Titration

  1. A simple technique for determining the unknown concentration of a solution is to react it with another solution of known concentration until all of the first solution has completed reacting.
  2. However, in order for this to work, you must overcome one problem - How do you know when you have added enough of one solution to the other such that the reaction is complete?
  3. For neutralization reactions (reaction between acids and bases) this is relatively easy because there are a number of substances which change color depending on the pH of the solution in which they are dissolved. Phenolphthalein is the substance we will use because it is colorless in acidic solution and bright pink in basic solution. Below is the colorless version:
    Phenolphthalein (colorless form)
  4. The phenolphthalein molecule changes slightly depending on the pH of the solution it is in.
    Colorless Phenolphthalein
    pH < 7
    Pink Phenolphthalein
    pH > 7
  5. If we add a few drops of phenolphthalein to an acidic solution, it should indicate (turn pink) the end of the reaction when all of the acid has been neutralized and extra base is added.
  6. Consider the following reaction:
    H2SO4(aq) + 2 NaOH(aq) --->2 H2O(l) + Na2SO4(aq)
  7. When we first add base, it reacts with some of the acid, forming water and sodium sulfate. But if not enough base is added the overall solution will still be acidic. Eventually, enough base has been added so that the only substance left in the beaker are water and sodium sulfate, giving you a neutral solution (pH 7). Once another drop of base is added the solution becomes basic. If phenolphthalein is present, a pink color will appear.
  8. Titration calculations are the same as any other type of stoichiometry problem. Using the equation above calculate the concentration of the unknown base if 35.0 mL of NaOH reacted with 53.0 mL of 0.25 M sulfuric acid?

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