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IV. 17 Indicators p. 159-163

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Indicator Weak organic acid or base with different colours for its conjugate acid & base forms HIn AcidIndicator

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HIn + H 2 O = In - + H 3 O + Example Indicator: Chlorophenol Red HIn - yellow In - - red

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When acid is added: [H 3 O + ] shifts Turns yellow HIn (for any indicator) is the form when in an acid

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When base is added: [H 3 O + ] (combines with OH - ) shifts Turns red In - (for any indicator) is the form when in a base

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When [Hin] = [In - ] End / Transition Point Ka= [H 3 O + ][In - ] / [HIn] = [H 3 O + ] End point = Ka for indicator pKa = - log Ka, since: »Ka = [H 3 O + ] »-log Ka = - log [H 3 O + ] »pKa = pH

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Chlorophenol Red 5.2 - 6.8 Yellow to red Turns yellow when pH < 5.2 Turns red when pH > 6.8 Turns orange between pH of 5.2 - 6.8

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At the midpoint of the colour change: pKa = pH = 6.0 for Chlorophenol Red Therefore, Ka = 1.0 x 10 -6

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