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Doctors now follow medicines and nutrients through the body by attaching a radioactive label and watching where it goes. The method began with a failed experiment. This post covers how George de Hevesy turned that failure into the tracer method, the arithmetic behind it, how he first used it on living plants, and what it showed about water in your own body.
Who was George de Hevesy?
His Nobel facts page records that he was born in Budapest on 1 August 1885 and died in Freiburg im Breisgau on 5 July 1966. His Nobel biography says he took his doctorate at Freiburg in 1908, went to Rutherford's laboratory in Manchester in 1910, and in 1913 carried out with Friedrich Paneth the first radioactive-tracer experiment in Vienna. In Copenhagen, with Dirk Coster, he discovered hafnium in 1923. He fled Nazi Germany in 1934 and settled in Stockholm in 1943.
The 1943 Nobel Prize in Chemistry went to him alone "for his work on the use of isotopes as tracers in the study of chemical processes". The prize was reserved that year and handed over in 1944.
How did a failure become a method?
In his Nobel lecture, Hevesy recalls that Rutherford had several hundred kilograms of "radio-lead" from pitchblende. It contained radium D, a radioactive isotope of lead with a half-life of about 20 years, but the ordinary lead absorbed its radiation and made it almost useless. In 1911 Rutherford told him: "My boy if you are worth your salt, you try to separate radium D from all that lead".
Hevesy worked at it for almost two years and failed completely. Then he turned the problem around. If radium D cannot be separated from lead, it must follow lead through every chemical step. An amount of it too small to weigh could label lead.
Worked example: counting lead you cannot weigh
Hevesy's lecture gives the logic. Dissolve 1 g of lead as lead nitrate and add radium D with an activity of one million relative units.
Each unit corresponds to 1 g ÷ 1,000,000 = 1 × 10⁻⁶ g, or 1/1000 mg, of lead.
If a later fraction shows 1 unit of activity, it contains 1/1000 mg of the original lead.
A fraction showing 2,500 units (a sample number) would hold 2,500 × 10⁻⁶ g = 2.5 mg.
His first use, with Paneth, measured the solubility of lead chromate, labelled with another lead isotope, thorium B. The lecture gives the result as 2 × 10⁻⁷ mol per litre. Since PbCrO4 gives one Pb²⁺ and one CrO4²⁻, the solubility product is Ksp = s² = (2 × 10⁻⁷)² = 4 × 10⁻¹⁴. The NCERT Class 11 Equilibrium unit covers Ksp, and our post on solubility product calculations has more.
Common mistakes to avoid:
Forgetting decay. Activity falls with time, so compare samples measured at the same time or correct using the half-life. Our post on radioactive decay and half-life shows how.
Ignoring daughter products. Hevesy's 1923 paper notes he waited about six hours for thorium B and its product thorium C to reach equilibrium before measuring.
Squaring the wrong thing. For a 1:1 salt Ksp = s²; for a salt like PbI2 it would be 4s³.
How did tracers first reach living things?
His paper The absorption and translocation of lead by plants (1923) used broad bean plants, *Vicia faba*. He dipped their roots for 1 to 48 hours in lead nitrate labelled with thorium B, then burned each part of the plant and measured the radioactivity of the ash with an electroscope.
From very dilute lead solutions, the roots took up more than half the lead, and almost none reached the stem and leaves. He then moved a labelled plant into solutions of other salts. After 24 hours in inactive lead nitrate, only 5% of the labelled lead remained in the roots; with sodium nitrate, 76% remained. Lead in the roots could be swapped for other lead atoms, so it had to be held as a loosely bound salt, perhaps on the cell walls, not locked into organic molecules.
What did heavy water show about your body?
Later, Hevesy used dilute heavy water as a label for water itself. His Nobel lecture reports that while a small fraction of water drunk is excreted within 26 minutes, the average life of a water molecule in the human body was found to be 13.5 days. The deuterium that made this possible was discovered by Harold Urey; see our post on Harold Urey and deuterium.
To practise the decay and tracer arithmetic, the Fermi Problem Library has worked problems with a step-by-step solver that explains the reasoning rather than printing an answer.
What can a student learn from Hevesy?
Turn an obstacle into a tool. Two years of failure gave him the method.
Choose a simple organism. A bean plant in a jar let him ask a clean question.
Measure what you cannot see. Radioactivity let him count far less lead than any balance could weigh.
Try it now: if radium D has a half-life of 20 years, what fraction of its activity is left after 60 years?
