The Analytical Column

When people picture a chemistry lab, they often picture a burette: the tall glass tube with a tap at the bottom, dripping liquid into a flask. It looks simple. But once the chemistry is right, much of the error left in a hand titration lives in that tube, in how it is read, in the last drop, and even in how warm the room is.

What the glass promises

A good burette comes with a promise: it will deliver what its marks say, within a small tolerance. For the best grade of 50 mL burette, called Class A, that tolerance is plus or minus five hundredths of a millilitre, about the size of a single drop. That sounds tiny, and it is, but what matters is how big it is compared with what you measure. Use 25 mL of the burette and that drop is two-tenths of a percent of your answer. Use only 10 mL and the same drop becomes half a percent. The same glass gives a better answer the more of it you use.

Reading it the same way every time

Liquid in a narrow tube does not sit flat; it curves, dipping in the middle. That curve is called the meniscus, and the convention is to read its bottom with your eye level with it. Look from above and the reading looks different than from below. That shift is called parallax, and it pushes the reading the same way every time you do it. The fix is simple: eye level, every reading, start and finish. A white card with a dark stripe held behind the tube makes the curve much easier to see.

One drop too many

Near the end of a titration, a single drop from a 50 mL burette is about the whole margin of error. One drop too many and you have overshot. Experienced analysts get around this by adding a fraction of a drop at a time: they let a drop hang from the tip, touch it to the inside of the flask, and rinse it down. And they never leave a drop hanging at the end, because a hanging drop has been counted as delivered without ever reaching the flask.

The room matters too

Liquids expand when they warm up. That means a millilitre of a warm liquid holds slightly less of the active ingredient than a millilitre of the same liquid when it is cool. For water-based liquids, the change is small, under three hundredths of a percent per degree. But a lab that runs ten degrees warmer in the afternoon than in the morning can push results a quarter of a percent high. For some solvent-based liquids, the effect is about three times bigger.

What machines change

Automatic titrators replace the glass tube with a motor-driven piston. There is no curve to read and no hand-delivered drop, which removes much of the human error. But they do not remove everything. A bubble in the tubing is counted as liquid that never arrived, and the temperature still matters just as much.

The lesson

The chemistry gets most of the attention, but the volume is where a careful analyst earns accuracy. Use enough of the burette, read it the same way every time, sneak up on the end with partial drops, and use the liquid near the temperature its strength was checked at. Small habits, measurable difference.

References

1. ASTM E287, Standard Specification for Laboratory Glass Graduated Burets, Table 1.
2. D. C. Harris, Quantitative Chemical Analysis (W. H. Freeman), burets.
3. F. Saes, "Potential sources of error in titration", Mettler Toledo UserCom Analytical Chemistry 9.