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Newsgroups: rec.pyrotechnics
From: (Randal Nelson)
Subject: Re: Sulfur Trioxide; Battery Acid
Date: Fri, 24 Apr 1998 13:44:34 GMT

In article <>,
Roger Fleming <> wrote:
>Seth <> wrote:
>>I thought SO3 was a white solid?
>SO3 is a white solid at room temperature, but it sublimes at only=20
>slightly elevated temps and has significant vapour pressure at RTP.

SO3 is actually an interesting compound

Sulfur trioxide exists in three forms: the gamma form, with a melting
point of 16.8 C, boiling point 44.8, vapor pressure of 433mm at 25 C;
a straight-chain polymerized form (beta) with a melting point of 32.5 C,
vapor pressure 344mm at 25 C; and a cross-linked (alpha)
form with a melting point of 62 C, vapor pressure 73mm at 25 C.
The gamma form tends to spontaneouly convert to the beta, and then to the
alpha form.
Melting the stable alpha form results in conversion back to the low-boiling
gamma form (the boiling point of the gamma form is below the melting
point of the alpha). This can produce a phenomenon known as the
"alpha explosion" if the gamma form is heated, causing sudden rupture of
closed or semi-closed vessels.
Can cause coughing, and severe discomfort at levels as low as 1ppm.

On Sulfuric acid vapor

Normal (non-asmatic) reactions to sulfuric acid vapor are as follows:
0.1-0.5 ppm is mildly annoying; 1.5-2.5 ppm is unpleasant; 10-20 ppm
is unbearable.

For mixed H2SO4 and SO3 vapors, one experiment determined the LC50 for
rats given 1 hr exposure to be 347ppm.

Sax has this to say about inhalation of vapors:
"Workers exposed to low concentrations of vapor tend to lose their
sensitivity to its irritating action. Inhalation of concentrated vapor
from hot acid can cause rapid loss of consciousness with serious damage to
lung tissue."

No chronic effects reported (sulfate is essentially non-toxic).

So it can be nasty stuff, but unless you put your head over the pan and
breathe deeply, you get quite a bit of warning before you are in real
trouble. (contrast the insidious effects of NO2).

  Randal Nelson			 716-275-8488	University of Rochester	Computer Science Department
..!{allegra,decvax,rutgers}!rochester!nelson	Rochester, New York,  14627

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