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 NH_3?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 NH_3?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