geometry, and that the target volume and extra reverse-pipetting reserve remain within the pipette/tip's working limits. If a pipette also leaks with water, inspect it using the manufacturer's leakage test and maintenance procedure.
For a quantitative assay, verify the delivered amount under the real ethanol workflow. Routine water calibration should not be treated as proof of equivalent performance with a volatile organic solvent.
Quick Troubleshooting: Ethanol Drips, What Should You Check?
| What you observe | Possible explanation | Practical next check |
|---|---|---|
| Water stays in the tip, but ethanol drips | Ethanol volatility affects the air cushion | Pre-wet, shorten handling time, evaluate reverse pipetting |
| Ethanol drips even without plunger movement | Vapor-pressure-driven expansion of the air cushion | Compare performance after pre-wetting; consider positive displacement |
| A visible film or droplets remain along the tip wall | Wetting and surface-interaction effects | Evaluate recovery with the actual liquid; do not assume a tip coating will stop vapor-driven leaks |
| Both water and ethanol drip | Tip fit, seal, or pipette condition may contribute | Check tip compatibility and run the manufacturer's leakage test |
| Dripping stops, but results still vary | A leak-free appearance does not establish correct volume | Verify delivered volume and consistency under assay conditions |
Do Low-Retention or Filter Tips Stop Ethanol from Dripping?
Not necessarily. These features address different problems.
Low-retention tips are designed to reduce residual liquid on the inner wall. They can be worth evaluating when wetting or sample recovery is a problem, but a low-retention surface does not remove the air cushion or eliminate ethanol vapor pressure. Read our guide to low-retention pipette tips for the distinction.
Filter tips provide a barrier intended to reduce contamination risk under their specified conditions. They do not automatically prevent gas movement or solvent-driven dripping. If using reverse pipetting, also ensure the extra aspirated liquid cannot contact the filter.
A Better Rule for the Lab Bench
If ethanol starts dripping from a tip, do not immediately replace the pipette—or assume that switching tips will solve it. Start with the liquid's properties, then check the technique and the instrument.
Try pre-wetting, minimize delays, and consider reverse pipetting when it fits the application. For work that requires tightly controlled solvent volumes, a positive-displacement method and a measured performance check are more defensible than relying on appearance alone.
Keep science moving forward.
Frequently Asked Questions
Why does ethanol drip from the tip even when I hold the pipette upright?
Ethanol evaporates into the air cushion inside a manual air-displacement pipette. The resulting pressure change can push liquid out of the tip even without plunger movement.
Does reverse pipetting completely prevent ethanol leakage?
No. It includes extra liquid as a reserve and can help with some volatile-liquid transfers, but dripping and volume errors may still occur. Validate the technique for your target volume and workflow.
Should I pre-wet the tip before transferring ethanol?
Pre-wetting with the same liquid two or three times can help condition the air cushion and reduce dripping. It is not a substitute for verifying accuracy when the assay requires it.
Can low-retention or filtered tips solve this problem?
Neither feature is a guaranteed fix for vapor-pressure-driven dripping. Low retention relates to residual liquid and wetting; filters relate to contamination control. Pipette technology and technique remain important.
When should I use a positive-displacement pipette?
Consider one for volatile liquids when routine air-displacement methods do not meet the required accuracy or reproducibility. Follow the manufacturer's solvent-compatibility and handling instructions.
Related D2L Liquid-Handling Resources
- Liquid Handling for Viscous and Volatile Liquids: Tips and Tricks — a broader guide to challenging liquids.
- Low-Retention Pipette Tips: When to Use Them — when sample recovery, rather than solvent volatility, is the main issue.
- Browse Universal Pipette Tips — select tips based on pipette fit, volume, sterility, and filtration requirements. Standard tips are not a substitute for positive-displacement systems.
Technical scope: general laboratory education. Always follow the instructions for the specific pipette and tip, solvent safety guidance, and your laboratory's validated procedures.
