A burette is a specialized piece of volumetric glassware designed for dispensing highly accurate, variable amounts of liquid, often used in chemical analysis like titration. Unlike other measuring vessels, it allows chemists to deliver a precisely known volume of a reagent into another solution. Achieving maximum accuracy requires careful attention to the instrument’s unique scale markings and the procedural steps for taking a reading.
Understanding Burette Scale and Calibration
The design of the burette scale is unique because it is inverted, with the zero mark, or 0.00 mL, located at the very top. This arrangement reflects the instrument’s function, as the volume reading increases as the liquid is dispensed downward toward the stopcock. A typical 50 mL burette features major division lines usually marked every 1 mL and minor, unlabeled graduation marks indicating 0.1 mL increments.
To obtain the necessary precision, the reading must be recorded to two decimal places. The first decimal place is read directly from the closest 0.1 mL mark below the liquid level. The second decimal place must be estimated visually by judging the position of the liquid level between two adjacent 0.1 mL marks. For example, if the liquid falls between the 25.4 mL and 25.5 mL lines, a reading of 25.43 mL would be an appropriate estimation.
Mastering the Meniscus and Parallax Error
Correctly identifying and reading the meniscus is essential for accurate measurement. The meniscus is the concave, U-shaped curve formed by the liquid surface inside the narrow glass tube due to surface tension. For clear liquids, the volume reading must always be taken from the bottom of this concave curve.
The correct alignment of the eye is equally important to prevent an error known as parallax. Parallax error occurs when the eye is positioned either above or below the level of the meniscus, causing the liquid level to appear higher or lower than its actual position on the scale. To eliminate this distortion, the reader’s eye must be exactly perpendicular and level with the bottom of the meniscus when taking the measurement.
Many burettes feature a white or light-colored band near the graduations to assist with accurate reading. A common technique involves holding a dark object, such as a burette reading card, directly behind and just below the meniscus. The dark background sharpens the view of the curve against the bright lines of the scale, making the bottom of the meniscus easier to pinpoint.
Calculating Dispensed Volume
The purpose of reading a burette is to determine the exact volume of liquid delivered, which requires two separate measurements. The process begins with the Initial Volume Reading (V_initial), which is the volume noted before the liquid is dispensed. Once the desired amount of liquid has been released, the final liquid level is recorded as the Final Volume Reading (V_final).
The actual volume dispensed is determined by a simple subtraction: V_dispensed = V_final – V_initial. For instance, if the initial reading was 1.25 mL and the final reading was 21.68 mL, the volume dispensed would be 20.43 mL.
Before taking the final reading, it is important to ensure that no partial drop of liquid is clinging to the burette’s tip. Any hanging drop, which is liquid that has been dispensed but has not yet fully detached, will cause the final volume reading to be lower than the true volume delivered. Tapping the tip lightly against the inside wall of the receiving vessel ensures that the full volume is transferred, leading to an accurate final calculation.