1.10.5 pH Curves, Titrations and Indicators

1.10.5 pH Curves, Titrations and Indicators: Interpret and sketch pH curves for the four acid–base combinations, calculate titration results, and select a suitable indicator.

The curve shows how pH changes as titrant is added to the solution in the flask.

Shapes of pH Curves

Strong acid + strong base

Starts at low pH, rises steeply through pH 7 with a vertical section of about pH 3 to 11, and finishes at high pH.

Weak acid + strong base

Starts at a higher pH (about 3), shows a buffering region, and the equivalence point is above 7.

Strong acid + weak base

Starts at low pH, has a steep section ending near pH 7, and the equivalence point is below 7.

Weak acid + weak base

Has no vertical section, so there is no sharp end point and no simple indicator works.

Titration	pH at equivalence	Suitable indicator
Strong acid + strong base	7	Methyl orange or phenolphthalein
Weak acid + strong base	above 7	Phenolphthalein
Strong acid + weak base	below 7	Methyl orange
Weak acid + weak base	no sharp change	None

An indicator is a weak acid that changes colour as the pH crosses its range.

Methyl orange

Red in acid and yellow in alkali over about pH 3.1 to 4.4.

Phenolphthalein

Colourless in acid and pink in alkali over about pH 8.3 to 10.0.

Choice

The colour change range must lie entirely within the steep part of the pH curve.

Few drops

A small amount of indicator is used so that it does not react with a significant amount of titrant.

Observation

State the colour at the end point and say that it must persist for a few seconds.

Write the balanced equation, such as CH₃COOH + NaOH → CH₃COONa + H₂O, to find the mole ratio.

Calculate moles of the solution in the burette from concentration × volume ÷ 1000.

Use the equation ratio to find moles of the solution in the flask.

Divide by the volume in dm³ of the flask solution to find its concentration.

Use concordant titres within 0.10 cm³ to calculate the mean titre.

Question: In a titration 25.0 cm³ of ethanoic acid required 22.50 cm³ of 0.100 mol dm⁻³ NaOH. Calculate the concentration of the acid and choose an indicator.

Moles NaOH = 0.100 × 22.50 ÷ 1000 = 2.25 × 10⁻³ mol.
CH₃COOH and NaOH react 1:1, so moles acid = 2.25 × 10⁻³ mol.
Concentration = 2.25 × 10⁻³ ÷ 0.0250 = 0.0900 mol dm⁻³.
A weak acid with strong base has an equivalence point above 7, so phenolphthalein is suitable.

Answer: The acid concentration is 0.0900 mol dm⁻³ and the indicator is phenolphthalein.

1.10.5 – Key Points for the Exam

Equivalence point: The point at which the reactants have been added in exactly the ratio shown by the equation.
End point: The point at which the indicator changes colour.
Concordant titres: Titres that agree within 0.10 cm³.
Buffer region: The relatively flat section of a weak acid curve where pH changes slowly.

Common Mistakes

Assuming that the equivalence point is always at pH 7.
Choosing an indicator by its colours rather than by whether its range lies within the steep section.
Using the mean of all titres including a rough titre instead of only concordant ones.
Forgetting to convert cm³ to dm³ when calculating concentration.