Simple Dilution
SUMMARY: Starting from a known concentration students learn to use dilutions to determine the concentration of an unknown. The following skills are used in this lesson:
  1. making dilutions
  2. reading the meter on a simple homemade spectrophotometer
  3. keeping careful laboratory records
  4. graphing on linear graph paper
  5. determining an unknown concentration from known
  6. using a colored filter to enhance contrast and sensitivity
Medium High Difficulty



See also Simple Dilution 2.

Dilution usually refers to a concentration of a material in a liquid and sometimes in a gas. Am I getting my moneys worth? Is it safe to use at this concentration? How much do I need to dilute before use? When we swallow a vitamin C tablet at breakfast we are diluting the vitamin C into our entire body. What is the proper amount of vitamin C for my body size? When we add bleach to a wash load or prepare a soap solution to wash our car, we are diluting a material to the proper concentration.

In later lessons we will be diluting a concentrated sperm suspension to fertilize sea urchin eggs. Too much and we prevent proper development, too little and they will not all fertilize. How much do we need to dilute the sperm to get the highest rate of survival of the embryos? In the experiment lab students will be diluting environmental "toxins" to see their effect on fertilization and development. They will need to understand dilution in order to be able to do these experiments in any kind of meaningful way.

In this lesson an attempt has been made to relate dilution to the real world. Scenario: You have purchased a "Coke" at the movie theater. Is it at the right concentration? Have I gotten my moneys worth or have they "watered down" the Coke to save money. What happens when the ice melts? How much more dilute is the Coke at the end?

Using Coke as our material of interest we will be performing a simple linear dilution to form a "standard curve" from which we will quantify our unknown samples (the Coke and ice mixtures). A simple device to measure concentration will be constructed using a light sensor, light source, filter and a meter.

What we see as white light is actually made up of light of different colors. The figure to the right represents the visible spectrum.
Coca Cola is not "clear" but has a distinctive color to it. Using a scanning spectrophotometer we can "see" in a more exact way what color Coke is. Coke absorbs more light in the blue end of the spectrum than in the green and red areas.

By adding a filter to our light sensor we can improve the "Coke" detecting ability of our sensor. Since Coke absorbs light in the blue we can get the most response from our sensor if we choose a filter that lets primarily blue light through. An inexpensive blue filter's spectrum looks like this:

This is not a perfect filter, letting some light through in the "Red" region, but it will work for our purposes.


Spectrophotometer parts: (The "spec" has the potential of being shared with the chemistry classes)


Setting Up the Spectophotometer:

The black tape is used to seal the CdS cell and filter against the chamber (shown in cross section, actual chamber is closed to room light). Meter is set to Ohms. These spectrophotometers can be made ahead of time or allow the students to make their own. If you make them, show how one is made by demo in front of class.

Standards Series:

% "Coke" 100 80 60 40 20 0
ml of Coke 10 8 6 4 2 0
ml water 0 2 4 6 8 10
total liquid 10 10 10 10 10 10
It is very important that the light and the chamber do not move during the experiment. Any movement will change the readings and nullify the curve!


Unknowns can be made up ahead of time or have students devise criteria. Examples:


Plot Readings vs concentration: (example shown)

% Coke K-Ohms
100 3.0
80 2.8
60 2.6
40 2.4
20 2.0
0 1.7
Unknowns can now be found by reading the K-Ohms reading of the unknown and finding the concentration on the graph. (or if they have had some algebra they could find the equation that fits the line and use that. Draw a line and use a two point substitution to solve for constants in standard linear equation y=mx+b)

Students may have some trouble with a graph that is not zero K-Ohms at zero Coke concentration. One way around this is to do a "blank subtraction". Just remember to subtract the blank from the unknowns also.


  1. This lesson did a linear dilution. How would you do a serial dilution based on a factor of 2? 10? (starting from the same 100% solution).
  2. In later lessons you will dilute sperm in a similar manner. How could you relate your optical density readings (the K-Ohms reading from the meter) with concentration? (Hint: use an hemacytometer to count sperm at a known dilution.)
  3. What about a solution that changed color with time. How would you record the results? [K-Ohms on Y axis and time on the X axis].
  4. How much do you weigh? in grams? (453 grams per pound) If you took a 1000 milligram vitamin C pill (1000 milligrams = 1 gram), how dilute would it be in your body? (assuming an even distribution)
  5. What would happen if the samples were not mixed well? How would this affect readings?


Class participation and lab report. On the lab report look for the following: