Measuring the Volume of Substances and Objects Using Appropriate Glassware
Objective
Upon completion of this laboratory exercise, students will be expected to:
demonstrate an ability to accurately identify common glassware used for measuring volume.
choose glassware of appropriate size and precision for a variety of uses and measurements in a chemistry laboratory.
accurately record volume measurements of liquids, regular solids (cubes, rectangular prisms, cylinders, etc.), and solid with irregular shapes using appropriate precision for the chosen glassware.
Introduction
The accurate measurement of volumes is a skill that is fundamental in the kitchen as well as a chemistry laboratory. In a kitchen, measuring the correct amount of ingredients is important for ensuring a recipe tastes just right; but in a chemistry laboratory, it is integral to many experiments involving the chemical and physical properties of matter such as density determinations, chemical reactions, and dilution of reagents, among others. In a kitchen, volume is measured using tools specialized for different applications and amounts; for example, large and small measuring cups and measuring spoons are used to measure relatively consistent amounts of ingredients. In much the same way, chemists use a variety of tools to measure volumes of chemicals in the laboratory. While some glassware in a chemistry laboratory such as standard pipet, beakers, and Erlenmeyer flasks (Figure 1) tend to be valued for their versatility, others such as volumetric pipets, volumetric flasks, burets, and graduated cylinders (Figure 2) are valued for more specific measurements.
Here we will describe some standard types of chemistry glassware and how to accurately use them to measure volume; we will learn which types of glassware should be used for certain types of tasks. Once we have learned how to identify and use appropriate glassware, we will measure the volumes of some solids and liquids.
Before we get into specifics, it is important to consider some common features that you will encounter with common laboratory glassware:
Graduation Marks
Graduation marks etched on glassware by the manufacturer enable a user to determine the volume contained by the glassware when filled to a certain level. The precision of the graduation marks on each type of glassware will depend on its size and intended use. For example, Erlenmeyer and beakers tend to have fewer marks, which indicate approximate volume. Other types of glassware such as graduated cylinders, graduated pipets, and burets have more frequent marks, which offer greater precision. Among the useful skills in measuring volumes is the ability to determine when approximate measurements will suffice or when greater precision is necessary for a certain measurement; more precise measurements tend to require more time and a more critical eye.
Cleaning Glassware
Glassware should always be clean before use, and you should never assume that someone else cleaned it before you use it. The proper technique for cleaning glassware is to wash it with soap and water, rinse with tap water, then rinse it with deionized water before letting it air dry. If the glassware is sufficiently clean, you will not see water bead up on the inside surface.
Volumetric Glassware
Some glassware is intended to deliver a specific and precise volume as indicated by markings by the manufacturer directly on the glassware. These types of glassware are called volumetric flasks or volumetric pipets. Alongside labels indicating the volume for which a piece of volumetric glassware is to be used are markings such as “TC 20” (to contain) or “TD 20” (to deliver). These tell the user that the glassware was individually calibrated “to contain” or “to deliver” the labeled volume at the temperature indicated by the number; for example, 20°C. The temperature is specified because the volume of a sample changes with temperature as the density changes. The difference between “TD” and “TC” is small, but important because confusing them can cause measured volumes to differ from desired or expected volumes. Glassware labeled “TC” will contain the desired volume, and can be used by directly pouring the measured liquid. Pipets labeled “TD” will contain a bit more than the indicated volume, and are intended to be used by allowing the liquid to drain out by gravity; once the liquid is dispensed, there may be a small amount remaining in the tip, which should not be added to the sample.
Reading Volumes Using Graduation Marks
When measuring volumes of samples using the graduation marks on the sides of the glassware, it is important to use proper technique to ensure an accurate reading. Whenever a liquid such as water is inside glassware, the surface of the liquid will not be perfectly flat. There will be a curve to the surface due to attractive forces acting within the liquid and between the liquid and the surface of the glassware. This curved surface is called the meniscus (Figure 3A). To read the volume of the liquid, the glassware should be held so that the meniscus is at eye level (Figure 3B); the recorded volume should be the level of the liquid at the bottom of the meniscus. Reading the volume when the eye is above the meniscus will result in a recorded volume that is too low while reading the volume when the eye is positioned below the meniscus will result in a recorded volume that is too high. The measured volume should include all of the certain digits, which are determined using the graduation marks on the container, plus one more uncertain digit, which is determined by estimating where the level of the liquid falls between the graduation marks.
Filling Glassware
Many pieces of glassware have large openings at the top into which liquids can be carefully poured; but some, such as burets, graduated cylinders, and Erlenmeyer flasks, may have smaller openings, which require use of a funnel (Figure 4). For this type of glassware, choose the appropriate size of funnel, and make sure to secure it properly before pouring the liquid as a funnel can make the glassware top-heavy and prone to tipping over.
