I expect most of you have heard this song, with its sad words.  It’s full of letters sad and blue, a photograph or two, roses, tokens ….

Not for me!  But as I am clearing out our chemical store, with materials being sent off for disposal prior to the closure of the Reading University Physics Department, some of those bottles do hold memories for me.

There, in an outside brick shed, far enough away to store large quantities of flammable chemicals, sits a jar of sodium nitrite.  But the use to which it was put was somewhat unorthodox. 

Laboratories often have a number of heating baths, and ours has been no exception.  Water baths are the most common, but with these you are restricted to an upper limit of 100°C, though if cooling is required the natural lower limit of 0° can be extended by the use of antifreeze.  Higher temperatures require the use of oil baths.  Now oil in this context often means liquid paraffin of the kind that is used as a laxative.  But if, after dunking it in the hot oil, you want to clean the apparatus or specimen afterwards, this often means use of organic solvents (did I hear the environment beginning to protest?)  Moreover, at high temperatures the oil does not last long in good condition, so people often tend to prefer silicone oil.  Silicone oil is stable and non-toxic [1] but it is more expensive and difficult to remove afterwards.  So yours truly defies convention [2] and uses glycerol instead.  This is very non-toxic and can be washed away with water.  After a few hours’ use at higher temperatures, though, it does discolour and start to caramelize, so it is effectively limited to a maximum of 200°C.  Wanna go higher?

My Ould Da (as we say in the city where I was born) was a chemical engineer, and used to relate to me tricks of the trade that you don’t get in the chemistry textbooks.  One time, for a particular application, he suggested the use of Dowtherm A, which is
a EUTECTIC mixture of two very stable compounds, biphenyl (C12H10) and diphenyl oxide (C12H10O). The fluid is dyed clear to light yellow to aid in leak detection. DOWTHERM A fluid may be used in systems employing either liquid phase or vapor phase heating. Suitable applications include indirect heat transfer.  
Now bi- (or di-) phenyl freezes at 69°, and diphenyl oxide freezes at 25° (which can be a nuisance even in temperate Britain when it’s kept in that outside brick shed, which I think is a wartime relic.)  However, mix the two together in a eutectic composition, and the freezing point is lowered drastically.  What you now have is a fluid composed of two relatively stable chemicals which has a large range of operating temperatures.  (Probably the best-known eutectic mixture is common solder, a mixture of tin and lead with which electronics enthusiasts are familiar.  Here is a page which explains the principle clearly.)

So, wanted – something stable, which can be washed off with water.  Why not a molten salt?  Most salts are much too high-melting for our purpose, but they do form eutectic mixtures which are much lower melting. 

To this end, I searched in our university library and found the Molten Salts Handbook by George J Janz, 1967, and found a suitable mixture.  As a more recent book, namely the
Handbook of Applied Thermal Design, Eric C. Guyer (Editor-in-Chief), 1999, states succinctly:
The most common mixture has been in use since the 1930s.  A blend of potassium nitrate, sodium nitrite, and sodium nitrate (53/40/7 percent by weight respectively), it melts at [about 142°C] and can be used up to [about 480°C]. 
So our technician (we had one in those days!), a most capable man, made up a brass pot surrounded by band heaters connected to a temperature controller, and there was our salt bath!

(Confession: I didn’t at the time spot the extra little bit of sodium nitrate, but even so, the mixture I did use was low-melting enough to be serviceable.)


[1] A colleague once spent a few months in a Japanese laboratory.  They wanted to cook an English breakfast, so after learning the recipe from him, they obtained the eggs, etc., and proceeded to fry them in silicone oil.

[2] The song My Way has an interesting history, and has been said to be suitable for dictators on trial for crimes against humanity or drunkards expiring in a ditch.  Put me in a laboratory, and you can add me to that list.