1.7.12 pH Titration Curves

What are pH titration curves

• Titration is a technique used in neutralisation reactions between acids and alkalis to determine the concentration of the unknown solution
• It involves adding a titrant of known concentration from a burette into a conical flask containing the analyte of unknown concentration
• An indicator is added which will change colour at the endpoint of the titration
• The endpoint is the point at which equal number of moles of titrant and analyte react with each other
• The equivalence point is halfway the vertical region of the curve

Equivalence point → moles of alkali = moles of acid

• This is also known as the equivalence point and this is the point at which neutralisation takes place

The diagram shows a pH titration curve where the equivalence point is the point at which equal number of moles of titrant and analyte have reacted

Sketching pH titration curve

• Draw axes with volume added (cm³) on the x-axis and pH on the y-axis

• Draw a horizontal line running parallel to the x-axis at pH 7
  • Everything below this line will be in the acidic region and everything above it in the alkaline region

• Determine which substance is in the conical flask
  • If it is a strong acid the initial pH is about 1 or 2
  • If it is a weak acid the initial pH is about 2-3
  • If it is a strong alkali the initial pH is about 13-14
  • If it is a weak alkali the initial pH is about 11

• Determine what type of acids and alkali are used
  • Strong acid + strong alkali
  • Strong acid + weak alkali
  • Weak acid + strong alkali
  • Weak acid + weak alkali

• Draw the pH titration curve

Strong acid + strong alkali pH titration curve

• Initially there are only H⁺ ions present in solution from the dissociation of the strong acid (HCl) (initial pH about 1-2)
• As the volume of strong alkali (NaOH) added increases, the pH of the HCl solution slightly increases too as more and more H⁺ ions react with the OH^- from the NaOH to form water
• The change in pH is not that much until the volume added gets close to the equivalence point
• The pH surges upwards very steeply
• The equivalence point is the point at which all H⁺ ions have been neutralised (therefore pH is 7 at equivalence point)
• Adding more NaOH will increase the pH as now there is an excess in OH^- ions (final pH about 13-14)

     The diagram shows a pH titration curve of 1.0 mol dm^-3 HCl (25 cm³) with NaOH

• The pH titration curve for HCl added to a NaOH has the same shape
• The initial pH and final pH are the other way around
• The equivalence point is still 7

    The diagram shows a pH titration curve of 1.0 mol dm^-3 NaOH (25 cm³) with HCl

Strong acid + weak alkali pH titration curve

• Initially, there are only H⁺ ions present in solution from the dissociation of the strong acid (HCl) (initial pH about 1-2)
• As the volume of weak alkali (NH₃) added increases, the pH of the analyte solution slightly increases too as more and more H⁺ ions react with the NH₃
• The change in pH is not that much until the volume added gets close to the equivalence point
• The equivalence point is the point at which all H⁺ ions have been neutralised by the NH₃ however the equivalence point is not neutral, but the solution is still acidic (pH about 5.5)
• This is because all H⁺ have reacted with NH₃ to form NH₄⁺ which is a relatively strong acid, causing the solution to be acidic
• As more of the NH₃ is added, the pH increases to above 7 but below that of a strong alkali as NH₃ is a weak alkali

        The diagram shows a pH titration curve of 1.0 mol dm^-3 HCl (25 cm³) with NH₃

• The pH titration curve for strong acid added to a weak alkali has the same shape
• The initial and final pH are the other way around
• The equivalence point is still about 5.5

Weak acid + strong alkali pH titration curve

• Initially there are only H⁺ ions present in solution from the dissociation of the weak acid (CH₃COOH, ethanoic acid) (initial pH about 2-3)
• As the volume of strong alkali (NaOH) added increases, the pH of the ethanoic acid solution slightly increases too as more and more H⁺ ions react with the OH^- from the NaOH to form water
• The change in pH is not that much until the volume added gets close to the equivalence point
• The pH surges upwards very steeply
• The equivalence point is the point at which all H⁺ ions have been neutralised by the OH^- ions however the equivalence points is not neutral, but the solution is slightly basic (pH about 9)
• This is because all H⁺ in CH₃COOH have reacted with OH^- however, CH₃COO^- is a relatively strong base, causing the solution to be basic
• As more of the NaOH is added, the pH increases to about 13-14

The diagram shows a pH titration curve of a weak acid with a strong base

• The pH titration curve for weak acid added to a strong alkali has the same shape
• The initial and final pH are the other way around
• The equivalence point is still about 9

Weak acid + weak alkali pH titration curve

• Initially there are only H⁺ ions present in solution from the dissociation of the weak acid (CH₃COOH, ethanoic acid) (initial pH about 2-3)
• In these pH titration curves, there is no vertical region
• There is a ‘point of inflexion’ at the equivalence point
• The curve does not provide much other information

The diagram shows a pH titration curve of 1.0 mol dm^-3 weak acid (25 cm³) with weak alkali

Answer

• The curve starts at pH 5.3
  • Mark on graph
• The volume of NaOH of added to reach the vertical section of the graph = 15.0 cm³
  • Vol acid x Concentration acid = Vol base x Concentration base
  • 10 x 0.150 = Vol base x 0.100
  •  = 15.0 cm³
  • There is no mark for the height of the vertical section, but the equivalence point must be above pH 7 for a weak acid - strong base titration
• The curve finishes at pH = 12.5 at 20 cm³.
  • Make sure the graph does not go above pH 12.5
    • This is the maximum pH value given in the question
  • Make sure that the volume does not exceed 20 cm³
    • This is the maximum volume of base added given in the question