A Stinky Colourful Chemical Mystery

I’m running an outreach activity and we’ll be making alginate worms. I was testing my sodium alginate solutions yesterday as they tend to go off a bit. Usually this is demonstrated by no longer forming beautiful worms or spheres on addition to calcium chloride solution but rather forming slime. I suspect the polymer chains are being clipped. I add food dye as well  – coloured worms are more fun.

One bottle yesterday was colourless and was a 2% solution of sodium alginate (food grade) that had been prepared a few months ago and was dyed with indigo carmine food dye (powdered). It had been stored in the dark (filing cabinet, my office)  at 22 deg C for 4 months. On opening it stank of rotten eggs so I quickly recapped the bottle and turfed it in the sink for cleaning. When I came back to it 30 mins later, it was a blue solution and still stinky!

The other solutions were the appropriate colour and all formed decent worms.

I thought it was quite a nice wee chemical puzzle – anyone want to take a guess at what was going on?  I have a working hypothesis which I’ll share later.





One Reply to “A Stinky Colourful Chemical Mystery”

  1. Well, somehow the sulfur is being released from the indigo carmine. Unfortunately my organic chemistry is a bit too rusty to figure out a mechanism, however.

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