Q: What is the fatal dose of chloral hydrate? Could it be detected in the body in the 1920s?
Carola Dunn, Author of the Daisy Dalrymple mysteries
A: Chloral hydrate was first discovered in 1832 and by mid-to-late 19th century was commonly used as a sedative and when mixed with alcohol as the classic Mickey Finn. It is a relatively safe drug unless mixed with alcohol, which is true of most sedatives and narcotics.
When a drug is evaluated for its lethal potential the term used is the LD50–this means the Lethal Dose 50% or the amount that would kill 50% of the people who took it. For chloral hydrate in humans this is still not well known but it appears to be somewhere between 300 and 500 mg per kilogram of weight. A 150 pound person weighs approximately 70 kgs and so the lethal dose for someone of this size would be between 21,000 and 35,000 mgs or 21 and 35 grams.
Chloral hydrate syrup, which is used as a children’s sedative, contains approximately 500 mg per teaspoon. So you can see that to kill an average size adult would require 40 teaspoons or more of the drug. This is if it is given alone. If you add alcohol to the mixture, everything changes. I could find no data on how lethal this combination is as far as the LD50 is concerned but if your victim was intoxicated and then chloral hydrate was given perhaps only five or 10 teaspoons mixed in some drink or food would work. It’s all very unpredictable so I would simply add some to an alcoholic beverage, have your victim drink two or three of them, become lethargic and sleepy, laps into a coma, stop breathing, and die. The reader will believe that whatever was given it was enough, so don’t sweat the math.
A method for testing for chloral hydrate, as well as chloroform, was first reported in 1923 by JH Ross who worked for Forest Products Laboratories in Montréal, Canada. The test was colorimetric in that an endpoint color represented the presence of the chemical tested for. Here the substance suspected of containing chloral hydrate was mixed with either aqueous sodium or potassium hydroxide and then pyridine. This was then heated and the formation of a deep red color would indicate the presence of chloral hydrate or chloroform–and a few other compounds.
Since this test was new and not widely available it might or might not have been useful in the time and location of your story. When something like this is discovered it’s usually several years before it gets out to the general public so your sleuth, toxicologist, or coroner might know nothing of it. Or they could be very cutting-edge and know about the test. It could go your way so use it as you need for your story. If he did know, he would probably test blood, urine, and stomach contents for the presence of chloral hydrate and, if present, make a best guess as to whether this was an incidental finding (maybe the person used to for sleep and therefore be expected to have it in his system) or was the cause of death. These are all judgment calls and even today are not black-and-white.