A Pasteur pipette is a simple glass or plastic tube with a narrow tip, used to transfer small amounts of liquid, typically around 1 to 2 mL at a time. You operate it by attaching a rubber or latex bulb to the wide end and using that bulb to draw up and release liquid. It takes a little practice to control the flow smoothly, but the technique itself is straightforward.
Attaching the Bulb
Start by pressing a rubber or latex bulb onto the top (wide) end of the pipette. Push it on just enough to create a seal, but don’t force the bulb down past the top segment of the glass. If the bulb sits too deep, it becomes harder to control suction and you risk liquid traveling up into the bulb itself, which contaminates both the bulb and your sample.
Drawing Up Liquid
Squeeze the bulb first, before you put the tip into the liquid. This is the key step most beginners get wrong. If you dip the tip in and then squeeze, you’ll blow air into the liquid and create bubbles or splash your sample.
With the bulb already compressed, lower the pipette tip into the liquid and slowly release pressure on the bulb. The suction draws liquid up into the glass stem. Release the bulb gently and steadily. If you let go too fast, you can pull in air bubbles or overshoot and suck liquid all the way up into the bulb. Keep the pipette upright the entire time. Tilting it lets liquid run sideways into the bulb or leak out the tip.
Watch the liquid level as it rises. You want it in the narrow stem section only, well below where the bulb connects. If you accidentally draw too much, hold the pipette over your source container and give the bulb a light squeeze to push some back out.
Transferring and Dispensing
Once you have liquid in the pipette, move it to your target container while keeping it vertical. Gently squeeze the bulb to push the liquid out. Use slow, consistent pressure. Squeezing too hard sends the liquid out in a fast stream, which can splash or introduce bubbles into your receiving container.
If you notice the liquid squirting unevenly or in bursts, try “conditioning” the pipette first: draw liquid in and push it back out a few times before making your actual transfer. This coats the inside of the glass with liquid and helps the flow behave more predictably. Pre-wetting is especially useful with viscous or sticky solutions, which tend to cling to dry glass and cause uneven dispensing.
Avoiding Common Mistakes
The most frequent problems with Pasteur pipettes come down to a few habits:
- Tilting the pipette. Even a slight angle can send liquid into the bulb, contaminating it and making it harder to control the next draw. Always keep the pipette vertical.
- Not immersing the tip deep enough. If the tip barely touches the surface of the liquid, you’ll aspirate air along with the sample, creating bubbles. Dip the tip a few millimeters below the surface.
- Releasing the bulb too quickly. This creates a sudden rush of suction that pulls in air and can overshoot the amount of liquid you want. Slow, steady release is the goal.
- Reusing a pipette between different solutions. Residue from the previous liquid clings to the inside of the glass. Even tiny amounts of carryover can contaminate your next sample or throw off measurements. Viscous and sticky fluids are the worst offenders, as they coat the inner surface and don’t fully drain. Use a fresh pipette for each different solution, or rinse thoroughly between uses.
Removing Supernatant and Other Delicate Tasks
One of the most common uses for a Pasteur pipette is carefully removing liquid from above a pellet or sediment at the bottom of a tube. The narrow tip gives you fine control that a larger pipette can’t match. The trick is to keep the tip near the surface of the liquid and follow it down as the level drops, rather than plunging the tip to the bottom. Touch the tip to the side of the tube wall if you need extra stability. Go slowly near the bottom, where you’re most likely to disturb the pellet and suck up material you want to keep.
Pasteur pipettes also work well for adding small amounts of reagent drop by drop, since a gentle squeeze on the bulb can release individual drops from the narrow tip. This makes them useful for tasks like adjusting pH or adding indicator solutions where precision matters more than speed.
Glass vs. Plastic Pasteur Pipettes
Glass Pasteur pipettes are the traditional version. They offer better chemical resistance, don’t leach compounds into your sample, and can be sterilized at high temperatures for reuse. The glass is also highly transparent, making it easy to see exactly how much liquid you’ve drawn up. The standard short version is 5.75 inches long and holds about 2 mL.
Plastic (disposable transfer) pipettes serve the same basic purpose but trade some precision for safety and convenience. They won’t shatter if dropped, they’re cheaper, and you don’t need to attach a separate bulb since the squeeze bulb is molded into the top. The downside is that plastic can interact with certain solvents, and viscous or low surface tension liquids tend to stick to plastic surfaces more than glass, leaving behind residue that affects accuracy.
For routine water-based transfers where breakage is a concern, plastic works fine. For organic solvents, precise volumes, or anything where chemical purity matters, glass is the better choice.
Safe Handling and Disposal
Glass Pasteur pipettes are fragile. The narrow stem snaps easily, and a broken tip creates a sharp point that can puncture skin. Hold the pipette by the wider upper portion, not the thin stem. When setting it down, place it somewhere it won’t roll off the bench.
Disposal depends on what you used the pipette for. Clean, uncontaminated glass pipettes go into designated glass waste boxes, not regular trash. This applies whether the pipette is broken or intact, since even unbroken glass can shatter in a trash compactor and injure waste handlers. If the pipette contacted biological materials, it goes into a sharps container and needs to be decontaminated (chemically or by autoclaving) before final disposal. Pipettes contaminated with radioactive or hazardous chemicals follow your facility’s specific waste protocols.
Never toss a glass Pasteur pipette into a regular trash bin. The thin glass easily punctures trash bags, creating a hazard for anyone who handles the waste downstream.