Choose Volumetric Apparatus: Fixed, Variable or Approximate

Choose apparatus from the volume task

A vessel can hold liquid without being suitable for measuring it accurately. Start with the job: do you need to transfer one fixed volume, deliver an adjustable volume, define a final contained volume, or make an approximate measurement? Only then compare the instrument’s stated range, calibration and tolerance. The examples below are fictional paper records, not instructions for handling chemicals.

“Exact volume” in a question means a specified target, not a measurement with zero uncertainty. A good answer names the apparatus and explains why its supplied specification meets the task. Naming the instrument with the most decimal places is not enough.

Four original specification cards

These specifications belong only to the example instruments. They are not universal tolerances for all apparatus of the same name. Assume the stated calibration conditions apply and the instruments are undamaged.

Containing and delivering are different claims. Card F defines what is inside the flask at the mark. It does not guarantee that pouring its contents elsewhere transfers that same volume: liquid can remain on the walls. Card P explicitly describes delivery under its calibration conditions. A single mark also cannot establish arbitrary smaller measured volumes.

Read a scale before subtracting

The scale detail represents a clear liquid with a concave meniscus. Its lowest point is the reading reference in this supplied drawing. On B, downward movement gives a larger reading; on C, upward movement gives a larger reading. The figure’s numeric alternative is: initial B reading 6.35 mL, final B reading 29.80 mL, and C reading 32 mL. The burette’s 6.35 mL estimate lies halfway between the 6.3 and 6.4 marks.

A view above or below the reference level can shift the apparent alignment of the meniscus and scale. That is parallax. The drawing represents an eye-level view; it does not give an extra uncertainty allowance. Nor can a drawing establish a real instrument’s calibration, temperature history or condition. Those need separate information.

Worked case 1: one specified transfer

The task is to transfer 25.00 mL with an allowable volume error no greater than 0.05 mL. Card P is the appropriate choice: its fixed delivered volume matches the target, and its ±0.03 mL tolerance is inside the allowed ±0.05 mL bound.

Card C is unsuitable for this requirement. Even if its scale appears to show 25 mL, its ±0.5 mL tolerance is ten times the permitted allowance. Card F fails on both capacity and purpose. Card B can describe a 25.00 mL delivery, but the supplied worst-case difference bound is ±0.10 mL, so it does not meet this particular requirement. The answer depends on these cards rather than a universal ranking of apparatus.

Worked case 2: a recorded variable delivery

A record gives B’s initial reading as 6.35 mL and final reading as 29.80 mL. The delivered volume is final minus initial:

29.80 mL − 6.35 mL = 23.45 mL.

The final scale reading is not itself the delivered volume because the starting reading was not zero. A negative result here would signal that the subtraction order had been reversed or the record needed checking. The difference also remains below the 50 mL scale capacity.

Under the explicitly supplied bounded-error model, the largest possible difference error is 0.05 + 0.05 = 0.10 mL. Thus the record is 23.45 mL with a worst-case bound of ±0.10 mL. Its interval is 23.35 to 23.55 mL. This is not a universal rule for every burette uncertainty budget: correlated calibration effects, reading estimates and statistical uncertainty require more information. Here the stated model makes the addition justified.

Worked case 3: a final contained volume

A worksheet asks which card defines a final volume of 250.0 mL, with an allowable contained-volume error of ±0.20 mL. Choose F: its purpose is to contain 250.0 mL at the calibration mark, and ±0.15 mL meets the requirement. This is an apparatus decision, not a preparation procedure.

Ten deliveries from P are not the same specification. Even ignoring transfer complications, ten individual ±0.03 mL bounds could accumulate to a worst-case ±0.30 mL. That exceeds the task’s allowance. Repetition does not automatically cancel a systematic error. A pipette consistently delivering slightly high would repeat that bias.

Precision, tolerance and honest digits

Resolution describes how finely the scale is divided. Precision concerns agreement among repeated results. Accuracy concerns closeness to the relevant true or reference value. Calibration tolerance is a specified allowable departure under stated conditions. These ideas are related, but none can be read reliably from the number of digits alone.

Writing the cylinder result as 32.000 mL would invent unsupported detail. Writing the burette difference as 23.45 mL retains the supplied readings’ decimal places, but does not claim an error of only 0.01 mL. The accompanying allowance matters. Similarly, “25.00 mL” is not proof that an unlabelled vessel is calibrated to deliver that amount.

Three practice tasks with solutions

1. Non-zero starting reading

A second B record starts at 11.20 mL and ends at 27.65 mL. What volume was delivered, and what bound follows from Card B?

Solution: 27.65 − 11.20 = 16.45 mL. The same model gives ±0.10 mL, or 16.35 to 16.55 mL. Using 27.65 mL alone would include the part of the scale already passed before this delivery.

2. Challenge the wrong tool

A learner chooses F for a single 40 mL measurement because “a volumetric flask is accurate.” Explain the error and choose a card for an approximate contained volume within ±1 mL.

Solution: F has a 250.0 mL mark, not a calibrated 40 mL mark. C has the required range and ±0.5 mL contained-volume tolerance, which meets the approximate task. An accurate mark at one volume does not calibrate every other level.

3. Evaluate a strict allowance

Would P meet a 25.00 mL transfer task allowing only ±0.02 mL?

Solution: No. Its ±0.03 mL supplied tolerance is larger than the allowed error. None of the cards establishes a suitable 25.00 mL transfer at that bound. “Insufficient suitable apparatus” is a valid conclusion when the specifications fail.

Frequently asked questions

Is a volumetric flask a delivery instrument?

Not on Card F. Its stated calibration concerns the volume contained at its mark; transferred volume needs a different specification.

Why does a burette scale increase downward?

The numbered scale records progressively lower liquid levels. A later, larger reading minus the initial reading gives the volume delivered in this model.

Must the initial burette reading be zero?

No. A valid difference can start at a non-zero reading, provided both readings and the applicable scale range are known.

Can I estimate between scale marks?

Sometimes the supplied reading model permits it, as at 6.35 mL here. Estimation does not change the instrument’s stated calibration tolerance.

Does a smaller tolerance guarantee a suitable choice?

No. The capacity, fixed or variable volume, and containing or delivering purpose must also match the task.

Does repeating a measurement remove bias?

No. Repetition can reveal scatter, but a consistent calibration error may persist through every reading or transfer.

Is a measuring cylinder always unsuitable?

No. Card C is suitable for the approximate 40 mL task. Suitability depends on the required allowance and the actual instrument’s specification.

Reference

The Royal Society of Chemistry’s measurement teaching guidance supports the distinctions between volume apparatus and meniscus/parallax interpretation. All specification cards, drawings and tasks here are original teaching examples.