The Golden Age of Poison

by Jasmine Cotton, MChem Chemistry. Originally published in issue seven of Resonance.

BooksPoison has long been a murder weapon of choice, both fictionally and in real life. There seems to be a morbid fascination surrounding the use of poison, one which has led to several murder mystery books adopting it as the murder weapon. But do they depict it accurately?

One of the great murder mystery authors was Agatha Christie. Across more than 80 detective novels, she has killed off over 300 people, and at least 100 of those were murdered using poison. Christie uses a vast range of poisons in her detective novels, the chemistry behind which is more often than not accurate in its description.

The accuracy behind Christie's chemistry can be traced back to her volunteer efforts during the First World War. At first, she volunteered as a nurse in Torquay, but later she was offered the chance to work in a pharmacy as a dispenser. During those times the prescriptions had to be prepared by hand, and in order to be qualified to do this, Christie had to pass a number of exams. For these, both a theoretical and practical knowledge of chemistry had to be studied. Thus, Christie would have had a vast knowledge of common medicines found in people’s homes, as well as how other prescription medicines or household chemicals would react with these. This knowledge allows Christie to spin a plausible plot, making her murder mystery books ever more thrilling.

One book in which Christie’s chemical knowledge is applied brilliantly, is her first-ever Poirot novel, 'Mysterious Affair at Styles', published in 1920. The basic plot line of this novel is the wealthy Mrs Inglethrop is found dead in her room one morning. The poison which kills her is found to be strychnine; a poison if given as a fatal dose, typically taking 15 minutes to act.

During the time that 'Mysterious Affair at Styles' was written, strychnine was a common medicine. It was prescribed as a remedy for heart and respiratory complaints. Thus, it was common to see in most households. In the novel, strychnine was dissolved in water to make a tonic, one which Mrs Inglethrop took regularly. The question therefore arises, how did she ingest a fatal amount of strychnine? As mentioned above, lethal doses take around 15 minutes to act, so how was it that Mrs Inglethrop had suddenly died one morning long after taking this particular batch? Surely if this contained a lethal amount, then she should have died much sooner. This is where Christie demonstrates her aptitude for chemistry.

Figure 1: Strychnine sulphateThrough her studies to become a dispenser, Christie knew that the addition of a large amount of potassium bromide to strychnine bromide tonic would lead to the precipitation of strychnine bromide cystals. This is due to the common ion effect; the reduction in the solubility of an ionic precipitate when a soluble compound containing one of the ions of the precipitate is added to the solution. This effect can be seen:a solution of strychnine sulphate in water is completely clear, but after addition of potassium bromide, crystals of strychnine bromide precipitate, see Figure 1.

Potassium bromide was used as a common prescription medicine as well, one which the novel stated Mrs Inglethrop would occasionally take as a sleeping pill. Thus, the murderer had all of the necessary compounds to hand already.

This is the chemistry behind the murder of Mrs Inglethrop. Once potassium bromide was added to the tonic, a lethal dose of strychnine would precipitate to the bottom of the solution. As long as the solution was not shaken, the lethal dose would be administered as Mrs Inglethrop reached the end of the tonic.

This is just one example of Christie exercising her chemistry prowess. Clues of her chemical knowledge can be found in her liberal poisons in other numerous novels. Just a few examples include white phosphorus in 'Dumb Witness'; thallium in 'The Pale Horse' and the infamous cyanide in several novels. 

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References:

  1. Agatha Christie’s murders are enmeshed with real chemistry, PRI
  2. Agatha Christie: The Chemistry of a (Nearly) Perfect Murder, Chemistry Views
  3. Poison tales: the chemistry of crime fiction – Science Weekly podcast, The Guardian
  4. Die tödliche Brechnuss. Strychnin – Von der Isolierung zur Totalsynthese, Klaus Roth, Chem. Unserer Zeit 2011, 45, 202–218. Chem. Unserer. Zeit. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission.