strong acid strong base pH CURVES & INDICATORS How pH indicators work methyl orange phenolphthalein Choosing an appropriate indicator pH curves strong acid strong base strong acid weak base weak acid strong base weak acid weak base SUMMARY diprotic acid H2C2O4 / NaOH diprotic base Na2CO3 / HCl

INDICATORS HIn  H+ + In- colour 1 colour 2 Indicators are weak acids which have a different colour to their conjugate base HIn  H+ + In- colour 1 colour 2 low pH: equilibrium pushed left = colour 1 high pH: equilibrium pushed right = colour 2

Methyl orange pH < 3.2  pH > 4.4

acid end-point alkali

Phenolphthalein  pH < 8.2 pH > 10.0

phenolphthalein acid alkali

INDICATORS For an indicator to work in a titration: The pH range of the indicator’s colour change must be within the range of the pH change at the end-point.

INDICATORS Indicator colour at low pH pH range of colour change high pH methyl orange red 3.2 – 4.4 orange phenolphthalein colourless 8.2 – 10.0 purple

Strong acid – Strong base pH at equivalence = 7

Strong acid – Strong base

Strong acid – Strong base

Strong acid – Weak base pH at equivalence < 7

Strong acid – Weak base

Strong acid – Weak base

Weak acid – Strong base pH at equivalence > 7

Weak acid – Strong base

Weak acid – Strong base

depends on relative strength of acid and base Weak acid – Weak base pH at equivalence depends on relative strength of acid and base

Weak acid – Weak base

Weak acid – Weak base

SUMMARY

SUMMARY

SUMMARY

H2C2O4 v NaOH H2C2O4 + OH- ↓ HC2O4- + H2O HC2O4- + OH- ↓ C2O42- + H2O

H2C2O4 v NaOH H2C2O4 + OH- ↓ HC2O4- + H2O HC2O4- + OH- ↓ C2O42- + H2O

H2C2O4 v NaOH H2C2O4 + OH- ↓ HC2O4- + H2O HC2O4- + OH- ↓ C2O42- + H2O

Na2CO3 v HCl CO32- + H+ ↓ HCO3- HCO3- + H+ ↓ H2O + CO2

Na2CO3 v HCl CO32- + H+ ↓ HCO3- HCO3- + H+ ↓ H2O + CO2

Na2CO3 v HCl CO32- + H+ ↓ HCO3- HCO3- + H+ ↓ H2O + CO2