How does a colorimeter work and other colorimetry questions...

I'm writing up the plan for my A2 chemistry coursework on my own synthesised aspirin in which I will be carrying out a back titration and colorimetry.

I've written everything apart from the colorimetry section, which I am UBERLY confused about. How exactly does a colourimeter work? I've tried looking everywhere but neither my books nor the internet are helping - They don't give me enough information!

So far I have:

Colorimetry is the measurement of the wavelength and the intensity of electromagnetic radiation in the visible region of the spectrum. Colorimetry can help find the concentration of substances, since the amount and colour of the light that is absorbed or transmitted depends on properties of the solution, including the concentration of particles in it. A colorimeter is an instrument that compares the amount of light getting through a solution with the amount that can get through a sample of pure solvent. A colorimeter contains a photocell is able to detect the amount of light which passes through the solution under investigation. The more light that hits the photocell, the higher the current it produces, hence showing the absorbance of light. A colorimeter takes 3 wideband readings along the visible spectrum to obtain a rough estimate of a color sample. Pigments absorb light at different wavelengths.

However, I'm not sure all of that is right.
Also, I don't understand the bit about '3 wideband readings', what does this mean?

Also, how do you choose your filter? I know it's stuff to do with the wavelength (the solution will be purple, and its been suggested in various plans I've found that the wavelength that will pass through the purple solution is 530nm) and i've seen in plans that the filter normally used is a yellow-green filter. I know this is supposed to be the complementary colour of purple so that only purple wavelengths pass through and nothing else. Is that right? Is there something else I need to be saying? And what's the best way to word it? (Because how I've written it here isn't the clearest, most scientific way of putting it XD)

What else do I need to write, about the science behind colorimeters?

And finally (sorry for all the questions) I have a rough plan for the colorimetry, but I don't know if it is correct or not:

- It is necessary to prepare a calibration curve before using the aspirin with the colorimeter. To create a calibration curve, a range of 2-hydroxybenzoic acid concentrations must be used.
- Measure out 2cm3 of each concentration and add 2 drops of iron (III) chloride to each using a pipette.
- Pour each solution into separate cuvettes and place in the colorimeter (under a yellow-green filter). Record each reading and draw a graph
- Next, dissolve 0.2g of the aspirin sample in a 50:50 water:ethanol mixture in a 100cm3 volumetric flask and add 1cm3 of a 5% aqueous iron (III) chloride solution before adjusting the volume to 100cm3 with more of the water/ethanol mixture
- Take a sample of this mixture and put it in a cuvette. Take a reading with the colorimeter.

Is this correct? Is there anything else I need to do?

Also, from my results, how exactly do you find out the concentration of salicylic (2-hydroxybenzoic acid) present in the sample. I know that the darker the purple colour of the solution, the more salicylic acid is present, but how do you calculate exactly how much is present?

Thanks you so much for any help you can give me! :biggrin:

Reply 1

Wangers

You're definatly along the right lines, but the paragraphs are not coherent. First explain the principles of colorimetry, then detail controls, then detail expected results, and thus deductions therein.

Reply 2

ChemistBoy

Raindroppe

Also, from my results, how exactly do you find out the concentration of salicylic (2-hydroxybenzoic acid) present in the sample. I know that the darker the purple colour of the solution, the more salicylic acid is present, but how do you calculate exactly how much is present?

That's what your calibration curve is for. You should have a nice straight line relationship between known concentrations of salicylic acid and the readings on your colorimeter. Simply look at the readings for your unknown analytical solution and read off the corresponding concentration on your calibration curve.

Reply 3

Extreme!

Your not expected to know the ins and outs of colorimetry, but it is worthy to state Beer's law (which shows that Absorbance is directly proportional to concentration; A=abc), what a calibration curve is and how you will use it, and perhaps a diagram to show how it works. You will also need to talk about using the complementry colour for the filter.

Additonally, your method needs to be more 'in depth' - I'm assuming your going to test at least 5 known concentrations. What concetrations? What type of pipette are you using? Also, mention about the meniscus and the calibration line but you may have talked about this elsewhere. How will get 0.2g - you need to state this!