Poison Chemistry - Thallium Sulfate

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Having already looked at arsenic and cyanide in the previous instalments in this series, our attention turns to thallium, another famed poison. Thallium perhaps doesn’t share quite the same profile as arsenic and cyanide, but despite this it’s perhaps an even more effective compound in poisonings. This graphic looks at its history and effects.

Thallium as an element was first discovered in 1861 by the British chemist William Crookes. He identified it from the bright green line in its emission spectrum, and consequently named it thallium, from the Greek word ‘thallos’, meaning ‘green shoot’. The toxicity of thallium and its compounds was quickly noticed; experiments in which the compound was fed to animals saw all of the test subjects die within a few days.

Despite its toxicity, thallium compounds saw use in medicinal treatment of a number of conditions. Thallium sulfate was used for the treatment of diseases and conditions including syphilis, gonorrhoea, gout, and dysentery.  The efficacy of these treatments isn’t noted, but they did inevitably come with significant side effects, and as such didn’t gain widespread use. It was also used cosmetically, as a depilatory agent to remove hair from the skin.

Later, in the early 1900s, use of thallium sulfate as a rodenticide and insecticide became more frequent. Of course, its availability as a rat poison also made it readily available to any would-be poisoners, and cases of both accidental and homicidal poisonings with thallium sulfate increased as a consequence over the decades after its introduction. Today, its use in rat poisons is banned in many countries, and has been since the 1970s, though some countries still allow its use.

Unless given in a high dose, thallium is an agent that exerts its effects slowly. Its early symptoms are very non-specific, including gastrointestinal problems, such as vomiting, or merely nausea. For up to four days after the initial symptoms, victims can actually feel and appear in normal health. At this stage, constipation can be a characteristic sign, but not one that is likely to cause undue concern in the victim. After this, more unpleasant symptoms can start to kick in. The victim starts to experience pains in their extremities as a consequence of nerve damage, along with excessive thirst and insomnia.

Two weeks into the poisoning, the patient starts to exhibit a symptom that is characteristically that of thallium poisoning: a dark pigmentation will begin to appear around the roots of the hair. This then rapidly progresses to complete loss of the hair and alopecia. In many cases, this is the symptom that alerts those providing treatment to patients with thallium poisoning. The skin of the victim becomes dry and scaly, and white lines, referred to as ‘Mee’s Lines’, appear on the fingernails. Increased heart rate and loss of some motor control can also occur, and in severe cases blindness and further nerve damage, eventually culminating in death due to respiratory failure or cardiac arrest.

The toxicity of thallium is due to its similarity in properties to potassium and sodium ions. Potassium ions have a number of roles in the body, including the transmission of messages through nerves; however, the body cannot distinguish between thallium and potassium, and the incorporation of thallium ions instead means that these roles cannot be carried out.

There is no real antidote for thallium poisoning, in the sense that there is no known agent that will remove thallium from the body once it has already been absorbed in the gastrointestinal tract. However, a number of agents can be administered to prevent it from being absorbed, or reabsorbed. The primary agent used in cases of poisonings is Prussian blue, though it itself is toxic so must be administered in moderate doses. Thallium can also bind to sulfhydryl groups, and so compounds such as N-acetylcysteine have also been suggested as a treatment method, with a degree of success.

In terms of detection, once spotted thallium poisoning can be confirmed via tests on the urine. A rapid test can be carried out on a urine sample using bismuth nitrate, nitric acid and sodium iodide, which gives a red precipitate in solution if thallium is present. However, on occasion false positives can be obtained from this test, so instead atomic absorption photospectrometry is more commonly used. Thallium gives a characteristic absorption of light using this method that can be used to identify its presence.

These days, thallium sulfate is much harder to come by, meaning that its use in poisonings is relatively rare. Nonetheless, the do still occur, and there was a high profile case as recently as a few years ago, when a computer engineer in New Jersey was admitted to hospital and later died, as a result of what was confirmed to be thallium. His wife was a research chemist who had access to thallium, and was later convicted of the murder.



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References & Further Reading