pH Curves, Titrations & Indicators (Oxford AQA International A Level Chemistry)

Titrations Calculations

• Titration calculations are used to find the concentration of unknown solutions

• They can also be used to calculate the pH after a given point during a titration

• The steps in a titration are:

  1. Measuring a known volume (usually 20 or 25 cm³) of one of the solutions with a volumetric or graduated pipette and placing it into a conical flask

  2. The other solution is placed in the burette

  3. A few drops of the indicator are added

  4. The tap on the burette is carefully opened and the solution added, portion by portion, to the conical flask until the indicator just changes colour (this is the end point)

  5. Multiple trials are carried out until concordant results are obtained

pH Curves & Indicators

• All pH curves show an s-shape curve

• Halfway along the vertical region of the curve is called the equivalence point and is the point at which neutralisation occurs

Equivalence point → moles of alkali = moles of acid

• From the curves you can:

  • Determine the pH of the acid by looking where the curve starts on the y-axis

  • Find the pH at the equivalence point

  • Find the volume of base at the equivalence point

  • Obtain the range of pH at the vertical section of the curve

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 0 - 2

  • If it is a weak acid the initial pH is about 2-4

  • 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)

• 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

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 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 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 relatively strong base, causing the solution to be basic

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

• 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 the 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

Indicators

• Indicators are substances that change colour when they are added to acidic or alkaline solutions

• When choosing the appropriate indicator, the pH of the equivalence point is very important

• The two most common indicators that are used in titrations are methyl orange and phenolphthalein

Indicator & pH range table

• Both indicators change colour over a specific pH range

  • Methyl orange changes from red to yellow over a pH range of 3.1 - 4.4

  • Phenolphthalein changes from colourless to pink over a pH range of 8.3 - 10.0

Choosing indicators for titrations

Strong acid and strong alkali

• The colour change for both indicators takes place at a pH range that falls within the vertical region of the curve

• Therefore, either indicator can be used

Strong acid and weak alkali

• Only methyl orange will change colour at a pH close to the equivalence point and within the vertical region of the curve 

Weak acid and strong alkali

• Now, only phenolphthalein will change colour at a pH close to the equivalence point and within the vertical region of the curve

• The pH range at which methyl orange changes colour falls below the curve

Weak acid and weak alkali

• Neither indicator is useful, and a different method should be considered