A <burette> delivers an adjustable measured <volume (thermodynamics)> of a <chemical solution>; the change in its reading gives the delivered <volume (thermodynamics)> in an <acid-base titration>.

Its downward-increasing scale measures a change in reading, not the initial liquid inventory. For initial reading $R_i$ and final reading $R_f$ without refilling, the nominal delivered <volume (thermodynamics)> is $v=R_f-R_i$. An illustrative $2.35\,\mathrm{mL}$ to $12.35\,\mathrm{mL}$ reading therefore represents $10.00\,\mathrm{mL}$. There is no requirement to start at zero. An air bubble disappearing from the tip uses part of the apparent delivery to fill the tip, and a hanging drop has not yet entered the sample; both break the intended volume account.

\Image[/titration-apparatus-and-burette-readings.png]
{title=A <pipette> selects the sample and a <burette> measures added <titrant> by two <liquid meniscus> readings}
{id=titration-apparatus-and-burette-readings}
{description=Original schematic, not a photograph or experimental readout. The apparatus shows a clamped <burette>, mixed sample, and optional <glass electrode> beside the stirring bar. Enlarged illustrative scale readings increase downward; the bottom of the curved <liquid meniscus> gives $R_i=2.35\,\mathrm{mL}$ and $R_f=12.35\,\mathrm{mL}$. Their difference is the added <volume (thermodynamics)>.}

For transparent aqueous samples, read the bottom of the <liquid meniscus> at eye level; viewing from above or below shifts its apparent alignment. The mark is a comparison between a curved liquid surface and a scale, not a direct measure of <amount of substance>. Eye-level reading and the distinction between contained and delivered <volumes> are part of <solution preparation and volumetric calibration>.<Calibration of Volumetric Glassware (Buffalo State laboratory handout)>[Laboratory Experiment 1. Calibration of Volumetric Glassware, p. 1, eye-level bottom-of-meniscus reading]{ref}

Both readings contribute <measurement uncertainty>. If each has independent <standard deviation> $\sigma_R$, their difference has <standard deviation> $\sqrt{2}\sigma_R$; a common additive reading offset cancels, while a scale-factor error remains. For the inventory $n=c_Bv$, uncertainty in <titrant> <concentration> remains even with perfect readings. This is why repeat delivery, independent volume calibration, and <titrant standardization> answer different questions.


 Burette (source code)