[GrahamO]

Hello,

First post, so please be gentle and I hope this question is within the remit of the forum. I crave your forgiveness if it is not.

Many years ago I was an analytical chemist, so my inorganic knowledge was limited then and almost non existent now. On the basis of my electronics knowledge, I was asked to solve a problem by a friend, but I think the solution may be chemical.

What I am looking for is a colour change reaction which is permanent and can be achieved by passing a small current through the "indicator tube" (a transparent tube in which the colour change occurs and is visible to the operator). The trigger for the reaction will make a circuit between a battery and the indicator tube. As the battery discharges, the current flowing through the indicator tube will effect a colour change which can be seen.

Rough specs for the system are:
Voltage - 1.5V preferred, (a single dry cell) but more could be put in series if required.
Current - a few mA
Lifetime untriggered - 10 years
Lifetime triggered - must be visible for 10 years
Environmental - must work from +60C to -20C

If anyone can give me any pointers for solutions or where to look for further information, it would be much appreciated.

Many thanks for your help,

Graham

[Borek]

In general it sounds doable, with one serious caveat. -20°C means easy to manage water electrochemistry is off the limits.

How much charge can you transfer through the cell? In other words - how many mAh can be used to switch the indicator?

[GrahamO]

Yes, the -20C does cause problems with aqueous media. I've been trying to think of non aqueous or anti freeze media? How about solid media? I was considering an electrochemical cell which is biased off by the dry cell but after triggering the battery is by passed, the cell can then react and there is a visible colour change? I'm full of ideas, but lacking in knowledge.
For the amount of current/charge, we are not limited within reason by the size of battery and could add extra voltage or charge as required. To meet 10 year lifetime (battery is not discharged until device triggers) and environmental, my first thoughts are something similar to Energiser Ultimate Lithium cells.

Thanks for your help and input.

Graham

[Borek]

Yes, addition of some antifreeze agent could do the trick.

I would think in terms of something rather simple - say, oxidation of Fe²⁺ to Fe³⁺ in the presence of thiocyanate (I am not saying this particular combination will work, it is just to give the idea, you need this kind of reaction). Solution sealed in a glass tube, just with two contacts/electrodes sticking out. Add some ethanol or methanol to water, to make sure it doesn't freeze, but doesn't interfere with the chemistry. Initially solution is colorless (perhaps not entirely, but mostly), once you pass the current and iron gets oxidized, it gets complexed and you observe color of the famous red/wine complex. You will need another redox system though for the cathode reaction.

In a way the problem is similar to the construction of chemical coulometers (compare this thread: http://www.chemicalforums.com/index.php?topic=63142.0). Could be the solution was described (or even commonly used) 80 years ago, just nobody remembers it today.

[GrahamO]

Thanks Borek, that is the sort of information I am after. I was thinking that FE²⁺ to FE³⁺ would do the job. Your mechanical suggestions are spot on and what I was thinking. I'll have a look at the coulometer link.

Thanks for your help. Any other suggestions from anyone else?

Graham

[Enthalpy]

Your project seems a perfect application of my old patent EP0564012
http://www.freepatentsonline.com/EP0564012.html
http://www.freepatentsonline.com/EP0564012.pdf
which has expired hence is available for free. The text must exist in other languages.

I considered the deposition of a metal by electrolysis because metal 100nm thin suffices and needs very little current, far less than changing the degree of oxidation of an ion in a thick solution. The display has a transparent upper electrode (consider a metal mesh also, not just ITO), a metal-containnig electrolyte few µm thin, and a back electrode of some colour. The current deposits the electrolyte's metal under the top transparent electrode. Au²⁺ reduced to Au for 100nm on 5cm², or 1mg, needs 1C or 10mA*100s for instance.

-20°C shouldn't be a worry as the dissolved salts prevent water from freezing. You might use some powder or fabric to hold the water in place, or gel it. Even solid electrolytes exist.

Changing the colour once must be easy, and if choosing the proper metal and ions, 10 years isn't a worry (except to test it quickly). Gold cyanide has the drawbacks of cyanides, but there are more options.

I wanted it as a depletion indicator for telephone prepaid cards when they still existed. Later, people wanted a reversible indicator, which would have been more difficult.

Beware I didn't test it! It looks reasonable to my eyes, the experts here may well have a different opinion, and engineering is always full of surprises.

[GrahamO]

Thank you Enthalpy for the information. I am quite intrigued to try it and possibly to register multiple triggers, i.e. have multiple colour change cells with each trigger changing the next cell along. There is some background information which means this could be useful to register if the trigger is a single event or multiple events.

Lots of help and ideas here, thank you to all.

Regards

Graham