Retiree's Lunch Update June 2021
Written by Dr Richard Thwaites, FRACI CChem
Published 9 July 2021
“ARSENIC” – a talk by Professor Ian Rae
Our Retirees’ Group was treated to a fascinating presentation, prepared at short notice by Professor Ian Rae, about arsenic at a virtual lunch on Tuesday June 1st, held on-line due to the recent COVID-19 lockdown in Victoria. Whilst we were deprived of Graduate House cuisine, we were able to feast on a bounteous supply of facts and anecdotes about arsenic, provided by Ian.
Arsenic (Atomic Number 33) is a most curious element which occurs quite widely throughout the world, comprising some 1.5 ppm or 0.00015% of the earth’s crust overall. It is slightly more abundant than the rare earths holmium, terbium and thulium, and the halogen bromine, and slightly less abundant than uranium, tantalum, germanium and molybdenum. (I wonder who actually goes round measuring these concentrations? And how do they do it? Obviously a topic for another day.)
Traditionally arsenic has been known as a metalloid, having properties resembling those of both a metal and a non-metal. It can exist in three oxidation states, -3 in the arsenides, which (according to Wikipedia, my main source of chemical knowledge these days as I have forgotten nearly everything else) are alloy-like intermediate compounds, +3 in the arsenites and +5 in the arsenates and most organo-arsenic compounds.
Arsenic ores usually contain iron and sulphur as well as arsenic, with compounds with the generic formula such as FeAsS arsenopyrite being among the most common although other arsenic bearing ores exist like realgar and orpiment, compounds of arsenic and sulfur.
Ian told the story of his visit to a small town in NSW, where he had heard there had been an old mine where the ore bodies included cobalt, but on enquiry, nobody in the town seemed to know anything about it. So, Ian set out to do some exploring for himself, and very soon came across a field surrounded by barbed wire fencing, with several holes dotted around and rocks strewn everywhere. Clearly the site of the old mine! Ian retrieved one of the rocks that showed the pink oxidation product, erythrite (cobalt arsenate), and when he went back to the town, they agreed that was where the mine had been, but it seemed that locals liked to keep quiet about it to deter strangers, just in case the value of the ore bodies increased and it would be worthwhile to start excavating again.
Incidentally, in doing some research for this report, I found a fascinating document produced by the NSW Department of Mines, part of the Geological Survey of NSW, entitled: “The Mineral Industry of NSW, No 3, Arsenic” compiled by D S Flack in 1967 which covers deposits, recovery, uses, production and substitutes for arsenic. A really interesting read, available on the internet. And NSW seems to be home to an incredible number of arsenical ore bodies! It wouldn’t surprise me if the As content of NSW’s soil is significantly above the global average.
Arsenic has played a major role in crime fiction. The play (described as a farcical black comedy) “Arsenic and Old Lace” had record runs on Broadway and the West End in the early 1940s, and the film version was equally successful at the Box Office.
Arsenic was one of Agatha Christie’s favourite poisons. Not that she actually used any herself in real life, it’s just that in many of her novels, characters went down with arsenic poisoning: the culprit responsible for the killing was a lot more difficult to trace than a villain relying on shooting or stabbing his (or her) victim. Ian noted that Ms Christie knew what she was writing about as she had been a laboratory assistant before turning to writing about crime. (Maybe more of her victims would have been shot if she had been a member of a gun club, and possibly more would have been stabbed if she had been a part-time butcher or a fencer? Who knows?)
But although Ian touched on some of arsenic’s literary fame, he mainly talked about where it occurred, what it has been used for (other than bumping people off) and the strange fact that in small quantities, the ingestion of arsenic compounds was thought in some cases to be beneficial and to have curative properties, but in larger quantities, it was lethal. Like so much of chemistry, it’s all a matter of dosage and concentration.
Arsenic was often used to improve the shininess of horses’ coats. Phar Lap had a very shiny coat, but following his death, the autopsy indicated that he might well have had a bit more arsenic in his diet than was really good for him. Whether feeding arsenic to Phar Lap was intentional or accidental remains one of the great mysteries of the 20th century.
Arsenic compounds were used for many years in sheep and cattle dips as insecticides. Arsenic compounds have also been used as weedicides, fungicides and in wood preservative solutions.
Thomas Fowler, a British physician in the 18th century, published a treatise: “Medical Reports of the Effects of Arsenic in the Cure of Agues, Remitting Fevers and Periodic Headaches”. This was the start of the therapeutic use of inorganic arsenic. Actually, Fowler discovered that arsenic was a key ingredient in a locally available patent medicine, and building on this observation, he produced his own “Liquor Arsenicals” or “Fowler’s Solution” which was listed in the London Pharmacopeia in 1809. Enough patients survived for the product to be widely prescribed for a variety of complaints, although you probably wouldn’t want to exceed the recommended dosage.
Arsphenamine, also known as Salvarsan or Compound 606, was first synthesised in Paul Ehrlich’s lab in Frankfurt by Alfred Bertheim in around 1907. It was introduced soon after as the first effective treatment for syphilis (just in time for the start of the First World War). This organo-arsenic compound with the chemical structure recently elucidated as cyclic (Ar-As), rather than the original Ar-As=As-Ar structure assigned to it by Ehrlich has been described as the first modern anti-microbial agent.
Although compounds of arsenic are mainly used to bump bugs (or people) off, its major application which does not make use of its poisonous qualities is in the glass industry where refined “white arsenic” (arsenious oxide) finds application as a decoloriser, as a constituent of opalescent glass and in enamels.
So the moral of the story is, if you walk into a bar and the bar tender asks “What’s your poison?” maybe don’t say “Arsenic”: just stick to beer (or Scotch, or G&T, or Vodka,…..).
Hope to see you soon!
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