Volumetric Flask Guide: Class A vs B, 10 to 100 ml Sizes and Accurate Solution Prep

A 100 mL standard can be off before the instrument reads it

A technician makes up a 100 mL calibration standard for AAS, ICP-OES, or a titration. The balance was calibrated, the reagent was in date, and the instrument passed its checks. Yet the calibration curve sits slightly off, and every sample result inherits the same bias. The cause can be the flask.

Concentration is amount divided by volume (C = n/V). If the flask holds more than its nominal volume at the mark, every solution made in it is more dilute than the label says. If it holds less, the solution is more concentrated. The instrument reports faithfully on a standard that was already wrong, so the fault looks like drift in the instrument or the samples.

A volumetric flask is glassware made to hold one stated volume when filled to its mark at a stated temperature. ISO 1042 sets that reference temperature at 20 °C, with 27 °C allowed where a country has adopted it. The flask is used for standard solutions and accurate dilutions. Eurachem lists flask calibration, repeatability, and temperature among the contributors to volume uncertainty.

This guide covers the preparation procedure first, then Class A and Class B tolerances, then sizes and what to check when buying.

How the shape of a volumetric flask controls accuracy

Each part of the flask helps hold one volume consistently.

• Flat bottom. The flask stands upright on the bench, leaving both hands free to add solvent and judge the meniscus. Globe Glass flasks have a flat base like this.
• Long, narrow neck. A small change in liquid height here equals a small change in volume, so the meniscus can be set against the mark with fine control. A wide vessel gives the same height change a much larger volume change.
• Single etched line. The calibrated capacity is marked once, usually by diamond etching, so you read only one fill point.
• Stopper. A ground-glass or polymer stopper closes the flask so you can invert it to mix. BRAND lists glass-stopper and PP-stopper versions of its Class A flasks.

In a narrow neck, a 1 mm slip in liquid level changes the volume only a sliver, while the same slip in a beaker changes it far more.

Most analytical flasks are borosilicate glass. According to SCHOTT's data, Boro 3.3 has a linear thermal expansion coefficient of about 3.3 × 10⁻⁶ K⁻¹. Low expansion does not eliminate temperature effects, so the flask still needs to be near its calibration temperature. Plastic flasks such as PP or PMP are an alternative, and amber glass versions are sold for light-sensitive solutions.

Flasks carry a marking for how they were calibrated. TC or In means the flask holds the stated volume at the mark. TD or Ex means the vessel was calibrated to deliver that volume, allowing liquid to be left behind, as BRAND explains. Volumetric flasks are normally TC, so the nominal volume is what the flask contains, not what pours out.

Making a 100 mL standard solution, step by step

The running example is a 100 mL calibration standard made from a weighed solid. Each step names the error it prevents.

1. Check the flask. Confirm it is clean, with no droplets on the neck, and at room temperature. A warm flask holds a different volume from the one printed on it.
2. Weigh the solute. Calculate the mass from the target concentration and final volume, then weigh it accurately on an analytical balance. Record the actual mass, not the target, because any difference goes straight into the concentration.
3. Transfer quantitatively. Pour the solute through a funnel into the flask, then rinse the weighing vessel, funnel, and stirring rod into it. One UBC procedure calls for three rinses. Solute left on the glassware makes the standard weaker than calculated.
4. Dissolve in part of the volume. Add solvent to roughly half to two-thirds of the final volume and swirl until the solid dissolves. Dissolving in a smaller portion first leaves room to reach the mark without overshooting.
5. Let it equilibrate. Dissolution can warm or cool the solution. Wait until it returns to the calibration temperature; otherwise, the volume set at the mark is wrong once it settles.
6. Approach the line. Add solvent to just below the line, then add dropwise with a dropper until the meniscus reaches the line, viewing at eye level. You cannot correct an overshot flask.
7. Stopper and invert. Hold the stopper firmly and invert repeatedly, letting the bubble rise through the liquid. Procedures range from about 10 to 20 inversions; UBC uses 20. Without this, the denser solution stays at the bottom, and the top is more dilute.
8. Transfer and label. Move the solution to a storage bottle labeled with identity, concentration, solvent, preparation date, and preparer. The flask is a measuring vessel and is needed for the next standard.

If the liquid is already near the line while solid remains, the flask is too small for the mass.

Meniscus, parallax, temperature and cleaning: where volumetric errors come from

Most volumetric error in the 100 mL standard enters during steps 1, 5, and 6 of the procedure above. Each has a specific cause and a specific fix.

• Parallax at the meniscus. Viewing from above or below makes the liquid appear displaced from the mark. ISO 4787:2021 requires holding the flask vertically, with your eye in the same horizontal plane as the meniscus. Set the flask on the bench or hang it from a clamp.
• Reading the meniscus. For aqueous solutions, the lowest point of the curve should sit on the upper edge of the graduation line. A white card with a dark band behind the neck, or a backlight, sharpens that point.
• Temperature mismatch. Flasks are calibrated at a stated temperature, commonly 20 °C. Let the flask and solution approach that temperature before the final adjustment, as in step 5. Water density falls from about 0.998207 g/ml at 20 °C to 0.997048 g/ml at 25 °C per calibration reference data, which matters in warm Malaysian laboratories.
• Dirty glass. Grease or residue distorts the meniscus, and droplets on the neck wall add liquid above the mark. Water should wet the inner surface uniformly; beading means the flask needs cleaning again. ISO 4787 advises against fluoride-containing cleaners.

NIST guidance says analytical glassware should not be oven-dried, because non-elastic expansion can shift the calibration. Rinse with the solvent or air-dry.

