PROTOCOL N° 4

DETERMINATION OF THE TOTAL HARDNESS
OF A WATER SAMPLE

ATTENTION: The ammonia/ ammonium chloride solution used in this experiment, for the determination of the water total hardness, looses the gas ammonium to the atmosphere. Care should be taken in manipulating the solution due to the unpleasant smell of the gas, which causes irritation of the eyes and skin.

Risk code(s): R 36, 37, 38, 41, 43
Safety code(s): S 18, 25, 26, 37

REAGENTS

  • Black Eriochrome T indicator
  • EDTA (Ethylene Diacetic Tetra Acid) standard solution, 0.01 mol/dm3 (prepared from the disodium salt)
  • Ammonia/ammonium chloride pH buffer standard solution, pH=10.0

PROCEDURE

  1. Place 10 mL of the water sample in an Erlenmeyer,
  2. Add 8 drops of the pH buffer solution to the water,
  3. Add 4 drops of the indicator,
  4. Drop by drop, counting the number of drops, keep adding EDTA solution and stir carefully, until the colour changes from red to blue,
  5. Record the results.

DATA TREATMENT

Values of total hardness are given by the following equation:

Total Hardness / mg/L CaCO3 = n x 5

where n is the number of drops of added EDTA solution.