Lab Techniques
Measurement Accuracy
The ability to repeat an experiment and get the same results, or reproducibility, is essential in scientific research. However, i…
In scientific research, the reproducibility of an experiment is extremely important. Thus, keeping a lab notebook with detailed procedural recordings along with proper lab techniques helps reproduce experimental findings.
For example, simple measurements, like liquid volume, must be performed using the proper glassware to ensure accuracy. Always measure volume using volumetric glassware, like a volumetric flask, graduated cylinder, or volumetric pipette. The volume is measured at the bottom of the meniscus.
Volume markings on beakers and Erlenmeyer flasks are not accurate and serve as guidelines only. When selecting the volumetric glassware, select the smallest container possible for the volume needed. Volumetric glassware is calibrated either to deliver or to contain.
Containers calibrated to deliver are designed to provide the volume stated with the understanding that a small amount of liquid will remain in the glassware after it is emptied. In this case, there is no need to remove the remaining liquid or more than the desired volume will be emptied. Glassware calibrated to contain will hold and deliver the volume stated but require that the remaining liquid will be poured out so that the full volume is received.
We measure to obtain a true value. However, there will always be some level of uncertainty and error. The measured value is our best estimate of the actual value, which is often unknown to us. Error is the difference between the measured and actual values. Measurement uncertainty describes the range in which we think it is likely that the actual value lies. When recording measurements, it is important to maintain the appropriate number of significant figures.
Significant figures are the digits in a measurement that carry meaning. The last digit recorded defines the level of uncertainty. All numbers other than leading and trailing zeros are significant. And trailing zeros are significant when there is a decimal point preceding them.
For example, in a measurement of length using a ruler, we see that the length is at least one inch, but certainly not 2 inches. So, the first significant digit is one. The next tick mark represents 0.1 inches and is also significant.
A recording of 1.1 inches has two significant digits and implies that the uncertainty lies in the tenths place. However, the true width lies between two tick marks. So, the uncertainty lies here in the hundredths place as the length is reported as 1.15 inches.
When conducting calculations using measured values, remember not to carry out calculations to a higher resolution than the original measurement. Those additional digits are not significant and should not be included. For example, when calculating the area of the square with a side length of 1.15, we see that the length has three significant figures. So, the answer should also have three significant figures.
The calculated area of 1.3225 inches squared has five figures and introduces certainty into the calculation that was not there in the original measurement. Thus, the correct area is 1.32 inches squared.
In this lab, you'll practice proper lab skills by measuring the density of an egg and utilizing significant figures in your calculations and recordings. In addition, you will practice record keeping in your lab notebook and examine the accuracy of measurements with volumetric glassware.
In scientific research, the reproducibility of an experiment is extremely important. Thus, keeping a lab notebook with detailed procedural recordings along with proper lab techniques helps reproduce experimental findings.
For example, simple measurements, like liquid volume, must be performed using the proper glassware to ensure accuracy. Always measure volume using volumetric glassware, like a volumetric flask, graduated cylinder, or volumetric pipette. The volume is measured at the bottom of the meniscus.
Volume markings on beakers and Erlenmeyer flasks are not accurate and serve as guidelines only. When selecting the volumetric glassware, select the smallest container possible for the volume needed. Volumetric glassware is calibrated either to deliver or to contain.
Containers calibrated to deliver are designed to provide the volume stated with the understanding that a small amount of liquid will remain in the glassware after it is emptied. In this case, there is no need to remove the remaining liquid or more than the desired volume will be emptied. Glassware calibrated to contain will hold and deliver the volume stated but require that the remaining liquid will be poured out so that the full volume is received.
We measure to obtain a true value. However, there will always be some level of uncertainty and error. The measured value is our best estimate of the actual value, which is often unknown to us. Error is the difference between the measured and actual values. Measurement uncertainty describes the range in which we think it is likely that the actual value lies. When recording measurements, it is important to maintain the appropriate number of significant figures.
Significant figures are the digits in a measurement that carry meaning. The last digit recorded defines the level of uncertainty. All numbers other than leading and trailing zeros are significant. And trailing zeros are significant when there is a decimal point preceding them.
For example, in a measurement of length using a ruler, we see that the length is at least one inch, but certainly not 2 inches. So, the first significant digit is one. The next tick mark represents 0.1 inches and is also significant.
A recording of 1.1 inches has two significant digits and implies that the uncertainty lies in the tenths place. However, the true width lies between two tick marks. So, the uncertainty lies here in the hundredths place as the length is reported as 1.15 inches.
When conducting calculations using measured values, remember not to carry out calculations to a higher resolution than the original measurement. Those additional digits are not significant and should not be included. For example, when calculating the area of the square with a side length of 1.15, we see that the length has three significant figures. So, the answer should also have three significant figures.
The calculated area of 1.3225 inches squared has five figures and introduces certainty into the calculation that was not there in the original measurement. Thus, the correct area is 1.32 inches squared.
In this lab, you'll practice proper lab skills by measuring the density of an egg and utilizing significant figures in your calculations and recordings. In addition, you will practice record keeping in your lab notebook and examine the accuracy of measurements with volumetric glassware.
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Q1: Why is it important to use the correct glassware when measuring liquid volume?
Using proper volumetric glassware like graduated cylinders and volumetric pipettes ensures accurate measurements essential for reproducibility. Beakers and Erlenmeyer flasks have volume markings that serve only as guidelines and are not calibrated for precision. Accurate measurements are critical because they directly affect experimental results and the ability to repeat findings.
Q2: How do you correctly read the volume of liquid in graduated glassware?
Volume is measured at the bottom of the meniscus, the curved liquid surface in glass containers. Read the measurement by viewing the liquid surface from the side at eye-level to avoid parallax error. Looking from above or below will cause the liquid level to appear higher or lower than it actually is, leading to inaccurate readings.
Q3: What is the difference between glassware calibrated to contain versus deliver?
Glassware calibrated to contain (TC) holds the specified volume when filled to the mark but leaves liquid behind when poured out; you must pour out all liquid to receive the full volume. Glassware calibrated to deliver (TD) dispenses only the specified volume and should not be emptied completely. Volumetric flasks are typically TC, while pipettes are typically TD.
Q4: What are significant figures and why do they matter in lab measurements?
Significant figures are the digits in a measurement that carry meaning and reflect the precision of your equipment. The last digit recorded defines the level of uncertainty. All numbers except leading and trailing zeros are significant; trailing zeros are significant when preceded by a decimal point. Proper significant figures prevent false certainty in calculations.
Q5: How many significant figures should a calculated result have?
A calculated value must have the same number of significant figures as the least precise measurement used in the calculation. For example, multiplying 1.15 inches by 1.15 inches yields 1.3225 square inches, but should be reported as 1.32 square inches to match the three significant figures in the original measurement. Reporting extra digits introduces false precision.
Q6: Why should you select the smallest volumetric container possible for your needed volume?
Selecting the smallest appropriate container maximizes measurement precision and accuracy. Larger containers have larger increments between markings, reducing the precision of your measurement. Using the smallest suitable volumetric glassware ensures you can measure your desired volume with the greatest possible accuracy for your experiment.
Q7: What role does a lab notebook play in ensuring experimental reproducibility?
A detailed lab notebook records procedural steps, measurements, and equipment used, making it possible to reproduce experiments and verify results. Without comprehensive records, other scientists cannot replicate your work or verify your findings. Proper record-keeping combined with correct lab techniques ensures that experimental results can be consistently reproduced.