Precision Glassware: Pipettes, Burettes, and Flasks

Precision (volumetric) glassware, which includes volumetric pipettes, burettes and volumetric flasks, is calibrated to measure or deliver exact volumes, usually with an uncertainty of a few hundredths of a milliliter. In brief: use a volumetric pipette to deliver a fixed volume such as 25.00 mL, a burette to deliver a variable volume in a titration, and a volumetric flask to prepare a solution of a known total volume.

When the concentration of a solution has to be known to three or four significant figures, ordinary beakers and cylinders are not enough. Precision glassware is designed for such work, but it only gives good results when used correctly. This article describes each type, shows typical tolerances, explains the technique for each, works through uncertainty calculations, describes the calibration idea and lists frequent errors.

What do Class A and Class B mean?

Volumetric glassware is made to tolerance classes. Class A items have the tightest tolerances and are used for analytical work. Class B items have tolerances roughly twice as large and are cheaper. Glassware is marked with the volume, the unit, the calibration temperature (usually 20 °C) and the type of calibration: TD (to deliver) means the instrument delivers the stated volume when emptied under defined conditions, and TC (to contain) means it holds the stated volume.

How do you use a volumetric pipette?

1. Rinse the pipette twice with a small amount of the solution to be measured, so that dilution by residual water is avoided.
2. Use a pipette filler (never your mouth) to draw liquid above the calibration mark.
3. Quickly place a finger or the filler valve to stop the flow, wipe the outside of the tip with a lint-free tissue and let the liquid drop until the bottom of the meniscus rests exactly on the mark at eye level.
4. Touch the tip to the inside wall of the receiving vessel and let the liquid drain freely, holding the pipette vertically.
5. Wait about 10 to 15 seconds after the flow stops, and touch the tip to the wall. Do not blow out the last drop unless the pipette is marked to blow out, since it was calibrated to leave it.

How do you use a burette?

1. Rinse with distilled water, then with two small portions of the titrant, letting each run through the tip.
2. Close the stopcock and fill the burette with the titrant above the zero mark, using a funnel, which is removed before reading.
3. Open the stopcock briefly to fill the tip and to remove air bubbles, and adjust the level to a reading near zero. It does not need to be exactly zero, as long as the exact start reading is recorded.
4. Read the bottom of the meniscus at eye level, with a white card with a black stripe behind it to make the meniscus clear. Read to the nearest 0.01 mL, estimating between the 0.1 mL marks.
5. During the titration, swirl the flask with one hand and use the other to control the stopcock. Add the titrant drop by drop near the end point, and record the final reading.
6. Volume delivered = final reading − initial reading.

How do you use a volumetric flask?

1. Dissolve the solid in a beaker in less than the final volume of solvent, then transfer the solution into the flask with a funnel, rinsing the beaker and funnel into the flask several times.
2. Add solvent until the level is a few centimeters below the mark and swirl to mix.
3. Add the last portion drop by drop with a dropper until the bottom of the meniscus is exactly at the mark.
4. Stopper the flask and invert it about ten times to mix thoroughly.
5. Do not heat volumetric flasks, and transfer the solution to a labeled storage bottle afterward.

Typical tolerances of Class A glassware

Exact tolerances depend on the standard and the manufacturer. Check the marking or the certificate of the glassware.

Worked examples with uncertainty

1. Titration volume. The initial burette reading is 0.45 mL and the final is 24.85 mL. The volume delivered is 24.85 − 0.45 = 24.40 mL. Each reading has an uncertainty of about ±0.05 mL (the readability and tolerance), so the volume is 24.40 ± 0.10 mL. The relative uncertainty is 0.10 ÷ 24.40 × 100% = 0.41%.
2. Making a diluted solution. A 25.00 mL sample of 0.1000 mol/L hydrochloric acid is transferred to a 250.0 mL volumetric flask and diluted to the mark. The new concentration is 0.1000 × 25.00 ÷ 250.0 = 0.01000 mol/L.
3. Uncertainty of the diluted solution. The pipette has a relative uncertainty of 0.03 ÷ 25.00 = 0.12%, and the flask has 0.12 ÷ 250.0 = 0.048%. Adding the relative uncertainties gives 0.168%, and the concentration is 0.01000 ± 0.00002 mol/L (0.168% of 0.01000 is 0.0000168).
4. Choosing the right size. If an expected titre is about 12 mL, a 25 mL burette gives a smaller relative uncertainty than a 50 mL burette, since the same reading error of ±0.05 mL affects a small volume more.
5. Standard solution. To make 250.0 mL of 0.1000 mol/L NaCl, the mass of NaCl (58.44 g/mol) needed is 0.1000 × 0.2500 × 58.44 = 1.461 g, weighed to ±0.001 g, dissolved and made up to the mark.

How can glassware be calibrated?

Glassware can be checked by weighing the water it delivers. A burette delivers 10 mL of water into a weighed flask, and the mass of water is measured with a balance. At 20 °C, the density of water is 0.9982 g/mL, so the volume is the mass divided by 0.9982. For example, if the burette delivers 9.985 g of water, the real volume is 9.985 ÷ 0.9982 = 10.003 mL, which is within the tolerance for a 10 mL delivery. The temperature has to be recorded because the density of water changes with it.

Where did volumetric analysis begin?

Volumetric analysis, in which the volume of a solution that reacts with a sample is measured, was developed in the late 18th and the 19th centuries. Francois Antoine Henri Descroizilles invented an early burette around 1791, and Joseph Louis Gay-Lussac improved the burette and pipette and introduced the term titration in the 1820s. Karl Friedrich Mohr developed the pinch-clamp burette and many methods in the middle of the 19th century. Standardized glassware followed in the 20th century.

Which mistakes are common?

Mistake 1: Skipping the rinse with the solution

Water left in a burette or pipette dilutes the solution. Rinse with the solution itself.

Mistake 2: Leaving an air bubble in the burette tip

The bubble gives a false volume when it is expelled. Remove it before the first reading.

Mistake 3: Adding solvent above the mark of a volumetric flask

You cannot remove it. The solution has to be remade.

Mistake 4: Blowing out the last drop of a pipette

Most pipettes are calibrated to leave a small amount in the tip.