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Pontifications on Poison

Being some ramblings on events associated with poisonous plants.

Wednesday 21st December 2011 

Someone told me they’d heard that rhubarb leaves, from Rheum x hybridum, had hundreds of years ago been used as a murder weapon and asked me if I could find out if it was true.

The story they were told was that rhubarb leaves were soaked in water to extract the oxalic acid and the resulting liquid was poured into drinking goblets where it dissolved the silver and produced a poison that was deadly for whoever drank from the goblet.

When I started to think about this my initial reaction was to rely on my arrogance. I’d never heard this story before and it would be easy to think that if I haven’t heard of a way of using a poisonous plant extract then it won’t be true. But, as I’ve said before, I do my best to be a scientist so I know that there are always new things to learn. Maybe, I was learning something new about the humble rhubarb leaf.

Rheum x hybridum, rhubarb

Rheum x hybridum, rhubarb leaves

So, setting my arrogance to one side, I began to think of the questions I would need to ask to determine if this was a practical way to kill someone. Obviously, I would need to find out if oxalic acid could be extracted from rhubarb leaves just by soaking them in water. Then, there was the question of how strong the oxalic acid solution would be because that would determine if it would react with enough silver to form a lethal amount of the alleged poison.

And, of course, I wanted to find out about the taste of such a liquid both before any reaction with the goblet, during the reaction and after the reaction was complete because a distinctive taste at any of those stages would be likely to arouse suspicions.

And, finally, I needed to know how poisonous the resulting silver oxalate would be to check to see if a lethal dose could be obtained from the contents of a drinking goblet.

That seemed like some quite tricky questions but I set about my quest and almost immediately found this; ‘silver oxalate is insoluble in water and forms a sludgy grey deposit in the bottom of the vessel’.

That seemed to pretty much be the answer to everything. You would hardly expect to get away with a murder weapon that made itself so obvious. Clearly, no-one would have tried this as a method of committing murder. I almost gave up looking for any further information but I tried checking on how much silver oxalate you would need and found that it is as near to being non-toxic as it is possible to be. That is to say, I found a safety data sheet for it saying that ‘To the best of our knowledge, the chemical, physical, and toxicological properties have not been thoroughly investigated’. It also classified it as ‘0’ on the various harm scales.

I think it is pretty reasonable to assume that if no-one has ‘thoroughly investigated’ the toxicity of a substance it is because there has never been any suggestion that it is toxic.

I’m quite comfortable to now fall back on my arrogance and say I’d never heard of this way of killing someone because it doesn’t exist.

What interests me, and it is a question to which I will never find the answer, is why did my contact’s contact say that this was so? People do get substances mixed up quite readily so perhaps there is something else that could be used in a similar, if not completely identical, way. Or, the story may have come from a work of fiction. Many writers take care to try and present information about poisonings as realistically as possible, but not all. There could be a book somewhere that relies on this totally impossible method to further its plot.

Rhubarb leaves are toxic, though it’s worth saying that the oxalic acid is not thought to be what makes them lethal because the concentration is not much different from something like spinach, Spinacia oleracea, but, if you’re getting out the really special tableware for your Christmas meal, you don’t need to worry about someone trying to use them to bump you off.