Indicators. Weak acids, HIn, that have different colours in the undissociated form, HIn and the disscociated form In -. HIn (aq) H + (aq) +In - (aq)

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Presentation transcript:

Indicators

Weak acids, HIn, that have different colours in the undissociated form, HIn and the disscociated form In -. HIn (aq) H + (aq) +In - (aq)

Add H + (aq) equilibrium to left red Methyl Orange Add OH - (aq) equilibrium to right yellow

Indicators Different indicators change colour at different pH values Investigate the colours of different indicators in different solutions IndicatorColour in Acid Colour in Alkali pH range for colour change

IndicatorColour in Acid Colour in Alkali pH range for colour change Methyl orangeRedYellow2.9 – 4.0 Methyl redRedYellow4.2 – 6.3 litmusRedBlue5.0 – 8.0 Bromothymol blue YellowBlue6.0 – 7.6 Phenolphthaleincolourlesspink8.2 – 10.0 Phenol redYellowRed6.6 – 8.0

Choosing Indicators for Titrations For an indicator to work the colour change must occur within the steep part of the pH curve.

Strong Acid / Strong Base phenolphthalein Methyl orange Any indicator in pH range 3 – 11 suitable

Weak Acid / Strong Base phenolphthalein Methyl orange Any indicator in pH range 7 – 11 suitable phenolphthalein

Strong Acid / Weak Base Methyl orange Any indicator in pH range 3 – 7 suitable Methyl orange

Weak Acid / Weak Base phenolphthalein Methyl orange No rapid pH change at equivalence – no suitable indicators