Glossary of Definitions and Instructions for Common Laboratory Techniques

Accurate Weighing

In this manual, the phrase “accurately weigh x grams of chemical y” will be used quite frequently. Although this is a clumsy construction, it does convey the following especially important concept: It is often either not necessary or not possible to weigh exactly the desired amount of chemical. When using the balance, which reads to 1 milligram (0.001 gram), measuring exactly 2.000 grams (for example) is nearly impossible. What the above phrase means is that you should weigh out approximately x grams of material, and you should record exactly how much you weighed. For example, if instructed to “accurately weigh 3 grams of calcium carbonate on the analytical balance” you might measure out 2.967 g and record this mass in your notebook. Note that whether you are just under or just over the required amount is not important, as long as you are close and record the exact amount accurately.

Beer’s Law Plot

Beer’s Law expresses the relationship between concentration of a colored species in solution and the measured absorbance of the solution at the wavelength of maximum absorption (λ max). The equation A = εℓc (where A is the absorbance, ε is the molar extinction coefficient, ℓ is the path length, and c is the concentration) is a mathematical statement of the law. ε is a property of the chemical at the wavelength and behaves as a constant in the equation. ℓ is held constant during an experiment by using identical cuvettes. With ε and ℓ reduced to constants, Beer’s Law becomes A= kc. A plot of absorbance on the y-axis versus concentration on the x-axis yields a straight line.

A typical experiment involves making a series of standard solutions of known concentration and measuring their absorbance at λ max. This information is plotted on a graph and a line of best fit is drawn. The concentration of an unknown solution of the compound may then be determined by recording its absorbance at λ max and reading the concentration off the graph.

Calibration Curve

Calibration curves are graphs of known data. A calibration curve is generated by measuring a property (such as density) of a series of standards (such as solutions of known concentration) and plotting the data on a graph. The same property of an unknown substance or solution then may be measured and compared to the graph to find the property plotted on the x-axis. Note that a “calibration curve” may be a straight or curved line, depending on the relationship between the two properties plotted on the graph.

Decanting a Liquid

To separate a liquid from a solid or a solution from a precipitate, you can pour the liquid from one vessel into another, being careful not to disturb or transfer any of the solid in the bottom of the first vessel. With good technique (gained through practice) it is possible to decant over 90% of the “supernatant” liquid above a solid without transferring any solid.

Successive Dilutions

Dilute solutions do not necessarily need to be made directly from the stock solution provided. If you are trying to make 50 mL of a 0.010M solution from a 2.0M stock solution, you would need to pipet 0.25 mL of the stock into a 50.00 mL volumetric flask to prepare it directly. This amount is so small as to be impossible to deliver accurately with a 10 mL Mohr pipet. Instead, you can utilize successive dilutions to make 50 mL of 0.20M solution first (which requires you to pipet 5.00 mL of the 2.0 M stock). Pipetting 2.50 mL of this 0.20M solution into a second 50.00 mL flask and making up the volume to the mark yields the desired 0.010M solution. Both volumes you were required to pipet were large enough to measure and deliver accurately with your 10 mL Mohr pipet. To prepare very dilute solutions, multiple successive dilutions may be necessary.

Types of Observations to Record

A general list of observations that you will need to make is described below, but you should realize that there might be other important observations that you should make.

• Observation of the Appearance: You should record the appearance of each sample before it is mixed with any other reagent.
• Observation of the pH: Approximate pH values can be established using pH indicator paper. Place a glass stirring rod in a solution of interest and touch it to a piece of pH paper. Compare the wet paper to the pH-color chart from the pH paper container. Acidic solutions have a pH less than seven (pH<7) and basic solutions have a pH greater than seven (pH>7). It is seldom useful to measure the pH after two solutions have been mixed. The pH values can vary several pH units upon the addition of even one extra drop of a reagent. (You will learn all about this in CHEM 104.) Since the addition of reagents is often approximate due to limiting reagents, the pH value for a reaction mixture may not reflect a change due to the reaction accurately.
• Observation of Color Change: Describe any color change you observe. If a solution turns brilliant orange, that change is important. Color can be subjective; therefore, the better the description, the easier the analysis will be. If a solution becomes cloudy or opaque, then a precipitate is likely to have formed. The formation of a precipitate is not classified as a color change.
• Observation of a Precipitate: The formation of a precipitate should be obvious. The solution should appear cloudy or opaque. In the lab, if the solution is cloudy and there is uncertainty about the formation of a precipitate, it may be helpful to change the color of the background. You can change the background by viewing the test tube in front of a sheet of white paper.
  • Sometimes incomplete mixing of reagents makes a liquid appear to have striations called schlieren. They can be removed by stirring the mixture with a glass rod.
• Observation of a Liberated Gas Liberated: Gases can appear as small bubbles that form after the reagents have been mixed. Be careful to describe the gas evolution as fully as possible. Sometimes bubbles are caused by the entrapment of air during mixing of the two solutions.
• Observation of a Temperature: Change Some reactions may produce or absorb heat. An observation of this effect may be made by touching the reaction flask. The change in thermal energy can also be quantified using a thermometer.
• Observation of “No Visible Change”: “No Visible Change” is an important observation. Many combinations of reagents will produce “No Visible Change”