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Science, Maths & Technology

Mike Bullivant's diary: Underwater torch

Updated Monday 28th January 2008

Mike Bullivant's Sun and Sea diary, from the BBC/OU series Rough Science 2

Day 1

Kathy and I are to work together on this programme. Our challenge is to come up with an 'underwater torch', a light source that will operate below sea level, so that we can light up an underwater reef. Kate tells us that we'll be allowed an electrical power supply (if Mike L can come up with some way of re-charging the 12V car battery - that's his challenge for this programme).

As a physicist, Kathy's quick to think of the experiments that Edison did in this field at the end of the 19th Century. She suggests that, like Edison, we try and make some sort of simple light bulb. We can surely make a filament from the selection of different gauge wires that we've been supplied with and we could use some of the well-stoppered, empty fruit juice bottles as (relatively) air-tight containers in which we can house the filament.

The big problem is: how do we create a vacuum or even a partial vacuum, so that the thin metal filament doesn't just burn up ('fuse') as soon as we pass an electric current through it and it gets hot? After a bit of thought, we agree that we don't have to create a partial vacuum after all: all we need to do is find some way of removing most or all of the (21%) oxygen in the air inside the bottle. If we ensure that the lid is sufficiently air- and water-tight for the minute or so when the 'light bulb' will be under water it should, in principle, work. Time to put our hypothesising to the test.

Kathy's first job is to test out some of the wires in our tool chest to find out which one works best with the power source we'll be using. When she's established which gauge works best, she'll also have to experiment to find out what length of that particular wire gives the brightest glow without 'fusing'. This seems straightforward enough. After a few failed attempts, Kathy and I agree that the easiest way to remove the oxygen would be to burn a candle inside the sealed bottle. I leave her to get on with it, while I think of another way of approaching this particular challenge.

I'm going to try a different tack altogether. As luck would have it, I've recently been reading a wonderful book by John Emsley, called The Shocking History of Phosphorus. In it the author writes of 17th Century 'chemists' trying to extract a white solid from human urine; a waxy substance that glows in the presence of air but which doesn't combust in it - they didn't know it at the time but they were attempting to isolate the element (white) phosphorus. This is one of those magical transformations that would be absolutely amazing if I could pull it off: just think of it - phosphorus (a chemical element, no less) that glows in the dark - from urine! Now that would be magic television.

It turns out that we humans excrete almost 2.5g of phosphorus a day. Problem is, from what I can remember, it's a difficult extraction, requiring a particularly high temperature in its final step. White phosphorus is also a very nasty chemical to handle. It can ignite spontaneously at room temperature. It's highly toxic and all contact with it should be avoided. But this is too good an opportunity to show what much of chemistry is about - the transformation of one thing into another and a quite startling transformation at that!

These days, phosphorus is extracted from bones, not urine. Bones are almost entirely composed of calcium phosphate and, as such, are a far richer source of phosphorus than urine. What's more, there are loads of old animal bones scattered around the lime factory. Besides, we don't have enough time to collect the litres of urine that I guess we'd need to produce enough phosphorus for a light source. Bones it is then.

Having collected up several kilos of dry animal bones, I need to grind them to a fine powder, so I leave them in the kiln for an hour or so, after which time, they're brittle and easily ground down. The next step is to treat this powdered calcium phosphate with some sulfuric acid from the car battery. This produces a precipitate of calcium sulfate (also known as gypsum: in a hydrated form, it's 'Plaster of Paris'). The other product of the reaction of calcium phosphate with sulfuric acid is orthophosphoric acid, which remains dissolved in the water. After filtering off the calcium sulfate, the clear filtrate is heated to boiling for a short while, which converts the orthophosphoric acid into metaphosphoric acid (a dehydration reaction). To extract the phosphorus from the metaphosphoric acid, we just need to heat the acid to drive off all the water, finely grind the solid that remains with some charcoal powder and heat the resulting solid mixture to as high a temperature as we can in the kiln.

 

Day 2

I know from what I've read in Emsley's book that this is a very difficult reaction to get to go successfully. It's one that takes place in the solid state, rather than in solution. As such, the reagents have to be very finely powdered and intimately mixed together so that they come into close contact with each other. In these Rough Science conditions, I doubt whether we'll actually be able to get a good enough powdered reaction mixture.

There are other concerns too. Although it's the way that phosphorus is still extracted these days, I believe that in industry electric arc furnaces are used in the final step, generating a temperature as high as 1500°C. Even when operating at its most efficient, our kiln is only capable of getting up to about 1100°C. I'd like to think the phosphorus extraction's going to work but on the quiet, I'm not all that confident. After all, even with the recipe, it took the famous 17th Century chemist Robert Boyle more than three years to get any phosphorus and he had a properly equipped laboratory. What chance do I stand? Still, if it comes off, it'll be quite spectacular, so it's worth trying. And after all, there's not a lot of help that I can give Kathy with her light bulb. She's doing fine by herself anyway. I'd only get in her way if we teamed up on the light bulb project. No, best to plough my own furrow and risk failure with the phosphorus...

Day 3

As anyone who's watched this programme will already know, I didn't in the end succeed in making any phosphorus. It's a pity in one sense but a relief in another. Had I actually isolated any I don't think I could have brought myself to put some in a bottle and lower it into the Caribbean Sea. Phosphorus is an extremely nasty chemical and even small quantities would be capable of polluting the local reefs and killing lots of the aquatic life on and around it. That wouldn't be right at all. Perhaps it's just as well that I'd failed.

Kathy on the other hand has succeeded marvellously. Bless her! She's justly proud of her 'underwater torch'. What a star!! Her light bulb is amazing and it works at an underwater depth of about 2 metres, without any water leaking into the bottle. What an achievement. Well done, Kathy!! At least one of us succeeded. I don't feel too bad though. I knew that making phosphorus was going to be difficult and that I'd be lucky to pull it off. Just goes to show that science isn't always easy. This time, I really was up against Nature and this time she won. I wonder what I'll fail to do on Programme 6, the last in the series...

 

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