Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: Zensation on May 18, 2014, 12:27:55 AM
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The sun has many times been a reliable source of UV for photochemical reactions, but how could it be quantitatively compared to a lamp designed specifically for UV?
I remember reading once on a UV curing forum that someone idiotically shined his UV light at his face for 5 minutes and it was horribly sun burned the next day. This never happens so fast in normal sunlight, which may be a (possibly not so reliable) indicator of how strong solar UV is.
Does anyone have any idea of how these would quantify on paper? Planning to do some photochemical reactions but I am still on the hunt for a good reasonably priced UV lamp. Do you think the sun would suffice? My biggest curiosity is what wattage lamp solar UV might compare to, on a reasonably sunny day.
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This will depend upon the reaction you are planning to do.
For normal photochemistry a Hg lamp in a quartz apparatus is the way to go, not sunlight.
You may find a reasonable apparatus on E-bay.
For lamp specifications you will need to find a manufacturer and see if they have any posted.
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You can buy lamps that are calibrated to emit an equivalent amount of solar UV radiation (at a specific wavelength, at a specific longitude/latitude) as the sun. A powerful UV lamp will give you more radiation flux than sun on a clear day (I wager), so the burn anecdote wouldn't surprise me, but even beside that - there is too much variability in solar output/reception (weather, solar cycles, diurnal cycles, seasonal cycles, longitude/latitude, etc.) to make it an experimentally reliable tool for controlled chemistry. I'm talking reproducibility here. For the hobbyist it may be ok, but not for anything that's intended to be submitted to a peer reviewed journal, where reproducibility is important. Even in studies that simulate solar photochemistry (weathering studies, say), light sources are carefully calibrated to ensure reproducibility of the experiment.
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discodermolide, can you find me a link to one on Ebay? The only ones I have been able to find are either fairly bulk, extremely pricy ($6000>), and even at the very low end range I found some quartz lamps for UV curing purposes (with the right uv spectrum of course), that cost a minimum $900-2000. Most of which was the cost of the ballast, oh, and those companies are all overseas so I would have to buy a transformer if I wish to run it in the US.
Not really sure why I can't find a 100w medium pressure mercury vapor bulb that will screw directly into an edison socket. There is an abundance of high pressure bulbs at the hardware store. I'm assuming the medium pressure bulbs just aren't manufactured as much due to less of a demand. If anything, they would be easier to manufacturer due to lesser pressure, whilst the design would remain the same. I looked at some germicidal (low pressure mercury) lamps online but they only really output in the 254nm spectrum and are really low wattage. Medium pressure lamps encompass the 254nm as well as some of the higher UVB and UVA.
Luckily the [SRN1] reactions I am interested in are largely radical reactions that only need to be [continually] initiated by light, or some other source. Some of these reactions have been reported finishing in 5 minutes - 1 hour, thanks to a radical chain reaction. So intense heat through quartz isn't fully a requirement since an equal molar amount of photons isn't required. More like 0.001% or less of photons can get the reaction going and send it to completion, depending on the substrates of course and how likely chain termination is to take place.
I plan to be experimenting with sunlight among other radical initiators. My main reason for asking this question was to find out if average solar UV on a sunny day is closer to a 100w uv lamp, or a 1w uv lamp. Exactitude can't always be measured as you have said because of atmospheric inconsistencies, but a good approximation may be able to be had.
If I had to take a wild guess... if that guy was sun burned in 5 minutes by a 250-400w uv lamp, then solar UV on a sunny day might be around 25-50w.
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You can buy lamps that are calibrated to emit an equivalent amount of solar UV radiation (at a specific wavelength, at a specific longitude/latitude) as the sun.
Furthermore, you can use a standard to measure photo-degradation of a particular substance. This is done all the time to determine the photo-liability of a pharmaceutical, as an example. The NIST standard for photo-degradation is quinine. It degrades, under the standardized UV lamp, at a known rate. You can compare, the loss of UV absorbance of a sample, at various time points for its exposure to the standard lamp, to the similar loss for a product. So you can certainly take a quinine sample, and use it to compare sunlight with various other light sources.