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.