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13 July, 08:21

What are the limitations of litmus paper and Phenolphthalein indicators? Name to other indicators that can be used that do not have such limitations

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  1. 13 July, 11:09
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    Here's what I find.

    Explanation:

    An indicator is usually is a weak acid in which the acid and base forms have different colours. Most indicators change colour over a narrow pH range.

    (a) Litmus

    Litmus is red in acid (pH 8).

    This is a rather wide pH range, so litmus is not much good in titrations.

    However, the range is which it changes colour includes pH 7 (neutral), so it is good for distinguishing between acids and bases.

    (b) Phenolphthalein

    Phenolphthalein is colourless in acid (pH 10).

    This is a narrow pH range, so phenolphthalein is good for titrating acids with strong bases ...

    However, it can't distinguish between acids and weakly basic solutions.

    It would be colourless in a strongly acid solution with pH = 1 and in a basic solution with pH = 8.

    (c) Other indicators

    Other acid-base indicators have the general limitations as phenolphthalein. Most of them have a small pH range, so they are useful in acid-base titrations.

    The only one that could serve as a general acid-base indicator is bromothymol blue, which has a pH range of 6.0 to 7.6.
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