How to perform serial dilutions

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How to perform serial dilutions
Serial dilutions are a way to make a dilute solution. (brightly coloured science test tubes image by Steve Johnson from Fotolia.com)

A serial dilution allows you to take a concentrated solution and make a dilute solution through a series of steps. In each step the solution becomes more dilute. You will often need to perform serial dilutions if you need a smaller amount of the solute than you can measure directly from the original solution. Serial dilutions are often used when measuring minute concentrations for chromatography equipment such as in liquid or gas chromatography. For example, you can dilute a solution from 1g/ml to 0.001g/ml.

Skill level:
Moderate

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Things you need

  • Original solution of 1g/ml concentration
  • Test tube rack
  • 3 New test tubes or sample vials with caps or seals
  • Permanent marker
  • Pipette with 1-10ml range
  • Pipette tips for 1-10ml pipette
  • Solvent (Water in this example)
  • Vortex mixer (optional but recommended)

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Instructions

  1. 1

    Place your original solution (the 1g/ml solution) in the test tube rack. Use the permanent marker to label the other test tubes or sample vials with the following labels (you can write directly on the tube or vial): "A 0.1g/ml," "B 0.01g/ml" and "C 0.001g/ml." Line these tubes or vials up in order in the test tube rack.

  2. 2

    Place a pipette tip on the pipette. Transfer 9ml of water into Tube A.

  3. 3

    Transfer 1ml of the original solution into Tube A. Cap or seal the tube and mix using the vortex mixer for a few seconds or by gently inverting the tube a few times. Tube A now contains a 0.1g/ml solution. Discard the pipette tip.

  4. 4

    Place a new pipette tip on the pipette. Transfer 9ml of water into Tube B. Transfer 1ml of the 0.1g/ml solution in Tube A into Tube B. Cap or seal the tube and mix using the vortex mixer for a few seconds or by gently inverting the tube a few times. Tube B now contains a 0.01g/ml solution. Discard the pipette tip.

  5. 5

    Place a new pipette tip on the pipette. Transfer 9ml of water into Tube C. Transfer 1ml of the 0.01g/ml solution in Tube B into Tube C. Cap or seal the tube and mix using the vortex mixer for a few seconds or by gently inverting the tube a few times. Tube C now contains a 0.001g/ml solution. Discard the pipette tip.

Tips and warnings

  • These measurements and concentrations can be adjusted for your needs - grams can change to micrograms, millilitres can change to microliters or you can use Molarity. The principle behind the dilution is the same regardless of the units you are using.
  • Continue following the same procedure if you want an even more dilute solution.
  • In this example, water is used as the diluting solvent. In practice, the solvent you use will vary depending on your lab protocol.
  • Use a new pipette tip for each dilution to prevent introducing a more concentrated solution into your dilution than you want.
  • Always pipette from the solvent (water in the example) first to avoid contaminating the solvent with your solute.
  • Make sure your cap or seal is tight before mixing the solution to avoid spills

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