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

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

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

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

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

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

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

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