Teacher's Notes

Outcomes Desired

The purpose of this experiment is to familiarize students with the uses and limitations of various indicators. After the students observe the colors of four specific indicators across a pH range, they will design a new indicator (the simplest combination of indicators they have already examined) to use for a pH value they have not yet tested. Finally, they might use their new indicator to test various rain or water samples.

Student Background Needed

Only a very basic (pun intended) understanding of pH and indicators is necessary for students to perform this experiment.

Detailed Chemistry Involved

Universal Indicator is made up of the following indicators: phenolphthalein, methyl red, bromthymol blue and thymol blue. We chose these four indicators for the students to work with in order to keep the experiment quite simple.

Materials (per student/group)

Approximately 20 mL each of

• methyl red (To make 100 mL use 0.02g methyl red salt dissolved in 60 mL ethanol, then add 40 mL deionized water.)
• bromthymol blue
• thymol blue
• phenolphthalein
• Buffered solutions of
       pH 4, pH 5, pH 6, pH 7, pH 8 (Simplest method - use buffer capsules.)
• Well plates (two or three, if possible)
• Micropipets or droppers

  Grading Advice

  We do not think accuracy should be graded in this experiment. A more important aspect might be the process by which they determined how to make their new indicators.

  Sample Data

  Colors of various pH solutions with one indicator:

  	Phenolphthalein	Bromthymol Blue	Methyl Red	Thymol Blue
  pH 4	colorless	yellow	dark pink	yellow
  pH 5	colorless	yellow	peach	yellow
  pH 6	colorless	green	yellow	yellow
  pH 7	colorless	blue	yellow	yellow
  pH 8	pink	blue	yellow	green

      (Table 1)

  Colors of various pH solutions with combinations of indicators:

  	Methyl Red & Bromthymol Blue	Methyl Red & Thymol Blue	Thymol Blue & Bromthymol Blue
  pH 4	red/orange	red/orange	yellow
  pH 5	orange	orange	yellow
  pH 6	green	yellow/orange	green
  pH 7	blue/green	yellow	blue/green
  pH 8	blue	yellow/green	blue

      (Table 2)

  Notes:

  • The combinations of methyl red and bromthymol blue seem to work the best for testing solutions of pH 5 and pH 7. The combination of thymol blue and bromthymol blue seems to work well for pH 7 solutions, as well. (Phenolphthalein is not necessary to include in order to test a solution less than pH 8.)
  • When students are making their new indicators, they could test different proportions of the indicators to achieve the sharpest colors.
  • To test the students' new indicators, you could use unknowns of pH 3, 5, 7 and 9 (only 5 and 7 will be in their range) or you could devise your own testing method.
  • Extension materials may include rain water, carbonated water, colas, vinegar, etc.

  Experimenting With Indicators

  This investigation will give you experience with four common acid/base indicators as applied to solutions of known pH. After this experience, you will design the simplest possible mixture of these indicators that will result in recognition of a pH of 5.

  Background Information

  The pH of a solution is a measure of its acidity or basicity with a range from zero (most acidic) to 14 (most basic). A pH of seven is neutral.
  Indicators show a color change over a specific part of this zero to fourteen range and, when used in combination, can narrow the determination of pH to a whole number in this range.

  Safety/Disposal

  Safety goggles must be worn during this lab.
  Disposal: Rinse all your well plate solutions down the drain.

  Basic Procedure

  In steps 1-5, you will be trying to determine the color of each indicator in each pH.
  

  1. Obtain a well plate(s) and the following four indicators: bromthymol blue, thymol blue, methyl red, and phenolphthalein.
  2. Using volumetric pipettes, add approximately 1 mL of pH 4 solution to the top row of
  3. Repeat step two using solutions of pH 6 (row 2) and pH 8 (row 3).
  4. Add two drops of methyl red indicator to each pH in the first column and record the color (use colored pencils, if available).
  5. Repeat step 4 with the other three indicators provided.

  Designing an Indicator:

  Design an indicator that will enable you to recognize a pH of 5. You may use the four indicators provided in any combination. Once you have made your indicator, your instructor will provide you with an opportunity to test it.

  Lab Report

  Your report should contain a table of your test results on the known pH solutions (4,6,8) and a similar table for the test results of your attempts to design an indicator for pH 5. Conclude with a discussion of how you arrived at the simplest possible mixture.

  Extension

  Construct a question that would relate your knowledge of pH indicators to new materials provided by your teacher. Design a simple experiment to answer your question and try it. Add this to the above report.