Chlorine dioxide

As a chemist, I feel I have to respond to Ellen McCrady's piece
about chlorine dioxide (Instance 12:79, April 6, 1999).

The first point is that chlorine dioxide is the same substance and
has the same properties, no matter how it is prepared.  The same
goes for hydrogen peroxide.  I guess that what is different in this
case is that only very small quantities of the reagent are prepared
at the point of use, and that there is a built-in mechanism for
neutralising any excess, so that there is no danger of any explosion
or harmful residues.

As Ellen says, we won't know what's in the sachets until they're on
the market, but I would guess that they contain a chlorate plus a
crystalline acid and maybe also a hygroscopic salt.  When the RH
rises, the salt deliquesces, the acid dissolves and ClO2 is formed.
When the RH falls, the salt dries out and the reaction stops.  As I
say, this is only a guess.

The second point is that there is no relation between the number of
different elements in a compound and its potential for causing
environmental damage.  It all depends on how the atoms are
connected, in other words, what compounds are formed.  The same
atoms can be combined one way to make a harmless compound, and
another way to make a dangerous one.

All this is not to say that "green chemistry" is not a valid
approach. It has to make sense to minimise the amounts of dangerous
substances used, and to ensure that any residues are decomposed
before they are released into the environment.

Dr Barry Knight
English Heritage
London

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                  Conservation DistList Instance 12:81
                  Distributed: Tuesday, April 20, 1999
                       Message Id: cdl-12-81-001
                                  ***