Glycerol is one of the trickiest liquids to pipette accurately because of its extreme viscosity. A standard air displacement pipette used normally will under-deliver by nearly 5%, which is enough to throw off buffer recipes, cryoprotectant mixes, and enzyme storage solutions. The key to pipetting glycerol well comes down to three things: using the right technique (reverse pipetting), going slowly, and choosing appropriate equipment for the concentration you’re working with.
Why Glycerol Is Hard to Pipette
Glycerol resists flow. At room temperature, even a 50% glycerol solution is far more viscous than water, and 100% glycerol barely moves at all. Cold temperatures make things worse: if your glycerol stock is at 4°C, it’s roughly twice as viscous as it is at room temperature. This viscosity creates two problems inside a standard air displacement pipette. First, the air cushion between the piston and the liquid compresses unpredictably, so you aspirate less than the set volume. Second, glycerol clings to the walls of the pipette tip during dispensing, leaving behind a significant residual volume.
Testing by Gilson found that a standard air displacement pipette dispensing 100% glycerol in normal (forward) mode had a systematic error of negative 4.67%, meaning it consistently delivered about 5% less than the target volume. The random error was 3.25%, so repeated pipettings also varied widely from each other. Switching to reverse mode on the same pipette only improved things slightly, to negative 4.38% systematic error and 2.54% random error. That tells you the problem isn’t just technique; it’s the fundamental mismatch between an air cushion mechanism and a thick liquid.
Reverse Pipetting: The Core Technique
Reverse pipetting is the single most important adjustment you can make. Unlike normal (forward) pipetting, where you press to the first stop to aspirate and blow out past it to dispense, reverse pipetting flips the logic. You aspirate extra liquid by pressing all the way to the second stop (the blowout position) before drawing up, then dispense only to the first stop. The small excess that remains in the tip acts as a buffer, ensuring you deliver the full target volume without needing a blowout that would introduce bubbles or inconsistency.
Here’s the step-by-step process:
- Set your volume on the pipette as usual.
- Press the plunger past the first stop all the way down to the second (blowout) stop before you touch the liquid.
- Immerse the tip in the glycerol and release the plunger slowly, letting it rise back to the resting position. Go much slower than you would with water.
- Pause after aspirating. Wait at least 1 to 2 seconds for dilute glycerol solutions (under 50%). For concentrated glycerol (above 80%), wait 2 full seconds or longer to let the liquid equilibrate inside the tip.
- Dispense by pressing to the first stop only. Again, press slowly. A small volume will remain in the tip, and that’s intentional.
- Pause after dispensing for at least 1 second before withdrawing the tip from the receiving vessel. For highly viscous glycerol, wait longer to let the liquid fully release.
- Discard the tip with the residual liquid still inside. Do not blow it out into your sample.
The slow speed matters as much as the technique itself. Aspirating or dispensing quickly creates negative pressure that pulls air into the liquid, forming bubbles that displace volume and ruin accuracy. Think of it as letting the glycerol come to you rather than forcing it.
Pipette Angle and Tip Position
Hold the pipette vertically (straight up and down, at 0 degrees) when aspirating. This minimizes the liquid film that coats the outside of the tip. When dispensing, tilt the pipette to about 30 to 45 degrees and touch the tip gently against the inside wall of the receiving tube. This contact point gives the glycerol a surface to flow along, helping it release from the tip more completely.
Use wide-bore pipette tips if they’re available for your pipette model. The wider opening reduces the resistance that glycerol encounters as it enters and exits the tip, which decreases aspiration time and limits the pressure differential that causes bubbles. Wide-bore tips are especially helpful at volumes above 200 microliters, where the slow flow rate of glycerol through a standard narrow tip becomes a real bottleneck.
Pre-Wetting the Tip
Before you pipette the volume you actually need, aspirate and dispense glycerol back into the source container two or three times using the same tip. This pre-wetting step coats the inner walls of the tip with glycerol, so the first “real” aspiration doesn’t lose volume to the dry plastic surface. Pre-wetting also helps condition the air cushion in an air displacement pipette, reducing the compression error on subsequent draws. Skip this step and your first pipetted volume will almost certainly be low.
Matching Equipment to Glycerol Concentration
Dilute Glycerol (Up to About 50%)
A standard air displacement pipette with reverse pipetting works well enough for glycerol solutions in the range of 10 to 50%. These concentrations are viscous but still flow reasonably. Pre-wet the tip, go slow, and pause after aspirating. You can expect acceptable accuracy for most routine lab work like making glycerol stocks for bacterial freezer storage.
Concentrated Glycerol (50 to 85%)
This is the range where standard pipettes start to struggle significantly. Eppendorf notes that solutions around 85% glycerol (roughly 200 millipascal-seconds viscosity) can still be transferred with an air displacement pipette using reverse pipetting, but you’re pushing the limits. Wide-bore tips become important here, and you should extend your pause times to at least 2 seconds after both aspiration and dispensing. If accuracy is critical, verify your volumes by weighing: pipette the glycerol onto a tared balance and compare the mass to what you’d expect based on glycerol’s density (about 1.26 grams per milliliter for pure glycerol).
Pure Glycerol (100%)
Neat glycerol at room temperature is extremely viscous, and at 4°C it’s nearly impossible to pipette accurately with a standard air displacement pipette. Even reverse pipetting on a standard pipette leaves you with roughly 4 to 5% error. For pure glycerol, a positive displacement pipette is the better tool. These pipettes use a piston that sits inside a disposable capillary tip and makes direct contact with the liquid, eliminating the air cushion entirely. Because there’s no air gap, viscosity and temperature don’t affect accuracy in the same way. Manufacturers like Eppendorf and Gilson make positive displacement pipettes and compatible tips designed for viscous liquids.
If you don’t have access to a positive displacement pipette, an alternative for pure glycerol is to weigh it. Set a tube on an analytical balance, tare it, and add glycerol with a standard pipette or even a serological pipette until you reach the target mass. For example, if you need 500 microliters of pure glycerol, aim for about 0.63 grams.
Warming Glycerol Before Pipetting
Letting cold glycerol come to room temperature before pipetting makes a noticeable difference. Glycerol stored at 4°C is roughly twice as viscous as glycerol at 20°C. Simply removing the bottle from the fridge 15 to 20 minutes before use, or briefly warming it in your hands, reduces the viscosity enough to improve flow into and out of the pipette tip. This won’t fix accuracy problems with 100% glycerol, but it helps substantially with solutions in the 50 to 85% range.
Common Mistakes to Avoid
- Pipetting too fast. Speed is the most common source of bubbles and volume errors. If you see any air bubbles in the tip after aspirating, eject the liquid back into the source and start over, more slowly.
- Using forward pipetting. Normal forward pipetting with a blowout forces air through the viscous liquid, creating foam and inconsistent volumes. Always reverse pipette glycerol.
- Skipping the pause. After aspirating, the glycerol inside the tip is still moving. If you pull out of the source liquid immediately, you’ll draw in air at the bottom of the tip. A 1 to 2 second pause for moderate concentrations (longer for pure glycerol) lets the liquid settle.
- Wiping the tip aggressively. It’s fine to gently touch the tip to the rim of the source container to remove excess liquid from the outside. But wiping it with a tissue can wick glycerol out of the tip through capillary action, reducing your volume.
- Reusing tips between solutions. Glycerol’s stickiness means cross-contamination is more likely than with aqueous solutions. Use a fresh tip for each transfer unless you’re pipetting from the same stock.