Acids, Bases, and Indicators

Edwin A. Metzl                 Lincoln Park High School
                               2001 North Orchard Street Mall
                               Chicago IL 60614
                               (312)534-8210

Objectives:

     This is a multileveled approach to learning some of the common 
characteristics of acidic and basic solutions and use of some of the
common indicators.
                            
Materials needed:

     Apron                                     Distilled water
     Safety goggles                            200 mL  0.1 M HCl
     13 small or medium sized test tubes       200 mL  0.01 M NaOH
     Test tube racks to hold 12 test tubes     a few leaves from
     1 glass stirring rod                        Red Cabbage
     12 250mm beakers                          Heat source such as
     1 glass marking pencil                      a hot plate
     1 10 ml graduate cylinder                 Pot or 1 L beaker
     1 100 ml graduate cylinder                Various solutions of
     At least 6 acid-base indicators which       unknown pH
     should include: Blue & Red Litmus Paper   13 Eye Droppers 
                     Bromothymol Blue
                     Universal indicator paper
                     Phenolphthalein
                     Phenol Red + others

Strategy:

     The teaching strategy is both multifacetted and multileveled.  At the 
lowest level, litmus paper testing of different household products might be an 
effective means of introducing the student to acid and bases.  However, for 
purposes of this discussion, it will be presumed that the class is very advanced 
and the teacher may decide how far and how fast to proceed. 
     Acid-base indicators are substances that change color as a function of pH, 
usually over a range of 1 to 2 pH units.  These indicators are weak acids or 
weak bases which disassociate and change colors when equilibrium shifts per     
Le Chatelier's principle.  Indicators are available that shift color for every 
value of the pH scale.  The earliest indicators were extracted from plants.  In 
this lab, we will compare indicator colors to known pH's; use the juice from red 
cabbage as an indicator; use red cabbage indicator to find the pH of unknowns; 
and lastly, do an acid-base titration. 

Procedure:

     Preparation of standard solutions
        -Obtain 12 250 mL beakers and label them 1 through 12.  To beaker 1,     
         add 100 mL of 0.1 M HCl.  This solution has a pH of 1.  Measure 10 mL   
         of this solution and add it to beaker 2 with 90 mL distilled water.   
         The resulting solution has a pH of 2.  Follow this procedure for 
         beakers 3, 4, 5, and 6.  Of course, beaker 7 will have only distilled
         water with a pH of 7.
        -To beaker 12, add 100 mL of 0.01 M NaOH solution.  This has a pH of 12.
         Successive dilutions by factors of ten will produce pH values of 11,
         10, 9, and 8.
     Comparing indicators with known pH's
        -Label 12 test tubes and take samples of each known pH solution.  Use
         whichever indicators are available to compare indicators with these
         knowns.  Note which indicators appear to be best for given ranges
         of pH.        
     Extraction of an indicator from red cabbage
        -Cut leaves from the red cabbage into small pieces and place them into 
         a clean large beaker with enough distilled water to cover the pieces.  
         Boil the cabbage and water until the solution is a deep purple.  Use
         the stirring rod to keep all leaf pieces in the boiling water.  Permit
         the solution to cool.
     Using red cabbage as an indicator
        -Place 5 mL of known pH solutions in each of your labelled test tubes.  
         To these, add 5 drops of red cabbage indicator.  You are now ready to 
         compare your unknowns with your knowns by placing 5 mL of an unknown 
         into a clean test tube, adding 5 drops of red cabbage juice and
         comparing the unknown with your knowns.
     Using red cabbage for titration
        -Only a few words on this subject ... As all titrations that this
         writer is familiar with use phenolphthalein as an indicator, there
         is no reason for not using red cabbage instead.  For those not 
         familiar with titrations, it is a method of determining quantities
         necessary to change a solution of one pH into a solution of a 
         different pH.

Performance Assessment:

     A comparison should be made by the students of the cabbage indicator with 
the "store bought" varieties plus a comparison between the various indicators.  
Questions, such as, "Which is the best indicator?" should be more specific with 
questions such as, "Which has the most discernible color change?" and "Which 
might be used for a specific range?" 
     Of course, with teacher approval only, students could try extracting other 
colored substances from leaves and flower petals to test as indicators...try tea 
and beet juice as examples. 

Conclusions:
     Cabbage juice appears to be a useful indicator, as do other some 
naturally colored materials.  Materials serve best within specific pH ranges.