Chemical Indicators

Chemical Indicators

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Unformatted text preview: O O phenolsulfonphthalein + H3O+ O O O phenolsulfonphthalein [H3O+] in this equilibrium is determined by the composition of the solution Effect of pH on In / HIn Ratio Effect - HIn + H2O H3O+ + In- Blue Yellow [In-] [In pKa = pH – log [HIn] At the midpoint of the indicator range… • • • • • [In-] = [HIn] Therefore, equal concentrations of blue & yellow = green [In-] / [HIn] = 1 log [In-] / [HIn] = log 1 = 0 pH (midpoint range) = pKa Indicator Color on pH Scale Indicator Mid-range xxxxxxx xxxxxxx xxxxxxx xxxxxxx x -2 -1 pKa +1 +2 Effect of pH on In / HIn Ratio Effect - HIn + H2O Blue [In-] [In pKa = pH – log [HIn] H3O+ + InYellow [In-] or pH = pKa + log [HIn] Consider the situation when… Consider [In-] > 10 x [HIn] The yellow of [In-] masks the blue of [HIn] so solution appears yellow [In-] / [HIn] = 10 log [In-] / [HIn] = log 10 = 1 So at 1 pH unit higher and above the solution appears yellow Indicator Color on pH Scale Indicator [In-] [In pH = pKa + log [HIn] pH 10 x [HIn] or pH = pKa + log [HIn] pH = pKa + 1 …and all pH’s > pKa + 1 pH Mid-range xxxxxxx xxxxxxx xxxxxxx xxxxxxx x -2 -1 pKa xxxxx xxxxx xxxxx xxxxx x xxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxxxxx +1 +2 Effect of pH on In / HIn Ratio Effect - HIn + H2O Blue [In-] [In pKa = pH – log [HIn] H3O+ + InYellow [In-] or pH = pKa + log [HIn] Consider the situation when… Consider [HIn] > 10 x [In-] The bl...
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This document was uploaded on 02/18/2014 for the course CHEM 1212 at Clayton.

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