A burette is a precise laboratory instrument for dispensing and measuring variable liquid volumes. It is commonly used in quantitative chemical analysis, particularly during titrations, to accurately measure one solution’s volume to determine another’s concentration. Mastering the correct reading technique is important for precise laboratory work.
Understanding Burette Markings
Burettes feature a calibrated scale along their length that indicates dispensed liquid volumes. Unlike measuring cylinders, the zero mark is at the top, and volume readings increase incrementally down the tube. This design shows the markings denote the volume of liquid delivered from the burette.
Primary graduations represent whole milliliters, with smaller lines subdividing these into 0.1 mL increments. For example, ten divisions exist between the 24 mL and 25 mL marks. Understanding these markings allows for precise readings, as the final volume is estimated to a further decimal place.
Identifying the Meniscus
When liquid is in a narrow tube like a burette, its surface forms a curve rather than remaining perfectly flat. This curved surface is known as the meniscus. This occurs due to the interplay of forces between the liquid and the glass.
For most aqueous solutions, the liquid “wets” the glass, causing a concave meniscus. To ensure consistency and accuracy, always measure from the very bottom of this curve. This provides a uniform reference point, preventing variations in readings.
The Reading Technique
Accurately reading a burette requires careful attention to prevent errors. Position your eye exactly level with the meniscus. Viewing it from above or below causes parallax error, making the reading appear higher or lower than its actual value.
Before taking a reading, ensure the burette is securely clamped upright on a stable surface. This prevents accidental movement that could shift the liquid level or cause tipping. A steady setup ensures consistent eye alignment with the meniscus.
To enhance meniscus visibility, place a white card or dark paper behind the burette, just below the meniscus. The contrast makes the curve clearer and easier to discern. This simple technique improves observation precision.
Once the meniscus is clearly visible and your eye is level, interpret the scale. Read the major milliliter mark above the meniscus, then count the 0.1 mL subdivisions down to its lowest point. The final digit, the hundredths place (e.g., 0.01 mL), must be estimated by visually interpolating between the smallest 0.1 mL markings. For example, if the meniscus falls exactly halfway between 24.3 mL and 24.4 mL, the reading would be 24.35 mL.
Calculating Volume Delivered
After liquid is dispensed from a burette, determining the precise volume delivered involves a simple calculation. This relies on two accurate readings: the initial volume at the start and the final volume at its conclusion. Both readings are taken using the careful technique described previously.
The volume of liquid dispensed from the burette is found by subtracting the initial reading from the final reading. Volume Delivered = Final Reading – Initial Reading. For instance, if the initial burette reading was 2.55 mL and the final reading was 28.70 mL, the volume delivered would be 28.70 mL – 2.55 mL, which equals 26.15 mL. Recording both the initial and final readings with precision is important for accurate volume determination.