INDICATORS EXPERIMENT

In previous science classes you have probably worked with litmus paper. The acid-base indicator is so widely used that many students can repeat, “If it turns blue it is a base”, from memory. But did you know that the dye used in litmus paper is a natural dye obtained from lichens? Most indicators can be tested using equally common household acids and bases.

The caustic nature of strong mineral acids, such as sulfuric acid, or strong bases, such as sodium hydroxide, has led to misconceptions and fears. All acids and bases wrongly become categorized as dangerous. Bases such as ammonia water and milk of magnesia are familiar products in most homes as are basic salts such as soap, trisodium phosphate, and bicarbonate of soda. Fruit, sour milk, and vinegar contain citric, lactic, and acetic acid, respectively. The soil of an area is rarely neutral. The success or failure of many home gardens can often be traced directly to the acidity of the soil. Even the color of some flowers is dependent on the soil’s acidity. Acid-base indicators work because the molecules of the dye change color according to the concentration of hydrogen ions.

In this experiment you will use a variety of dyes from indicators some of which are commercially prepared and others come from natural sources such as fruits, vegetables, or flowers. Both types can be used to help as an indicator for the acidity of a solution.

Materials:

12 solutions with pHs which range from 1 to 1296 well microplate

8 indicator solutionscolored pencils or crayons

Procedures:

Part 1:

You will be testing each of the 12 pH solutions with all of the 8 indicators. This will be done in the microplate. If you look at the microplate you will see there is an easy way to set this up. When testing the solutions use 4 drops of the pH solutions and 1 drop of the indicator with the pH solution. Record your observations with the colored pencils or crayons. Once you are finished clean out the microplate.

Part 2:

Using small test tubes, 0.1 M HCl, 0.1 M KOH, and universal indicator your job is to create at least 5 different colors in separate test tubes and have them sitting side by side in a test tube rack. Once you have achieved this have the teacher check it off.

Results and Conclusions:

  1. What is the significance of the color change of the different indicators?
  2. What problem is there with using an indicator to determine the pH of a solution?
  3. Which would be a good indicator for use with a wide range of pHs? Why?
  4. A solution is found to be clear with phenolphthalein added and green with universal indicator added. What is the pH of this solution?
  1. Natural substances such as red cabbage are sometimes used as chemical indicators, why may they not produce as dark of color changes as other indicators you used?