Indicators and What they tell you?

[Spina]

Hi guys,

Have been doing a fair few exams and this is the only topic that still confusing me. I'm consistently getting these types of questions wrong so if anyone could provide some light on what they do that'd be great.

Okay, so obviously I know if they have a colour represented by the "acid" it means they have lowish pH, depending on the indicator, and if they have a colour represented by the "base" they have a high-ish indicator.

But can someone please tell me what other information an indicator can give? I've had questions where you've had to determine the strength of the base/acid in a titration which completely confused me. So an overview of what they do would be great.

Thanks in advance.

[jyce]

Hey,

So acid-base indicators are basically weak acids/bases that experience a colour change over a certain pH range. We use them in acid-base titrations to approximate the equivalence point, because solutions of acids and bases, and the products of acid-base reactions, are usually colourless. If you take a look at the last page of the data book, it'll tell you the range over which each of a series of indicators change colour. For example, the indicator 'bromophenol blue' changes colour over the pH range 3.0-4.6: it is yellow when the pH is below 3.0, and it is blue when the pH is above 4.6, and it is a greenish colour in between. This indicator would be suitable for an acid-base titration where the equivalence point occurs at a pH between 3.0 and 4.6. Such a titration would involve a strong acid and a weak base. On the other hand, a titration between a weak acid and a strong base would have an equivalence point above 7. So, phenolphthalein might be a suitable indicator in this case. Note that indicators are not usually used in redox titrations, as oxidants and reductants themselves tend to change colour when their oxidation state changes. Also, since indicators are weak acids, you can use equilibrium principles to calculate the pH and the percentage ionisations of indicator solutions. I can't really think of any other ways you need to apply indicators in the VCE Chemistry course.

Does this clarify things for you?

[Spina]

Hey,

So acid-base indicators are basically weak acids/bases that experience a colour change over a certain pH range. We use them in acid-base titrations to approximate the equivalence point, because solutions of acids and bases, and the products of acid-base reactions, are usually colourless. If you take a look at the last page of the data book, it'll tell you the range over which each of a series of indicators change colour. For example, the indicator 'bromophenol blue' changes colour over the pH range 3.0-4.6: it is yellow when the pH is below 3.0, and it is blue when the pH is above 4.6, and it is a greenish colour in between. This indicator would be suitable for an acid-base titration where the equivalence point occurs at a pH between 3.0 and 4.6. Such a titration would involve a strong acid and a weak base. On the other hand, a titration between a weak acid and a strong base would have an equivalence point above 7. So, phenolphthalein might be a suitable indicator in this case. Note that indicators are not usually used in redox titrations, as oxidants and reductants themselves tend to change colour when their oxidation state changes. Also, since indicators are weak acids, you can use equilibrium principles to calculate the pH and the percentage ionisations of indicator solutions. I can't really think of any other ways you need to apply indicators in the VCE Chemistry course.

Does this clarify things for you?

Yes it does!!

Thank you so so so so much

[jyce]

You're welcome