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. With flat bottom, threaded top and long neck. The standard phenolic 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.