A buret is a long, graduated glass tube equipped with a stopcock valve at the bottom, used to dispense precise, measured volumes of liquid. It is primarily used in titration, where a solution of known concentration is slowly added to another solution to determine its unknown concentration. The scale is numbered in a way that feels backward compared to common measuring devices. Reading a buret requires a precise understanding of its unique scale orientation and the physical properties of the liquid being measured.
Understanding the Buret’s Graduated Scale
The answer to whether you read a buret from top to bottom is yes, which is contrary to how most people are taught to read volumetric glassware. Standard measuring cylinders have the zero mark at the bottom, with numbers increasing as you move up the cylinder to indicate the total volume contained. A buret, however, is designed to measure the volume delivered or dispensed, not the volume remaining inside the tube.
The zero mark (\(0.00 \text{ mL}\)) is located at the very top of the buret, and the numbers increase incrementally down the tube toward the stopcock. For example, a typical \(50 \text{ mL}\) buret runs from \(0.00 \text{ mL}\) at the top down to \(50.00 \text{ mL}\) near the bottom. This orientation means that as liquid is released, the scale reading increases. Most standard burets have major markings for every milliliter and smaller tick marks for every tenth of a milliliter (\(0.1 \text{ mL}\)).
The Mechanics of Taking an Accurate Reading
Achieving the accuracy required for a titration depends heavily on correctly reading the liquid level on the graduated scale. The liquid’s surface inside the narrow glass tube forms a curve called the meniscus, which is concave for most aqueous solutions. The standard procedure requires the measurement to be taken at the lowest point of this curved surface, which is the consistent reference for measuring the liquid’s volume.
Accurate measurement requires eliminating an optical distortion known as parallax error. This error occurs if the eye is not perfectly level with the meniscus, causing the liquid level to appear either higher or lower than its true position. To avoid this, the observer must align their eye horizontally with the bottom of the meniscus. Many technicians use a buret reading card, a white card with a black strip, placed behind the buret just below the meniscus to sharpen the contrast and make the bottom curve easier to see.
Since the smallest marked lines are usually \(0.1 \text{ mL}\), the measurement must be estimated one decimal place further to achieve the buret’s full precision. This estimation is to the nearest one-hundredth of a milliliter (\(0.01 \text{ mL}\)), meaning the final reading will always be recorded to two decimal places. For instance, if the meniscus falls between the \(24.2 \text{ mL}\) and \(24.3 \text{ mL}\) marks, the reading might be estimated as \(24.25 \text{ mL}\). This final estimated digit represents the uncertainty inherent in reading any graduated scale.
Calculating the Delivered Volume
The final step is to calculate the precise volume of liquid dispensed during the experiment. This calculation is necessary because the buret is rarely filled exactly to the \(0.00 \text{ mL}\) mark before starting. To determine the volume delivered, two readings are taken: an Initial Reading (\(V_i\)) before the experiment begins and a Final Reading (\(V_f\)) after the liquid has been dispensed.
The actual volume delivered is found by subtracting the initial reading from the final reading. This relationship is expressed by the formula: Volume Delivered = Final Reading – Initial Reading. For example, if the starting liquid level was \(1.50 \text{ mL}\) and the final level was \(25.65 \text{ mL}\), the delivered volume is calculated as \(25.65 \text{ mL} – 1.50 \text{ mL}\), which equals \(24.15 \text{ mL}\).