Pipets are filled from the bottom using suction. Never pipet by mouth! There are a few common types of tools that can be used to provide the suction necessary to pull liquid into a pipet: (A) a simple suction bulb, (B) a valve-type bulb, and (C) pipet pumps. In fancier laboratories, you may also encounter automated devices for providing the suction to pull liquids into pipets.
Small amounts of water or other liquid on the inside of a pipet can contaminate a sample. Therefore, it is important that each time a new liquid delivered from a buret or pipet, the barrel should be rinsed with the solution to be used. Fill the buret or pipet with a small amount of the desired liquid such that the solution covers and coats the entire inside surface, then dispose of the rinse liquid; repeat this process 2-3 times.
Simple Suction Bulb
Do not place the barrel of a pipet fully onto a simple suction bulb (Figure 5A); doing so may cause liquid to be pulled all the way into the bulb, which will contaminate it and require it to be cleaned and dried before further use. Instead, squeeze the bulb and press the opening of the bulb firmly but gently to the opening in the barrel of the pipet to apply the suction force. When you are ready to draw the liquid into the pipet, place the tip below the surface of the liquid, but avoid placing it on the bottom of the container as this can obstruct the flow of the liquid into and out of the pipet. Allow the suction to pull the liquid into the pipet just past the mark for the desired volume, then quickly remove the bulb from the barrel and replace it with the tip of your index finger. By gently releasing pressure on your index finger, the liquid can be allowed to fall to the desired level. Once at the desired level, the tip of the pipet can be inserted into the receiving flask and the finger can be removed to allow the liquid to be delivered by flow of gravity. If a pipet is labeled “TC,” then you may need to use the pipet bulb to expel any remaining liquid; with the tip of the pipet in the receiving flask, place the bulb on the barrel and gently squeeze it to blow out the remaining liquid.
Valve-Type Bulb
Valve-type pipet bulbs (Figure 5B) are a little trickier to use, but once you understand how they work, they can make pipetting much easier. There are three valves, which are to be used one-at-a-time. The first valve is at the top, labeled A for aspirate; while squeezing the A-valve, also squeeze the bulb to evacuate air from it. Attach the bulb to the barrel of the pipet on the opposite end of the A-valve; only insert it far enough so that the bulb and barrel form a snug seal. Insert the pipet tip into the liquid to be drawn into the pipet such that the tip is below the surface, but above the bottom of the glassware. Gently squeeze the S-valve to provide suction of the liquid into the pipet until the liquid is slightly above the desired level, then let go of the S-valve. Raise the tip of the pipet so that it is just above the level of the liquid, then gently squeeze the E-valve to empty out excess liquid until it is at the desired level, then let go of the E-valve. When the liquid is at the desired level, place the tip of the pipet over the receiving flask and squeeze the E-valve to empty the pipet’s contents.
Pipet Pump
Another common tool for filling pipets is a pipet pump (Figure 5C). To use this, gently, but firmly place the barrel of the pipet into the pump opening. Place the tip of the pipet below the level of the liquid and use the gear-like scroll-wheel to raise the plunger and draw the liquid into the pipet. This can be used to raise or lower the level of the liquid in the pipet to the desired height, but it is not as sensitive as the bulbs described above. Once filled with the desired volume, place the tip above the receiving flask, and empty the pipet by squeezing the lever on the side of the pump.
Measuring Volumes of Liquids
There are a variety of types of glassware that are used in chemistry laboratories, each with benefits and drawbacks. The features of some of the most common types are listed below.
Beakers and Erlenmeyer Flasks
Beakers and Erlenmeyer Flasks tend to be the most common types of glassware used in a laboratory. These are generally used when obtaining approximate volumes of liquid. They are easy to use, easy to clean, and they are the least likely to be knocked over or broken. However, beakers and Erlenmeyer flasks tend to have few graduation marks that are well-spaced apart, which limits their use to approximate volumes rather than very precise volume measurements.
Graduated Cylinders
Graduated Cylinders are the workhorse of the laboratory when it comes to volume measurements where a reasonable amount of precision is desired. Although they do not generally offer as high precision as burets and volumetric glassware, they are easy to use, easy to clean, and have considerably better precision than beakers and Erlenmeyer flasks. Smaller volume graduated cylinders tend to have a smaller diameter which allows for better precision than larger graduated cylinders.
