pH curves and Titrations

• • pH meters have a probe that you put into your solution and a digital display that shows the reading for the pH of a solution.
  • Must be calibrated before use.
• Calibrating a pH meter:

  1. Place the bulb of the pH meter into distilled water and allow the reading to settle to pH 7, then shake to remove the excess water.
  2. Place the tip of the pH probe in pH 4 buffer solution, ensuring the bulb is fully immersed and allow to sit until the pH stabilises.
  3. Do the same with a standard solution of pH 10.0.
  4. Make sure you rinse the probe with distilled water in-between each reading.
  5. Make sure to store the data.
• pH curves

  • pH curves show the results of titration experiments.
  • They can be made by plotting the pH of the titration mixture against the amount of base added as the titration goes on.
  • The pH of the mixture can be measured using a pH meter and the scale on the burette can be used to see how much base has been added.
• Strong acid and strong base

  The pH starts around 1, as there's an excess of strong acid.

  It finishes up around pH 13, when you have an excess of strong base.

  
• Strong acid and weak base

  The pH starts around 1, as there's an excess of strong acid.

  It finishes up around pH 9, when you have an excess of weak base.

  
• Weak acid and strong base


  The pH starts around 5, as there's an excess of weak acid.

  It finishes up around pH 13, when you have an excess of strong base.

  
• Weak acid and weak base

  The pH starts around 5, as there's an excess of weak acid.

  It finishes up around pH 9, when you have an excess of weak base.

  
• Why each graph has a particular shape (1):

  • The initial pH depends on the strength of the acid. So a strong acid titration will start at a much lower pH than a weak acid titration.
  • To start with, addition of small amounts of base have little impact on the pH of the solution.
  • All the graphs (apart from the weak acid/weak base graph) have a bit that's almost vertical -the mid-point of this vertical section is the equivalence point or end point.
  • At this point [H+]≈[OH-] — all the acid is just neutralised.
  • When this is the case, a tiny amount of base causes a sudden, big change in pH.
• Why each graph has a particular shape (2):


  • You need to add more weak base than strong base to a strong acid to cause a ph change, and the change is less pronounced. On the other hand, you need to add less strong base to a weak acid to see a large change in pH.
  • The final pH depends on the strength of the base — the stronger the base, the higher the final pH.
• Indicators

  • You can use titrations instead of pH meters to work out the concentration of an acid or base.
  • You'll need an indicator that changes colour to show you when your sample has been neutralised.
  • Indicators can be thought of as weak acids that partially dissociate in aqueous solution.
  • They work because they have differently coloured conjugate pairs. As the pH of the solution changes during a titration, the equilibrium concentrations of the conjugate pairs will also change. The colour will change depending on whether the indicator is mainly protonated or deprotonated.

  
• In an acid:

  • Increase of [H+] equilibrium moves to the left to give a red undissociated form.

  In an alkali:

  • Increase of [OH-]
  • OH- ions remove H+ ions to form water
  • Equilibrium will move to the right to produce a blue colour

  
• How to choose the right indicator:

  • You need to pick an indicator that changes colour over a narrow pH range that lies entirely on the vertical part of the pH curve.

  ”The range of the indicator coincides with the pH change during the sharp rise”
• Methyl orange and Phenolphthalein:

  • For a strong acid/strong alkali titration, you can use either of these indicators.
  • For a strong acid/weak alkali only methyl orange will do. The pH changes rapidly across the range for methyl orange, but not for phenolphthalein.
  • For a weak acid/strong alkali, phenolphthalein is the one to use. The pH changes rapidly over phenolphthalein's range, but not over methyl orange's.
  • For weak acid/weak alkali titrations there's no sharp pH change, so no indicator will work.