Basic Buffer Demonstration
Compare the results of adding acid and base to buffered
and non-buffered solutions by seeing relative changes in
color.
Materials
40 mL 6M acetic acid
4 g sodium acetate
1M HCl
1M NaOH
4 2” glass Petri dishes
Methyl orange indicator
Bromothymol blue indicator
Where to Find
Shelf 5F
Shelf 4D
Shelf 5F
Shelf 4F
Drawer N2
Drawer L3
Drawer L3
Set Up
Add buffer solution: 40 mL 6M acetic acid and 4 g sodium acetate to two petri dishes,
and water to the other two dishes. Add 2 drops methyl orange indicator and 6 drops
bromothymol blue indicator to each petri dish.
Safety
Use caution and wear goggles and gloves when working with acids and bases.
Disposal
Pour solutions down sink with water.
Professor/Lecturer
Chemistry
Buffers lessen or absorb the drastic changes in pH that occur when small amounts of
acids and bases are added to solution. In this case, the buffer solution is made of water,
acetic acid, and sodium acetate. The acetate ions shift the equilibrium, depressing the
ionization of the acetic acid. The pH will remain essentially constant as long as the ratio
of the concentrations of acids and bases are more or less constant. When enough acid or
base is added to exceed the buffer capacity of the solution, the pH will change
significantly along with its color.
OAc - (aq) + H3O+(aq) ---> HOAc (aq) + H2O (l) (addition of acid)
HOAc (aq) + OH - (aq) ----> OAc - (aq) + H2O (l) (addition of base)
Presentation
This is best done on the overhead or using Wolf-vision. Add a few drops of acid to one
of the water dishes and a few drops of base to the other water dish. There should be a
change in the indicator’s color showing the pH change. Then repeat this process with the
buffer solutions. There should be no color change.
Search Items
Buffer
pH
Suitable for Recitation
Yes
Reference
http://chemed.chem.purdue.edu/demos/demosheets/17.4.html