Sciencemadness Discussion Board

mixing gasses

dapper - 29-7-2008 at 17:38

Is the best way to mix two gasses in a given proportion to set the two regulators at say 1psi and 3psi respectively into a chamber for a 25/75% mix?

kclo4 - 29-7-2008 at 20:03

Nope I don't think so but I could be wrong.
the 1PSI wouldn't let off any gas after the chamber pressure reached to 3PSI. Why would it?
What you probably would have to do is figure out a way to control the amount of volume being let off of each valve.

one way that I might try is to bubble air into a long tall glass tube full of water, and take several pictures to figure out how much water the gas is displacing at any given time.

Once you figured that out you could then figure out how far to turn the valve for the amount of gas needed.

However, a problem I can see already is the water pressure is going to make the gas bubbles smaller, however, IFF the gases compress the same, it wouldn't matter I don't think. .. also if the gas is soluble might have saturate the solution with something.

heh.. of course I've never tried this!

not_important - 29-7-2008 at 22:10

Do you mean as a continuous flow, or simple to fill a vessel with a 1:3 mix?

For the 2nd case, taking inverted bottles filled with water (or some non-reactive liquid) and displacing enough water with the gases to give the 1:3 ratio work OK. You cam use a single vessel, adding the two gases in turn, or two separate vessels and then mix their contents.

For continuous flow use a pair of bubblers, not too deep and again filled with and non-reactive liquid that the gases are not too soluble in, and adjust the flow rates so one bubbles three times as fast as the other. For faster flow rates, flow meters make more sense. You can build your own, but there's some work in calibrating them.

kclo4 - 29-7-2008 at 22:42

What gases are you mixing?
Perhaps their may be an easier way if we know what you are doing?

dapper - 30-7-2008 at 04:08

I was planning to experiment with different mixes of CO2 and O2. - as far as rate of continuous flow without a flowmeter...I'd use oil for the co2 but for o2 it strikes me as a bad idea.

The_Davster - 30-7-2008 at 04:25

The best way:
You will need two reactors/vessels in series, with a valve between. And liquid nitrogen. You pressurize the first vessel with your pressure corresponding to the correct molar ratio, then open the valve to the second vessel and condense the gas by placing this vessel in LN2. After the pressure in the first vessel has dropped, you close the valve between the two vessels and fill the first with the desired pressure of the second gas and repeat.
The first vessel can be made even more fancy to purge/evacuate between fills. You then let the second vessel warm up and let the gasses react. The advantage here is you can do your reactions under pressure.

The fun and easy way:
not_important pretty much covered it. Thats how I did my gas phase hydrocarbon chlorination experiments on butane.

not_important - 30-7-2008 at 06:18

For O2 and CO2 water would be OK if saturated with the gases before the actual run, provided that the evaporated water would cause no problems. If this is a life sciences type experiment, you'll likely want to humidify the gases anyway, so the bubblers could be designed with large reservoirs to keep the water level in them constant. A smallish bottle for the bubbler and a several liter beverage bottle plus a couple of stoppers and a bit of tubing and peg board for support would do for each bubbler.

Note that breathing such mixtures can be interesting, but also a bit tricky. It's one thing for a grad student to suck in a lungful when sticking their head in the dry ice chest when Shulgin is standing next to them, or for someone to breath such a mixture under the care of an inhalation therapist; quite another to do so by yourself. Medical O2+CO2 mixtures generally use 4 to 5 percent CO2 in order to avoid hypoxic pulmonary hypertension, higher concentrations begin having side effects.

unionised - 30-7-2008 at 12:31

Empty the vessel with a vac pump. Fill it with the first gas to 1 PSI then with the second gas to 4PSI (yes, 4- the total is 4 so the added gas is 3PSI.)

kclo4 - 30-7-2008 at 14:58

If you add 100mls of alcohol to 100mls of water, you do not get 200mls of liquid, you get a little less, do gases act the same? I don't think they do because 22.4 liters of gas is the volume of 1 mole of gas at STP and It doesn't matter what sort of gas it is, right?
But I still think there is the possibility if someone were to mix 1 liter of He, with 1 liter of Nitrogen, that perhaps the helium could fill in the spaces between the nitrogen, like the water fills in the spaces of the alcohols and you could end up with less then 2 liters of gas.

If it needs to be really accurate, that may be a problem.

unionised - 31-7-2008 at 11:07

The volume of a gas is defined by the volume of the container it is in.

kclo4 - 31-7-2008 at 11:33

Yes, but what does that have to do with my question, or is it a response to another? :P

not_important - 31-7-2008 at 17:02

In a gas there's a lot of space, on the average, between molecules; the simple model is elastic collisions between sphere with much more space that spheres. So long as the gases don't interact much through reactive processes - think HCl & NH3 - they'll act similar to a single pure gas. As He and N2 fall into that category, there shouldn't any major difference in the volume taken up - a simple summation of the individual volumes. CO2 and O2 should behave similarly.

kclo4 - 31-7-2008 at 22:19

Oh right! I forgot how much space is between gas molecules, thanks for your reply!