When using a column purification system, it is important not to draw down the column completely empty. Bubbling or splattering as the product is drawn from the column is an indication of breakthrough of argon. For the column to be functional again, a lengthy priming operation may be needed.
7.C.7 Ultrasonicators, Centrifuges, and Other Electrical Equipment
The use of high-intensity ultrasound in the chemical laboratory has grown substantially during the past decade. Human exposure to ultrasound with frequencies of between 16 and 100 kHz can be divided into three distinct categories: airborne conduction, direct contact through a liquid-coupling medium, and direct contact with a vibrating solid.
Ultrasound through airborne conduction does not appear to pose a significant health hazard to humans. However, exposure to the associated high volumes of audible sound can produce a variety of effects, including fatigue, headaches, nausea, and tinnitus. When ultrasonic equipment is operated in the laboratory, the apparatus must be enclosed in a 2-cm-thick wooden box or in a box lined with acoustically absorbing foam or tiles to substantially reduce acoustic emissions (most of which are inaudible).
Avoid direct contact of the body with liquids or solids subjected to high-intensity ultrasound that promotes chemical reactions. Under some chemical conditions, cavitation is created in liquids that induces high-energy chemistry in liquids and tissues. Cell death from membrane disruption can occur even at relatively low acoustic intensities. Exposure to ultrasonically vibrating solids, such as an acoustic horn, can lead to rapid frictional heating and potentially severe burns.
High-speed centrifuges and ultracentrifuges rely on rotors designed Specifically for the particular make and model. These rotors are subject to high mechanical stresses from the forces of the rotation speed. Rotors are rated for a maximum speed and a load of Specific weight. Improper loading and balancing can cause the rotors to dislodge while spinning. Failure of the rotors may present a number of hazards: violent movement of the unit itself may cause injury or damage to equipment, electrical lines, gas lines, etc.; flying shrapnel may cause personal injury or facility damage; and some units are susceptible to explosions due to the configuration and materials of construction. (See Vignette 7.3.)
The following precautions should be taken when operating and inspecting centrifuge rotors:
• Balance the load each time the centrifuge is used. The disconnect switch should automatically shut off the equipment when the top is opened.
• Do not overfill the centrifuge tubes. Ensure that they are hung properly.
• Ensure that the lid is closed before starting the centrifuge.
• Do not overload a rotor beyond the rotor’s maximum mass without reducing the rated rotor speed.
• Follow the manufacturer’s instructions for safe
Lab workers had left samples running unattended in an ultracentrifuge using a large aluminum rotor that previously had been used multiple times without incident. The rotor dislodged while spinning at 20,000 rpm. Friction generated heat and finely divided aluminum powder while at the same time, the refrigeration lines ruptured and released Freon. The mixture of aluminum powder, heat, and Freon confined in a large airtight area resulted in an explosion. The safety shielding within the unit did not contain all of the metal fragments. The flying shrapnel damaged a refrigerator and freezer and gouged holes in the walls and ceiling. The movement of the unit itself damaged cabinets and shelving that held more than 100 containers of chemicals. Fortunately, the cabinets had sliding doors that prevented the chemical containers from falling and breaking. The shock wave from the explosion shattered all four windows in the lab and caused structural damage to the walls. Fortunately, because the lab was unoccupied, no one was injured.
The cause of the incident was the use of a rotor that was not approved for the particular unit. There was a warning decal on the unit explaining which model rotors were acceptable. The unit was more than 25 years old and not designed to current safety standards, resulting in more physical damage than what would be expected. There was no use log or derating of the rotor, and the operator had not been fully trained. The manufacturer’s instruction guide for the unit described similar incidents.