Plutonium

20 August 1942: a droplet in the Jones Chemical Laboratory

At the University of Chicago's Jones Chemical Laboratory on 20 August 1942, Burris B. Cunningham, Louis B. Werner and Michael Cefola concentrated a 0.015-millilitre solution containing purified plutonium until plutonium fluoride precipitated.

This was the first isolation of plutonium in a visible, macroscopic amount — not its first synthesis, its discovery, or its first tracer-scale chemical separation.

What they isolated was a plutonium compound, not elemental plutonium metal.

Seen before it could be weighed

The sample was not weighed on 20 August. The first weighing of a pure plutonium compound followed on 10 September 1942.

Historical accounts differ over the mass of that first weighed sample, giving figures such as 2.77 or 2.11 micrograms; the later weighing should not be conflated with the August precipitation.

Why the wording matters

Element 94 had already been synthesised and identified at Berkeley in February 1941 by Glenn Seaborg, Edwin McMillan, Joseph Kennedy and Arthur Wahl, and tracer quantities had been chemically separated there the same year.

By August 1942, cyclotron irradiation had finally produced enough plutonium-239, and Cunningham's group had developed ultramicrochemical equipment capable of manipulating pure compounds in extremely small volumes. Seaborg led the Met Lab's plutonium-chemistry group and later described the procedure.

The work took place in the Jones Chemical Laboratory, not at the Chicago Pile-1 site under Stagg Field.