Sodium acetate forms a basic solution because the acetate ion reacts with water to form hydroxide ion:
Similarly, ammonium chloride forms an acidic solution because only the cation hydrolyzes:
Five 0.1 M solutions selected from: sodium bicarbonate, NaHCO3; ammonium
chloride, NH4Cl; sodium bisulfate, NaHSO4; sodium chloride, NaCl; sodium
carbonate, Na2CO3; ammonium sulfate, (NH4)2SO4; aluminum nitrate, Al(NO3)3); sodium nitrate, NaNO3; ammonium acetate, NH4C2H3O2; and potassium phosphate, K3PO4. Select solutions that will give acidic, basic, and at least one neutral solution.
5 Petri dishes or small beakers
Add solutions to vessels placed on the overhead projector. Add 1-3 drops of
universal indicator solution to each container. Alternatively, large test tubes
in a test tube rack may be used with a diffuse light source behind them.
A buffer is a system that maintains a nearly constant pH in a solution
when relatively small amounts of acid or base are added to the solution. A
buffer is a mixture of a weak acid and its conjugate base, or a weak base and
its conjugate acid.
A buffer can be prepared by mixing a weak acid (for example, carbonic acid,
H2CO3) with one of its salts (for example, sodium bicarbonate, NaHCO3), since the anion (HCO3-) of the acid is the conjugate base. This buffer is very important in maintaining a nearly constant pH of the blood. A weak base (NH3) mixed with one of its salts (NH4Cl) can also function as a buffer.
3 M or 6 M Hydrochloric acid, HCl
3 M or 6 M Sodium hydroxide, NaOH
9 Large (about 180 mm) test tubes
Test tube rack with illuminated lightbox or white background
Universal indicator solution
Buffer solution A: prepare by mixing equal volumes of 4 M acetic acid (HC2H3O2) and 4 M sodium acetate (NaC2H3O2).
Buffer solution B: prepare by mixing equal volumes of 4 M aqueous ammonia (NH3) and 4 M ammonium chloride (NH4Cl).
Acids and Bases