Pipets
When an even more precise volume is desired, pipets are often the tool of choice. There are two main types of pipets that are most commonly used in laboratories, Mohr pipets and volumetric transfer pipets. Mohr pipets have graduation marks on the sides, which allow for transfer of any volume of liquid. Each volumetric transfer pipet is only used to deliver one specific volume for which it is individually calibrated. This leads to easy use and better reproducibility.
Burets
A buret is a tall graduated glass tube with a stopcock; it is particularly useful for dispensing different volumes of liquid with good precision and accuracy. Burets should be cleaned before and after each use to ensure precision, which can be as good as ±0.02 mL. Cleaning burets should be done at a bench because they are fragile and generally too long to fit into a sink. After cleaning with soap and water, it should be rinsed with deionized water by filling it and letting the water drain through the tip. Never fill or rinse a buret directly using the faucet. After use, the buret should be cleaned and filled with deionized water, then a stopper should be placed into the top; this helps to keep the buret clean helps to prevent it from breaking during storage.
To prepare to use a buret, a clean buret should be rinsed with a few small aliquots of the desired liquid before being filled to avoid dilution, then the stopcock should be closed. After it is rinsed, it can be filled with the desired liquid to a level near the top (it does not need to be to exactly to the top because of how the volume is dispensed and calculated), then it should be clamped into a buret holder attached to a ring stand. A waste beaker should be placed below the tip, and a small amount of liquid should be dispensed through the tip to ensure that the volume of liquid in the tip is accounted for. When this is complete, the buret will be ready to use.
The level of the liquid in the buret should be recorded. The receiving flask can be placed below the tip of the buret, and the liquid can be dispensed through the stopcock. A fully opened stopcock allows the liquid to flow freely, and the rate of flow can be slowed by partially closing it. Once the desired volume of liquid has been dispensed, the stopcock should be closed. The level of the liquid should be recorded again, and the dispensed volume is calculated by taking the difference between the two recorded volume.
Measuring Volume of Solids
Measuring the volume of solids in a chemistry laboratory generally falls into one of three categories: regular-shape objects, irregular-shape objects, and granular solids.
Regular-Shape Objects
To measure the volume of solid objects with regular shapes such as cubes, rectangular prisms, spheres, cylinders, etc., use a ruler to measure the dimensions. Then calculate the volume using the appropriate geometric formula.
Irregular-Shape Objects
Objects with irregular shape can be harder to measure, but this can be done using glassware with graduation marks and water. Fill an appropriate container with graduation marks using enough water to submerge the object. Record the level of the liquid. Then place the object into the container ensuring that it is fully submerged; if the object is not fully submerged, then the volume measurement will not account for the object’s entire volume. Once submerged, make sure there are no air bubbles stuck to the sided of the object, then record the new level of the liquid. The object will have displaced water in the container and caused water level to rise by an amount equal to the volume of the object. The volume of the object can be calculated by taking the difference between the two recorded volumes. This is called the displacement method for measuring volume.
Granular Solids
Granular solids like sand or powders are measured using scoops, spatulas, beakers, and graduated cylinders, however these measurements are considered to be approximate measurements due to space that can exist between particles depending on how they are packed together.
Required Materials
A set of beakers with the following volumes: ~1000mL, ~500 mL, ~100 mL, ~50 mL
A set of graduated cylinders with the following volumes: ~100 mL, ~50 mL, ~25 mL
Water
An object with a regular shape such as a book, a ball, a box, etc.
A small solid object with irregular shape that is small enough that it can be placed into a graduated cylinder, but large enough that it will displace sufficient volume (such as a rock).
Safety Precautions
If you choose objects that are breakable, be careful not to drop them.
Be careful with the solid object and water; keep them away from your eyes.
Note: all of the materials used should be safe to use in a typical kitchen.
Procedure
A. Measuring the Volume of Liquids Using Beakers
Fill the 50 mL beaker with water as full as it can possibly be without spilling. Transfer the liquid to the 100 mL beaker and record the volume on your report sheet. Be careful to record the volume to the proper number of significant figures; Remember: the measured volume should include all of the certain digits, which are determined using the graduation marks on the container, plus one more uncertain digit, which is determined by estimating where the level of the liquid falls between the graduation marks. Then transfer the water to the 500 mL beaker and record the volume on the report sheet. Transfer the water one last time, this time to the 1000 mL beaker and record the volume on the report sheet. . Take a picture to upload with your report.
B. Measuring the volumes of Liquids using Graduated Cylinders
Fill the 50 mL beaker with water to approximately 20 mL. Transfer the liquid to the 25 mL graduated cylinder and record the volume on your report sheet. Be careful to record the volume to the proper number of significant figures; Remember: the measured volume should include all of the certain digits, which are determined using the graduation marks on the container, plus one more uncertain digit, which is determined by estimating where the level of the liquid falls between the graduation marks. Then transfer the water to the 50 mL graduated cylinder and record the volume on the report sheet. Transfer the water one last time, this time to the 100 mL graduated cylinder and record the volume on the report sheet. Take a picture to upload with your report.
