BIOL 1406

PreLab 2.6

How can I prepare solutions with specific solute concentrations using the serial dilution technique?

Let’s return to our example where you have a 2 M stock solution of KMnO4 and you want to make 15 mL of each of the following concentrations of KMnO4: 0.2 M, 20 mM, 2 mM, and 0.2 mM.  In this case you must make a total of 4 dilutions. 

  1. First, the 2 M stock solution is diluted to make the 0.2 M solution.

  2. Next, the 0.2 M solution is diluted to make the 20 mM solution.

  3. Next, the 20 mM solution is diluted to make the 2 mM solution.

  4. Finally, the 2 mM solution is diluted to make the 0.2 mM solution.

How would you make these 4 dilutions using the serial dilution technique?  Before you begin, you must determine the values of 3 variables: df, v2, and v1.

df For each dilution, df is the dilution factor.  We have already seen that this dilution series has a constant dilution factor of 10.
v2 For each dilution, v2 is the volume of diluted solution that you want to make.  Since we want to make 15 mL of each diluted solution, v2 = 15 mL
v1 For each dilution, v1 is the volume of more concentrated solution that you mix with solvent to make the more dilute solution.  If you know df and v2, you can calculate the value of v1 using the following formula:

 

When making serial dilutions, use the following formula to calculate the value of v1:

(v1 + v2) / v1 = df

In our example:

  1. (v1 + 15 mL) / v1 = 10

  2. v1 + 15 mL = 10 v1

  3. 15 mL = 10 v1 - v1

  4. 15 mL = 9 v1

  5. v1 = 15 mL / 9

  6. v1 = 1.67 mL

Once you know the values of df, v2, and v1, set up 5 tubes (one for the stock solution and one for each of the dilute solutions) as shown in the following diagram and then follow the procedure described below the diagram:

NOTE:  The letters below correspond to the letters on the arrows in the diagram above.

 
  1. Place the correct amount of stock solution (V1 + V2) into tube 1. In our example, place 16.67 mL of 2 M KMnO4 into tube 1.
     
  2. Place the correct amount of solvent (V2) into the remaining tubes. In our example, place 15 mL of dH20 into tubes 2 through 5.
     
  3. Transfer the correct amount of solution (V1) from tube 1 to tube 2 and then thoroughly mix the contents of tube 2. In our example, transfer 1.67 mL of solution from tube 1 to tube 2 and thoroughly mix. This will leave 15 mL of 2 M KMnO4 in tube 1.
     
  4. Transfer the correct amount of solution (V1) from tube 2 to tube 3 and then thoroughly mix the contents of tube 3. In our example, transfer 1.67 mL of solution from tube 2 to tube 3 and thoroughly mix. This will leave 15 mL of 0.2 M KMnO4 in tube 2.
     
  5. Transfer the correct amount of solution (V1) from tube 3 to tube 4 and then thoroughly mix the contents of tube 4. In our example, transfer 1.67 mL of solution from tube 3 to tube 4 and thoroughly mix. This will leave 15 mL of 20 mM KMnO4 in tube 3.
     
  6. Transfer the correct amount of solution (V1) from tube 4 to tube 5 and then thoroughly mix the contents of tube 5. In our example, transfer 1.67 mL of solution from tube 4 to tube 5 and thoroughly mix. This will leave 15 mL of 2 mM KMnO4 in tube 4.
     
  7. Remove the correct amount of solution (V1) from tube 5 and discard it. In our example, remove 1.67 mL of solution from tube 5 and discard it. This will leave 15 mL of 0.2 mM KMnO4 in tube 5.

 

YOUR TURN

Starting with a stock solution of 2 mM KMnO4, you plan to make 6 mL of each of the following concentrations of KMnO4: 1.0 mM, 0.5 mM, 250 μM, 125 μM, 62.5 μM , 31.3 μM , and 15.6 μM
What is df for this dilution series? Hint Check your answer.
What is V2 for this dilution series? Hint Check your answer.
What is V1 for this dilution series? Hint Check your answer.
Examine the figure below and answer the questions in the spaces provided. You will need this information during lab, so you should also record it in your lab notebook.

 
What is the volume of "A" placed into tube 1? Hint Check your answer.
What is the volume of solvent "B" placed into tubes 2-8? Hint Check your answer.
What is the volume of "C to I" transferred between tubes 1 - 8? Hint Check your answer.
What is the final volume of tubes 1-8? Hint Check your answer.
What is the volume of "J" that is discarded? Hint Check your answer.

 

You have now learned three methods for preparing solutions. Use the interactive exercise below to test yourself and see if you can choose the best method and correctly carry out the procedure to make the required aqueous solutions.

 

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