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.

    next > Page 1 of 2

Content last updated: 21/07/2006