C. Measuring the Volume of an Object with Regular Shape
Choose an object with a regular shape such as a book, a box, a ball, etc., and describe it on your report sheet. Measure the dimensions of the object in centimeters, then use the appropriate formula to calculate the volume of the object in cubic centimeters. Convert this volume into milliliters (1 mL = 1 cm3), liters (1 L = 1 dm3), cubic meters, and cubic inches (1 in = exactly 2.54 cm). Show all of your calculations.
D. Measuring the Volume of an Object with Irregular Shape
Choose an object with irregular shape such as a rock, a small piece of metal, etc. that can fit easily into the 100 mL graduated cylinder, and describe it on your report sheet. Partially fill the 100 mL graduated cylinder with water such that the object will be completely submerged when placed into it, but the level of the water will remain below the maximum 100 mL mark. Record the initial volume. Place the irregular shaped object into the graduated cylinder and record the new volume. Take a picture of the object in the graduated cylinder and upload this with your Report Sheet. Calculate the volume of the object by taking the difference between the two recorded volumes, and record the final volume on your report sheet.
Use the same irregular shaped object to perform the volume measurement using the 100 mL beaker. Partially fill the 100 mL beaker with water such that the object will be completely submerged when placed into it, but the level of the water will remain below the maximum 100 mL mark. Record the volume. Place the irregular shaped object into the graduated cylinder and record the new volume. Calculate the volume of the object by taking the difference between the two recorded volumes, and record the volume on your report sheet.
Pre-Laboratory Questions
What is a meniscus? Sketch a 10 mL graduated cylinder filled to a volume of 5.24 Ml.
How do we typically read the liquid level when dealing with a liquid that forms a meniscus? Describe how to read a volume using a graduated cylinder with graduation marks every 1 mL.
Why is a rubber safety bulb always used when working with a pipet?
Why are the calibration marks on laboratory beakers taken only to be an approximate guide to volume?
Precision volumetric glassware (pipets and burets) may be marked with the legend “TC” or “TD” along with a temperature. To what do each of these notations refer?
Results and Observations
A. Measuring the Volume of Liquids Using Beakers
Volume of water measured in the 100 mL beaker: _______________
Volume of water measured in the 500 mL beaker: _______________
Volume of water measured in the 1000 mL beaker: _______________
B. Measuring the volumes of Liquids using Graduated Cylinders
Volume of water measured in the 25 mL graduated cylinder: _______________
Volume of water measured in the 50 mL graduated cylinder: _______________
Volume of water measured in the 100 mL graduated cylinder: _______________
C. Measuring the Volume of an Object with Regular Shape
Description of the object:
Dimensions of the object in centimeters (cm): Volume in cubic centimeters (cm3):
Volume in milliliters (mL): Volume in liters (L):
Volume in cubic meters (m3): Volume in cubic inches (in3):
D. Measuring the Volume of an Object with Irregular Shape
Description of the object:
Volume Using a 100 mL Graduated Cylinder
Initial volume: _______________
Final volume: _______________
Volume of the object: _______________
Volume Using a 100 mL Beaker
Initial volume: _______________
Final volume: _______________
Volume of the object: _______________
* Upload one picture for each part of the lab (A, B, and C) with your report sheet
Post-Laboratory Questions
How does the ~20 mL volume measured by the graduated cylinders compare to the volumes as determined in using a beaker? Do beakers tend to be more or less precise than graduated cylinders?
Which measuring device (beaker graduations or graduated cylinder) and size gives the greatest precision?
On the basis of your experience in this experiment, briefly discuss the relative precision permitted by a graduated cylinder and a beaker. Give several circumstances under which you would use each instrument, in reference to the other two.
Pipets used for the transfer of samples of aqueous solutions are always rinsed with a small portion of the solution to be used before the sample is taken. Calculate the percentage error arising in an experiment when 1 mL, 5 mL, and 10 mL pipets are used for transfer and each pipet contains 5 drops of water adhering to the inside of the barrel. Note: A single drop of water has an approximate volume of 0.05 mL.
It is important to make certain that there is no air bubble in the tip of the buret below the stopcock before the initial reading of the liquid level in the buret is taken. If a 0.5-mL air bubble is present in the tip of a buret, what percent error in 10 mL, 20 mL, and 40 mL samples will result if the air bubble is dislodged during the dispensing of the samples?
