This document specifies a) a method for determining the hydrolytic resistance of glass grains at 121 °C. The resistance is measured and expressed by the volume of acid required for titration of the alkali extracted from the unit mass of glass, and can also be expressed by the amount of sodium oxide equivalent to this volume of acid, and b) a classification of glass according to the hydrolytic resistance determined by the method of this document. This document is intended for use on the more resistant types of glass, e.g. borosilicate glass. NOTE 1 – For the less resistant glasses, e.g. soda-lime, the method specified in ISO 719 is more suited. NOTE 2 – It is emphasized that there is no exact correlation between the classification laid down in this document and that laid down in ISO 719, and it is, therefore, essential to identify which classification is being used.