These Class A Pyrex® mixing volumetric flasks have an upper bulb, located between the ST stopper and the capacity graduation to assist in the mixing of liquids or powders when shaken. This bulb is approximately one-tenth the flask rated capacity of the flask. The 2L size upper bulb is only one-twentieth capacity. The strength of these flasks has been increased appreciably through machine-blown bodies to which are sealed heavy-tubing necks tooled for ST glass stoppers. For replacement stoppers see Cat.No. 1-7650. These volumetric flasks have been manufactured to Class A tolerances as established by ASTM E- 694 for volumetric ware, ASTM E-542 for calibration of volumetric ware and ASTM E-288 for volumetric flasks.