Matrix-dependent instability of selenium(IV) stored in teflon containers

Solutions of selenium(IV) standards with different acid matrices were stored in containers constructed of boosilicate glass, conventional polyethylene, and fluorinated ethylenepropylene (teflon FEP). After 50 days of storage in FEP, there were highly significant losses of Se(IV) from standards in either 5% HCl/5% H₂SO₄, or 5% H₂SO₄. Increasing the hydrochloric acid concentration, e.g., 15% HCl/5% H₂SO₄, greatly reduced this loss. Addition of selenium-75 (selenate-free) indicated that the losses did not result from physical adsorption onto container surfaces. It is shown that the losses were caused by oxidation of Se(IV) to Se(VI).