Indicators (1 Viewer)

[horndog huang 2]

Why are Phenophthalein, bromothymol blue and methyl orange unsuitable for testing whether a substance is neutral?

 

[BlueGas]

Phenolphthalein has a range of 8.3-10.0 (basic), and that's obviously outside the netural pH of 7, so it is unsuitable, and so as Methyl Orange because it has a range of 3.1-4.4 (acidic). Bromothymol blue is also unsuitable because it has a range of 6.0-7.6 (sightly acidic).

 

[leehuan]

I agree completely with BlueGas' response to phenolphthalein and methyl orange (the major flaw is that you can mistake a basic substance for neutral with methyl orange, and an acidic substance for neutral with phenolphthalein).

However, I will quite argue that bromothymol blue CAN be used to determine neutrality. The range of 6.0-7.6 gives more room for barely acidic substances to be identified more easily, however at around the pH range of 6.9-7.0 bromothymol blue will be GREEN. Traces of blue and yellow can be easily picked up to determine if something is actually barely acidic/basic.

Whilst by definition pH=7.0 is neutral, it is not simple to determine this using judgements simply from the naked eye.

(Litmus is only superior due to the extremely large range of 4.5-8.3, revealing a wider range of colour changes and allowing neutral substances to be more easily distinguished by the purple.)

 

[BlueGas]

I agree completely with BlueGas' response to phenolphthalein and methyl orange (the major flaw is that you can mistake a basic substance for neutral with methyl orange, and an acidic substance for neutral with phenolphthalein).

However, I will quite argue that bromothymol blue CAN be used to determine neutrality. The range of 6.0-7.6 gives more room for barely acidic substances to be identified more easily, however at around the pH range of 6.9-7.0 bromothymol blue will be GREEN. Traces of blue and yellow can be easily picked up to determine if something is actually barely acidic/basic.

Whilst by definition pH=7.0 is neutral, it is not simple to determine this using judgements simply from the naked eye.

(Litmus is only superior due to the extremely large range of 4.5-8.3, revealing a wider range of colour changes and allowing neutral substances to be more easily distinguished by the purple.)

But the question was asking why is it unsuitable, so wouldn't it be better to go along with the question and agree that it's unsuitable?

 

[leehuan]

But the question was asking why is it unsuitable, so wouldn't it be better to go along with the question and agree that it's unsuitable?

The question is partly wrong by nature.