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Author Topic: How do indicators work in chemistry?  (Read 19630 times)

Offline cuso4

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How do indicators work in chemistry?
« on: 23/05/2003 08:00:48 »
How do indicators work?

For example, why is the universal indicator able to change into different colours according to the acidity of the substance? I'm guessing that this is to do with the redox of the chemical in the universal indicator, probably one of the transition metals because they are the only ones which can have varied oxidation states and produce different colours.

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« Last Edit: 04/07/2004 08:08:23 by Exodus »


 

Offline chris

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Re: How do indicators work in chemistry?
« Reply #1 on: 23/05/2003 23:58:24 »
Universal indicator is actually a mixture of indicators which all respond differently to the hydrogen ion concentration, producing a fingerprint colour representative of any given pH.

You can make a simple indicator at home by buying some red cabbage and grinding it up to get the juice out. The pigment is anthocyanin (a flavanoid) which gains an -OH at high (basic) pH, but looses it at low (acidic) pH.

This alters the wavelengths of light reflected by the compound, creating the color change at different pH's.

This link has some anthocyanins drawn out so you can see their composition :

http://www.micro-ox.com/chem_antho.htm

Chris


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« Last Edit: 23/05/2003 23:59:41 by chris »
 

Offline Quantumcat

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Re: How do indicators work in chemistry?
« Reply #2 on: 04/07/2004 09:17:10 »
Indicators are indicators because they change their structure to reflect different wavelengths of light when they react with the hydrogen in acids.

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« Last Edit: 04/07/2004 09:20:45 by Quantumcat »
 

Offline Roju

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Re: How do indicators work in chemistry?
« Reply #3 on: 07/08/2016 22:00:22 »
Given the structural formula of the active substance in red cabbage indicator. Is it possible to determine what colours it will absorb and reemit from first principles? I am guessing if it did it would involve alot of quantum mechanics, but surely it is possible to use the shrodinger equation to figure out all the allowed electron energy states ?
 

Offline chiralSPO

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Re: How do indicators work in chemistry?
« Reply #4 on: 08/08/2016 02:58:44 »
Given the structural formula of the active substance in red cabbage indicator. Is it possible to determine what colours it will absorb and reemit from first principles? I am guessing if it did it would involve alot of quantum mechanics, but surely it is possible to use the shrodinger equation to figure out all the allowed electron energy states ?

Yes, it is possible to calculate the absorption spectrum (and so much more) of the different species involved in red cabbage indicator from first principles. Density functional theory (DFT) calculations would probably be the way to go. It is entirely possible to optimize the structure of each of the relevant molecules, and at the same time calculate the energies and symmetries of the orbitals (occupied and virtual), which will directly determine the wavelengths of the photons it will interact with (and a rough estimate of the intensity of the absorption), which is exactly the info you would need.

The actual mathematics is pretty gnarly (lots of very large matrices being multiplied together, and subjected to operators, and eigenvectors being found), but there are computer programs that do all the crunching for you as long as you know what to ask them for.
 

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Re: How do indicators work in chemistry?
« Reply #4 on: 08/08/2016 02:58:44 »

 

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