How can we collect ammonia gas by water displacement?

1 Answer
Feb 14, 2016

I would not advize the use of ammonia gas (A NOXIOUS GAS AND QUITE WATER SOLUBLE!) in the laboratory; it is unsuitable for use in this context at any rate.


Other gases, i.e. dinitrogen, or dihydrogen CAN be collected by water displacement. And I outline a procedure for collecting such a gas.

In a deep sink or basin filled with water you can immerse a large graduated cylinder, that is ALREADY filled with water (it depends on how much gas you want to measure (100 mL, 1 L ??). You can of course raise this cylinder up and down WITHOUT losing the water. Atmospheric pressure keeps the water IN.

Meanwhile, you connect your (gas tight) reaction vessel with a hose in the vessel, and the exit of the hose in the inverted cylinder, and commence the reaction. As long as your reaction vessel is gas-tight, you can measure the gas by displacement of water in the cylinder. Are you with me?

Note that to do this safely, you need the help of 1 or 2 colleagues, so you have to train them.

At the end of the reaction, AND WHEN YOU ALIGN THE GAS VOLUME, the graduation, WITH THE TOP OF THE WATER IN THE SINK!) the pressure of gas in THE CYLINDER will be the same pressure of gas as in the room PLUS the saturated vapour pressure of water (this will be 20 mm Hg or so, it is tabulated extensively., in any case your teacher should provide it). Of course, you need to know the atmospheric pressure that day.

I reiterate that you cannot use ammonia in this measurement. It is typically done to collect (say) the hydrogen from a metal acid reaction. Oxygen, carbon dioxide, nitrogen, could be collected in this way (as a matter of fact, hydrogen would be a very good choice, because it is VERY water-insoluble, probably due to its size). As to ammonia, it is (i) a noxious gas of which you don't want to get a snort; and (ii) it is highly soluble in water.

If you are unclear on what I have mentioned, state your query, and I will give it another attempt. Note that this method usually gives good results. While you can only read to 1/2 mL on a graduated cylinder, because gases have such volume, you can get an excellent measurement of stoichiometry by these means.