Chemical & Chemical Engineering News (80th Anniversary Issue), Vol. 81, No. 36, 2003, Sept. Edited by X. Lu Introduction

Two competitive analyses were to be developed by this group: a rapid analysis for radium via the precipitation route and an emanation (radon) analysis method. Although the rapid radium analysis (RRA) was my main assignment, I assisted the emanation analysis (EA) development. RRA involved barium (radium) chloride precipitation; its dissolution was followed by direct -counting of barium (radium) sulfate on Pyrex glass disks. During this method, development of the effect of other -emitter cations (uranium, protoactinium, thorium, actinium, polonium, bismuth) on barium (radium) chloride precipitation was examined. Although actinium chloride and bismuth chloride were coprecipitated with barium chloride, they did not interfere with -counting, as these members of radium disintegration emit electrons (-rays). The results of this RRA were consistent with those of the EA method; consequently, Mallinckrodt requested it for the analysis because of its simplicity and ease of performance.

Following the successful completion of the radium analyses, it became apparent that the radium half-life had not been determined by direct -counting because of the -particle growth from radon-222 and three polonium isotopes: 210, 214, and 218. The National Bureau of Standards (now the National Institute of Standards & Technology), the repository for radioactivity data, reported 1,590 years for the half-life from an average of several indirect measurements. From an exhaustive literature search, I found 23 indirect measurements of this half-life. I was assigned the measurement of the radium-226 half-life by direct counting in November 1945.

By using the Bateman equations plus a Monroe calculator, I determined the -growth curve for radium based on known half-life values. -Particle growth curves on separate samples were counted for approximately five radon half-lives to determine whether radon diffusion and/or nuclide recoil after -emission was important; the nuclide recoil was found to be important. By using a constant diffusion rate plus variable recoil rates, modified growth curves were constructed. By using these curves with the -growth count for eight hours, the count rate of a known weight of radium after all daughters had been removed could be determined via extrapolation. Then the radium half-life could be determined from this count rate for a known radium weight.

Accurate radium half-life measurements required that I develop micro techniques before proceeding with milligram weights. These techniques included purifying radium by fractional crystallization from aqueous hydrochloric acid, drying purified RaCl₂, quartz fiber microbalance weighing, and preparing deposits for counting, as well as the procedure and counters for eight-hour -counting and extrapolation to zero time. Using arc-spectrograph analysis, the barium content was determined to be less than 0.02%. Then milligram weights of purified RaCl₂ were used to prepare solutions for the half-life determination. Fifty-four different RaSO₄ deposits from the microgram and milligram RaCl₂ solutions were prepared to obtain a half-life of 1,622 years, with an error estimate of 13 years. I completed this measurement in September 1946.

Donald P. Ames is president of Fluotech, a company he formed after spending 30 years as staff vice president/general manager of McDonnell Douglas Research Laboratories. He was a member of the Special Engineering Detachment, U.S. Army Corps of Engineers, assigned to the Manhattan Project at the University of Chicago.

RADIUM AT A GLANCE

Name: From the Latin radius, ray.

Atomic mass: (226).

History: Discovered in 1898 by Marie and Pierre Curie in pitchblende.

Occurrence: Occurs naturally in all uranium and thorium minerals.

Appearance: Brilliant white soft solid that blackens on exposure to air.

Behavior: Reacts with oxygen and decomposes in water. The surface of radium metal is covered with a thin layer of oxide that helps protect the metal. Radium is highly radiotoxic and exhibits luminescence, as do its salts.

Uses: Once used to treat cancer. Radium was used in the mid-1900s in a luminous paint to make the hands and numbers on watches glow in the dark.

SCANDIUM

GUNNAR RAADE, UNIVERSITY OF OSLO

Regarded by most people as a very exotic element, scandium is not all that rare. Its average abundance in Earth's crust is 22 ppm, which can be compared with 25 ppm for cobalt and 13 ppm for lead. Other familiar metals that we use, such as molybdenum, tin, tungsten, silver, and gold, are far below in crustal abundances. While these other metals tend to be concentrated in economically exploitable deposits, the problem with scandium is that it is dispersed in common rock-forming minerals. Accordingly, minerals with scandium as a main constituent are few and rare. This makes scandium an exciting element for the mineralogist and geochemist. Although today's total market is relatively small, an increasing demand for scandium calls for an increased effort to better understand the distribution of the element and to locate new deposits.

Only nine major scandium minerals are known so far (year of first description in parentheses): thortveitite (1911), bazzite (1915), kolbeckite (1926; not originally recognized as a scandium mineral), jervisite (1982), cascandite (1982), juonniite (1997), pretulite (1998), scandiobabingtonite (1998), and kristiansenite (2002). Kolbeckite, juonniite, and pretulite are phosphates; the rest are silicates. A few more new species are in the pipeline for formal description. Thortveitite, Sc₂Si₂O₇, was once the world's most expensive mineral and was produced in small amounts from granite pegmatites in south Norway. Radioactive calcium isotopes were produced from it for use in medicine.

ROCK SOLID Thortveitite, Sc2Si2O7, such as these crystals from Norway, was once the world's most expensive mineral.