[ActivationEnergy]

I am doing an investigation for chemistry on the pKa values of natural indicator and my teacher suggested using a colorimeter. I have searched online and can't find a method. The topics I got when I searched here didn't help, so sorry about making a new topic. Can anyone explain how I would go about this investigation?  Thanks anyways.

[fledarmus]

I'm not sure exactly what you're asking. Are you looking for information on what a colorimeter does, are you trying to develop a method for measuring pK_a of a specific compound using a colorimeter, or are you looking for a published step-by-step lab procedure that uses a colorimeter to measure pK_a? Or something completely different?

[ActivationEnergy]

I am looking for a method / procedure, general or step-by-step that uses a colorimeter to measure pKa. Sorry if that wasn't clear.

[Borek]

Best approach is to find pH at which concentrations of both acid and conjugate base are identical.

[ActivationEnergy]

how would you do that?

[Borek]

Do you know how concentrations of acid and conjugate base change with pH?

Imagine solution with very low pH - does it contain acid, or conjugate base?

Imagine solution with very high pH - does it contain acid, or conjugate base?

Can you use these solutions to measure extinction coefficient for pure acid and base?

[fledarmus]

Here are some general procedures for using a colorimeter to follow the course of various reactions. Once you have actually defined what the reaction is that you are trying to follow and have established the colorimetric characteristics of the various species in the reaction, the rest is just a matter of taking measurements and making graphs.

http://www.mystrica.com/files/mystrica%20investigations.pdf

[ActivationEnergy]

So I get the solution to be tested ready i.e. blueberries,
then test with different pH buffers to find the pH at which it changes colour?

[fledarmus]

It helps if you have some idea of what you are looking for. As an example, phenolphthalein is colorless - it has no absorbance in the visible spectrum. It is also acidic (can donate a proton) and if you remove that proton, it forms a salt which has a very strong absorbance at ~550 nm, making a solution of that salt appear purple. With a colorimeter and Beer's law, you can measure the absorbance of a solution of phenolphthalein at various pHs, calculate the proportion of protonated to deprotonated phenolphthalein at various pHs, and use that information to calculate the K_a.

You might need to take some visible light spectra of your blueberry extract at high and low pH to determine what wavelengths change before you can use your colorimeter. If you know the visible absorbance characteristics of a component of your extract at both high and low pHs, you should be able to follow the appearance or disappearance of characteristic absorbance maxima as you adjust the pH of the solution.

[ActivationEnergy]

OK. Thanks guys. I think I will be doing turmeric, blueberries and rose petals. Although I'm not sure where I will get rose petals.

[fledarmus]

Find a large florist shop - when they sell roses, they always peel off the outer few petals so they look more budlike and last better. If you leave them a container, you might be able to sweet-talk them into saving them for you.

If worse comes to worst, you might have to go to someplace like Wal-Mart or BJ's and buy some cheap bouquets. Assuming you live in the US - not sure what the equivalent in other countries would be.

[ActivationEnergy]

I live in the UK, the equivalent is Asda... How would i make the sample for the rose petals?

[ActivationEnergy]

Does anyone know if, for example, rose petals will dissolve in water (if heated)?

[Arkcon]

Not really, warm water won't really extract color, and hot water may damage the color.  Household rubbing alcohol is a good solvent for plant colors.  Consider, the pH indicator phenolphthalein, it is likewise dissolved in alcohol, and then a few drops used to perform a pH titration.  You should do the same with all of your plant based pH indicators.