Redox Titrations: MCQ 2

Potassium Permanganate in Redox Titrations

• Potassium permanganate (KMnO₄) is a useful oxidizing agent in redox titrations due to its complete reaction and ability to act as its own indicator, showing a clear color change.
• However, it has disadvantages, such as being unstable and requiring standardization against a standard solution of ammonium iron (II) sulfate prior to titration.

Preparing Ammonium Iron (II) Sulfate Solution

• Ammonium iron (II) sulfate is a primary standard that can be weighed and diluted to make a standard solution.
• Dilute sulfuric acid is added to prevent the iron (II) in the solution from reacting with air to form iron (III).
• The solution is prepared by washing the powder into a beaker of dilute sulfuric acid, stirring well, and then transferring it to a volumetric flask.

Potassium Permanganate Solution

• Potassium permanganate contains Mn in the +7 oxidation state, which is purple in color.
• In acidic conditions, Mn is reduced completely, and no indicators are needed for the titration.
• However, if acidic conditions are not maintained, an intermediate oxidation state will be reached, resulting in a brown color.
• To prevent this, dilute sulfuric acid is added to the KMnO₄ solution.

Titration of Potassium Permanganate with Ammonium Iron (II) Sulfate

• The conical flask contains 25 ml of iron solution and 10 ml of dilute sulfuric acid.
• Prior to titration, the following washings should be carried out: washing the pipette, burette, and conical flask with DI water, rinsing the burette with KMnO₄, and rinsing the pipette with iron (II) solution.
• The end-point is reached when a pale pink color persists.
• The concentration of the potassium permanganate solution can be calculated using the volume of KMnO₄ required for a complete reaction with the ammonium iron (II) sulfate solution.

Potassium Permanganate vs. Ammonium Iron (II) Titration

• The reaction between potassium permanganate and ammonium iron (II) sulfate occurs in a 1:5 ratio.
• The concentration of the potassium permanganate solution can be calculated using the volume of KMnO₄ required for a complete reaction with the ammonium iron (II) sulfate solution.

Potassium Permanganate vs. Iron Tablet Titration

• A standardized solution of potassium permanganate can be used to determine the amount of iron in an iron tablet.
• The KMnO₄ solution must be standardized immediately before the titration since it is unstable.
• The iron tablets are ground up and dissolved in dilute sulfuric acid, and the solution is transferred to a volumetric flask.
• The concentration of the iron solution can be calculated using the volume of KMnO₄ required for a complete reaction with the iron solution.

Iodine vs. Thiosulphate Titration

• This is a redox titration that occurs between iodine and thiosulphate.
• The reaction is used to standardize the thiosulphate solution using a known concentration of iodine.
• DI water must be used during the experiment as tap water contains chloride ions that would displace iodine from iodine solutions.
• The iodine solution is prepared by adding potassium iodate, excess dilute sulfuric acid, and excess potassium iodide to water.
• The colour changes during the reaction are: red/brown prior to titration, pale yellow during titration, and colourless at the end-point.

Application of Iodine-Thiosulphate Reactions

• These titrations can be used to:
  • Find the amount of dissolved oxygen in water (Winkler method).
  • Determine the % hypochlorite in bleach.

Determining % Hypochlorite in Bleach

• The hypochlorite ion (OCl⁻) is the active ingredient in bleaches.
• Household bleach is very concentrated and must be diluted prior to titration.
• The diluted bleach is reacted with excess potassium iodide (KI) in the presence of acid, and the liberated iodine is titrated against a known thiosulphate solution.
• The same colour changes occur as before.
• The concentration of sodium hypochlorite in bleach and the % w/v of hypochlorite in bleach can be calculated using the volume of thiosulphate required for a complete reaction with the iodine solution.