From: Allen Niemi <anniemi**At_Symbol_Here**MTU.EDU>
Subject: Re: [DCHAS-L] Gas Leak Training Input
Date: Tue, 5 Jan 2016 10:01:02 -0500
Reply-To: DCHAS-L <DCHAS-L**At_Symbol_Here**MED.CORNELL.EDU>
Message-ID: CAN0bzO77d4A-opdCipTQd_U2=5Ve0V2dZVDyYgFZ=ba71cLS8A**At_Symbol_Here**
In-Reply-To <34163510-0A62-45CF-808B-7C1438EBE4CA**At_Symbol_Here**>

I think there is a general misunderstanding about gas phase miscibility among many. Substances which exist in the vapor phase under ambient (existing) conditions are infinitely miscible. They will eventually form a complete gaseous solution (vial convectional mixing and Brownian motion) and will not separate or "settle out" after mixing unless they react to form a compound that is no longer a gas under the existing temperature and pressure (precipitation) or unless the existing temperature or pressure are changed, for example, in fractional distillation. One or more gas species in a gaseous solution could also preferentially adsorb onto a solid or liquid surface but I do not think this is something most people consider when dealing with gas releases. I have read that there are conditions of temperature and pressure under which gases may become immiscible but it does not appear to me that these would exist outside of an atmospheric chamber.

If someone can point me to a text or other source of information to the contrary I would greatly appreciate it.


On Sat, Dec 26, 2015 at 8:42 AM, Ralph Stuart <rstuartcih**At_Symbol_Here**> wrote:
> It also makes the point I have trouble getting people to understand--that heavier than air does not mean put the ventilation exhaust at the floor! Once airborne and mixed well with air, vapors will disperse and will not resettle quickly.. It's why heavy chlorinated solvents like the Freons end up in the stratosphere.
This is an important point. I heard a recent discussion with a PhD chemist who intuitively assumed that acetone and chloroform would move differently in the lab atmosphere. After all, he said, acetone is lighter than air (it isn't - it's vapor density is around 2) and chloroform isn't. My guess is that this sort of ad hoc reasoning is common among chemists who haven't had the occasion to think through macro properties of the chemicals they work with.

- Ralph

Ralph Stuart, CIH

Allen Niemi, PhD
Occupational Safety and Health Services
Room 322 Lakeshore Center
Michigan Technological University
Phone: 906-487-2118
Fax: 906-487-3048

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