Serial Dilutions Teresita Alicea, Sarah Anderson, Barb Hammond, Beverley Headley-Ayotunde, Sharonoa Moss, Kira Taylor

Objective: The objective of this lab is to demonstrate how to dilute solutions to a specific concentration.

Background: Frequently in lab you may need several concentrations of the same solution.  The most accurate and user-friendly way of making many concentrations of a single solution is to perform SERIAL DILUTIONS, or making many sequential dilutions from a single stock solution.  This is faster and more accurate than making every solution from scratch.

 Materials Distilled water Five 250 mL Erlenmeyer flasks 0.5 M solution NaCl (our stock solution) One small vial of blue dye 25 mL graduated cylinder 5 or 10 mL pipettes Labeling tape Goggles & gloves

PROCEDURE:

In your lab, you will have a stock (concentrated) solution that you wish to dilute.  For example, you may have a 0.5 M solution of NaCl that you need to dilute to make a 0.1 M, 0.01 M, and 0.001 M solutions.

1.  Decide how much of the dilute solution you want to make.  For example, let's say you want to make 20 mL of a    0.1 M solution from your 0.5 M stock solution.

2.  Use the following equation to calculate the volume of your concentrated stock solution that you will need:

Dilution Equation:  M1V1 = M2V2

where M1 = Molarity of initial solution, V1 = volume of initial solution to be used, M2 = Molarity of final (diluted) solution, and V2 = desired volume of final dilute solution.

 Thinking Question:  Why does this equation work when performing dilutions? Click here for an explanation

In our example:

M1 = 0.5 M

M2 = 0.1 M

V = 20 mL

solve for V1:   M2V2/M1 = V1 = 4 mL of stock solution needed!

3.  Obtain the precise amount of stock solution. Add two drops of blue dye to the 0.5 M solution so that you can track the dilution of the color during your experiment. Then, measure 4 mL of the 0.5 M solution with a 10 mL pipette  and dispense it into a small Erlenmeyer flask (see Figure 1 below).

Figure 1:  Bev is pipetting 4 mL of our 0.5 M stock solution. She is a very experienced medical technologist.

4.  Dilute with the appropriate amount of water.  Next you will need to add 16 mL of water to attain 20 mL.  Measure 16 mL of distilled water with an appropriately sized graduated cylinder. Make sure that you view the volume at eye level, as seen in the photograph of Barb below in Figure 2.  Transfer the water to the solution in the Erlenmeyer flask and swirl to mix thoroughly.  Voila, you have a 0.1 M solution!

Figure 2:  Barb is measuring water and viewing the meniscus at eye level to measure the volume accurately. Notice the safety goggles and gloves.

5.  Repeat steps 1-4 to make the next dilution, using the first dilution (the 0.1 M solution in this case) as your new stock solution.  In our example, we now have 20 mL of a 0.1 M solution.  If we want to make 20 mL of a 0.01 M solution, how much of the 0.1 M solution will we need?  We could calculate the required amount using the dilution equation again.  These calculations are shown in italics next to Figure 3 below.  Alternatively, we could simply think:  we are diluting our new solution by a factor of 10, so we need 2 mL of our concentrated solution to make 20 mL of the new solution (notice the ratio of 1:10).  So we take 2 mL of our 0.1 M solution, using a volumetric pipette, as seen below in Figure 3 and dispense it into a new flask.  Carefully add exactly 18 mL of water to make a total volume of 20 mL of solution.

 Use the dilution equation: M1V1 = M2V2 M1=0.1 M V1 = unknown M2= 0.01 M V2 = 20 mL Solve the equation for V1.    V1 =  2 mL.  This is the amount of the 0.1 M solution that we need.  The total volume is 20 mL, so we added 18 mL of water.

Figure 3: Teresita and Kira are demonstrating how to measure specific volumes of solutions using a volumetric pipette.

6. Continue serial dilutions, repeating steps 1-4, always using the most recent dilution as the stock solution. Never go back to the original stock solution. (Why not?) Okay, now say that we want to continue diluting our solution, so that we have 20 mL of a 0.001 M solution. Take the following SELF QUIZ to test your knowledge of performing dilutions:

What volume of our 0.01 M solution do we need to make 20 mL of a 0.001 M solution?