1. Acids and Bases (HL) - Lesson 12
Indicators

2. Refresh
A 25.0 cm3 solution of a weak monoprotic acid, HA(aq), is titrated with mol dm–3 sodium hydroxide, NaOH(aq), and the following graph is obtained.
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.

3. We Are Here

4. Lesson 11: Indicators Objectives: Understand how indicators work
Determine suitable indicators for a reaction
Make indicators from a range of natural products

5. Indicators An indicator is a compound whose colour depends on pH
For example:
Phenolphthalein
pH< 8: COLOURLESS pH>10: PINK
Methyl orange
pH<3.2: RED pH>4.4: ORANGE
Bromothymol blue:
pH<6.0: YELLOW pH>7.6: BLUE
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.

6. How Indicators Work (what you need):
Indicators are weak acids/bases in their own right
In solution indicators form an equilibrium:
In H ⇌ InH+
COULOUR COLOUR 2
Where: ‘In’ stands for indicator
As [H+] changes, the equilibrium moves to the left or right, thus changing the colour
The structure of indicators change depending on the pH:
Higher pH can cause weak-acid groups to deprotonate
Low pH can cause weak-base groups to protonate
This can have knock-on effects on the structure, of the indicator molecule which changes its colour

7. How Indicators Work (in detail):
Colour in many organic compounds comes from having overlapping π-systems, with many delocalised electrons
For example: phenolphthalein
The colour derives from conjugated π-systems which are radically altered by the changes in structure
The double bond on the central carbon atom allows the π-systems in the three benzene rings to interact with each other, leading to the pink colour
This is more detail than required by the IB!!! 
+ 2H+
pink
colourless

8. pH range and pKa Most indicators change range within ±1.0 of their pKa
pKa data for indicators can be found in your data booklet
Indicator
pKa
pH Range
Colour Change
Acid Alkali
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
methyl red
5.00
4.8–6.0
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

9. Choosing an indicator Check out the simulation here:
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

10. Using Indicators Indicators are NOT USED to measure pH
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

11. Making Indicators
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 on the sheet
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.

12. 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