I prepared my ‘red cabbage’ pH indicator for the titration experiment planned for the second half of Why Scotland, Why East Kilbride….and it stinks! In fact the kitchen and lounge – where I’m typing this up – both have that low level, but unmistakable, rotten egg smell. Why? And whoopee! (Well maybe, whoopee!)
Analytical chemistry using a cabbage?
The colour in red cabbage comes from the anthocyanin pigments in the leaves. The general chemical structure of the compounds is shown in the figure above. One or more of these ‘R groups’ are usually a hydroxyl (a -OH group) where the H (hydrogen) is either on, or off, the molecule depending on how acidic the surrounding environment is. When this happens the colour of the molecule changes. In techno-science jargon – the protons dissociate as pH increases, changing the charge on the conjugated chromophore and so altering the UV/vis absorbance characteristics!
These anthocyanin pigments are easily extracted using hot water (according to the procedure here); under alkali conditions they are blue, and under acid conditions red (as in the picture of the test tubes).
And the smell?
Here’s the bonus! In the last two posts I covered the risk assessment of using a man-made thiol (dimercaptopropanol) and the issues we faced when I discovered it produced reproductive effects in lab animals (Art, Science and Safety and Action and Reaction). After my anthocyanin extraction this evening the smell of cabbage-derived thiols was really strong. After a bit of literature digging it appears that the technique I used is a published methodology for extracting the thiols and sulphoxides from vegetables. It’s not ‘full-smell-ahead’ for Cryptic Nights yet, the thiol needs to be dispersed properly into the atmosphere, and I’m not sure if we could extract enough for it to be useful. But maybe, just maybe, we might be back on track with a natural thiol product.