Titration curves

In this lesson, we will learn:

• To recall the types of titration curves and their general shapes.
• How to choose an appropriate indicator for the type of titration being performed.

• Titration is an important technique in finding the concentration of a chemical sample.
  Depending on what combination of strong/weak acid/base you are using, when plotting volume of added titre (the chemical with known concentration) against pH from a titration run, you will get distinct graph shapes.
  
  
• You need to be careful with the descriptions of a titration: the chemical with unknown concentration is being titrated by the titer, which is the chemical with known concentration.
  For example in the sentence “The titration of chemical A by chemical B”, A is the sample (with unknown concentration) in the beaker that you want to investigate, and B is the known chemical being added from the burette.
  
  
• The simplest titration curve is a titration of a strong acid/base by a strong base/acid. The following phases occur in order as titer volume increases:
  • An initial ‘horizontal’ phase of extremely little change in pH as titer volume increases.
  • A ‘vertical’ phase when close to the equivalence point, where a very small addition of titer causes a large change in pH. Around the equivalence point, pH changes nearly ‘vertically’ on the graph (rising in acid titrated by base and dropping in base titrated by acid) as opposed to nearly ‘horizontally’ outside this region. The equivalence point in a strong acid and strong base titration, whichever way around, should occur at pH 7.
  • A second horizontal phase with a levelling-off, like the beginning, after the equivalence point.
  • See the graph below, where:
    • V_a / V_b = volume of acid or base required to reach the equivalence point:
  • The titration of a weak acid by a strong base has a graph with these phases in sequence as titer volume increases:
    • An initial rise in pH seen with the first small amounts of titer,
    • A horizontal levelling off phase (like in strong acid by strong base), where very little pH change occurs. This is known as the buffer region, where pH is staying nearly constant due to the weak base equilibria being disturbed and re-established as per Le Chatelier’s principle.
    • A nearly vertical rise near the equivalence point, like the curve with strong acid/strong base titrations. The equivalence point in these titrations will occur at a pH greater than 7 (basic conditions).
    • Another near horizontal level-off after the equivalence point where very little pH change occurs.
    
    See the graph below, where:
    • V_B = volume of strong base required to reach equivalence point.
    • V_1/2 = half the value of V_B
    • pH_1/2 = pH at V_1/2:
  • The titration of a weak base with strong acid has all the same features as the curve of a weak acid with strong base except it is has been turned upside down
    • An initial drop in pH seen with the first small amounts of titer,
    • A horizontal levelling off phase, where very little pH change occurs with titer being added. This is known as the buffer region, where pH is staying nearly constant due to the weak base equilibria being disturbed and re-established as per Le Chatelier’s principle.
    • A nearly vertical pH drop around the equivalence point, similar to the curve with strong acid/strong base titrations. The equivalence point in these titrations will occur at a pH lower than 7 (acidic conditions).
    • Another near horizontal level-off after the equivalence point where very little pH change (a very slow drop) occurs.
    
    See the graph below, where:
    • V_A = volume of strong acid required to reach equivalence point.
    • V_1/2 = half the value of V_A
    • pH_1/2 = pH at V_1/2:

• The V_a / V_b measurements, along with can be used to find the concentration of the unknown acid
  
  
• Knowing which type of titration you’re doing is important for your choice of pH indicator. When performing a titration you will need to use an indicator that has a ‘pH range’ that changes color through the equivalence point. Depending on whether you have a strong acid and base or strong/weak acid/base, this gives you specific options.
  • Titrating a strong acid with a strong base needs an indicator with a range covering pH 7, for example bromothymol blue which has a pH range of 6.0 – 7.6. This ensures the equivalence point (where a massive change in pH occurs with minimal titer being added) will be flagged by the color.
  • Titrating a weak acid with a strong base means the equivalence point will be at a pH above 7, so pick an indicator with a pH range above 7. Phenolphthalein is a very good choice for titrating acids with strong bases as it has a pH range from 8.2-12.
  • Titrating a weak base with a strong acid means your equivalence point will be at a pH below 7, so you need an indicator with a pH range below 7 too. Methyl red is a good choice with a pH range of 6.2-4.4.