Safety during ultrasonication

James Bond in Casino Royale shoots a propane tank with a handgun. You didn’t miss that did you? If you did, do you at least remember what happens when a gas tank explodes after a crash in Terminator 2: Judgment Day? It’s okay if you don’t, because I’m going to tell  you what happened when the famous ‘rainbow experiment’ went wrong in a lab. Not once but twice.

Chemicals are as nasty as they are shown in films. Chemicals are also very noxious in a chemical laboratory as much as they are at any place else. That’s why wherever you are, whatever you do, chemicals should be handled with care. (Girls, even the acetone that you use to remove nail polish from your nails can catch fire as soon as it comes in touch with an ignition source, i.e. fire.)

Coming to the rainbow experiment. It’s really fascinating to look at. What happens is, when elements such as Na, Sr, K, Li and Cu are mixed with methanol and ignited, they all burn in the colors of a rainbow. You can see its video here.

What went wrong with the rainbow experiment?

A teacher’s chemistry experiment exploded during a demonstration at Beacon High School in Manhattan on Thursday, creating a fireball that burned two 10th graders, one severely, according to Fire Department and school officials. – Chemjobber

It is better to be safe than sorry. As you can see, horrible things have happened, not only to grown-ups but also to children. This doesn’t meant you should avoid doing things that involve risks. Instead, you can do it in a safe way.

As students who had to work in a laboratory, we were told by our professor (Prof. Bhujle) to learn the safety aspects of our respective projects. For those who do not know me or what I was up to during my Masters degree, here’s what I did:

Process intensification using alternative energy source i.e. ultrasound irradiation (sonochemistry), which leads to decrease in energy consumption and waste reduction. Also investigated a Lewis acid catalyzed homogeneous organic condensation reaction and an ultrasound-assisted Pd-catalyzed heterogeneous transfer hydrogenation reaction.

Safety aspects associated with the project:

Ultrasound usage can be categorized as:

  • Low frequency, high power ultrasound (20–100 kHz)
  • High frequency, medium power ultrasound (100 kHz–1 MHz)
  • High frequency, low power ultrasound (1–10 MHz)

The equipment I used to generate ultrasound i.e. ultrasonic bath, runs on a 33 kHz frequency. Hence, it can be taken as low frequency, high power ultrasound.

Contact Exposures:

Contact exposure is exposure for which there is no intervening air gap between the transducer and the tissue. This may be via direct and intimate contact between the transducer and the tissue or it may be mediated by a solid or liquid. Contact exposure can in some cases provide nearly 100% energy transfer to tissue. [1] 33 kHz frequency ultrasonic bath can cause observable effects.

Airborne ultrasound:

The most plausible mechanisms for non-auditory effects of airborne ultrasound on a human are heating and cavitation. [1] An exposure limit for the general public to airborne ultrasound sound pressure levels (SPL) of 70 dB (at 20 kHz), and 100 dB (at 25 kHz and above). [2] The major effects of airborne ultrasound of concern in practice are the result of reception by the ear. To summarize, exposure to ultrasonic radiation, when sufficiently intense, appears to result in a syndrome involving manifestations of nausea, headache, tinnitus, pain, dizziness, and fatigue. The type of symptom and the degree of severity appear to vary depending upon the actual spectrum of the ultrasonic radiation and the individual susceptibility of the exposed persons, particularly their hearing acuity at high frequencies. A concise summary of the physiological effects of ultrasound with specific stated exposure conditions has been given by Acton.

Measures to be taken for safety:

  • Contact exposure to high-power ultrasound must be avoided at all times. [1]
  • Only operators qualified to use the equipment or persons under strict supervision should be allowed within the boundaries of the controlled area while the equipment is operating. [1]
  • Personnel using high-power ultrasound, and safety inspectors in industry should be knowledgeable about the possible harmful effects of ultrasound and necessary protective measures. [1]
  • Warning signs should be placed at the entrance to any area which contains high power ultrasound equipment or applied to each high power ultrasound device. Accompanying each warning sign there should also be a statement indicating the precautionary measures to be taken while the ultrasound power is on. [1]
  • Safety procedures for the protection of personnel are similar to those used for audible noise. The protection for ultrasonic frequencies is expected to be at least 14 dB for ear muffs and rubber ear plugs, and 24 dB for foam ear plugs. [1]

References:
1. Guidelines for the Safe Use of Ultrasound Part II – Industrial & Commercial
Applications – Safety Code 24. Health Canada. ISBN 0-660-13741-0, (1991).
2. AGNIR (2010). Health Effects of Exposure to Ultrasound and Infrasound. Health
Protection Agency, UK, 167–170.

I’ll discuss transfer hydrogenation in subsequent blog posts.

Stay safe. ;)

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