As a casual experiment I was seeing whether I could make hydrogen from sodium hydroxide, aluminium and water. Rather than buy sodium hydroxide or go through a load of cleaning products to find it, I thought it would be much more fun to produce it through electrolysis. I set it up, and the electrolysis was going very well. After about 20 minutes I noticed a small build up of a hard white substance around the sellotape on the hydrogen electrode. Is it sodium hydroxide or is it more likely to be something to do with the sellotape (or indeed somthing else)? Thanks, Rowan.