How To Clean Red Phosphorus

Phosphorus! The word brings to mind war, fire, and Thomas Edison. In all of these cases, phosphorus is memorable for its dangerous properties. Specifically, the allotrope white phosphorus autoignites in air and burns, spraying toxic droplets of white phosphorus everywhere. Its less-dangerous brother, red phosphorus, can be found in nearly every home.

I wanted some phosphorus for my element collection, so I chose red phosphorus over the extremely deadly white phosphorus. Red phosphorus (with crushed glass) is on the dark red striker pads on boxes of matches, so I collected 14 of these and cut off the striker pads. I placed these into acetone to dissolve the glue binding the red phosphorus to the cardboard. This went rather quickly, and after some stirring to dislodge any remaining red phosphorus, I removed each soggy strip of cardboard and washed it with more acetone. This helped catch any remaining red phosphorus and reduce losses. There isn't much red phosphorus on each strip to begin with, so careful handling is critical in this experiment.

After I had collected all the red phosphorus in the acetone, I let the mixture settle and then decanted off most of the acetone. I let the rest evaporate outside. To purify the red phosphorus from the annoying fibers of cardboard in the matted red cake, I poured in some concentrated sulfuric acid and heated the mixture outside on a hotplate. Needless to say, this step is dangerous and personal protective equipment is absolutely necessary. The hot sulfuric acid will release fumes as it chars the paper bits into carbon powder, so be mindful of that if you do this experiment yourself. Additionally, the mixture may "burp" a little, so a watch glass over the beaker might be helpful keeping the acid in the beaker.

Once I was satisfied that all the paper had been completely destroyed, I let the mix cool and placed the beaker in cold water. Then, I carefully added cold water to the beaker's contents, drop by drop, until I filled it to the top. This must be done slowly, as the dissolution of sulfuric acid into water releases a lot of heat. I let the mixture settle and decanted off the top portion of water. Then, I diluted and decanted the mix six more times so that no sulfuric acid remained in the beaker. Finally, I let the remaining water evaporate and then scraped the purified red phosphorus onto a sheet of paper.

To finish the experiment, I made another glass ampoule (Experiment 28: DIY scientific ampoules) and mushed the rounded base of the test tube into a flat bottom so the ampoule could stand upright. I then carefully tapped the red phosphorus powder into the amoule and sealed it with a torch (I first cleaned all residue off the neck of the ampoule). The resulting red powder is a pretty color and makes a very interesting sample for my collection.