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Before pride comes a fall


Testing the properties of iodine monochloride
Norman Greenwood Scientist
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So, he was dealing with halogens, as you know, that's chlorine, bromine and iodine are the common ones, but at the top of the table is fluorine and at the bottom is astatine. Now, Emmy was an expert in fluorine chemistry, and he was, and his students, were studying some very reactive compounds, particularly bromine trifluoride, and other fluorides of the halogens, they are the interhalogen compounds where more than one halogen is involved. So, a rather easier one, and I suspect that Harry was leading me in gently, we decided on iodine monochloride, which has exactly the same atomic weight as bromine because it's in between the two of them, but iodine monochloride is a most beautiful compound. I had to make it and crystallise it, and then measure its properties. And to do that, I had to pass chlorine over iodine, that was easy enough to do, and then I got these beautiful, ruby-coloured crystals.

And to make sure that they were pure, and not contaminated with any solvent, I had to purify them in an all-glass apparatus which I'd constructed, and fortunately they melted when pure, at 27.2 degrees Celsius, roughly. And that meant that they melted just above room temperature, it was an easy temperature to handle in a thermostat, and then you could freeze it. And you froze it slowly, only the pure material would freeze out, and the impure would remain in the solution; it's really just a bit like zone refining of silicon crystals. And so, you could pour the impure liquid off, and keep the crystals out of the way, and then you re-freeze it again partially, and pour it off, and after successive recrystallisations, you could grow crystals which were, oh, perhaps 2in long, 5cm long; thin, transparent, needle-like crystals. And you could see right through them, it really was like shining a light through a ruby, they were beautiful. But the trick, of course, was you couldn't then take them out, because they'd be contaminated, you had to do all the measurements there.

So I had to design an apparatus, which was not too difficult. I did... with the crystallisation balls, and then underneath that, the conductivity apparatus. And when it was completely pure, I'd fuse, melt, the compound that was pure, pour it by upending it into the conductivity cell, and then just seal off. And by this time of course I was a good glass blower, so it would all be done, untouched by human hands. So I did that, and got an interesting result, actually, first experiment. I did it as a function of temperature, so I was doing it between room temperature, say 15 to 20 degrees, because I could super-cool it a little bit, up to about 80 degrees. The conductivity increased, and that wasn't unexpected, because I thought, well, if it's slightly higher... it was highly conducting, which was the first surprise, and the conductivity increased. And... but as you heated it, and I did this several times, it went through a maximum, at about 45 degrees, and then diminished again, so that was a peculiar thing.

Two other technical things; it had no decomposition potential when I used direct current, and there was no electrolysis with... passage of electric current. So this was really a very interesting result, Emmy got very excited about it, JS Anderson, who was at Harwell at the time, had come over to give a lecture, he was excited about it. NV Sidgwick, who was the Oxford professor, and a venerable old man at this stage who'd written another enormous book which was published post-war, The Chemical Elements, he was actually an authority on interhalogens, and he just loved this, he was perched on a lab stool, I can still see him, talking about this. So this was all very interesting.

Norman Greenwood (1925-2012) was born in Australia and graduated from Melbourne University before going to Cambridge. His wide-ranging research in inorganic and structural chemistry made major advances in the chemistry of boron hydrides and other main-group element compounds. He also pioneered the application of Mössbauer spectroscopy to problems in chemistry. He was a prolific writer and inspirational lecturer on chemical and educational themes, and held numerous visiting professorships throughout the world.

Listeners: Brian Johnson

Professor Brian FG Johnson FRS, FRSE, FRS Chem, FAcad Eu, FAS. Professor of Inorganic Chemistry University of Edinburgh 1991-1995, Professor of Inorganic Chemistry University of Cambridge 1995-2005, Master Fitzwilliam College Cambridge 1999-2005. Research interests include studies of transition metal carbonyls, organometallic chemistry, nano- particles and homogeneous catalysis. Professor Johnson is the author of over 1000 research articles and papers.

Tags: The Chemical Elements, Harry Emeléus, JS Anderson, NV Sidgwick

Duration: 5 minutes, 18 seconds

Date story recorded: May 2011

Date story went live: 25 November 2011