The US Environmental Protection Agency (USEPA) maximum contaminant level for combined 226Ra and 228Ra in drinking water is 5 picocuries (37 mBq) per liter. Sensitive radioanalytical methods for both 226Ra and 228Ra are therefore required, and individual standards with well-known concentrations of each radium isotope are needed for method development, testing and quality control.
Because of its relatively short half life of 5.75 years, 228Ra standards must be periodically prepared. The first step towards the production of the future 228Ra standard, NIST SRM 4339B, was its chemical separation from ca. 2 MBq of its long-lived 232Th parent after 25 years of in-growth (i.e., from time of last separation). Co-precipitation of the 228Ra with two successive lead nitrate crystallizations from a concentrated nitric acid solution of the thorium parent achieved the primary Ra-Th separation. Subsequent separation of the 228Ra from bulk lead was carried out by precipitation of lead sulfide, PbS, from dilute nitric acid solution (228Ra remains in solution). Further purification of the 228Ra relied on removal of remaining thorium by thorium peroxide precipitation from dilute HNO3 and anion exchange sorption of thorium from 8M HNO3. A final elimination of dissolved organic matter from the radium solution using activated carbon yielded a 228Ra product with less than 10 mg of associated non-active mass. The solution of this purified 228Ra then proceeded to the next stage of its journey towards becoming SRM 4339B.