Glassware Tolerances and Uncertainty

Look closely at a volumetric pipette and you’ll see small print etched into the glass: something like “25 ml ±0.03 A 20 °C Ex”. Those few characters carry a lot of information. They tell you how accurate the glassware is, how it should be used, and at what temperature it’s valid. Understanding them is essential for serious quantitative work and for calculating the uncertainty of your results.

What a tolerance is

A tolerance is the maximum amount by which the true volume of a piece of glassware is allowed to differ from its nominal (stated) volume, under standard conditions, according to the manufacturing standard.

For example, a 25 cm³ Class A pipette with a tolerance of ±0.03 cm³ is guaranteed to deliver between 24.97 and 25.03 cm³ when used correctly at the calibration temperature.

Tolerances are set by international standards, such as those published by ISO (the International Organization for Standardization) and national equivalents like ASTM in the United States.

Class A and Class B

Volumetric glassware comes in two main accuracy grades:

• Class A: tighter tolerances, used for accurate analytical work. Often individually tested and may come with a certificate.
• Class B: tolerances roughly twice as wide as Class A. Cheaper, suitable for routine teaching and less demanding work.

Item and size	Typical Class A tolerance	Typical Class B tolerance
Volumetric pipette, 10 cm³	±0.02 cm³	±0.04 cm³
Volumetric pipette, 25 cm³	±0.03 cm³	±0.06 cm³
Volumetric pipette, 50 cm³	±0.05 cm³	±0.10 cm³
Burette, 50 cm³	±0.05 cm³	±0.10 cm³
Volumetric flask, 100 cm³	±0.10 cm³	±0.20 cm³
Volumetric flask, 250 cm³	±0.15 cm³	±0.30 cm³
Volumetric flask, 1000 cm³	±0.40 cm³	±0.80 cm³
Measuring cylinder, 100 cm³	±0.5 cm³	±1.0 cm³

These are representative values; exact tolerances depend on the standard and manufacturer. Always use the value printed on your glassware or given by your course.

Notice that the percentage tolerance is much smaller for pipettes and volumetric flasks (around 0.1%) than for measuring cylinders (around 0.5–1%).

The markings decoded

Marking	Meaning
25 ml (or cm³)	nominal volume
±0.03	tolerance
A or B	accuracy class
20 °C	calibration temperature
Ex or TD	calibrated “to deliver”
In or TC	calibrated “to contain”
5 s or similar	waiting time after draining (for some pipettes)
Coloured band	colour code for the volume (e.g. on pipettes)
AS	Class A with a faster (“swift”) delivery design

TD (to deliver) vs TC (to contain)

• TD / Ex glassware (pipettes, burettes) is calibrated so that the stated volume flows out when used correctly. A thin film of liquid remains on the inside walls and a small drop in a pipette tip; the calibration already allows for this. That’s why you must not blow out a pipette.
• TC / In glassware (volumetric flasks) is calibrated so that it holds the stated volume when filled to the mark. If you pour it out, you’ll get slightly less.

Calibration temperature

Glassware is calibrated at a reference temperature, usually 20 °C (sometimes 27 °C in hot climates). At other temperatures, both the glass and the liquid expand or contract slightly. For water near room temperature, a 5 °C difference changes the volume of the liquid by roughly 0.1%, which matters only in very precise work. Hot solutions should never go into volumetric glassware; besides the volume error, repeated heating can permanently distort the glass.

Tolerance vs reading uncertainty

There are two different kinds of uncertainty to think about:

1. Tolerance (calibration uncertainty): how far the glassware itself might be from its nominal value.
2. Reading uncertainty: how precisely you can read a scale.

For a pipette or volumetric flask, there’s only one mark, so the tolerance is the main uncertainty.

For a burette, you take two readings (start and end). Each is usually read to the nearest 0.05 cm³, so each reading has an uncertainty of ±0.05 cm³, and a titre has ±0.10 cm³. In many school courses, this reading uncertainty is used as the burette’s uncertainty, rather than the manufacturer’s tolerance.

For a measuring cylinder, the uncertainty is usually taken as half the smallest scale division.

Different courses have slightly different conventions. Use the one your exam board specifies.

Using tolerances in calculations

Percentage uncertainty = (absolute uncertainty ÷ measured value) × 100

Example 1: pipette. Class B 25.00 cm³ pipette, ±0.06 cm³: 0.06 ÷ 25.00 × 100 = 0.24%

Example 2: volumetric flask. Class B 250.0 cm³ flask, ±0.30 cm³: 0.30 ÷ 250.0 × 100 = 0.12%

Example 3: burette. Titre = 24.35 cm³, uncertainty ±0.10 cm³ (two readings): 0.10 ÷ 24.35 × 100 = 0.41%

Example 4: total for a titration. If a standard solution is made in the flask, then pipetted and titrated, a simple estimate of the overall percentage uncertainty is the sum of the individual percentage uncertainties: 0.24 + 0.12 + 0.41 + (balance uncertainty, say 0.20) = about 0.97%

So a calculated concentration of 0.0987 mol/dm³ would be reported as 0.0987 ± 0.0010 mol/dm³. More on combining uncertainties is in propagating uncertainty and calculating percentage uncertainty.

Which piece of equipment limits accuracy?

In most titrations, the burette contributes the largest percentage uncertainty, especially if titres are small. That’s why:

• experiments are designed to give titres of at least 20 cm³
• reading precisely to ±0.05 cm³ matters
• a Class A burette helps for demanding work

When a question asks “Suggest one change to reduce the percentage uncertainty”, look for the measurement with the largest percentage uncertainty and suggest either a more precise instrument or a larger measured quantity.

Calibrating your own glassware

In high-precision analysis, chemists calibrate pipettes and burettes themselves by weighing water:

1. Deliver water from the pipette into a weighed container.
2. Weigh the water accurately.
3. Convert mass to volume using the density of water at that temperature (about 0.9982 g/cm³ at 20 °C).
4. Repeat several times and average.

This gives the pipette’s actual delivered volume, which can then be used instead of the nominal value.

Caring for volumetric glassware

• Clean thoroughly; grease or residue on the inside causes droplets to cling and changes the delivered volume. Clean glassware drains in an even film with no droplets.
• Never heat volumetric flasks or pipettes, or dry them in a hot oven.
• Store pipettes safely; chipped tips change the delivered volume and should be replaced.
• Keep burette taps clean and properly lubricated (or use PTFE taps, which don’t need grease).

Key takeaways

• A tolerance is the maximum allowed deviation of the glassware’s true volume from its nominal volume.
• Class A glassware has roughly half the tolerance of Class B.
• TD/Ex glassware (pipettes, burettes) is calibrated to deliver; TC/In glassware (volumetric flasks) to contain.
• Calibration is at a stated temperature, usually 20 °C.
• Percentage uncertainty = uncertainty ÷ value × 100; sum the percentages to estimate an overall uncertainty.
• For practical technique, see measuring volume accurately.