5.1.1: Volume measurements

Lab #1, CHEM 0103: Chemistry of the life sciences

Date:                                                                          Group leader:

Report due:                                                                 Technician:

                                                                                  Record keeper:

Measuring small volumes and making solutions

Many drugs are given in liquid form, either for oral intake, injection or intravenous drip. It is important that these liquids contain the correct concentration, and that the correct volume is given. If you give too little, there might not be any therapeutic effect. If you give too much, it might be toxic.

Today, we will explore how we can measure volumes in the range of 10 µL to 10 mL. Some of the solutions will be colored, making it easy to see if we forgot to add something, or are way off in our measurements. To check volume measurements, we will use an analytical balance (why would a balance be more accurate, and how can we arrive at volumes from measuring mass?).

Preparation

The group leader and the record keeper will read this lab manual, create a data table for task 1 and plan the lab. Meanwhile, technicians will meet with the instructor at the balances to learn how to use the adjustable pipettes (units of measurement, range and choosing which one to use, picking up and ejecting pipette tips, first and second stop of the plunger, setting the volume, decimal point that sometimes is almost invisible) and balances (cleanliness, weighing container, zeroing the balance, closing the sliding doors before reading off result, units of measurement). You will see that mixtures of liquids do not mix readily on their own, and will learn different techniques to speed up mixing (swirling, inverting, vortexing).

Groups Hydrogen, Phosphorous and Sulfur will start with task 1 and then do task 2. Groups Nitrogen, Carbon and Oxygen will start with task 2 and then do task 1. This way, every group will have access to an analytical balance. You will need equipment from your drawers (the technician should is responsible for the equipment, including making sure it gets returned to its place at the end of the lab).

Task 1

A.    Figure out the best way to measure 0.8 ml

You will compare how reproducibly we can measure volumes using a syringe, an adjustable pipette or a transfer pipette. Each measurement will be done at least three times (in triplicate). Place a weighing dish on the balance and zero (“tare”) it. Measure out 0.8 ml of deionized water (it is convenient to fill a beaker with water and take sample from it rather than fetching water for each single measurement) and transfer it into the weighing cup. Read off the mass of the water and record it in a table in a spreadsheet. With three ways of measuring and triplicate measurements, you should have 9 measurements in the end. For each triplicate, calculate the average mass and the standard deviation (Typically, you would do the calculation with a data table in a spreadsheet, but you can also use an online calculator). Discuss the pros and cons of each method, making reference to expected values vs. averages ("accuracy") and standard deviations ("precision").

B.    Figure out the best way to measure 0.05 mL.

For the second part of this task, you will measure 0.05 ml using the syringe or the appropriate size adjustable pipette. How much is 0.05 mL in µL? What is the appropriate adjustable pipettor (orange, blue or pink)? Again, make a data table of your 6 raw measurements, and calculate averages and standard deviations.

Discuss how the 0.8 ml measurements compare to the 0.05 ml measurements. What problems did you encounter? Which sources of error can you avoid by better technique, and which are unavoidable?

Task 2

A. Make a large volume of a mixture

Using the 1 mL adjustable pipette, place the following volumes of solutions into a clean and dry  10 mL graduated cylinder:

1.     2 ml of colored solution (first color, see below)

2.     0.4 ml of colored solution (second color, see below)

Fill to 10 mL with deionized water (using a transfer pipette for the last couple of drops).

Mix until uniform, and transfer 250 µL into an empty well of a microplate.

Group	First Color	Second color
Carbon,  Hydrogen	red	yellow
Nitrogen, Sulfur	yellow	blue
Oxygen, Phosphorous	red	blue

B.  Make mixtures in small volumes

Using the appropriate adjustable pipette, combine the following volumes of solutions directly in a single empty well in the microplate:

1.     50 µL of colored solution (first color, see table)

2.     10 µL of colored solution (second color, see table)

3.     190 µL of deionized water

Mix by pipetting up and down. Make two more of the same mixture (use other empty wells in the microplate). Compare the colors and volumes of the four solutions you just made. They should look identical (why?). If you get strange results, discuss within your group and, if necessary, with another group. Decide which of the tasks you might want to repeat. If you are satisfied with your results and there is time, use other empty wells to make the other groups’ mixtures. When you are done, display your results on the front bench and take a picture of the microplate for the lab report (put a white sheet under the plate, and take the photo from the top so that colors are clearly visible).

Clean up:

Discard solutions in sink, rinse the microplate and shake out the water. Invert to dry. Make sure your lab bench is clean, adjustable pipettors and tips are in the common drawer, and clean glassware is back in your group drawer. Lock your drawer and place the key in the key box near the entrance.