22.1    Acid-base Titrations

Titration curves

• Titration curves or pH curves are curves that show the pH changes during an acid-base titration.

• Titration curve of a strong acid and strong base titration, take hydrochloric acid and sodium hydroxide as an example:
  1. Running the acid into alkali:
     • pH changes by a small amount until the equivalence point, that is, the point where the solutions have been mixed in exactly the right proportions according to the equation.
     • This is followed by a sharp decrease in pH when small amount of acid is added.
  2. Running the alkali into acid:
     • This is the same as above except the curves starts at pH 0, because the curve shows the pH in the conical flask

• Titration curve of a strong acid and weak base titration, take hydrochloric acid and ammonia as an example:
• Running the acid into alkali:
  • The curve starts at pH 12 because a weak base is present
  • Initially, the pH falls As more acid is added, the curve becomes less steep because a buffer solution composed of excess ammonia and ammonium chloride is set up. Buffer solutions resist changes pH when small amount of acid is added.
  • The equivalence point now is a bit acidic
  • When excess acid is added, the curve is the same as before
• Running the alkali into acid:
  • The beginning of curve is the same as before
  • After the equivalence point(which is also a bit acidic), the pH changes by a small This is because a buffer solution composed of excess ammonia and ammonium chloride is set up. Buffer solutions resist changes in pH when small amount of  alkali is added.

• Titration curve of a weak acid and strong base titration, take ethanoic acid and sodium hydroxide as an example:
  1. Running the acid into alkali:
     • The beginning of curve is the same as before
     • After the equivalence point(which is a bit alkaline), the pH changes by a small This is because a buffer solution composed of excess sodium ethanoate and ethanoic acid is set up. Buffer solutions resists pH changes when small amount of acid is added.
  2. Running the alkali into acid:
     – The curve starts at pH 3 because a weak acid is present
     – Initially, the pH increases As more alkali is added, the curve becomes less steep because a buffer solution composed of excess sodium ethanoate and ethanoic acid is set up. Buffer solutions resist changes pH when small amount of alkali is added.
     – The equivalence point now is a bit alkaline
     – When excess alkali is added, the curve is the same as before

• Titration curve of a weak acid and weak base titration, take ethanoic acid and ammonia as an example:
  1. 1. Running the acid into alkali
        • The curve starts at pH 12 because a weak base is present
        • Initially, the pH falls As more acid is added, the curve becomes less steep because a buffer solution composed of excess ammonia and ammonium ethanoate is set up. Buffer solutions resist changes pH when small amount of acid is added.
        • There is no sharp decrease in pH at any volume
     2. Running the alkali into acid
        – The curve starts at pH 3 because a weak acid is
        – Initially, the pH increases As more alkali is added, the curve becomes less steep because a buffer solution composed of excess ammonium ethanoate and ethanoic acid is set up. Buffer solutions resist changes pH when small amount of alkali is added.

  

• Titration curve of a polyprotic acid and base titration, take ethanedioic acid and sodium hydroxide as an example:
• Running the alkali into acid:
  • Ethanedioic acid is a diprotic acid, it donates the two protons in two This is because one proton is more easier to remove than the other.
  • The curve will therefore show two sharp increase in pH

 

Acid-base indicator

 

• An indicator is a substance that changes colour as the pH of the solution which it dissolves changes

 

• In an acid-base titration, an indicator is used to mark the end point of the titration, that is, the point where the indicator changes colour

 

• Most indicators are weak acids. They have an acid colour and a base colour

• Consider a general indicator with the formula HIn. HIn has a different colour from In⁻. The colour of  HIn is called the acid colour while the colour of  In⁻ is called  the  base colour.
  HIn ⇌ H⁺ + In⁻

1. When acid is added, the equilibrium position shifts to the left due to the increasing concentration of H⁺ The indicator exists predominantly as HIn. Hence, the solution shows the acid colour.
2. When base is added, the equilibrium position shifts to the right due to the removal of H⁺ ions. The indicator exists predominantly as In⁻. Hence, the solution shows the base colour

• Take methyl orange as an example, it has an acid colour of red and a base colour of yellow
  1. When acid is added, the equilibrium position shifts to the left and the solution looks red
  2. When base is added, the equilibrium position shifts to the right and the solution looks yellow

 

Importance of pftIn

• Since the indicator is a weak acid, an expression of fta can be written for it. However, the fta now is called ftIn.

• As acid/base is added, the colour changes. At half-way through the colour changes, there will be equal amount of HIn and In⁻ present. The equilibrium expression now becomes:

• This means that the end point depends entirely on the pftIn of the At pH = pftIn, the indicator is changing its colour.

 

• However, the indicator usually changes its colour over a range of pH, usually around pKIn ± 1, this is called the pH range of am indicator
• Some common examples:

 

Selecting a suitable indicator for titration

 

• An indicator should be chosen such that it changes colour close to the equivalence point of the titration to give an accurate titration result

 

• The indicator therefore should have a pH range close to the equivalence point of the titration

 

• A guide to choose a suitable indicator:

 

Finding pKa  from titration curves

 

• Suppose during an acid-base titration, 25 cm³ of alkali is required to neutralise a weak acid. Therefore at half-neutralisation, half-volume of the alkali has been added, that is 12.5 cm³.

 

• Half of the acid has been neutralised and half of the salt has been formed. Therefore, [HA] = [A⁻].

 

• According to the formula, log([HA]/[A⁻]) = 1 and pfta = pH.

 

• Conclusion is, pKa is the pH of the solution at half-neutralisation.