6.1.14Additional lessons
Acid-Base Indicators (HL) — IB Chemistry HL Revision Notes & Topical Questions
5 min read 4 parts Lesson 128 of 172
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 4How an Indicator Works
The colour change of an indicator is a result of a shift in a weak acid equilibrium.
Weak acid
- Weak acid An indicator is a weak acid, written HIn, whose molecules and conjugate base ions have different colours.
Equilibrium
- Equilibrium The equilibrium is HIn(aq) ⇌ H⁺(aq) + In⁻(aq), with colour A on the left and colour B on the right.
In acid
- In acid Added acid raises [H⁺] and moves the equilibrium to the left, so the colour of HIn is seen.
In alkali
- In alkali Added alkali removes H⁺ and shifts the equilibrium to the right, so the colour of In⁻ is seen.
Part 2 of 4The Working Range
The Working Range
Definition
- Definition Kin = [H⁺][In⁻] / [HIn], and pKin = −log₁₀ Kin.
Mid-point
- Mid-point At pH = pKin, [HIn] = [In⁻] and the colour is a mixture of the two forms.
Range
- Range The colour change takes place over about pKin ± 1 pH units, because one form must be roughly ten times the other to dominate.
Narrow
- Narrow A good indicator changes colour sharply over a small pH range, ideally with one drop of titrant.
| Indicator | Colour in acid | Colour in alkali |
|---|---|---|
| Methyl orange | Red | Yellow |
| Bromothymol blue | Yellow | Blue |
| Phenolphthalein | Colourless | Pink |
Part 3 of 4Using Indicators
1
Add only a few drops to the solution so that the indicator does not affect the pH being measured.
2
The indicator changes colour when the pH is within its working range, which is close to its pKin.
3
Methyl orange changes in an acidic range, at about pH 3 to 4.5, so it is suited to strong acid with weak base titrations.
4
Phenolphthalein changes in an alkaline range of about pH 8 to 10, and bromothymol blue near neutral, at about pH 6 to 7.6.
5
Universal indicator is a mixture of indicators, giving a range of colours across the pH scale but not a sharp end point.
Part 4 of 4Exam Checklist & Tips
6.1.14 – Key Points for the Exam
- Indicator A weak acid or base whose conjugate forms have different colours.
- pKin The −log₁₀ of the acid dissociation constant of the indicator.
- Working range The pH interval, roughly pKin ± 1, over which the colour changes.
- End point The point at which the indicator changes colour.
Common Mistakes
- Saying the indicator changes colour at exactly pH 7.
- Using large quantities of indicator, which can alter the amount of acid or base present.
- Thinking that the colour changes at one pH rather than over a range.
- Writing the equilibrium with the colours the wrong way round.
Last updated:
\n