Sciencemadness Discussion Board

temperature dependent color change of a solution?

DubaiAmateurRocketry - 6-10-2018 at 18:58

Anyone aware of color change of solutions where the color change starts at around a temperature of 40 celsius ?

Sulaiman - 7-10-2018 at 01:46

for many of the thermochromic pigments available via eBay,
31oC is a common transition temperature
some transition at 13oC
I've not noticed any at 40oC.

Most of the "thermometer strip" via eBay have one 40oC patch.

(I was looking for a thermochromic dye to use in my recirculating water-cooled condenser system
but only found insoluble pigments that I thought could cause blockages,
while searching I did come across a pigment manufacturer with a range of transition temperatures,
too expensive for me so I did not keep a link )

[Edited on 7-10-2018 by Sulaiman]

fusso - 7-10-2018 at 06:52

Can adjusting cobalt & chloride concentration change the transition temp?

DubaiAmateurRocketry - 7-10-2018 at 14:37

Sulaiman, theyre not in solutions though? do they still behave the same in a solution ?

Sulaiman - 7-10-2018 at 14:46

I'm not sure but it seems that the pigments are insoluble in anything that does not destroy the thermochromic effect,
so the pigment is used like those in oil paints, lacquers etc. a slurry/colloid/suspension or embedded in a polymer.
I think.

DraconicAcid - 7-10-2018 at 14:57

Cobalt chloride in solution will change colour with temperature. You could probably play with the concentrations of cobalt and chloride ion so that 40 oC is your transition point. Not sure how broad of a range it will change over, though.

Sulaiman - 8-10-2018 at 02:03

I believe that the thermochromic change in cobalt chloride is practically irreversible ?

woelen - 8-10-2018 at 03:55

A pinch of cobaltous chloride or cobaltous sulfate, dissolved in 10 ml of appr. 3 M HCl makes a good temperature dependent solution. You need to carefully prepare the solution. Dissolve some of the cobalt salt in cold 5M HCl. You get a purple/blueish solution. Drip in cold water while swirling, until the liquid becomes purely pink. At that point the acid will be around 3M.

If you heat this pink solution then you can get it dark royal blue when it is hot and it is pink when it is at room temperature again. This effect is 100% reversible as often as you want. I put some of this solution in a glass ampoule and use it for demo purposes. I just put the ampoule with pink liquid in a cup of hot water (e.g. from a coffee machine) such that the top half of the ampoule remains above the hot water so that you can handle it without hurting your hand. The effect is really striking. I think that the transition occurs over a range of appr. 30 C (purely pink) to 70 C (deep royal blue).