A graduated cylinder is a common piece of laboratory equipment designed specifically for measuring the volume of a liquid. It is a long, narrow container used in chemistry and biology to accurately measure liquids for experiments. It offers a good balance of speed and precision, intended for moderately accurate measurements, distinguishing it from less precise beakers and more precise volumetric flasks.
Physical Characteristics and Materials
The graduated cylinder is tall and uniformly cylindrical, a shape engineered to improve the accuracy of volume readings. It features a broad, stable base, often hexagonal or round, allowing it to stand upright on a laboratory bench. A small, indented pouring spout is typically present at the top rim to facilitate the controlled transfer of liquid without spillage.
Cylinders are commonly constructed from glass or plastic. Glass versions are often made from borosilicate, which provides excellent chemical resistance and stability against temperature changes. Plastic cylinders, frequently made from polypropylene (PP) or polymethylpentene, are valued for their durability, lower cost, and resistance to breakage. Plastic is a preferable option where fragility is a concern, as polypropylene offers chemical resistance and polymethylpentene is transparent and lighter than glass.
Interpreting the Scale and Precision
A series of horizontal lines, known as graduations, are etched or printed along the side of the cylinder, representing the volume scale. The unit of measurement is universally the milliliter (mL), though larger cylinders may also be marked in liters (L). Heavy markings denote major volume increments, with smaller, finer lines indicating smaller divisions.
The precision of the cylinder is directly tied to its capacity, which can range from 5 mL up to 2,000 mL. Smaller cylinders, such as a 10 mL model, have much finer divisions (e.g., 0.1 mL) for more precise measurement of small volumes. Larger cylinders have wider spacing, indicating a lower degree of precision. Precision is further categorized by Class A and Class B, where Class A cylinders offer a higher tolerance and are considered twice as accurate as Class B.
How to Measure Volume Accurately
To ensure an accurate measurement, the cylinder must be placed on a flat, level surface, and the liquid must be allowed to settle completely. When liquid is poured into the cylinder, surface tension causes it to adhere to the walls, creating a slight curve known as the meniscus. For water and most aqueous solutions, this curve is concave, meaning it dips down in the center.
The volume measurement must always be read at the lowest point of this concave curve. To read the volume correctly, align your eye exactly level with the bottom of the meniscus. Failing to read the measurement at eye level introduces a distortion called parallax error, which can cause the reading to be artificially high or low. Scientists choose the smallest possible cylinder size that can hold the desired volume to ensure the highest possible precision.