A buret delivers precise volumes of liquid, and reading one correctly comes down to eye position, meniscus alignment, and recording to the right decimal place. A standard 50 mL buret has graduation marks every 0.1 mL, and you’re expected to estimate one digit beyond that, giving readings to the nearest 0.01 mL. Here’s how to get accurate, repeatable measurements every time.
Preparing the Buret Before Use
A clean buret is the foundation of an accurate reading. Start by washing the inside with soap and water, then rinse thoroughly with distilled water. Look at the inner glass surface as water drains: if it sheets evenly without forming droplets, the buret is clean. Droplets clinging to the walls mean residue is still present, which will interfere with your volume measurements.
Once the buret is clean, you need to prime it with whatever solution you’ll be dispensing. Place about 5 mL of that solution in the buret, hold it horizontally over a sink, and rotate it so the liquid coats the entire inner surface. Let the solution drain out through the stopcock into a waste beaker. Repeat this priming step at least two more times. This ensures the concentration of the solution is uniform throughout the buret, with no leftover water diluting it.
Removing Air Bubbles
Air bubbles trapped in the tip or near the stopcock will throw off your volume measurement. Before you take any readings, drain several milliliters of solution into a waste beaker and check the tip for bubbles. If you still see one, try opening the stopcock fully and running solution through quickly. You can also give the buret a short, firm downward jerk while the stopcock is open. The combination of fast flow and a quick shake usually dislodges stubborn bubbles. Don’t skip this step: a single bubble expanding or releasing mid-measurement can add an error of several hundredths of a milliliter.
Reading the Meniscus
The meniscus is the curved surface that forms at the top of a liquid in a narrow tube. For water and most aqueous solutions, this curve dips downward in the center. You always read the volume at the bottom of that curve.
Position your eyes so they’re exactly level with the liquid surface. If you look down at the meniscus, it appears higher than it actually is. If you look up, it appears lower. This is called parallax error, and it’s the single most common source of inaccuracy with a buret. One helpful trick: line up the front edge of a graduation mark with the back edge of the same mark as seen through the glass. When those two edges appear as a single line, your eyes are at the correct height.
A buret card (a small card that’s white on one side and dark on the other) can also help. Hold the dark side just behind the meniscus. The dark background makes the bottom of the curve sharper and easier to pinpoint.
Recording to the Correct Decimal Place
On a standard 50 mL buret, the smallest graduation marks are 0.1 mL apart. You can read the mark closest to the meniscus directly, but you’re also expected to estimate one additional digit between the marks. For example, if the meniscus sits between the 25.2 mL and 25.3 mL lines and appears to be about eight-tenths of the way past 25.2, you would record the reading as 25.28 mL.
That last estimated digit carries an assumed uncertainty of plus or minus one in the final place, so 25.28 mL really means somewhere between 25.27 and 25.29 mL. This gives you four significant figures, which is the standard precision for buret work. Never round your reading to just one decimal place. If the meniscus sits right on a line, record it with a zero in the hundredths place (for example, 25.20 mL, not 25.2 mL). That trailing zero communicates that you looked at the hundredths place and found nothing extra.
Note that buret scales read from top to bottom: 0.00 mL is at the top, and 50.00 mL is at the bottom. This is the opposite of a graduated cylinder, and it trips up many first-time users.
Calculating Volume Delivered
A buret measures how much liquid you dispensed, not how much is left. To find that, you need two readings: one before you start dispensing (the initial reading) and one after (the final reading). The volume delivered is simply the final reading minus the initial reading.
You don’t need to start at exactly 0.00 mL. If your initial reading is 1.34 mL and your final reading is 26.78 mL, the volume delivered is 25.44 mL. Starting at a convenient point rather than forcing the meniscus to 0.00 saves time and avoids wasting solution.
Understanding Buret Tolerances
Not all burets are equally precise. Laboratory glassware is graded into two classes. A Class A 50 mL buret has a tolerance of plus or minus 0.05 mL, meaning the actual volume could differ from the reading by up to that amount. A Class B 50 mL buret has double the tolerance, plus or minus 0.10 mL. For smaller burets, the tolerances tighten: a 10 mL Class A buret is accurate to plus or minus 0.02 mL.
For most general chemistry and analytical work, Class A burets are standard. If you’re doing rough measurements or student demonstrations, Class B is adequate. The tolerance is usually printed or etched on the buret itself, near the top.
Common Mistakes That Affect Accuracy
Beyond parallax error and air bubbles, a few other pitfalls can quietly ruin your measurements. Forgetting to prime the buret is one of the most frequent: residual water from rinsing dilutes your solution, making the first few milliliters less concentrated than the rest. Overly generous stopcock grease is another issue. If grease migrates into the tip and partially blocks it, liquid delivery becomes uneven. You can clear a blocked tip by opening the stopcock and dipping the tip into a small beaker of a grease-dissolving solvent, repeating until the opening is clear. If the blockage is severe, a fine piece of wire threaded through the tip first helps the solvent reach the grease.
Wet fingers on the stopcock area can also cause problems. Moisture or oils from your hands may make the stopcock slip, releasing liquid unexpectedly. Handle the stopcock with dry hands, and turn it with gentle, controlled pressure rather than gripping tightly.
Step-by-Step Summary
- Clean the buret with soap and water, then rinse with distilled water until the glass surface sheets evenly.
- Prime with 5 mL of your solution, rotating to coat the inside. Repeat at least twice, discarding the rinse each time.
- Fill the buret above the 0.00 mL mark, then drain to remove air bubbles from the tip.
- Record the initial reading to two decimal places, with your eyes level to the bottom of the meniscus.
- Dispense the desired amount of liquid.
- Record the final reading using the same technique.
- Calculate the volume delivered by subtracting the initial reading from the final reading.