Compound Interest: The Chemistry of Glow Sticks

The chemistry of glow stick colours — Click to enlarge

Everyone’s familiar with glow sticks, but it’s likely that fewer are familiar with the chemistry behind their glow. You may have wondered what happens when you snap a glow stick to activate it; by doing this, you’re actually kicking off a chemical process that eventually leads to the production of the coloured light. But how does this process work, and why do you need to bend the glow stick to initiate it?

Glow sticks actually contain two separate compartments, with two different chemical solutions. One solution, in the case of most glow sticks, contains a diphenyl oxalate compound, along with a dye whose identity varies depending on the desired colour. The other solution is one of hydrogen peroxide, a chemical recently featured in the ‘Everyday Compounds’ series, and is contained within an inner glass cylinder. This cylinder keeps the two solutions separate from each other, and prevents them from reacting. The action of snapping the glow stick breaks the glass cylinder, allowing the two solutions to mix and kicking of the reaction leading to the glow.

The reaction that takes place is between the hydrogen peroxide solution and the diphenyl oxalate. The diphenyl oxalate is oxidised by the hydrogen peroxide, which, amongst other products, produces the unstable compound 1,2-dioxetanedione. This compound is so unstable that it readily decomposes into carbon dioxide, also releasing energy as it does so. It’s at this point that the compound being used as a dye comes into play. Although uninvolved in the reaction, the electrons in the molecules of the dye can absorb the energy given off by the decomposition of 1,2-dioxetanedione, and in doing so they are promoted to an ‘excited state’. When the electrons fall back to their ‘ground state’ (i.e., their original energy), they lose their excess energy, in the form of photons of light. This process is known as chemiluminescence.

The exact energy of the light given off is dependent on the structure of the molecule, and this allows different colours to be achieved. A range of different chemicals can be used, including those shown in the graphic, as well as one or two additional dyes. Whilst the molecules of the dye are always present in the solution, the hydrogen peroxide and the diphenyl oxalate are slowly used up by the reaction, until one runs out and the reaction ceases – and it’s at this point that the glow stick will stop emitting its glow.

You may also have noticed that the packaging of glow sticks often advises that they shouldn’t be cut open. There’s also a chemical reason for this. As well as the hydrogen peroxide contained within the glass cylinder inside the glow stick, the reaction of the diphenyl oxalate with the hydrogen peroxide can also produce small amounts of phenol as a byproduct. Skin contact with the solution contained within glow sticks can therefore cause some unwanted effects, including irritation and dermatitis.

On a final note, glow stick reactions, like many chemical processes, can be influenced by temperature. Warmer temperatures will accelerate the rate of reaction, whilst cooler temperatures will decrease it. Want to make your glow sticks last longer? Stick them in the freezer!