Colorimetry

I love carrying out colorimetry, it is one of my favorite types of labs to carry out and I do believe that as teachers, it is a really underused type of lab and undervalued type of lab.

I always teach the theory first, covering the ideas behind the Beer-Lambert Law and the way that a colorimeter works. We will usually start off by getting the class to make up standard solutions of copper sulfate (0.1 mol dm-3 to 1.0 mol dm-3 in 0.1 mol dm-3 increments). This is itself is a good exercise to recap several concepts that are taught earlier in the course (ie, making standard solutions. It is amazing how many students struggle to make up a 100cm3 solution of say, 0.300 mol dm-3 CuSO4 – but that is another story!)

Once the standard solutions are made we put the 1.00 mol dm-3 solution into the colorimeter (I always ask students why we use the most concentrated) and then measure its absorbance at different wavelengths. This gives the idea of needing to use the maximum wavelength of absorption.

By GYassineMrabetTalk. This W3C-unspecified vector image was created with Inkscape. – self-made, inspiration from a real model of Spectronic 21D UV-Vis spectrophotometer., CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=3301409

Once lambda max has been found, it is a case of putting in samples of the standard solutions that the students have prepared to produce the calibration curve (line).  The calibration curve always gives such a good line it is great for the students to do so they can clearly see the relationship is linear. 

Finally, a solution of unknown concentration is given to the students for them to determine the concentration. At this point, they don’t really need to be told what to do – it makes sense to them what to do and they can usually achieve a fairly accurate result.

Colorimetry can also be used to determine the order of a reaction. Propanone and iodine seems to be a common one but I’m wondering if a halogenoakane (probably a bromoalkane – iodoalkanes would be too fast and chloroalkanes too slow) and sodium hydroxide could be used. Silver nitrate could be added to precipitate the bromide ion and hence stop the reaction).

Using in colorimetry in the lab has also become more important since the November exams. Paper 3, question 1 was all about colorimetry so exposure to this sort of lab would have undoubtedly helped students to answer it. There was still a tricky question that asked students to determine the equation for the straight line of a calibration curve. The answer is straight forward enough but I know it would cause difficulty for many students, especially SL students who maybe are only studying one science. Do you teach students how to do this? I didn’t …… but will certainly do so in the future!

Do you use colorimetry in the lab? If you do, how do you use it? Or do you have any comments on the November paper 3 questions? Please feel free to post your comments and ideas below – it would be great to hear from you and to hear about some alternative labs.