- Students are given an unknown dye solution and asked to determine its concentration by comparing it with standards they create.
- Repeated "serial" dilutions will be necessary to determine accurate concentration readings.
- Varying the color of the filters and the path length will aid in seeing lower concentrations.
45-50 minute class period
This lesson follows Simple Dilution.
The ability to make dilutions is essential to working in biology, chemistry, cooking and many other fields. Students who have not mastered the skills involved in making dilutions will have difficulty with the Experiments sea urchin lab.
In Simple Dilution we saw how one could construct a standard curve and use this to determine concentrations of unknown dilutions. Life does not always hand you all of the tools to solve problems in a "cookbook" manner. In this experiment the students use the tools they learned to solve a slightly more difficult problem. In this lesson we will use household food coloring. The students will have a much wider color intensity range to work with, due to the highly concentrated nature of the dyes.
Students may not see the relevancy of using a dye, but a dye in this experiment could represent an environmentally toxic poison. A dye is used because it can be safely seen and measured without expensive equipment. As a teacher you could invent a scenario of "pretending" it is the toxin.
- one small package of assorted food colors for each lab group. (Larger size bottles should be purchased for refills for future classes.)
- a light table, students working in groups of 2. (A sheet of translucent white plastic over a sunlit window works great!)
use the spectrophotometers from the Simple Dilution lesson, students working in groups of 4-5
- set of colored filters (Edmund Scientific #A60,373 $16.75), colored cellophane, or colored transparencies. Make sure they have several colors, so they are forced to make a choice.
- set of 10 clean test tubes of the same size & test tube rack
- set of unknowns made up ahead of time, dilute but still showing visible color. Too high a concentration and the students may think they have solved the problem too easily.
- graduated cylinders (10ml) and eye droppers
- water, paper towels, etc. This lab can be messy!
- Hand out materials and one unknown per group
- Instruct students that:
a) They are to determine the concentration of the unknown to the highest accuracy possible (Assume that the dye in the food color bottles is 100%.)
b) Their grade depends on the accuracy of their answer, their reasoning, and conservation of resources.
- Suggest that their first dilution might be 1-2 drops in 10 ml. Serial dilutions will need to follow.
- They may talk among themselves and with other groups. This is not a competition. They can all get A's or all get F's.
- Go! Answer no questions of a technical nature until everyone is done. Do not hand out any more dye or materials during the experiment.
They will need to figure out:
- which colored filter to use
- which dye to make the dilutions from
- how many serial and linear dilutions (quite a few, as the dyes are VERY concentrated)
- record their procedures and results
- use their records to calculate the concentration of their unknown
- cooperate in a group in determining strategies, assigning duties, responsibilities and doing the work
Some students will find this experiment frustrating, because there is no specific procedure given. Don't give in! They have all the tools they need. They will have to think!
Example senario without a spectrophotometer:
- The group is handed an unknown that has a "pink" color.
- Placing the unknown and a blank (zero dye concentration, same water as used later to make dilutions) against the light table they try the three colored filters to see which gives the highest contrast between the two. Green seems to work the best.
- Looking at the color choices of dye, they select the red dye as being the closest to the color of the unknown.
- Opening the dye bottle they note that there is only a small quantity of dye to use and it only comes out in drops.
- As an experiment, two drops are added to 10ml of water and mixed. The resultant solution is more intense than the unknown. (They try one drop also, but this is still too intense)
- They decide that the two drop solution can be used as a starting point to make further dilutions. 1 drop = 0.05ml so 2 drops = 0.1 ml. 0.1ml/10ml = 0.01 or a 1% concentration of the stock dye bottle.
- A linear dilution is made of the "new stock", making up more stock as needed. Using the 10ml graduated cylinder they add 10ml of stock to the first tube and no water. To the next they add 8ml stock and 2ml of water and so on.
- Calculating, they find the dye concentrations are now 1.0%, 0.8%, 0.6%, 0.4%, 0.2%, and blank.
- It appears that even 0.2% is more intense than the unknown.
- 0.2% is selected as the "new stock" and is diluted using the simple dilution method to the calculated values of: 0.2%, 0.16%, 0.12%, 0.08%, 0.04% & 0.
Note: they could have chosen a different dilution series to make the numbers easier to deal with, such as 0.2%, 0.15%, 0.10% 0.05%, & 0.
- It appears that the dye is somewhere between 0.04% and 0.08%.
- The differences are so subtle at this point they decide not to try further refinements in their dilution series. They make a best guess of 0.06% and record this as their answer.
- They clean up their lab space and return the materials to the appropriate locations.
- They spend the rest of the period laying out their lab reports together. Each student presents their part of the process and discusses the problems and solutions. (Each student might be required to turn in a separate lab report, but they are allowed to discuss possible layouts and what should be included.)
As you can see from the above scenario, careful record keeping and math are very important to successful completion of this lab. The major math concept introduced in this lesson is repeated multiplication of percentages and/or fractions (depending on how students decide to follow the dilutions).
- How did you choose the dilutions that you used? Could you have used a different series?
- Using one drop of concentrated dye, how much water would you have to add to dilute the dye to the concentration of the unknown?
- How dilute a solution is it possible to mix and still detect above the blank? With and without the colored filter?
- Look down the end of the tube, from top to bottom. Notice the intensity of the color. What causes this? How could you make use of this fact?
In this lab it is very important to assess the thinking that went on as well as getting to the answer:
- Did the group follow logical steps to arrive at a solution?
- What were the problems encountered?
- Did they recognize the problems and were they overcome?
- Did any group come up with a unique way of solving the problem that went beyond prior learning?
- Did everyone in the group understand what was done and why? (Individual lab notes and reports should tell you this.)
- Did the group achieve an accurate answer?
- Was there a good division of labor? Did everyone participate?
- Implication questions