Welcome back to Chemistry 101: hBARSCI’s newest blog series designed to explain fundamental science in simple, yet thorough, ways, making use of basic how-tos and activities to get you on the path to exploring science wherever you are, whether that’s the classroom, your kitchen, or the great outdoors. This week, we’re taking a closer look at the multiple ways you can effectively measure liquids as you engage in scientific experimentation.
(If you missed Chemistry 101: Acids and Bases, check it out here!)
A successful chemistry experiment relies on reactants and reagents coming together precisely to produce an expected outcome. In order to safely control this outcome, using exact amounts of these materials is key. When it comes to measuring liquids, many types of devices are used in a laboratory; here, we’re going to discuss three: graduated cylinders, burettes, and pipettes with a bulb. Read on to learn the how, when, and why of using each.
When students think of measuring solutions, the graduated cylinder is usually what they imagine first. Graduated cylinders are generally more accurate and precise than laboratory flasks and beakers, but they are not as precise as volumetric glassware, such as a volumetric flask or volumetric pipette.
Graduated cylinders commonly range in size from 5 mL to 1000 mL, marked in white or blue along the side of the cylinder . They can be made of glass or plastic. Because the cylinder is long and narrow, it can be at risk for tipping over (and breaking, if made of glass!), so care must be taken when working around a filled graduated cylinder.
Hold the cylinder at eye level to take a reading. If resting the cylinder on a table, be careful not to jostle the table as you read. Look at the horizontal lines on the side of the cylinder and determine the increments of measurement on the tube. For example, if the area between the 40ml mark and the 50ml mark is divided into ten segments, each segment represents 1ml. Locate the closest whole measurement below the surface of the liquid.
Count the number of segments up to the line nearest the meniscus, or the curve in the upper surface of a liquid (for most liquids, this will be the bottom-most point in the curve). Calculate the volume of the liquid by adding the whole measurement to the sum of the segments.
Graduated cylinders are also used to determine displacement, or the measurement of the change in a volume of water when additional materials are added to it. This means that the volume of solid objects and non-aqueous solutions (those lacking water) can be determined using a graduated cylinder by recording the difference between the original (or V1) and final (or V2) volume measurements: V2 - V1.
Burettes come in a variety of sizes that usually range from 5 ml up to 100 ml. Burettes are used primarily for titration—to deliver one reactant until the precise end point of the reaction is reached. Burettes can be made of glass or plastic.
A burette is similar to a graduated cylinder in that it is a tube with an opening on the top and graduated measurements on the side; however, a burette stays in place, usually by fixing it to a retort stand and rod with a burette clamp, and has a valve near the bottom (the stopcock) that controls the liquid that drips out of an opening at the bottom of the tube, which goes over a liquid-filled flask, allowing its liquid to mix slowly and exactly with the liquid in the flask.
The purpose of burette reading is to tell you how much solution has been dispensed rather than how much the burette contains. Unlike other volumetric glassware, the zero scale on a burette is written on top.
As when reading graduated cylinders, when reading the amount of liquid in a burette, it is important that you make sure your eye is at the same level as the meniscus to avoid an error. The difference in volume is calculated by taking the difference of the initial recorded volume (before you release liquid using the stopcock) and final recorded volume (once you have finished releasing liquid).
Because using the burette with a light or colorless solution may make it difficult to observe the bottom of the meniscus, the black strip technique can make it easier to accurately observe and record measurements. Use a black marker and color a black strip on a white piece of paper, such as an index card. Place the black strip behind the burette to make the meniscus easier to see and the volume easier to read. Schellbach burettes, a specific type of burette, make observing light and colorless solutions much easier.
A pipette, or pipet, is another common piece of laboratory equipment. It is a small tube that can transfer liquids from one container to another. There are three kinds of pipettes typically used in labs: Pasteur pipettes, volumetric pipettes, and micropipettes. Below, we’ll be looking at how to measure using volumetric pipettes, which are more commonly found in chemistry laboratories.
Volumetric pipettes transfer a single, predetermined volume of liquid. They are often called a bulb pipette for their shape: long and tube-like and interrupted in the middle by a bulb in which the bulk of the solution is held. The name is also indicative of the bulb that attaches to the top of the pipette, often made of rubber, which is squeezed to create a vacuum.
To use a volumetric pipette, first look at the side and notice a number and a line or mark on the side of each. This number indicates the number of milliliters the pipette holds and dispenses when the pipette is filled all the way to that line or the mark. Volumetric pipettes are calibrated to have a very high level of accuracy, so when you dispense a certain volume with a volumetric pipette, you can report that volume in your notes with up to two figures after the decimal point (e.g. 5.00 mL).
Place your rubber bulb (which looks a little like a turkey baster) at the top of the pipette and squeeze it to empty it of air. Press the rubber bulb against the pipette to create a seal. The pipette should not be forced into the hole in the pipette bulb; the rubber bulb and the pipette only need to come into contact with one another.
With the tip of the pipette submerged in your solution, gently relax the bulb to allow it to be drawn up into the pipette. Allow the level of the fluid in the pipette to rise a couple centimeters above the line or mark on the side of the pipette. Do not allow the fluid to rise up into the rubber bulb.
Remove the rubber bulb and quickly cap the open top of the pipette with your finger. By tilting your finger to one side, allow a little air into the pipette so the fluid drains out until the bottom of the meniscus reaches the fill mark or line. With practice, you’ll be able to do this slowly and precisely! Remove the pipette from the reagent solution and transfer it to the receiving beaker or flask. Allow the pipette to drain into the receiving beaker or flask.
For larger volumes, manual or automated pipette pumps and fillers are often used.
Using labware safely often requires multiple pieces of equipment for proper use or storage. For instance, all labware, once cleaned, must dry properly to make sure no liquid remains that will skew future measurements. Check out this draining rack, for instance. Also, burettes require a stand during use, and pipettes need to be stored properly so they don’t break while not in use. Keeping your labware clean, dry, and properly stored ensures a long life.
