Of all the common laboratory test methods, the Kjeldahl method for determining nitrogen subjects the boiling flask to the most severe thermal and chemical abuse. These round bottom Kjeldahl Flasks, specially styled, have long tooled finished necks for extra strength and uniform stopper fit (stoppers not included ). Wall thickness of the borosilicate glass is controlled to offer the optimum balance between thermal shock and chemical resistance. Can be used in ASTM E-258 procedure (“Standard Test Method for Total Nitrogen in Organic Materials by Modified Kjeldahl Method”). Care must be taken to ensure the bottoms of the Kjeldahl flasks do not come in contact with the heating elements. Most Kjeldahl equipment manufacturers have ring supports for this purpose. Reference: ASTM E-1377 (Specs for laboratory glass Kjeldahl flasks)