Class limits, stopcock choice & drainage
Where the error actually comes from in a titration.
A burette measures by difference: it is read before and after delivering, so its accuracy depends on two readings and on what stays behind on the wall between them. ISO 385 sets the tolerance of the glass, but the film left by a slow drain, a greased stopcock or a hurried reading can exceed that tolerance without any fault in the instrument.
Class A limits are tighter than most people assume
ISO 385 puts Class A at ±0.020 mL for a 10 mL burette, ±0.030 mL at 25 mL, ±0.050 mL at 50 mL and ±0.100 mL at 100 mL. Since a drop from a standard tip is roughly 0.05 mL, the tolerance of a 50 mL Class A burette is about one drop across its whole length. That is the scale on which everything else has to be controlled.
PTFE and ground-glass stopcocks fail differently
A ground-glass key needs grease, and grease creeps into the bore, contaminates the titrant and eventually blocks the tip. A PTFE key runs dry, cannot seize and is unaffected by alkali, but it creeps under load, so the nut needs occasional adjustment. Laboratories running strong bases almost always move to PTFE, since alkali fuses ground glass shut.
Delivery is calibrated with a waiting time
A burette is calibrated to deliver, and the delivered volume assumes the film on the wall has been given time to drain down. Reading immediately after closing the stopcock reports a volume that is still partly on the glass. Standard practice allows a short settling period — commonly around 30 seconds — before taking the final reading, and applies it consistently across every titration.
The tip does as much as the barrel
A jet drawn too fine slows the delivery until an analyst starts rushing; too coarse and half-drops become impossible to split. Tip geometry also decides whether the last drop hangs and has to be rinsed into the flask. When burettes from two suppliers give different endpoints on the same titration, the tip is where to look before questioning the calibration.
The Schellbach stripe is a reading aid, not decoration
A blue stripe fused into the back of the barrel appears through the meniscus as two arrowheads meeting at the liquid surface, which removes much of the judgment from reading a curved surface in poor light. It costs more to make and it is the single most useful option on a burette used by students or by anyone reading at speed.
Alkali will destroy a burette quietly
Sodium hydroxide attacks glass surfaces slowly and etches the graduation lines and the stopcock seat first. A burette used for base titrations and left standing full will develop a stopcock that seizes and a scale that becomes hard to read within months. Draining and rinsing after every session is what keeps a burette in class rather than in a drawer.
