Lab Techniques

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.