For years, my wife and I have had lots of alcohol around the house that we haven’t drunk. A few months ago, I realized that very small amounts make an excellent solvent to wash off the writing from our calendar. But yesterday I came up with an even better idea.
On Friday she brought home a digital colorimeter that can be attached via USB to a laptop, or it could transmit data with Bluetooth to an app on a phone or tablet.
I started with a brown-colored German liqueur (35% alcohol)
Using successive dilutions with water, I prepared, from the original, three other standard solutions of 17.5%, 8.75% and 2.19% alcohol. I also made a 4.38% solution that we used later as our unknown.
We calibrated the instrument with a blank (water) and then used the 35% solution to find the optimum wavelength. Out of the 4 choices, a 565 nm wavelength gave us the best results, which looked like the following:
By interpolating (see black line), the absorbance reading of 0.11 for the “unknown” corresponded to an alcohol concentration of 4.47%, only off by 0.09% (a 2 percent experimental error) from the 4.38 that it was supposed to be.
When you try different wavelengths with the colorimeter, you are reminded that absorbance has to be near the range of 0.2 to 0.5 to get a linear relationship. The alcohol solution works well because it is homogenous, free of turbidity, and it does not chemically react in the presence of light. Other factors are taken care of by the instrument itself—it comes with identical cuvettes, which provide the same path length for the light trying to get through. For example the same colored solution will look darker in a wider glass. Also, the light beamed through is also approximately monochromatic, that is of a single frequency, which ensures that the Beer-Lambert Law will hold.
If you can think of better things to do with alcohol than colorimetry experiments( after all, it’s a good fire-starter; good for calorimetry experiments and good fuel for an alcohol cannon) you can use the beautiful blue copper-ammonia complex. The intensity of the blue will be proportional to the concentration of copper ions, and can be used to determine the concentration of Cu2+ in an unknown. These concentrations work well:
The procedure from which I took the table mentions the use of a spectrophotometer, but I am willing to bet that it would work with the Vernier probe or its equivalent.