UNIVERSITY of DELAWARE
DEPARTMENT of CHEMISTRY and BIOCHEMISTRY

 

Important Safety Issues
 

Krytox Vacuum Grease Hazards

If you are using Krytox Fluorinated Vacuum Grease on any items PLEASE let me know, even if you feel you cleaned it well! Krytox greases require cleaning procedures involving special solvents to clean all residue "before glassware can safely be heated". This grease releases deadly fluorine compounds at high temperatures. I would prefer that you do NOT use any Krytox greases if not absolutely necessary for a particular application.

Safe Glassware /Apparatus Disposal

It is important to know the proper UD procedure for disposal of "clean" as well as "contaminated" glass. Below is a direct link to the instructions. Please read them and if you have any questions, let me know or contact UD Occupational Health and Safety at 831-8475.

http://www.udel.edu/OHS/glassdisp.html

 

Potential Vacuum System Hazard

Hopefully this will explain that frequent cleaning / greasing of joints, stopcocks and connectors does more than just prevent annoying leaks. It also helps insure that you and your vacuum system remain healthy…

Leaking manifolds allow air to enter a vacuum system. The liquid nitrogen trap can condense the oxygen into a liquid state. The danger presents itself when the N2 either runs dry or the dewar is removed. The liquid oxygen can quickly form back into gas with enough internal pressure to cause the vacuum manifold system to literally explode. Although it rarely occurs to that extent (if vacuum lines are maintained well) all lab workers should be aware of the hazard and course of action below.

 

If you remove a filled N2 dewar from your system and notice liquid inside the trap:

1. Assume it is oxygen and first “REPLACE DEWAR IMMEDIATELY”!

2. Turn off vacuum pumps

3. Place a shield in between you and the dewar

4. Release vacuum and open system to atmosphere through largest (and closest) ports available.

5. Leave shielded and untouched until all N2 and oxygen has returned to gas form


NOTE:
If it can be done easily (while still in filled N2 dewar with ports open fully) disconnect trap from the system and move all to shielded fume hood. Post a sign indicating the situation and let warm until no liquid remains.

 

Over-Pressurizing Hazard - Sealing off Samples

Small amounts of cryogenic liquids can evaporate quickly into very large volumes of gas. Closed containers of cryogens can explode violently when allowed to warm.

 

Example:
A 10mm glass tube has an “Maximum Allowable Internal Pressure of about 200 psi (See MAIP Table).

Even very small amounts of liquid oxygen, nitrogen, argon, etc can quickly produce in excess of 1000 psi when liquid warms to gas in a sealed vessel.

The same explosion hazard can also exist when sealing off frozen samples that are open to atmosphere during the freezing process. Oxygen can condense in the tube when immersed in the liquid nitrogen. This seemingly small amount of oxygen can cause the tube to explode during warming process. When in doubt, if possible, evacuate sample before freezing with liquid nitrogen (or use Low Temperature Bath). Always wear goggles and use appropriate shield at all times (even during thawing process).

Contact the Glass Shop for more information on your particular application.

The Low Temperature Baths below prevent freezing of oxygen:

System
Degrees C
Ethylene glycol / CO2
-15
o-Xylene / N2
-29
Acetonitrile / CO2
-42
Chloroform / CO2
-61
Ethanol / CO2
-72
Acetone / CO2
-77
Methanol / N2
-98


Two types of systems are shown in table above. One involves pouring liquid nitrogen (boiling point -196 C) into a solvent by stirring until slush is formed. The temperature may be maintained by periodically adding nitrogen to maintain the slush. The second system involves addition of small lumps of dry ice to the solvent until a slight excess of dry ice coated with frozen solvent remains. Again temperatures can be maintained by periodically adding more dry ice.


Ref: ASGS- Fusion Nov 2001

 

Hydrofluoric or Ammonium Bifluoride- which is safer??

Many believe that Ammonium Bifluoride Acid based products are harmless or inherently "safer" than the better known Hydrofluoric acid. This is a potentially dangerous misconception. Ammonium Bifluoride presents virtually the same risks as hydrofluoric acid and in fact forms varying concentrations of hydrofluoric acid when dissolved. Because many people treat ammonium bifluoride products as if they are safe, it sometimes makes it even more dangerous.

For the non-chemists, essentually an ammonium bifluoride solution is what you would get if you started with hydrofluoric acid and added enough ammonium hydroxide to neutralize 1/2 of the acid present.

Always treat either one with the same precautions and use safety garments/ goggles!

 

High Vacuum and Schlenk Line Precautions

Below are some possible hazards that everyone should be aware of.

1. Explosion - Explosions can occur in a number of ways, including:

A.. The use of pressurized gases. High vacuum manifolds are often connected to an inert or reactant gas supply line. One must ensure that the vacuum system is not closed when the gas supply is opened - there MUST be a source of pressure relief such as a bubbler. The pressure must be monitored with an electronic gauge, manometer or bubbler; make sure the valve to the pressure reading device is open to the manifold!

Always TRIPLE CHECK that the manifold and supply line are connected to pressure relief (and your pressure sensor) before opening the gas supply. ALWAYS use an appropriate pressure regulator to avoid opening the line to more than 1 atm of pressure at any time.

B. Condensed gases- Some gases, such as carbon monoxide, oxygen and ethylene, are easily condensed into a liquid nitrogen-cooled trap. If the coolant level drops or you remove the nitrogen dewar without providing a means of pressure relief, the liquid may convert rapidly back to vapor. For example, just 10 mL of liquid CO (b.p. -191.5 °C) corresponds to 6.5 liters of gas. In a vacuum line with an internal volume of 500 ml the internal pressure would be 13 atm, more than enough to shatter the manifold with explosive force!

C. Runaway reactions- Some reactions can occur violently and evolve large quantities of gas. Stay alert and watch for this situation and always provide for a quick source of pressure relief!

D. Heating a closed system- Never heat a vessel on a closed line without being open to a bubbler.

Explosions of glass vacuum lines have lead to death and serious injuries when used improperly. Always wear your safety glasses/goggles to protect your eyes. Consider placing your vacuum manifold in a fume hood or behind a sliding blast shield to further protect yourself when possible.

 

2. Implosion - An unseen star crack or stress in a glass manifold can cause a catastrophic failure of the line while under vacuum. Also accidently bumping the line can cause a failure. While not usually as serious as an explosion, implosions generally involve sharp pieces of flying glass. And if the materials you are using are flammable or pyrophoric - watch out!

 

NOTE: The section above discussed a few general hazards associated with high vacuum and Schlenk lines and is not meant to be a comprehensive reference to dealing with or assessing all possible hazards.

 

Is it Safe to Store Wine in Crystal Decanters?

No. Many crystal pieces contain over 20% lead (to make it sparkle) and can leach into stored liquids. All new crystal should be soaked for 24 hours in vinegar and thoroughly cleaned to remove surface lead residue from the manufacturing process. You can feel safe using cleaned crystal glasses and decanters "for serving" at dinner, because the lead levels will be negligible. However never leave wine in lead crystal longer than necessary, as the levels can reach dangerous concentrations in a relatively short period of time. Acidic liquids such as fruit juice, wine or vinegar increase the leaching effect.

Scientists have found lead concentrations over 20 parts per million (ppm) in wines kept for only a couple weeks in crystal containers. Under the Food and Drugs Act, the maximum allowable lead concentration in beverages is 200 parts per billion.

 

 

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