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Indicators

Indicators. Indicators. Weak acids, HIn, that have different colours in the undissociated form, HIn and the disscociated form In - . HIn (aq) H + (aq) + In - (aq). Methyl Orange. HIn (aq) H + (aq) + In - (aq). Add H + (aq) equilibrium to left red.

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Indicators

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  1. Indicators

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

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

  4. Indicators • Different indicators change colour at different pH values • Investigate the colours of different indicators in different solutions

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

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

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

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

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

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