ABSOLUTE ISOTOPIC ABUNDANCE AND THE ATOMIC WEIGHT OF A REFERENCE SAMPLE OF THALLIUM.

Abstract

The accepted atomic weight of thallium has remained at a value of 204. 37 plus or minus 0. 03 since 1962. At this level of uncertainty, however, the atomic weight becomes a limiting factor to high accuracy analysis. The new mass spectrometric determination of the atomic weight of thallium has been completed. A high precison assay technique was developed so that accurately known quantities of the **2**0**3Tl and **2**0**5Tl separated isotopes could be mixed to produce standards for calibration of the mass spectrometer. This assay technique involved the gravimetric determination of 99. 3 percent of the thallium as Tl//2CrO//4. The soluble thallium was then aliquoted and determined by isotope dilution mass spectrometry. Before making up the final solutions from which the assay and calibration samples would be withdrawn, the separated isotopes were purified by solvent extraction and electrodeposition. The **2**0**5Tl/**2**0**3Tl absolute isotopic abundance ratio of the reference sample is found to be 2. 38714 plus or minus 0. 00101, yielding an atomic weight of 204. 38333 plus 0. 00018.