Burettes are used to perform titrations in which they accurately deliver a measurable volume of a liquid. The photo to the right shows a burette clamped to a retort stand. Always ensure that the burette is resting in the indentations in the clamp so that it will secure and vertically aligned. (If the burette is tilted, it will yield erroneous readings for the volumes.)
Cleaning, Filling and Dispensing
Before using a burette, rinse it twice with small portions of the titrant solution. (This cleaning procedure is similar to the one used for pipettes.) After adding titrant to the burette, check for air bubbles in the tip - see photographs to the left. If a bubble is present, as seen in Figure 1, it must be flushed out before starting the titration. Once the tip is full of solution, as in Figure 2, fill the burette to slightly below the zero mark. Do not attempt to set the titrant level at exactly 0.00 mL. It is unnecessary, and practically impossible. The initial titrant level should be recorded and the difference between this and the final titrant level will yield the volume dispensed.
To dispense the titrant, cup your left hand (if right-handed) around the tip so that you can control the valve. As the titrant is released, use your right hand to swirl the titration flask.
The 50 mL burettes used in the CHEM 1000 labs have graduations every 0.1 mL, but volumes can be estimated to ± 0.02 mL. The thickness of a line on the burette is approximately equivalent to 0.02 mL as it is about one fifth of the distance between one line and the next. Use this to estimate the level of the titrant when it lies between two lines. When an aqueous solution is placed in a narrow glass container, the liquid surface curves at the glass walls because of interactions between the water and glass. This curved surface is known as a meniscus. Generally*, using the bottom of the meniscus as the reference point for volume measurements is best. Figures A, B and C below illustrate a common error that arises when reading a burette. This error, known as parallax, occurs if the eye is either above or below the level of the meniscus. In Figure A, when the meniscus is viewed from below, the volume appears to be 25.74 mL. In Figure B, the same meniscus is viewed at the proper eye level and the volume appears to be 25.68 mL. If viewed from above, as in Figure C, this meniscus appears to be at 25.64 mL. In this example, the difference between viewing the meniscus from above and below is 0.10 mL that may seem like a small difference but it has increased the uncertainty in the measurement five fold from the ± 0.02 mL that can be achieved. The parallax error can be minimized by aligning the encircling marks on the burette. This will take some practice, but will ensure that the eye level and meniscus are the same.
* For some solutions, such as those containing permanganate, the bottom of the meniscus may be difficult to see. For these solutions, use the top of the meniscus.