E785: Prep Notes

Introduction

Strong base is added to three different buffers and water and the effect on pH is observed.

  Supplies

Short List

  •   Goggle and gloves
  •   Reference pH solutions in lighted rack
  •   10 mL graduated cylinder
  •   4 x 150 mL beakers

Chemicals

  •   1.0 M NaOH
  •   Universal indicator
  •   Sodium acetate
  •   Acetic acid

 Prior to Lecture  (prep time ~ 5 min.)

1.  Prepare 100 mL each of buffers 1, 2, and 3:  Boil 700 mL of distilled water (to remove dissolved CO2), cover, and cool.  Weigh out and add NaC2H3O2.3H2O (sodium acetate) as indicated.  Add concentrated acetic acid.  Gently mix and dilute to 100 mL with boiled water.  Do not shake --- CO2 will re-dissolve into solution and alter pH.  

                                                            Sodium acetate          Conc. acetic acid

                              Buffer 1                        13.60g                       5.76 mL

                              Buffer 2                         13.60g                    0.0057 mL*

                              Buffer 3                         1.36g                         0.58 mL

                                    *  Dilute 0.58 mL acetic acid to 100 mL and take 1.0 mL of that solution.

2.  Assemble the following chemicals:

  1.  1.0 M NaOH (sodium hydroxide)
  2.  Universal indicator

3. Assemble the following equipment:

  1.   four labeled 150 mL beakers
  2. 10 mL graduated cylinder (for NaOH)
  3.   reference pH solutions with indicator in lighted rack

To Conduct Demonstration:

1.    Place 150 mL beakers containing 100 mL of each of three buffers and distilled water on the overhead projector.  Add a few drops of universal indicator so that the color is sufficiently intense.

                                           Buffer                             pH                             Color       

                                               1                               4.74                          orange

                                               2                                7.9                             green

                                               3                               4.69                          orange

                                           Water                            5.5                            orange

2.  Add 2 mL of 1 M NaOH to each beaker.  Results should be:

  1.   Water goes from orange to blue.
  2. Buffers 1 and 3 remain orange; equimolar concentrations of acid and conjugate base withstand small DpH.
  3. Buffer 2 goes from green to blue; 1000:1 HOAc:OAc- ratio is ineffective buffer, pH goes from 7.9 to 12.8.

3.  Add 8 mL of 1 M NaOH to buffer 1 and buffer 3.

  1. Buffer 1 remains orange; large (1 M) equimolar concentrations of acid and conjugate base give large buffer  capacity.
  2. Buffer 3 goes from orange to blue; smaller (0.1 M) concentrations of acid and conjugate base lead to lower buffer capacity than A.

Safety and Disposal

Goggles should be worn. 

All solutions may be sink-disposed.