Issues concerning the measurement of borate in occupational environments

Borates are susceptible to weight change due to uptake or loss of water and this hydration instability can lead to gravimetric and interpretation errors in occupational hygiene field sampling of dust. The hydration stability for inhalable borate dust particles (mean diameter 7-22 microns) was characterized over a range of ambient temperature and relative humidity conditions simulating field sampling. Borax 10 mol (Na2O.2B2O3.10H2O), a fully hydrated borate, has a relatively high vapor pressure to water that led to rapid dehydration with significant weight change. Low hydrate borates, Neobor borax 5 mol (Na2O.2B2O3.5H2O), anhydrous boric acid (B2O3), and anhydrous borax (Na2O.2B2O3) were found to hydrate rapidly with an increase in weight. In contrast, boric acid (B[OH]3) and borax 5 mol were found to be stable to dehydration under all conditions. Boron can be measured with high analytical accuracy, but because the specific borate species or borate compounds collected in a 37-mm dust sampler cannot be accurately identified, it is argued that occupational exposure values should be revised to reflect exposure to boron and exposure values for these borates should be the same based on equivalent boron content.