1. Theory of Indicators Indicators are mainly weak acids or bases
When dissolved in water an equilibrium is reached
HIn + H2O = H3O+ + In-
HIn is one colour and In- is another
In litmus HIn is red and In- is blue
When both present it is a mixture of red and blue i.e. purple

2. Addition of Acid H3O+ goes up
System tries to get it down - Le Chatelier
By using up H3O+ I.e going left - forming HIn
HIn conc goes up. Red colour gets stronger.
In- goes down so blue gets weaker
Eventually only HIn colour can be seen
Litmus is now red

3. Addition of Base Adding OH- OH- reacts with H3O+ forming 2 H2O
H3O+ goes down
System tries to get it back up - Le Chatelier
By forming H3O+ and In- [going to right]
In- conc goes up so blue colour gets stronger
HIn goes down so red gets weaker
Eventually only In- can be seen - so litmus blue

4. Indicator Colour [In-] / [HIn] = 0.1 or less HIn colour shows
[In-] / [HIn] = 10 or more
In- colour shows
Whichever concentration is 10 times greater - its colour will show
Otherwise intermediate colour

5. Which Indicator? SAWBMO Strong Acid Weak Base Methyl Orange WASBPH
Weak Acid Strong Base Phenolphthalein
SASBANY
Strong Acid Strong Base ANY
WAWBNONE
Weak Acid Weak Base NONE

6. Indicator Colours Alkaline Colour Acid Colour Indicator Changes in
pH Range
Methyl Orange
Orange / Red
3 - 5
Yellow
Methyl Red
Red
4 - 7
Yellow
Bromothymol Blue
Yellow
6 - 8
Blue
Purple
Phenolphthalein
Colourless
8 - 10

7. Strong Acids Weak acids Strong Bases Weak Bases HCl HNO3 H2SO4 H3PO4
HCOOH
CH3COOH
HCN
Strong Bases
NaOH
KOH
Weak Bases
NH4OH
Na2CO3

8. THE END