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Name: Victor
Status:  educator
Age: 40s
Location: N/A
Country: N/A
Date: 5/7/2004


Question:
In alcohol thermometers which alcohol is used. And the red color, is it specific to alcohol or some coloring agent is added.


Replies:
Hi Victor!

The alcohol used in thermometers is naturally without color (non colored) so some colorant (blue or red) is added to help the vision of the separation meniscus and the correct temperature reading

Thanks for asking NEWTON!

Mabel
(Dr. Mabel M. Rodrigues)


I am sure it is a dye. All the normal alcohols are water-clear. The simple, small molecule with low freezing temperature and reasonably high boiling point / low vapor pressure, has no large electron clouds capable of electron excitations at energies as low as a visible photon. Alcohol is one of the best solvents for many organic dyes, which by themselves are solids. Only about a percent is needed to make the alcohol red.

I do not know which alcohol, off hand. Methanol (CH3OH) is the strongest solvent for dyes, but its boiling temperature is pretty low. Maybe that does not matter much when trapped inside the capillary tube, but it is preferable to make the thin bulb difficult to burst by overheating, and to make separated segments difficult to form by evaporation. Ethanol (CH3CH2OH) is probably often used. Heavier alcohols ( CH3(CH2)nOH ) have higher boiling points, but become progressively weaker solvents. So Ethylene Glycol ( antifreeze, HOCH2CH2OH ) would be a good choice, high-boiling, low-melting, and strongly solvent of many dyes. Glycerin is less toxic, but it can freeze at 20C. Propylene glycol and poly-ethylene-glycols HO-(C2H4-O-)n-H are probably good. Mineral oil will not dissolve any dyes you will commonly find.

I guess there are lots of choices, depending on what you value most: color, temperature range, non-toxicity, age-immutability. In my Fisher Scientific catalog here are some "Red-Spirit" and "Blue-Spirit" thermometers; says the blue is iso-amyl benzoate, red is kerosene. Neither is even an alcohol. Slightly surprising to me that isoamyl benzoate has a melting point below -30F. Melting point of 2-Propyl benzoate is +58C, 4-propyl benzoate, +141C, and ethyl benzoate, -35C. Melting points can be unpredictable.

To find out about the thermometer you have in hand, you would have to find claims on the package or in the catalog or asking the manufacturer. Or break it open and figure out the unknown liquid.

If you are making your own thermometers, you have lots of fluids to choose from. Then add any dye that is soluble in each particular fluid. This could be a search in itself, admittedly, but I think you can find something. There are some findings at Google: thermometer alcohol dye. A small amount of Iodine would darken your alcohol pretty well, straighforwardly. I think it may fade in the long run.

Jim Swenson


Good question. I searched the Internet and every "hit" just mentioned "alcohol" without specifying. Given the nominal range, I think it is safe to assume that the fluid is anhydrous ethanol + small amount of red food coloring. Sometimes a yellow dye is used.

Vince Calder



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