Dewer flask

Suitable low-temperature freezer or Dewer flask for tissue storage. 

The furnace is then replaced by a Dewer flask containing liquid nitrogen, the level of which is kept constant throughout the analysis. 

When the addition is completed, the three outlets of flask B are closed, the vessel is detached from the assembly, while kept immersed in the cooling bath at - 78° C. The hydrogen iodide solution thus prepared is then transferred into nitrogen-filled small brown ampoules via dry and cold syringe under dry nitrogen, immediately sealed, and kept in a deep freezer or a Dewer filled with crashed dry ice. These ampoules must be sealed as quickly as possible while the solution is cool, otherwise evolving... 

Experimentally, particularly with the McBain spring balances, some difficulties may be experienced. The tube containing the sample must be well immersed in the liquid nitrogen. Thermal transpiration effects can operate when radiation from the equipment above the sample can radiate energy and raise the temperature of the sample. Similarly, as the containing tube has the characteristics of a Dewer vessel (thermos flask) sufficient time should be allowed for thermal equilibration. The carbon sample may never reach 77 K but is a litde higher as experimental limitations become controlling factors. 

---