The last rinse before placing glassware into a drying oven should be with distilled or deionized water. Regular tap water may leave remains (seen as spots) that will contaminate the glassware. If possible, rotate the glassware so that liquid can drain during the drying process. Although it is acceptable to use technical grade acetone or methanol for a final rinse when not using a drying oven, never use either of them in conjunction with a drying oven. Flames or even an explosion can result from such a combination.

Drying ovens are typically set at 120°-130°C, which is well below the recommended temperature of about 400°C needed to remove all adsorbed water. Contrary to misconception, it is completely safe to place any borosilicate volumetric ware in a drying oven. The temperature is way below what is needed to cause any distortion and resultant volume changes (see Sec. 2.3.5). Objects placed in drying ovens should remain three to four hours, so that the drying of the glass (the adsorbed water) can reach an equilibrium. Otherwise, if items look dry, they probably are dry.

Never place plastic items into a drying oven, especially items made of Teflon. Typically, there is enough heat leaking through the top of a drying oven for plastic items to be placed on the top for drying. To prevent the plastic items from scratches or contamination, place them into an Erlenmeyer flask or beaker. When assembling Teflon stopcocks where the barrel was just removed from a drying oven, wait until the item is completely at room temperature before final assembly. A Teflon stopcock that is snug when warm is likely to leak when cooled to room temperature.

6.1.9 Alternatives to Heat

Heat is typically used to bring a solution to its vapor point (boiling) so that it may be distilled, reduced, or purified. Although this procedure works in most cases, not all solutions can be safely heated. Some materials break down in heated conditions, and some do so violently. Fortunately, there are two alternative methods to

Fig. 6.4 A static vacuum can be used to remove solvents from samples.