Burette Accuracy: Class A vs Class B, Reading Errors, and Calibration

Short answer: burette accuracy depends on three things — tolerance class (A or B), reading technique at the meniscus, and how much of the volume you deliver relative to the full capacity. A Class A 50 mL burette is accurate to ±0.05 mL at full delivery; the same burette used for a 2 mL delivery loses relative accuracy fast. Know your class, read the meniscus correctly, and use the burette in its accurate upper range.

Pick by the job, not by habit

  • Regulated QC or compliance titration
    Class A burette
    Tightest ISO 385 tolerance; traceable calibration available
  • Teaching lab or routine non-critical titration
    Class B burette
    Twice the tolerance, lower cost, adequate for non-regulated work
  • Small deliveries < 5 mL needed accurately
    Smaller-range burette or volumetric pipette
    Absolute tolerance stays fixed; relative error rises on small volumes
  • Consistent high or low results
    Check meniscus reading technique
    Parallax error is systematic — always in the same direction
  • Results vary between operators
    Standardise reading and rinse procedure
    Technique variation is the most common source of inter-operator error
  • Burette delivering slowly or inconsistently
    Check and clean the stopcock
    Grease contamination or worn PTFE plug causes flow and seal problems

Burette vs pipette, side by side

  • ISO 385 tolerance (50 mL)
    ±0.05 mL
    ±0.10 mL
  • Relative error at 50 mL
    0.10 %
    0.20 %
  • Relative error at 5 mL
    1.0 %
    2.0 %
  • Typical use
    QC, regulated, reference methods
    Teaching, routine, non-critical
  • Verification
    Gravimetric test recommended
    Visual check usually sufficient
  • Cost
    Higher; tighter manufacturing spec
    Lower; wider tolerance accepted

Class A vs Class B — what the tolerance numbers mean in practice

ISO 385 defines two accuracy classes for burettes. Class A has the tighter tolerance: a 50 mL Class A burette carries ±0.05 mL absolute error. Class B allows twice that: ±0.10 mL. For a single titration that difference is small; across hundreds of titrations in a QC or regulated environment it accumulates into a systematic offset. Use Class A wherever the result feeds a calculation or a compliance record.

The volume-range accuracy trap

A burette's tolerance is stated as an absolute value (±0.05 mL), not a percentage. At 50 mL delivery that is 0.1 % relative error — excellent. At 5 mL delivery it is 1 % relative error — ten times worse. At 2 mL it is 2.5 %. The burette did not change; the maths did. For small deliveries, a volumetric pipette or a smaller-range burette gives better relative accuracy.

Three reading errors that cost accuracy

  1. Parallax at the meniscus. The eye must be exactly level with the bottom of the meniscus. Reading from above gives a lower reading; from below, higher — a consistent systematic offset every titration.
  2. Not rinsing with titrant before use. Rinsing with distilled water and then filling with titrant leaves a dilution layer on the glass walls. Rinse the burette two or three times with the titrant itself before starting.
  3. Ignoring the stopcock dead volume. The volume between the stopcock and the tip is not on the scale. A drop hanging on the tip at the endpoint is not accounted for. Touch the tip to the vessel wall to include it in the delivery.

Standards

  • ISO 385:2005
    Burettes — Class A and Class B tolerance limits and test methods
  • ISO 4787:2021
    Laboratory glassware — volumetric instruments; methods for testing and use
  • ISO 8655-6
    Gravimetric calibration method — applicable as reference for burette verification

Frequently asked questions

  • What is the accuracy of a Class A burette?
    A 50 mL Class A burette (ISO 385) has an absolute tolerance of ±0.05 mL. Relative accuracy depends on the volume delivered — best near full capacity, worst on small deliveries.
  • How do I read a burette correctly?
    Read the bottom of the meniscus with your eye level with the liquid surface. Any other angle introduces a parallax error that is consistent and systematic.
  • Why does my burette give different results each time?
    Variable results usually point to technique: inconsistent meniscus reading, residual rinse water diluting the titrant, or a stopcock that does not close fully between readings.
  • When should I use a burette instead of a pipette?
    Use a burette when you need to deliver a variable volume with drop-by-drop control — titrations, finding an endpoint. Use a pipette when you need to transfer one fixed volume repeatably.
Sourcing Class A burettes or certified titration equipment at scale? Contact us for factory-direct supply from ISO-certified manufacturers.