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    The color change principle of acid-base indicator

    • Last Update: 2021-06-18
    • Source: Internet
    • Author: User
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    1.
    Discoloration principle of indicator

    Substances that can indicate the acidity or alkalinity of the solution through color changes are called acid-base indicators, such as litmus , phenolphthalein, and methyl orange .
    They show different colors in acid and alkali to indicate the acidity or alkalinity of the solution

    .

    The acid-base indicator is generally a weak organic acid, which can indicate the acid-base of the solution because the dissociation equilibrium of the indicator molecules does not move at the same time, and the undissociated molecules and the dissociated products have different colors.
    , So that the solution has different colors due to the different degrees of dissociation of the indicator

    .

    If HIn is used to represent the weak organic acid indicator molecule, its dissociation equilibrium is

    The molecule HIn and acid radical In - show different colors
    .
    When the concentration of H
    + in the system is large, the indicator is mostly in the form of HIn, and the color of the molecule is displayed at this time, for example, the methyl orange molecule is red; when the concentration of OH - in the system is large, the indicator is mostly in the form of In - .
    At this time, the color of acid radicals appears, for example, the acid radical of methyl orange appears yellow

    .

    2.
    Indicator discoloration point and discoloration range

    Different indicators indicate different pH ranges
    .
    For example, the discoloration range of methyl orange is pH 3.
    2-4.
    4, and the discoloration range of phenolphthalein is pH 8.
    2-10

    .
    The discoloration range of the indicator is determined by the dissociation equilibrium constant of the indicator

    .
    E.
    g

    Equilibrium constant

    When the amount of molecules in solution and the acid groups are equal, [HIn] = [the In - ], the solution was mixed color of the two
    .
    At this time, Ka
    Θ =[H + ] in the solution, and the theoretical discoloration point of the indicator is
    pH=pKa Θ

    When product ≥ 0, the indicator mainly exists in the form of acid radical In-, and the solution shows the color of acid radical;

    When the product is less than or equal to 0, the indicator mainly exists in the form of molecule HIn, and the solution shows the color of the molecule
    .

    Therefore, the theoretical discoloration range of the indicator is

    pH=pKa Θ ±1

    Due to the difference in color concealment ability and the sensitivity of human eyes to different colors, the actual range of discoloration has to be a little deviation
    .
    For example, the theoretical discoloration range of methyl orange is pH 2.
    4-4.
    4

    .
    Since red (the color of molecules) has a much stronger hiding power than yellow (the color of acid radicals), the actual discoloration range is pH 3.
    2-4.
    4
    .

    When selecting an indicator, you should carefully search the manual and select an indicator with a proper range of discoloration according to the actual situation
    .

    Related link: Dissociation of multiple weak acids

     

     

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