Microwave Chemistry

Frequently asked questions

Contents

 If microwaves are a type of radiation, how dangerous are they?

 Why does the gold rim on a plate spark in a microwave oven?

 Why do pickles glow when you heat them in a microwave oven?

 How dangerous is microwave radiation?

 Mention the word radiation and, for most people, the alarm bells start ringing. True, microwaves are a type of radiation, but so is visible light, and the risk should be evaluated objectively. As regards the risks from chronic low-level microwave exposure, the jury is still out, I'm afraid, and seems set to be out for some time. Intense debate and research is ongoing amongst radiation safety experts and it is not yet known whether chronic exposure to low levels are harmful.
The recognised physiological effects result mainly from hyperthermia (particularly in acute, high level exposures), and whilst there is little consensus on the levels involved, it is generally accepted that the most sensitive organs are the eyes and the testes. Cataracts in the cornea resulting from microwave exposure have long been recognised as a result of poor heat transfer from this organ, although the risk from unmodified ovens is not thought to be significant.
It has been suggested that microwaves may be responsible for miscarriages, alopecia, and haematological changes. However, evidence for these claims is weak. Likewise, evidence of a linking microwaves to the formation of cancer is at present too indefinite for detailed conclusions to be drawn. This suggests that if any health risks do exist, they are so small as to be insignificant when set against the normal risks of everyday life.

 The current UK limits allow exposure of up to 10mWcm^-2 at a distance of 5cms from the source for up to 2 minutes in any one hour period, or short-term exposure of up to 25mWcm^-2. The exposure limits are controversial, and have been heavily revised as evidence of the effects of long-term exposure has accumulated. In view of the downward trend in exposure levels, care should be taken to work well within these limits, and to use a commercial microwave leakage detector when working with microwave equipment.

 For authoritative information on the dangers of non-ionising radiation, take a look at the National Radiological Protection board web site.

Why does the gold rim on a plate spark in a microwave oven?

 The easiest way to visualise what is going on is to think of microwaves for what they really are - alternating high voltage electric fields. Furthermore, because the microwaves are held inside a closed cavity, standing waves can develop and the field strengths found inside a microwave oven can easily be of the order of several tens of kVm^-1. This huge electric field polarises the metal rim of the plate in about 10^-18 seconds, giving ries to very large voltages between different parts of the rim. These voltages may discharge through the air as the air breaks down and ionises, in much the same way as we see in the familiar Van der Graaf generator.

 Two things favour the efficiency of the 'AC' microwave system over that of the 'DC' Van der Graaf generator though. Firstly, alternating voltages are much more efficient at initiating electrical breakdown of gases than a static voltage for the same RMS potential. Secondly, once the air has ionised, the microwave field continues to provide energy to the electrical discharge by accelerating the ionised particles. This releases yet more ions which expands the discharge in a positive feedback process. As this 'plasma' cloud is very hot, it tends to expand upwards and outwards from the original source, providing a very dramatic visual diplay.

 Although we are continually told that we should not put metals into microwaves, partly for this reason, it is possible to use the ability of metal powders to 'couple' with microwave fields to drive chemical reactions (see, for example, J. Chem Soc. Dalton Trans. p2073 (1995))

Why do pickles glow when you heat them in a microwave oven?

 In conventional chemistry, demonstrations of glowing pickles are familiar to those involved with electrochemistry and science education.  By placing a high voltage (typically 110 or 230 volts ac) across electrodes in either end of a pickled cucumber, it can be made to emit a yellowish glow, as the current passes through. The glow is only observed after the pickle has been significantly heated by the electrical current - typically a couple of minutes - which should give the reader some clue as to the mechanism. What is being observed asnot a luminescent glow, but a process of electrical arcing through pockets of vapour in the heated pickle.
In the microwave oven, a pickle is likewise subjected to an alternating electric field and currents are induced within it (as with metals ). Dielectric heating effects ensure that the pickle is rapidly heated and, in contrast to its electrically activated counterpart, the pickle begins glowing very soon after the demonstration is started. The microwave heating ensures that the pickle soon cooks, and if performed for extended periods creates a rather obnoxious stench.
The demonstration may be enhanced by pre-soaking the vegetable in a solution of group I or group II ions. This provides metal ions which alter the colour of the discharge. Hence, soaking in a concentrated strontium nitrate solution will result in a red glow. Likewise, barium nitrate solution results in a green glow, etc.Note Barium and Strontium salts are toxic.

 © Gavin Whittaker, 1997