Selecting Indicators

Learning Objectives (OCR Spec 5.1.3)

By the end of this lesson you should be able to:

• Explain how an acid-base indicator works in terms of its own Ka
• Define pKIn and relate it to the colour-change range
• Select an appropriate indicator for a given titration from its pKIn and the titration curve shape
• Recognise why universal indicator is unsuitable for titration endpoints
• Link indicator choice to the shape of the pH titration curve
• Discuss neutralisation enthalpy in context

What Is an Indicator?

An acid-base indicator is a weak acid (or weak base) whose conjugate forms have different colours. Writing the indicator as HIn:

HIn(aq) <=> H+(aq) + In-(aq) colour A colour B

HIn has one colour (colour A), In- has another (colour B). The equilibrium position depends on [H+]:

• In acidic solution (high [H+]), Le Chatelier pushes the equilibrium to the left -> HIn predominates -> colour A.
• In alkaline solution (low [H+]), the equilibrium shifts to the right -> In- predominates -> colour B.

The indicator therefore changes colour as pH passes through a particular range.

Quantifying the Change: KIn and pKIn

The indicator has its own dissociation constant:

KIn = [H+][In-] / [HIn]

The colour is most obviously mixed when [HIn] = [In-]. At that point:

KIn = [H+] -> pH = pKIn

So the midpoint of the colour change is at pH = pKIn. The eye detects the dominant colour once one species is about 10x more abundant than the other, so the practical colour-change range is approximately:

pKIn - 1 <= pH <= pKIn + 1

A typical indicator therefore changes colour over about 2 pH units.

Common Indicators

Indicator	pKIn (approx)	Colour change range	Colour acid -> alkali
Methyl orange	3.7	3.1 - 4.4	red -> yellow
Bromophenol blue	4.0	3.0 - 4.6	yellow -> blue
Bromocresol green	4.7	3.8 - 5.4	yellow -> blue
Methyl red	5.1	4.4 - 6.2	red -> yellow
Bromothymol blue	7.0	6.0 - 7.6	yellow -> blue
Phenol red	7.9	6.8 - 8.4	yellow -> red
Phenolphthalein	9.3	8.3 - 10.0	colourless -> pink
Thymolphthalein	9.9	9.3 - 10.5	colourless -> blue

You are not required to memorise the full table, but you should know methyl orange and phenolphthalein in detail and be able to use a supplied table.

Matching Indicators to Titration Curves

The rule is simple: the pKIn (and the full range) must fall entirely within the vertical jump of the titration curve. If the range overlaps the jump, the colour change happens at (or near) the equivalence point and the endpoint is sharp.

Strong acid + strong base

The vertical jump spans roughly pH 3.5 to 10.5. Both methyl orange (3.1-4.4) and phenolphthalein (8.3-10.0) fall within this range, so either indicator works. In practice phenolphthalein is preferred for visual clarity (colourless -> bright pink).

Weak acid + strong base (e.g. CH3COOH + NaOH)

Equivalence is above pH 7 (~8.7 for ethanoic acid). The vertical jump spans roughly pH 7 to 11. Phenolphthalein (8.3-10.0) falls within this jump and is the correct choice. Methyl orange (3.1-4.4) would change colour far too early, during the buffer region when barely any NaOH has been added - it is unsuitable.

Strong acid + weak base (e.g. HCl + NH3)

Equivalence is below pH 7 (~5.3 for NH3). The vertical jump spans roughly pH 3 to 7. Methyl orange (3.1-4.4) falls within and is the correct choice. Phenolphthalein (8.3-10.0) would change much too late - unsuitable.

Weak acid + weak base

There is no vertical jump. No indicator can cover the gradual curve because the colour change would spread over a wide volume range, giving no sharp endpoint. For such analyses, chemists use a pH meter instead.

Worked Example 1: Choosing an Indicator

You are titrating 25.0 cm3 of 0.100 mol dm-3 ethanoic acid with 0.100 mol dm-3 NaOH. Which indicator should you use?

The equivalence point is at pH ~8.72 (lesson 9). The vertical jump runs from about pH 7 to pH 11. An indicator with pKIn ~ 9 and range roughly 8-10 is needed. Phenolphthalein (range 8.3-10.0) is ideal.

Worked Example 2: Explaining Why Methyl Orange Does Not Work

Why is methyl orange unsuitable for a weak acid + strong base titration?

Methyl orange changes colour between pH 3.1 and 4.4. In a weak acid / strong base titration the pH at equivalence is above 7; the region around 3-5 is in the buffer region of the curve, where the pH changes only slowly. The colour change would be gradual and would happen when only a small fraction of the NaOH had been added - long before the equivalence point. The endpoint would be premature and inaccurate.

Worked Example 3: Strong Acid + Weak Base

You are titrating 25.0 cm3 of 0.100 mol dm-3 HCl with 0.100 mol dm-3 NH3. Which indicator should you use?

Equivalence is at pH ~5.28. The vertical jump runs from about pH 3 to pH 7. Methyl orange (3.1-4.4) falls within this region and is the correct choice. Phenolphthalein (8.3-10.0) changes long after equivalence - unsuitable.

Why Universal Indicator Is Not Used for Endpoints

Universal indicator is a mixture of dyes that gives a different colour for each pH unit between about 2 and 12. It is wonderful for estimating the pH of a solution to the nearest unit or two, but it does not give a sharp colour change at any specific pH. In a titration it would drift gradually through the rainbow with no definite endpoint.