Chemical Traffic Light

Useful information
Kit List: 

Glass flask with lid, suitable for vigorous shaking
Reagents - These should be kept mixed in the flask however there are some spares if it needs replacing. It contains:
5g Sodium Hydroxide
3g Dextrose (or probably Glucose)
250ml Distilled Water
Indigo Carmine
White paper to use as background for colour changes.

Packing Away: 

Secure flask so no risk of destruction.


You should before beginning carefully pour the solution from the flask into a plastic screw top bottle (e.g. 500ml pepsi bottle) and tape the lid. At the end of the day, cut the tape, pour the solution back into the flask. If possible replace the bottle the next day. The solution seems to have limited effects on plastic over the course of a day but long term storage could have effects (we don't intend to find out) [source: Royal Society of Chemistry, they also do this experiment this way].

Every so often while demonstrating you'll need to add a pinch of indigo carmine dye. You can show people the colour before it's added (unsurprisingly blue/indigo). Indigo Carmine is both a pH and redox indicator.
Tech info has a range of 11.6-13 with blue acid and yellow alkali.

Start by shaking the flask, the solution should go green after a good shake. Then observe, the solution goes from red to yellow. If you shake again it goes back to green!

What is happening? When we shake there must be some reaction happening, however the flask is sealed, so what is causing the reaction. When we shake we're mixing the air into the solution, we then react with the oxygen in the air. What's causing the colour change when we're not shaking then?

Starting from yellow we are in a fully reduced state, a small swirl introduce some oxygen and transitions to red. More oxygen takes us to Green, this is an oxidation.

From green the dye gets reduced by the dextrose, following the same backwards.

PLUS: From paper by Anderson et al, University of Winconsin-Madison. Under alkaline conditions, the glucose is ionised and tautomerises to the straight chain aldehyde and then to the key intermediate, an enediolate anion. This anion can reduce the dye, forming a keto-aldehyde, glycosulose, in the process. Shaking the flask introduces oxygen into the solution, which reoxidises the dye to the blue form, ready to repeat the process. The hydroperoxide anions this produces can cleave the keto-aldehyde in the presence of excess hydroxide to finally give the sodium salt of arabinonic acid.

There are several different colour changes if you shift pH a little bit, this should be placed for a nice traffic light. Lower pH gives a blue->purple->red->orange->yellow change but is slower due to less molarity (<11.4)
Higher pH gives yellow->red->yellow (>13). The one we have is in between, note green=blue+yellow. It's a bit of a pain in our set up our thing to change pH frequently and conserve resources.

You can also use different indicators, methylene blue in the same solution gives a classic 'blue bottle' reaction

There are endo and exothermic directions with some noticeable heat or lack there of.

This is a site with lots of info on this reaction

Risk Assessment
Date risk assesment last checked: 
Thu, 26/12/2019
Risk assesment checked by: 
Date risk assesment double checked: 
Fri, 17/01/2020
Risk assesment double-checked by: 
Polly Hooton
Risk Assessment: 
Hazard Risk Affected Person(s) Likelihood Severity Overall Mitigation Likelihood Severity Overall
Flask breaking or leaking Glass may end up on floor as a hazard to feet and crawling children. See below for risk of solution contact. All 4 4 16 Transfer solution to a plastic bottle before demonstrating to mitigate risk of breaking. Regularly change the plastic bottle to minimise impact of solution on plastic integrity and prevent leakage. Prevent children scrunching plastic and tape on lid. Keep glass container away during demonstration. If breakage occurs, clear area, solution can be safely cleared from floor with water or weak acid (vinegar). 1 4 4
Human contact with solution Solution is basic and can irritate skin, eyes and internal bits. All 2 2 4 If solution spilt on skin, run area under running water, if eyes use eye wash. If drank phone 999. All chemicals are safe but these things should be avoided. Tape lid onto bottle to prevent kids getting in. 1 1 1