Balances and volumetric glassware

Awareness level only. This lesson explains the ideas so you recognise them. It does not qualify you to do this work. Before you use lab equipment or handle chemicals, biological materials or patient samples for real, you need the training your employer or institution requires, and you must work under supervision until they sign you off.

Why it matters

Almost every lab result starts with a mass or a volume. If the 250.0 mg you weighed was really 247 mg, or the 100 mL flask was filled past the line, every number downstream carries that error: the concentration of your standard, the calibration curve built from it, and the patient or product result read from that curve. Balances and volumetric glassware are the first instruments you will be trained on, and the habits you build with them follow you through your whole career.

Accuracy, precision and readability

• Accuracy is how close a measurement is to the true value.
• Precision is how close repeated measurements are to each other.
• Readability is the smallest step the display shows.

A balance can be precise but inaccurate (it always reads 0.3 mg high) or have fine readability but poor precision (the last digit jumps around in a draught). Calibration and good technique address different parts of this.

Balances

Good weighing practice on an analytical balance:

1. Check it is level. Most balances have a bubble level. An unlevel balance reads wrong.
2. Check the daily verification. Many labs weigh a certified check weight each day and log it. If the log shows a failure, the balance is out of service.
3. Close the draught shield doors before reading. Air currents move the last digit.
4. Let warm or cold items reach room temperature. A warm beaker creates convection currents and reads light.
5. Use a weighing boat or paper, and tare it. Taring sets the display to zero with the container on the pan.
6. Weigh by difference for hygroscopic or valuable material: weigh the closed vial, remove some material, weigh the vial again. The mass transferred is the difference, and nothing left on a weighing boat is lost from the calculation.
7. Keep the pan clean. Spilled material changes later readings and can damage the mechanism.

Worked example: weighing by difference

You report 0.2494 g (249.4 mg), with all four decimal places the balance gave you.

Volumetric glassware

Glassware falls into two groups:

• Volumetric (precise): volumetric flasks, volumetric (transfer) pipettes and burettes. They are calibrated to a single volume or a fine scale.
• Approximate: beakers, Erlenmeyer flasks and, to a lesser extent, graduated cylinders. Beaker markings are only rough guides, often within about 5 percent. Never use a beaker to measure a volume that matters.

Volumetric glassware is made to a class. Class A has the tightest tolerances, and Class B tolerances are roughly twice as wide. The tolerance is printed on the glass. For example, a 100 mL Class A volumetric flask is typically marked ±0.08 mL.

Two markings tell you how the glass was calibrated:

• TC (to contain): the flask holds the stated volume when filled to the line. Volumetric flasks are TC.
• TD (to deliver): the stated volume comes out when the pipette drains, leaving a small film behind. Most transfer pipettes are TD, and you do not blow out the last drop unless the pipette is marked for it.

Reading a meniscus

Water forms a curved surface (a meniscus) that dips in the middle. Read the bottom of the meniscus with your eye level with the line. Looking from above or below causes parallax error. A white card with a dark stripe held behind the glass makes the bottom of the curve easier to see.

Significant figures

Record every certain digit plus one estimated digit. A 50 mL burette has 0.1 mL graduations, so you read it to 0.01 mL, for example 23.46 mL. Writing "23.5 mL" throws away information you actually measured.

Common mistakes

• Measuring an important volume with a beaker or graduated cylinder instead of a volumetric pipette or flask.
• Filling a volumetric flask to the line before the solid has fully dissolved, then topping up later.
• Reading the meniscus from above.
• Weighing a sample straight from the oven.
• Rounding a balance reading in the notebook.
• Using a balance that failed its daily check.

Beyond this course

This course teaches the knowledge behind bench work. Real bench skills come from supervised practice, and the careers it serves have formal requirements:

• Medical laboratory technologist / scientist: a NAACLS-accredited program with clinical rotations (https://www.naacls.org), ASCP Board of Certification MLS certification (https://www.ascp.org/content/board-of-certification), and a state licence in states that require one.
• Biological technician: an associate or bachelor's degree in biology or biotechnology with supervised lab hours. In regulated manufacturing, GMP training happens on the job (FDA CGMP: https://www.fda.gov/drugs/pharmaceutical-quality-resources/current-good-manufacturing-practice-cgmp-regulations).
• Chemist: a bachelor's in chemistry with hands-on lab courses; ACS-approved programs are preferred by many employers (https://www.acs.org/education/policies/acs-approval-program.html).
• Biochemist or materials scientist: a bachelor's with real lab or instrument coursework for technician roles; an MS or PhD for independent research.

Requirements vary by state and employer, so check the official sources. (Checked on 2026-10-23.)

Quick recap

• Accuracy, precision and readability are different properties.
• Level, verify, close the doors, tare, and weigh by difference when it matters.
• Use Class A volumetric glassware for exact volumes, and read the bottom of the meniscus at eye level.
• Record every digit the instrument gives you, plus one estimated digit for scales you read by eye.

Real lab training covers your institution's balances and glassware in person. For lab safety rules, see OSHA's laboratory page, https://www.osha.gov/laboratories (Checked on 2026-10-23.)

Try it

Open the Do tab to practise accuracy, precision and meniscus reading with a kitchen scale, a measuring jug and plain tap water.