These PYREX® standard volumetric flasks conform to Class A capacity tolerances prescribed by ASTM. However, to facilitate transfer of crystals, salts and powders, the open end has been expanded on the 25mL and 50mL sizes. The standard 18-410 G.P.I. thread screw caps have a durable cone-shaped polyethylene liner. This Poly-seal cap insures a leak-resistant closure and no contamination when working with most common reagents.