Reading a Buret Properly
Most people get this wrong on their first attempt and keep carrying the error through an entire titration. The buret is a graduated glass tube with a stopcock at the bottom, usually marked in 0.1 mL increments up to 50 mL. That sounds simple, but the way you actually extract a number from it involves a handful of mechanical steps that most lab manuals gloss over. Fill the buret past the zero mark first, then open the stopcock and let liquid flow through the tip until the meniscus settles somewhere below zero. That flushes out any air bubbles trapped in the nozzle. Close the stopcock, wait ten seconds for drainage, then note where the bottom of the meniscus sits. Adjust to exactly 0.00 mL or whatever your protocol requires. Write down that initial reading before you start dispensing into your flask. When it comes time to take the final reading, close the stopcock the moment you hit your endpoint. Wait another ten seconds. The glass walls drain slowly and the level will drop a bit more as liquid slides back down the sides. If you read immediately, your volume will be artificially low by anywhere from 0.02 to 0.05 mL, which matters when you are trying to hit three significant figures.
The meniscus is the curved surface of the liquid inside the tube. For aqueous solutions, you read the bottom of the curve, not the top. Position your eye level with the meniscus. If you look from above, the reading shifts higher than it actually is. From below, it shifts lower. This parallax error is probably the single most common source of mistake in undergraduate labs. I have seen students consistently read 0.1 mL too high because they stood while everyone else was sitting at the bench. To make the reading cleaner, use a card with a dark band across the middle. Hold it behind the buret so the band aligns just below the meniscus. The contrast makes the bottom of the curve sharply defined instead of a blurry curve against the graduation marks. You can buy these cards for a few dollars or just cut one from index cardstock. The graduated markings go from 0.00 at the top to 50.00 at the bottom. This is backwards from everything else in the world, which trips people up constantly. The zero is at the top because the buret measures how much has come out, not how much remains inside. When the liquid level is at the 12.50 mL mark, you have dispensed 12.50 mL, assuming you started at 0.00. Always subtract the initial reading from the final reading, regardless of where those numbers fall on the scale.
Reading Between the Lines
The smallest division on a standard 50 mL buret is 0.1 mL. You should estimate one digit beyond that, to 0.01 mL. That means if the meniscus sits between 24.30 and 24.40, you record it as something like 24.34 or 24.36 depending on where it looks like it lands. This estimated digit is why buret readings always have two decimal places. Here is a practical nuance nobody emphasizes enough: the meniscus changes shape depending on the liquid. In a concentrated sodium hydroxide solution, the meniscus can appear nearly flat or even slightly convex because of the liquid's interaction with the glass. In that case, reading the top edge instead of the bottom becomes reasonable. For standard acid titrations with phenolphthalein indicator, the meniscus is well-defined and you stick with the bottom. If you are working with something unusual like a viscous organic solution, forget about using a 50 mL glass buret and switch to a glass pipette or a digital dispenser. The drainage characteristics are completely different and your readings will be unreliable no matter how carefully you approach them. I spent an entire semester dealing with titration results that were consistently off by about 0.08 mL until I realized the buret I was using had a manufacturing defect in the tip. The stopcock sealed fine but there was a tiny irregularity in the glass just above the aperture that held back a consistent amount of liquid on every drain cycle. The readings looked normal but the actual delivered volume was lower than calculated. I switched to a different buret and the problem vanished immediately. This is why you should run a calibration check at the start of any new experiment, even if your lab kit seems brand new. Fill to 0.00, dispense 25.00 mL into a tared weigh boat, and weigh it. At room temperature, 25.00 mL of water should weigh approximately 25.00 grams. If it is off by more than 0.03 g, your buret needs attention or you need a different one.
Common Failure Modes
Air bubbles in the tip are the most frequent practical problem. If you fill the buret quickly without running liquid through the tip first, an air pocket can get trapped below the stopcock. When you dispense, that bubble comes out and the meniscus drops, but no liquid actually entered your flask. The volume difference between your initial and final reading includes the bubble volume, so your calculated titrant volume is too high. The fix is straightforward: after filling, keep the stopcock open and let liquid gush through the tip for a second or two to purge any trapped air, then re-read and reset the initial volume. Another issue is that the glass itself is not dimensionally stable across temperatures. A buret calibrated at 20°C will deliver slightly different volumes at 25°C because the glass expands. For most routine work this is negligible, maybe 0.01 mL over a full 50 mL range. In analytical work where you are chasing four significant figures, you need to either control the lab temperature or apply a correction factor based on the liquid's temperature and coefficient of expansion. The stopcock is also a wear item. A PTFE stopcock will last years with standard use and a thin film of grease. A ground glass stopcock needs regular cleaning and repacking with the right lubricant, and if it starts to stick, you are looking at inconsistent flow rates that make endpoint detection unreliable. I once had a ground glass stopcock that was barely perceptibly sticking during the last 0.5 mL of a delivery. The titration curve flattened out differently than it should have, and the endpoint came out about 0.15 mL early. Replaced the stopcock and the problem disappeared entirely. So if your results seem jagged or your titration curves look wrong, check the mechanical function of the stopcock before you blame the technique.
Finally, if you are doing high-throughput work where you need to deliver many aliquots rapidly, a manual glass buret is simply not the right tool. The process of reading, resetting, and dispensing takes too long and introduces too much variability. An automated buret or a calibrated pipette system will give you better precision in a fraction of the time. The glass buret has its place in teaching labs and routine qualitative work, but it is not built for efficiency.
