Cyanide, a deadly compound found in bitter almonds and apple seeds, consists of simply two atoms (Carbon and Nitrogen) bonded together as a unit. Cyanide is typically found in ionic compounds where it is bonded to one other positive ion (a cation); hydrogen cyanide and potassium cyanide are notorious examples.
The toxicity of cyanide depends on how easily the bond between the cyanide unit and the cation can be broken. For instance, in hydrogen cyanide, the bond between the H+ and the cyanide unit are easily broken so hydrogen cyanide is extremely toxic.
When cyanide enters the bloodstream, it attaches to the iron in haemoglobin – displacing oxygen. Once bonded to haemoglobin, cyanide can travel rapidly around the body and reach the mitochondria of the cells. Within the mitochondria, cyanide can bind irreversibly to cytochrome c oxidase (an important respiration enzyme) – preventing energy from being released into the cell and, eventually, killing the cell.
If you’ve ever had the pleasure of experimenting in the kitchen and making curries, you are probably already familiar with turmeric – you might even have a jar of the golden spice hiding in your kitchen cupboards right now. Although not usually the first substance to come to mind when thinking about chemistry, turmeric (and its active agent curcumin) have some truly fascinating chemical properties.
As with all spices, turmeric is a mix of chemical compounds, but its instantly recognisable bright yellow colour is due to the presence of a particular compound: curcumin.
Curcumin is a polyphenol (an organic compound with multiple phenol units). The phenol units and aromatic structures of curcurim mean that the electrons in the curcumin molecules can absorb (and, hence, gain energy from) UV light. The energy gained promotes them to a higher energy level (an ‘excited state’). This is the same excitation that occurs during flame tests of metal ions. This process doesn’t last long though; they quickly lose some of this energy as vibrational energy, before dropping back down to their original energy level, emitting their excess energy as visible light. This means that curcumin can fluoresce!
In order to demonstrate this amazing ability of curcumin, I swirled turmeric into a beaker of ethanol (as the spice is more soluble in organic solvents) and viewed the solution under a blacklight- the results were almost mesmerising.
Another exciting property of curcumin is its ability to act as a chemical pH indicator (like universal indicator solution or the natural compound anthocyanin). Curcumin changes colour in alkali solutions because of a subtle change in its chemical structure.
Curcumin’s chemical structure contains a sequence of alternating single and double bonds between atoms, what chemists refer to as ‘conjugation’. The length of this sequence of alternating bond types affects the wavelengths of light that the compound absorbs. In curcumin’s case, when it’s added to an alkaline solution, a subtle change in structure alters the length of the conjugated part of the molecule, in turn altering the wavelengths of light that it absorbs. This causes the colour change from yellow to red.
I also demonstrated this property by adding turmeric to a beaker of dilute HCl and a beaker of dilute KOH. In a more informal (kitchen) setting, this experiment could also be replicated with lemon juice (which is weakly acidic) and bleach.
Finally, much to the delight of the attendees, we created pH indicator paper from the turmeric-ethanol solution and filter paper; the homemade indicator paper works within seconds, quickly providing answers to some of life’s greatest mysteries such as what is the pH of coffee, seawater, and even saliva!
Thank you very much to Compound Interest for the brilliant article and presentation which this session of Exploring Everyday Bio(Chemistry) was based on.
Check out their video (with more details of the experiment and great animations) below.
This blog post is based on the course content I created for a series of sessions on the chemistry of spices as part of the Exploring Everyday (Bio)Chemistry Society which I run at The Tiffin Girls’ School. The sessions focus on connecting chemistry with everyday products and phenomena to encourage KS3 engagement with science and they have been a great success so far. More blog posts based on this society can be found here.
A close-up sketch of the porous and undulating bone formations of the deer mandible that I found and examined a few years back – white pencil on black cartridge paper.
Curcumin is a polyphenol (an organic compound with multiple phenol units). 
Curiosity Killed The Cation 