asked in 科學化學 · 1 decade ago

why indicators give color chan

In acid base titration. why indicators give color chan?

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  • 1 decade ago
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    Indicators are weak acids or weak bases. Take a weak acid indicator as example, and denote the indicator as HIn. The weak acid undergoes dissociation in water, and a state of equilibrium is established.

    HIn(aq) = H+(aq) + In-(aq)

    Being an acid-base indicator, either the molecular form (HIn) or the anionic form (In-), or both, are coloured.

    For example, methyl orange indicator is a weak acid, which undergoes dissociation in water. Denote the molecular form of methyl orange as HMe and the anionic form as Me-. HMe is red in colour, and Me- is yellow.

    HMe(aq) = H+(aq) + Me-(aq)

    Denote [HMe] as the molar concentration of HMe, and [Me-] as the molar concentration of Me-.

    In a strongly acidic solution, the concentration of H+(aq) is very high. Much H+(aq) reacts Me- to give HMe, and thus [HMe] >> [Me-]. Human eyes can only see the colour of HMe, and thus the methyl orange solution looks red.

    In a strongly alkaline solution, the concentration of H+(aq) is very low. Much HMe(aq) dissociates to give [H+] and [Me-], , and thus [Me-] >> [HMe]. Human eyes can only see the colour of Me-, and thus the methyl orange solution looks yellow.

    In a solution of pH of colour change, HMe and Me- have comparable concentrations. Human eyes cannot distinguish between the colours of HMe and Me-, and thus the methyl orange solution looks a mixed colour of red and yellow, i.e. orange.

    Conclusively, the colour of an indicator changes according to the pH value of the solution. In a titration, the pH of the solution changes, and thus the colour of the indicator changes.

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