Howdy everyone,
        A couple of years ago there were several postings to this list
about KBr salt crystals and their use as an F2 getter. These
postings in the isogeochem archives came to my attention about a
month ago during a search for information on KBr, and I have been
a member of the list since then. I am researching an intriguing
possibility concerning the chemistry of Io, and it might turn out
that the exchange of ions in KBr crystals plays a critical role
in the investigation I describe below. I would like to ask
for help and advice from anyone who might be interested.
        The 1979 Voyager images of volcanos of Jupiter's satellite
Io put new pizzazz into the meaning of astrochemistry. SO2,
being a major component of terrestrial volcanos, was viewed as
a good candidate, and the very good fit between the features near
4  $B&L (Bm in lab spectra of solid SO2 on one hand and those in the
observed spectrum of Io on the other made it virtually certain that
SO2 is present on Io. Although there were suggestions back in 1979
that other molecules present in terrestrial volcanos might also
be present on Io, apparently none have been detected.
    The terrestrial volcano analogue might cause us to wonder if
hydrogen halides are among the ejected constituents of Io's volcanos.
While thinking about this possibility, I discovered that there is a
striking correspondence between the details of the 4- $B&L (Bm
features of matrix-isolated HBr with those of both solid SO2
and Io. An exact comparison is not possible with the information
available at present, because the HBr spectra were taken under
different conditions than those of SO2. Nevertheless, taking into consideration temperature, matrix species, concentration of
sample to matrix, and a few others, the HBr spectrum at 4  $B&L (Bm
cannot be distinguished from the nu1 + nu3 combination band
of SO2. One, of course, is prepared to encounter such magnificent
coincidences occasionally, but several factors cause me to
suspect that this is not a coincidence. I suspect that HBr
has contaminated the SO2 spectra and that the features near
4  $B&L (Bm seen in SO2 spectra belong not to SO2 but to HBr.
        The factors that have influenced me include several
independent lines of evidence, which I shall not go into, that
argue in favor of the halogens being at more than one
location in the solar system. The primary factor,
however, is that KBr salt crystals are almost universal in
spectroscopy, either as windows, as substrate, or as
reference material. KBr has been in contact with
SO2 gas as well as with traces of H2SO4 and H2O in all SO2
spectroscopic experiments. Under these circumstances, and given
the striking similarity of the HBr spectrum with the 2457 cm^-1
band of SO2, would it be prudent to ask whether the KBr could
have been a source of HBr and whether the HBr might have
been condensed along with SO2 onto the substrate during
spectroscopic experiments with SO2?
        In order to find out whether my imagination is overactive
concerning this contamination hypothesis, I would like to ask
members of this list if HBr could possibly be produced by the
action of sulfuric acid, HF, and / or HCl on KBr crystals. If it
could then be shown that one of more of these species might
have been present inside the experimental systems in
previous experiments with SO2, this would provide a rationale
for doing new experiments to confirm or rule out HBr
contamination.
        H2SO4 is sometimes a contaminant in bottled SO2, and in
1994, Schmitt et al reported that it was a certified contaminant
in their SO2 lecture bottle at 35 ppm. According to the solubility
values for KBr and K2SO4, if there were to be a small speck of
sulfuric acid in contact with a KBr crystal, such as might form
if there were to be a tiny leak at the edge of one of the KBr
windows, sulfate anions would precipitate with the K+ while at
the same time KBr would dissolve, and the speck of solution
would be quickly transformed from sulfuric acid into
hydrobromic acid. Under vacuum, which is how Schmitt et al
deposited their samples, HBr, whose vapor pressure is much higher
than H2O, might be expected to sublimate faster than H2O
from concentrated hydrobromic acid. Because the 4  $B&L (Bm band
of HBr is a strong fundamental, it would take a quantity measured
only in micromoles to produce detectable absorption at 4  $B&L (Bm.
      Sulfuric acid would not be the only thing that might
displace Br from the KBr crystal. Cl and F anions will
almost always displace Br. So any rogue HF or HCl reaching
the KBr window (or reaching an aqueous solution leaking in
at the edge of it) could possibly cause the release of HBr.
Schmitt's spectrum of the 2457 cm^-1 band of SO2 was
taken of a sample deposited on a MgF2 substrate for 13
hours at 1  $B&L (Bm/hour. How much F and Cl might be expected to
have come into the sample chamber during that time period from
the vacuum system, from the substrate, or that outgassed from
the cryostat housing, the deposition lines, etc?
        Do you know of any other mechanism by which HBr might be
produced by the action of SO2 on KBr? Is there anything else
about the experimental details I have mentioned that would cause
one to be on the lookout for contaminant HBr?
        This investigation must be viewed as a long-shot (or as
crazy),  because planetary specialists are very much convinced that
Io's surface material is 'really' SO2. However, there have been a very
few times during the course of scientific investigation that what was
thought to 'really' be true was not, in fact, true. One of the topics on
this list a few weeks ago concerned the dogma about Ag3PO4 darkening
if exposed to light, and a consensus developed that the dogma
was simply not true. In the present case, it is my feeling that the
proximity of a potential source of bromine atoms to experimental
samples during SO2 spectroscopy and the correspondence
between the 4- $B&L (Bm spectral features of HBr and SO2 leave
room for reasonable doubt about the interpretation of the 4  $B&L (Bm
absorption features seen in IR spectroscopy of SO2.
    If HBr is one of the main molecular species on Io, whether or
not SO2 is also present, we will have made a significant
advance in understanding the chemistry of the solar system.
    Please let me know your reactions and your comments.
Your expertise, especially regarding ion exchanges in KBr
crystals with the possible production of HBr, will be greatly
appreciated.

With best regards,
Martin

Reference:
Schmitt, Bernard et al, Icarus 111, 79-105, 1994. "Identification of three
absorption bands in the 2- $B&L (Bm spectrum of Io"

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Martin Peters
Kanagawa Dental College
Inaoka-cho 82
Yokosuka-shi, Kanagawa 238
JAPAN
Phone: 0468 25 1500  ext 260
alternate email: [log in to unmask]
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