Beading on the neck wall means residue is present, and a clean flask gives a smooth meniscus you can read against the line.

Class A and Class B flasks: how far apart the tolerances are

Class A is the tighter grade. Class B allows twice the capacity error for the same nominal volume, a relationship described by both UKAS guidance and ASTM E694. The figures below are the ISO 1042 limits for the sizes most labs buy.

Source: ISO 1042:1998. ASTM E288 uses its own limits, which are slightly different for the same sizes. For 100 ml, it gives about ±0.08 ml for Class A and ±0.16 ml for Class B, so check which standard your supplier works to.

Two flasks of the same nominal volume can differ by a factor of two in permitted error, and the markings on the glass tell you which one you hold.

In the running example, a Class B 100 ml flask can be off by 0.2 ml, or 0.2% of the volume, before any handling error is added. Class A suits calibration standards and accredited analysis. Class B is adequate for rough dilutions and teaching.

A batch certificate identifies the production batch and typically reports the batch mean volume, standard deviation, reference temperature, applicable standard, and equipment traceability. It supports routine use when batch-level evidence is enough for the method.

Class sets a permitted manufacturing tolerance. It does not give the full measurement uncertainty or replace a lab's own calibration checks. ISO/IEC 17025 laboratories need traceable calibration of volumetric glassware regardless of class.

Choosing a flask size: 10, 25, 50 and 100 mL and beyond

ISO 1042:1998 lists nominal capacities of 1, 2, 5, 10, 20, 25, 50, 100, 200, 250, 500, 1,000, 2,000 and 5,000 ml. Choose the smallest flask that gives enough solution without forcing an impractically small sample mass.

Smaller flasks save reagent until weighing error takes over. A common rule is to weigh at least 1,000 times the balance readability, about 100 mg on a 0.1 mg balance. When the target concentration requires less than that in a 10 ml flask, NIST guidance on measurement uncertainty often recommends a 100 ml flask. That is why 100 ml suits the running example.

Flasks of 5 ml and below suit very small volumes. Sizes of 200 mL and above suit bulk stock or mobile phases, at the cost of more glass and solvent per batch.

Amber Class A flasks from Wittek and Kimble Ray-Sorber protect light-sensitive analytes.

Volumetric flask vs beaker, graduated cylinder and Erlenmeyer or Florence flask

A 100 mL Class B cylinder typically carries a ±1.0 mL tolerance with 1 mL graduations, though models vary.

In the running example, dissolve the solute in a beaker or conical flask, transfer it quantitatively, and bring it to the line in the volumetric flask. Beakers, cylinders, and boiling flasks are used for approximate volumes, mixing, and heating. The volumetric flask sets the final volume.

Buying checklist for volumetric flasks

• Class and volume: Confirm Class A or B, nominal volume, and the marked standard.
• Stopper: It should seat firmly and fit the neck.
• Material: Borosilicate suits most work. For HF or trace metals, PMP is rated excellent for strong acids, and FEP suits trace-metal work.
• Amber: Use it for light-sensitive solutions.
• Certificate: Accredited labs need batch or UKAS-type certificates.
• Packaging: Check breakage protection.

D&D Laboratory can match glassware to your method and accreditation needs.

Sources

• Laboratory glassware — One-mark volumetric flasks — International Organization for Standardization
• Quantifying Uncertainty in Analytical Measurement — Eurachem
• 500mL Volumetric Flask, Globe Glass, Class B, 6/Box, 12/Case ... — Globe Scientific
• BLAUBRAND® Class A volumetric flasks with glass stopper — BRAND
• BORO-8330™ technical data — SCHOTT
• Amber BLAUBRAND® Class A volumetric flask — BRAND
• Volumetric Measuring Instruments | Glass & Plastic - brand.de — BRAND GMBH + CO KG
• Preparing a Solution — Harper College
• Technique: Standard Solutions — University of British Columbia
• Preparing Solutions from Solids — University of York
• Volumetric Flask Operation Guide — Kyoto University
• INTERNATIONAL STANDARD ISO 4787 — ISO
• ISO 4787:2021 — ISO / standards.iteh.ai
• ISO/FDIS 4787 — International Organization for Standardization
• NISTIR 7383 — National Institute of Standards and Technology
• SIST EN ISO 4787:2022 — Slovenian Institute for Standardization / standards.iteh.ai
• Selected Procedures for Volumetric Calibrations (2017 Ed) — National Institute of Standards and Technology (NIST)
• LAB 15: Traceability of Volumetric Apparatus — UKAS
• ISO 1042:1998 — ISO / iTeh Standards
• Certification | DWK Life Sciences — DWK Life Sciences
• Analytical Lab Balances: How to Choose, Calibrate, and Weigh Accurately — Lab Manager
• QUANTIFYING MEASUREMENT UNCERTAINTY IN ANALYTICAL CHEMISTRY – A SIMPLIFIED PRACTICAL APPROACH — National Institute of Standards and Technology (NIST)
• Amber-Stained Volumetric Flasks — witeg
• KIMBLE RAY-SORB Amber Class A Volumetric Flasks — DWK Life Sciences
• PP Class B Tall-Form Graduated Cylinders with Molded Graduations — VITLAB
• Transferring and Weighing Methods — Chemistry LibreTexts
• Volumetric Flasks | Thermo Fisher Scientific - US — Thermo Fisher Scientific
• DURAN Volumetric Flask Class A, Amber, with UKAS Certificate, 25 mL — DWK Life Sciences