2.5.5

pH Curves & Titrations (A2 Only)

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Key Titration Techniques

Here's a quick recap of key titration information.

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Glassware

  • Above are labelled images of a burette and a pipette.
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Technique

  • In a titration, you want to calculate the concentration of an acid or base by reacting it with a known amount of base or acid. To do this:
    • Add a measured amount of one to the other.
    • Use a balanced equation to calculate the concentrations.
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Accuracy

  • In a titration, you want to make sure you have accurate results.
  • Accurate results are achieved by repeating your titration until you get concordant results.
    • Concordant results are values that are within 0.1 cm3 of each other.
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Indicators

  • To find the endpoint of a titration, you use an indicator.
    • An indicator changes colour when all of the unknown reactant is used up.

Strong Acid/Base Titrations

Titrations of a strong acid with a strong base are the simplest to understand.

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pH curve

  • The pH curve pictured shows the change in pH when a strong acid is titrated with a strong base.
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The beginning

  • In the beginning, you can see that the pH is very low, and doesn’t change very quickly.
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The middle

  • In the middle of the graph, there is a sharp change and the pH rises very rapidly. This is known as the equivalence point.
    • At the equivalence point, the acid is just about neutralised. Adding any more base causes a rapid shift to a basic solution.
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The end

  • The curve ends at a very high pH because the solution has excess base present.
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Indicator

  • When doing an acid/base titration, you need to use an indicator to find the equivalence point.
  • A good indicator will change colour dramatically over a small pH range.
    • If the small pH range includes the range of the equivalence point, it’s a great indicator for the reaction.
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Examples of good indicators

  • The two main indicators are phenolphthalein and methyl orange.
    • Phenolphthalein changes from colourless to pink from pH 8.3 to pH 10.
    • Methyl orange changes colour from red to yellow between pH 3.1 and pH 4.4.
  • Either of these are well suited for a strong acid/base titration.

Calculations of Titrations

We have seen before how to carry out titration calculations using titre values and balanced equations. We shall recap these calculations.

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Information provided

  • Before starting your calculation you will be given:
    • The balanced equation for the reaction.
    • The volume and concentration of the titre.
    • The volume of the other reactant.
  • You will, most often, be asked to find the concentration of this other reactant.
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First step

  • Calculate the moles of the titre using the two pieces of information we know about the titre: the volume and the concentration. Use the equation:
    • Moles = volume x concentration
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Second step

  • Find the number of moles of the other component:
    • A titration is stopped at the exact point of neutralisation so we can say that the mole ratio will be the same as in the balanced equation.
  • Examples:
    • If the mole ratio (from equation) is 1:1, the component will have the same number of moles as the titre.
    • If the mole ratio (from equation) is 2:1, the component will have the twice as many moles as the titre.
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Third step

  • Find the concentration of the other component:
  • We can rearrange the equation:
    • Moles = concentration x volume
  • To get:
    • Concentration = moles ÷ volume
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Units

  • Units are very important in titration calculations!
    • Moles → mol
    • Volume → dm3
    • Concentration → moldm−3
  • Forgetting to convert the volume from cm3 to dm3 is where most people make mistakes!
    • 1 cm3 = 0.001 dm3

Jump to other topics

1Physical Chemistry

2Physical Chemistry 2 (A2 Only)

3Inorganic Chemistry

4Inorganic Chemistry 2 (A2 Only)

5Organic Chemistry 1

6Organic Chemistry 2 (A2 Only)

6.1Optical Isomerism (A2 Only)

6.2Aldehydes & Ketones (A2 Only)

6.3Carboxylic Acids & Esters (A2 Only)

6.4Aromatic Chemistry (A2 Only)

6.5Amines (A2 Only)

6.6Polymers (A2 Only)

6.7Biological Organic (A2 Only)

6.8Organic Synthesis (A2 Only)

6.9NMR Spectroscopy (A2 Only)

6.10Chromatography (A2 Only)

6.11A-A* (AO3/4) - Organic 2

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