Coulometric Titrations

December 6, 2003

• Titration is a common analytical technique
  • a reagent is added, usually with a buret
  • the reagent is called the titrant
  • the reagent consumes the analyte
    • the species we are analyzing for
  • addition continues until the analyte is completely consumes
  • we then determine the amount of analyte
    • by determining how much of the titrant we added
    • usually by multiplying the volume delivered by the concentration of the titrant

• There are advantages in generating the titrant electrically
  • we don't need carefully standardized solutions of reagents
  • we don't need to worry if an old solution of still the same concentration
  • we can deal with much smaller or much more dilute samples

• We will titrate thiosulfate ion, S₂O₃^2-
  • the titrant is I₂ (iodine)
  • We will produce the iodine by the electrolysis of Iodide ion
  • It then reacts I₂ + 2 S₂O₃^2- ----> 2 I- + S₄O₆^2-

• The apparatus consist of
  • a. A DC electrical power supply
  • b. a clock that records the time the current is on
  • c. a meter that measures the electrical current
  • d. a cell that contains
    • 1. the sample to be analyzed
    • 2. potassium iodide
    • 3. a phosphate ion buffer (the pH must be controlled)
    • 4. a Pt electrode that generates I2
    • 5. A second electrode (shielded so it can't react with the I2)
    • 6. starch (turns blue if there is a slight excess of I2)
  • e. a micropipet for introducing the thiosulfate samples

• Note-- many samples can be titrated in the same solution and we determine the endpoint visually

• we usually begin by titrating the solution to a faint blue color
  • we then add the sample and the color clears
  • we turn on the power and titrate until the faint blue color reappears
• Overall, the stoichiometry is 1:1 (2 electrons per I2 and 2 thiosulfates per iodine... or one electron per thiosulfate)

example: the sample is 350. microliters

titration takes 1 minute and 17 seconds at a current of 90.2 milliamperes

[S2O3^2-] = { 0.0902 amp x 77 seconds /96500 amp-sec/mol } / 350.0 x 10^-6 liters = x.xxx Moles/liter

Many laboratories generate H+ and OH- electrochemically for the titration of acid and base