Producing a serial dilution is a really useful skill for all sort of biology experiments. In this article we will look at how to prepare a serial dilution, and how it could be used to produce a calibration curve for a colorimeter experiment.
How to prepare a serial dilution
In A-Level biology, you will probably need to produce a serial dilution of either glucose or sucrose solution. We will use glucose solution as our example.
To produce a serial dilution, we need to start with a glucose solution of known concentration e.g. 2M glucose. We also need a few test tubes containing distilled water. The amount of distilled water will depend what dilution factor you are aiming for, but in this example we will use 5cm3 of water because we are going to dilute by a factor of two each time, making the total volume to 10cm3.
- Measure out 10cm3 of the 2M glucose solution into an empty test tube.
- Transfer 5cm3 of this solution into a test tube containing 5cm3 of distilled water and mix thoroughly. You now have a 1M glucose solution because we have diluted the original solution by a factor of two.
- Transfer 5cm3 of the 1M glucose solution into another test tube containing 5cm3 of distilled water and mix thoroughly. You now have a 0.5M glucose solution because you have diluted by a factor of two again.
- Keep repeating this until you have diluted for the desired number of times.
In the above diagram, we now have five concentrations of glucose solution. You would probably also need to prepare a tube containing distilled water only to act as a negative control.
If you wanted to dilute by a factor of ten, you would want to start with 9cm3 of distilled water in each tube and transfer 1cm3 of each solution.
Producing a calibration curve
Serial dilutions are often used to produce calibration curves. An example would be using the Benedict’s test and a colorimeter to find the concentration of a glucose solution.
During the Benedict’s test, a coloured precipitate is formed. A higher concentration of glucose means that more precipitate will form, and the less blue the remaining solution will be. If we filter the precipitate out of the solution, we can then measure the absorbance of the solution using a colorimeter. Carrying out the Benedict’s test on all the serial dilution tubes will enable us to produce a calibration curve that looks something like this:
Yes it’s a straight line, not a curve! For some reason they are called calibration curves and not calibration lines. It’s probably a maths thing.
The red line represents how we could use the calibration curve: measure the absorbance after performing the Benedict’s test on a glucose solution of unknown concentration, then read across to the calibration curve and down to find the glucose concentration.
- A serial dilution is prepared from a solution of known concentration, diluting by the same factor each time.
- Thorough mixing is important.
- A calibration curve can be produced from a serial dilution, allowing the concentration of an unknown solution to be determined.