Question

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

Answer 1

Answer: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.

Explanation:

(a) Litmus

Litmus is red in acid (< pH 5) and blue in base (> 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 8.3) and red in base (> 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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Answer 2

False reading may be obtained if there is a run over from adjacent reagent area in excessively wetted strips

Phenolphthalein only works efficiently from 4 to 10 ph

Litmus and phenolphthalein and methyl orange indicators can only allow you to tell if solution is alkaline,neutral or acidic

The two indicators with less limitations are methyl orange and thymol blue