These Pyrex® microvolumetric flasks are calibrated to Class A tolerances. They are designed for microchemical work in accordance with the recommendations of the Committee of Microchemical Apparatus, Analytical Division of the A.C.S. To improve the stability of this small flask, a hexagonal base, similar to those on all cylinders, has been added. Uses a ST glass stopper. For replacement stoppers, see Cat.No. 1-7650. The graduation line is sharp and permanent. The white markings are easy to read. Reference: ASTM E-237.