lafemmefatale

So let's say you have the indicator
Orange IV

and it's pH range which colour occurs is
1.4 - 2.8

and the colour change as pH increases
red to yellow


So my question is if the pH was 1.3 or 1.2 or 1...would the colour be red? Likewise, at 2.9 or 3, would the colour be yellow?

Also, if the pH is 2, is the colour something like...orange?


Argh! Chemistry books and teachers provide an inadequate amount of information on indicators...

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lordjeebus

yes and yes

You have the right idea. Think of the litmus paper, which is a pH indicator with a very wide range -- it's the gradual change in color that enables you to determine the pH. With other indicators, the change is more abrupt, so you're less likely to see the intermediate colors.

I'm surprised you haven't had some sort of demonstration of pH indicators in class...

Lebell

An indicator (e.g. methyl orange) goes directly to whatever color whenever it's pH is reached, no partial color change (at least, that's my experience). Now pH paper will have gradiations and a scale for them on the case.

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lordjeebus

I've seen half-way points with indicators in chem lab although only when adding base or acid drop by drop. They do change very quickly, but I think that in principle (because the indicator is in equilibrium between two different forms) that there is a point where half of the indicator is in one form and half is in the other. In such a situation (which may be difficult to achieve) light of both colors should be reflected by half of the indicators, and an intermediate color should be observed.

rsl12

something like phenophthalein (sp?) goes from clear to pink very quickly, but there is a moment when it is a very faint pink. there are indicator index strips available to determine the ph for the intermediaries.

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