Every number you will defend this semester came out of a piece of glassware or off a balance. How well you know that number is decided before you write anything down — by which instrument you reached for and how you read it.
This is the page that turns “we measured 25 mL” into a claim you can say how much to trust.
The instrument decides the answer, not the calculation
A calculation cannot improve a measurement. If you delivered your acid with a beaker, no amount of careful arithmetic afterwards makes the result better than the beaker was, and quoting the answer to four figures does not hide that — it advertises it.
So the first question in designing any procedure is not “what do I do?” It is “how well do I need to know this quantity, and which piece of glassware gives me that?” Sometimes the answer is a beaker, and reaching for a volumetric pipet would be a waste of everyone’s afternoon. Knowing which situation you are in is the skill.
Reading a meniscus
Water and aqueous solutions climb the glass slightly, so the surface curves into a dish. That curve is the meniscus, and there are three rules.
- Read the bottom of the curve, at its lowest point.
- Get your eye level with it. Looking down at the mark makes the volume read low; looking up at it makes it read high. That error is called parallax, it is invisible while you are making it, and it is the reason a group reads a burette twice and gets two different numbers with nobody having done anything wrong.
- Estimate one digit beyond the finest graduation. If the marks are every 1 mL, you report tenths. If they are every 0.1 mL, you report hundredths. That last digit is uncertain by definition — it is meant to be — and leaving it off throws away real information.
The dark card trick
Hold a white card with a thick black band on it behind the burette, with the top of the black band just under the meniscus. The curve goes from a faint grey line to an obvious dark crescent. It costs nothing, it takes two seconds, and it is what people who titrate for a living do.
The glassware, and when each one is the right choice
| Glassware | How well it knows the volume | Reach for it when |
|---|---|---|
| Beaker or flask graduations | Roughly ±5% — decorative, essentially | Never for a measurement. Rinse water, waste, “about 50 mL of solvent” |
| Graduated cylinder | Around 1% | The volume needs to be about right: dissolving, diluting, filling a water bath |
| Volumetric pipet | Around a tenth of a percent | Delivering one exact, fixed portion — the aliquot you titrate |
| Burette | Around a tenth of a percent, on a volume you choose | You need an exact volume but do not know in advance what it is |
| Volumetric flask | A few hundredths of a percent | Making a solution up to an exact total volume |
The exact figure is printed on the glass — a tolerance and a temperature, usually 20 °C, and often a class letter. Go and look at a real one before your first titration. Most students have used volumetric glassware for a year before noticing the numbers etched on it.
Two consequences of that etched temperature that catch people out:
- Never heat volumetric glassware, and never dry it in an oven. It was calibrated at one temperature, and heating can change the glass permanently. A volumetric flask you dried in an oven is now a flask of unknown volume that still has a number written on it, which is worse than no flask.
- A pipet and a burette are calibrated to deliver. A small film of liquid is meant to stay behind. Let the pipet drain, touch the tip to the inside wall of the receiving flask to take off the drop that wants to come, and do not blow out the rest — it was never part of the volume. The exception announces itself: a pipet with a frosted ring near the mouthpiece is a blow-out pipet and wants the opposite.
And, as ever: never pipette by mouth, with anything, including water. A bulb or a pump, every time.
Why “to the mark” means one thing in a volumetric flask and nothing in a beaker
A volumetric flask has one mark, and it is on the narrowest part of the glass. That is the entire design.
In a narrow neck, being a millimetre off vertically is a tiny error in volume, because a millimetre of a narrow tube holds very little. In a wide beaker, a millimetre of height is a large volume, which is why beaker graduations cannot be precise no matter how carefully they are printed. The flask is not more accurate because it is better made; it is more accurate because of its shape.
A volumetric flask is also calibrated to contain its stated volume, not to deliver it. Pour it out and some stays behind, which does not matter, because you use it as a container: everything you dissolved is in there, in exactly that volume, so you know the concentration.
Dissolve first, make up to the mark second
Add your solid, add water to perhaps two-thirds full, swirl until every grain is gone, and only then fill to the mark — the last few drops from a dropper, with your eye level with it. Volume changes when things dissolve, so a solution made up to the mark and then dissolved is not the concentration you wrote on the label. Stopper it and invert it fifteen or twenty times; a flask that has been swirled rather than inverted is layered, and the first sample you draw off will not represent it. See Preparing a Standard Solution.
Using the balance
- Check it reads zero with nothing on it before you start. Ten seconds, and it catches the error that would otherwise be in every mass you take today.
- Nothing goes directly on the pan. Weighing paper, a weighing boat, or a container — and never a corrosive on a bare pan.
- Close the draught shield if there is one, and do not lean on the bench while it settles.
- Let hot things cool to room temperature. A warm object sets up rising air currents that push up on it, and it reads light. It is also a fine way to damage the balance.
- Handle with tongs or a paper strip. Fingerprints have mass, and on a four-figure balance they are visible.
- Use the same balance for every mass in one experiment. A balance that is consistently 0.01 g out gives you a wrong mass and a right difference, and most of what you want is a difference.
Taring, and weighing by difference
Taring zeroes the balance with the empty container on the pan, so the display shows only what you add. One reading, one uncertainty, and it is the right choice when the sample stays where you weighed it.
Weighing by difference means weighing the container with the solid, transferring, and weighing the container again. Two readings, so their uncertainties combine and the difference is less precise than either one. It is still usually the better method when you are transferring, because it measures what actually left the container rather than what you intended to leave it — solid stuck in the weighing boat is counted correctly by the difference and counted wrongly by the tare.
That trade is worth understanding rather than memorising: you accept a larger uncertainty in exchange for removing a systematic error. It is the same bargain What Counts as Evidence keeps making.
Burettes, specifically
- The scale runs downward: zero at the top. You take an initial reading and a final one, and the volume delivered is the difference. There is no requirement to start at exactly 0.00.
- Expel the air bubble in the tip before the initial reading. If it comes out during the titration, the burette level falls without anything reaching the flask, and your delivered volume is recorded too large — a mistake that looks exactly like a real result.
- Read to two decimal places, both times, at eye level, with the card behind it.
- Run the tap with one hand from behind the burette, so the same hand never has to hold and swirl.
Rinsing: the rule that decides a titration
This looks fussy and is not. It is the difference between a result and a number.
| Glassware | Rinse with | Because |
|---|---|---|
| Burette | The solution it will hold | Water left inside dilutes it, and its true concentration is no longer the one on the label |
| Pipet | The solution it will hold | Same reason, and you cannot see the film that is doing it |
| Conical flask | Distilled water only | The amount of substance you delivered into it is fixed; extra water changes the volume but not the moles, so it cannot change the titre |
| Volumetric flask | Distilled water only | It is going to be filled to the mark anyway |
If you can explain the third row to somebody else, you understand concentration better than the rule does.
Before you leave the bench
- Burette drained and rinsed, left with the tap open
- Pipets rinsed and returned to the rack, tips up
- Volumetric flasks rinsed, not oven-dried
- Balance area brushed clean and the pan wiped
- Every reading in your notebook in ink, in the order you took it
Related: Significant Figures in Practice for what to do with the digits you have just collected, Writing a Lab Report for where they go, and Lab Safety and WHMIS for handling what is in the glassware.
Curriculum connection
A1.6
compile accurate data from laboratory and other sources, and organize and record the data, using appropriate formats, including tables, flow charts, graphs, and/or diagrams
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