Step 10: Hydrogen Production
You can do the math yourself and figure out how many grams of each reactant you need, but it's hard (or at least time-consuming) to estimate the volume of an envelope. I've found I get the best results with about 120g Al and 170 g NaOH -- the exact figures are 116g and 172 g respectively, but you won't notice the difference. When producing gas. you have three main concerns: generating the necessary volume, controlling reaction rate/ temperature, and preventing atmospheric contamination (letting air seep into your hydrogen gas). The less pure your gas is, the less its buoyant force. I always fill the reaction vessel up to about an inch below the spout, so that when the reaction begins there are only a few cubic inches of air left at the top, which you can let bleed out before filling. In my case, this means a lot of water-- several liters -- so the reaction starts slowly at first as the water absorbs the heat. It will get out of control if you let it. It might take a bit of of trial and error depending on the size of your container, but always err on the side of using more water, and always bring more ice than you think you need, and one or two large plastic basins. Maybe you cool it too much, and you have a slow, boring fill, but it's much better than having your basement floor plastered in steaming hot caustic slime releasing clouds of hydrogen gas all at once ( yes, I'm speaking from experience here). Personally, I like a long, cool "slow burn" reaction with lots of ice -- it takes hours, but you minimize the amount of water vapor that might enter your envelope and maximize the purity of your gas.
Anyway, with the amounts of reactants previously mentioned, you should get enough gas to fill up your envelope completely -- maybe you need a little less, or a little more. If it's your first time, halve the number of ingredients and use a lot of water to get a feel for it before you go for the full amount. If you want to store the excess gas, be aware that any sort of container you put it in (hospital pouches, in my case), if it's not made of mylar or material of a similar quality, will leak relatively quickly and will not maintain the purity of the gas. In general, it's best to make only as much as you need.
When setting up your reaction, you should consider putting in some sort of buffer between the reaction vessel and the envelope -- mine is the pouch with the two tubes in it. If you start the reaction with the water line close to the spout, even at low temperatures, the sheer volume of bubbles produced will cause water to bubble into the tube leading from the spout. This is normal, and will not do any serious harm as long as you sufficiently separate your balloon from the reaction.
Never leave a reaction unattended, and check the temperature regularly -- if you let it get too hot, your envelope could be ruined.
Set up your reaction like in the two pictures: add the aluminum first, then water. At this point, attach your rubber tubing to the spout, and when you're ready, add the lye. Wait for about ten seconds to see how many bubbles form -- if the water level rises too much, just pour a bit out into your basin. Once you've got the level right, wipe the inside of the neck of the vessel with a rag to remove moisture (otherwise your plug will slip out) and then plug it with a rubber stopper. Use duct tape if necessary.
Within a few moments, you should feel/see a flow of gas from your tube.