pH Titration Curves (Cambridge (CIE) A Level Chemistry): Revision Note

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 the indicator changes colour

• The equivalence point is halfway along the vertical region of the curve

Equivalence point → moles of alkali = moles of acid

• The equivalence point is the point at which equal numbers of moles of titrant and analyte have reacted, and is the point at which neutralisation takes place; a well-chosen indicator changes colour at the endpoint very close to it

Example pH titration curve

Sketching a 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 acid 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 the 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, the pH is 7 at the equivalence point

• Adding more NaOH will increase the pH as now there is an excess in OH^- ions (final pH about 13-14)

pH titration curve for a strong acid + strong alkali

• 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

pH titration curve for a strong alkali + strong acid

Strong acid + weak alkali pH titration curve

• Initially, there are only H⁺ ions present in the 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 weak acid that partially dissociates to release H⁺, 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

pH titration curve for a strong acid + weak alkali

• 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 the 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 point 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 weak base that partially reacts with water to form OH^-, causing the solution to be basic

• As more of the NaOH is added, the pH increases to about 13-14

 pH titration curve for a weak acid + strong alkali

• 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, the solution contains mostly undissociated weak acid, with a low concentration of H⁺ ions from its partial dissociation (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

pH titration curve for a weak acid + weak alkali