Aim: To compare the concentration of chloride ions in pool and tap water respectively.
Background: Tap water contains a variety of dissolved ions such as Ca2+, Mg2+, Na+, HCO3- and Cl-. In this experiment we will determine the concentration of the Cl- ions, and comparing it to the concentration found in a sample of pool water. The method used is the standard one for determining the concentration of chloride ions: titration with silver nitrate solution of known concentration. Silver ions form insoluble silver chloride when added to a solution containing chloride ions:
Ag+(aq) + Cl-(aq) -> AgCl(s)
By adding silver ions until silver chloride is no longer precipitated, the amount of chloride in a solution can be found.
Potassium chromate (VI), K2CrO4, can be used to indicate the end-point of the titration, the point at which all chloride ions have been precipitated. Silver ions combine with chromate (VI) ions to form a red precipitate of silver chromate (VI):
2Ag+(aq) + CrO2-4(aq) -> Ag2CrO4(s)
When both chloride ions and chromate (VI) ions are present, however, no silver chromate (VI) is precipitated until all the chloride ions have been removed. The sudden appearance of red silver chromate (VI) therefore indicates the end-point of the titration.
Hypothesis: The concentration of chloride ions in pool water should be significantly higher than the concentration of chloride ions in tap water.
Independent: Water sample
Dependent: Concentration of chloride ions present in solution. Controlled: Amount of each water sample used, concentration of silver nitrate, room temperature and pressure.
A burette was filled with Silver Nitrate solution of concentration 0.02 mol dm-3. 0.01 dm-3 of tap water was pipetted into a conical flask. 10 drops of potassium chromate (VI) indicator were added to the flask. The initial...