Volumetric apparatus and correct technique

The three pieces of volumetric glassware

Each has one job, and using the wrong one for that job is a systematic error that no amount of repeating will fix.

Apparatus	What it delivers	Typical uncertainty
Volumetric pipette	One fixed volume, very accurately (the aliquot)	±0.06 mL on 20 mL
Burette	A variable volume, read as a difference (the titre)	±0.05 mL per reading
Volumetric flask	Makes a solution up to one fixed total volume	±0.2 mL on 250 mL

• A measuring cylinder has an uncertainty around ±0.5 mL — ten times worse. It is fine for the water you rinse with, and never for a volume that enters the calculation.
• A conical flask measures nothing. Its volume markings are decorative.

Setting up the titration

• Titrant (the standard solution of known concentration) goes in the burette.
• Aliquot (the measured sample of unknown concentration) goes in the conical flask, delivered by pipette.
• Indicator — two or three drops only — goes into the conical flask.
• A white tile under the flask makes the colour change visible.

Rinsing — the rule that catches everyone

Each piece of glassware is rinsed with something different, and the reasoning is the same each time: rinse with whatever solution will be in it, so that residual water cannot dilute it.

Apparatus	Rinse with	Why
Burette	the titrant	water left inside would dilute the titrant, so a larger titre would be needed
Pipette	the solution being pipetted	water would dilute the aliquot, so a smaller titre would be needed
Conical flask	distilled water only	the amount of water present does not change the moles of substance in the aliquot
Volumetric flask	distilled water only	it is going to be filled to the mark with water anyway

• The conical flask is the one students get wrong. Rinsing it with the sample solution adds extra moles of the substance being analysed and makes the titre too large — a systematic error every single time.

Reading the volumes

• Read the bottom of the meniscus for colourless solutions, at eye level, to the nearest 0.05 mL.
• For dark solutions such as permanganate, read the top of the meniscus, and be consistent.
• Record both the initial and final readings, not just the difference — the initial reading need not be 0.00 mL.

• Reading from above or below eye level causes parallax error, which shifts every reading in the same direction — a systematic error.

Running the titration

• Trial (rough) titration first — add titrant quickly to find the approximate endpoint.
• Then the accurate ones — run in quickly to about 1 mL before the rough value, then add dropwise.
• Swirl continuously so the reagents mix, otherwise the indicator changes locally and you stop early.
• Rinse the flask walls with distilled water near the endpoint, so any splashes are washed back in. This does not dilute the result, because the moles are unchanged.
• Stop at the first permanent colour change that lasts on swirling.

Concordant titres

• Concordant titres are those agreeing within ±0.05 mL of each other (some schools accept ±0.1 mL — use your teacher's rule and state it).
• Keep titrating until you have at least three concordant titres.
• Average only the concordant ones. The rough titration and any outlier are excluded from the mean.