Title: Lesson 12: Indicators
Learning Objectives:
– Understand how indicators work
– Determine suitable indicators for a reaction
– Make indicators from a range of natural products
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
A 25.0 cm3 solution of a weak
monoprotic acid, HA(aq), is
titrated with 0.155 mol dm–3
sodium hydroxide, NaOH(aq),
and the following graph is
obtained.
a)
b)
c)
Determine the pH at the
equivalence point.
Explain, using an equation, why
the equivalence point is not at
pH = 7.
Estimate, using data from the
graph, the dissociation
constant, Ka, of the weak acid,
HA, showing your working.
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Indicators
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An indicator is a compound whose colour depends on pH
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For example:

Phenolphthalein


pH>10: PINK
Methyl orange


pH< 8: COLOURLESS
pH<3.2: RED
pH>4.4: ORANGE
Bromothymol blue:

pH<6.0: YELLOW
pH>7.6: BLUE
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Most indicators change colour only once (sometimes twice). The obvious exception is universal
indicator which is actually a mixture of several indicators with different colour change ranges.

Note: the pH generally changes over a small range, but the logarithmic nature of pH means often
equates to a single drop.
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How Indicators Work (what you need):
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Indicators are weak acids/bases in their own right
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In solution indicators form an equilibrium:

Where: ‘In’ stands for indicator
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The undissociated and dissociated forms have different colours

As [H+] changes, the equilibrium moves to the left or right, thus
changing the colour

By applying Le Chatelier’s principle, we can predict how this
equilibrium will respond to a change in the pH of the medium:


Increasing [H+]: equilibrium will shift to the left in favour of HIn (Colour A –
low pH)
Decreasing [H+]: equilibrium will shift to the right in favour of In- (Colour B –
High pH)
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Example – Methyl Orange


Red in acid (Decrease in pH)
Yellow in Alkali (Increase in pH)
Weak Acid
Conjugate Base
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Indicators change colour when the ppH is equal to their
pKa

What determines the pH at which this occurs for an indicator?

Consider the equilibrium expression:

When the equilibrium is balanced between the acid and the
conjugate base, [In-] = [HIn], the indicator is in the middle of its
colour change.

Expression becomes simplified as

Any small addition of acid or base will shift the equilibrium and
the colour will change  this is the end point
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Indicators and pH curves
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© Boardworks Ltd 2010
pH range and pKa
Different indicators have
different pKa values.
Hence different end
points and colour change
at different pH values.

Most indicators change range within ±1.0 of their pKa

pKa data for indicators can be found in section 22 of your data booklet
Indicator
pKa
pH
Range
methyl orange
3.46
3.2–4.4
Red
Yellow
bromophenol blue
4.10
3.0–4.6
Yellow
Blue
bromocresol green
4.90
3.8–5.4
Yellow
Blue
methyl red
5.00
4.8–6.0
Red
Yellow
bromothymol blue
7.30
6.0–7.6
Yellow
Blue
phenol red
8.00
6.6–8.0
Yellow
Red
phenolphthalein
9.50
8.2–10.0
Colourless
Pink
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Colour Change
Acid
Alkali
Indicators can be used to signal the equivalence point
in titrations

Because indicators give us a visible cue when pH changes, they can be used to identify the equivalence
points in titrations (where the pH changes most dramatically)

It will signal the equivalence point when the end point (pH of colour change) coincides with the pH at
the equivalence point.

This means that different indicators must be used for different titrations, depending on pH of the
equivalence point.
Steps for choosing an appropriate indicator:
1. Determine the combo of weak and strong acid and bases are reacting together.
2. Deduce the pH of the salt solution at equivalence from the nature of the parent acid and
base (look up ‘salt hydrolysis!’)
3. Choose indicator with an end point in the range of the equivalence point by consulting
data tables.
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Selecting an appropriate indicator
Around the equivalence point of a titration, the pH changes
very rapidly. Indicators change colour over a narrow pH
range approximately centred around the pKa of the indicator.
An indicator will be appropriate for a titration if the pH range of
the indicator falls within the rapid pH change for that titration.
Indicator
thymol blue
methyl orange
bromophenol blue
methyl red
bromothymol blue
phenolphthalein
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Colour in acid pH range Colour in alkali
red
red
yellow
red
yellow
colourless
1.2–2.8
3.1–4.4
3.0–4.6
4.4–6.2
6.0–7.6
8.3–10.0
yellow
yellow
purple
yellow
blue
pink
© Boardworks Ltd 2010
Weak acid with strong base
titration:
Equivalence point 7.0-11.0
End point range ± 1 because
the ratios of one form to
another (e.g. HIn:In-) is 10:1
and then switches to 1:10.
This is a range of 2 pH units!
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Using Indicators

Indicators are NOT USED to measure pH



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Colour change is not an accurate measurement
Range of colour change limits the measurements that could be taken
pH probes are very accurate
Indicators ARE USED:


To determine the end-point of reactions in titrations
To give a ‘rough and ready’ idea of pH
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Solutions
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Choosing an indicator

Check out the simulation here:


http://chem-ilp.net/labTechniques/AcidBaseIdicatorSimulation.htm
From the simulation you can see the indicator should be chosen such that the
colour change happens as close as possible to the equivalence point
of the titration
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Making Indicators
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In this experiment you will make an indicator from a natural product and
design an experiment to determine the pH range over which it changes
colour.

Follow the instructions here
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BONUS EXPERIMENT (IF YOU FINISH EARLY):



Chop up some onion as finely as possible
Place it in a small beaker with ~25 cm3 sodium hydroxide
Add 50 cm3 hydrochloric acid in 10 cm3 portions, smelling before and after each one.
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Key Points

Indicators change colour over a narrow pH range

The colour change generally occurs at a pH within ± 1.0 of the indicator’s pKa

Indicators should be chosen with a pH range that matches the expected
equivalence point as closely as possible
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