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:

Suggested Demonstration

• Materials
  Universal indicator
  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

• Directions
  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.

Buffers

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.

Suggested Demonstration

• Materials
  
  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
  Distilled water
  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).

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