If a longer period elapses between collection and examination the time
should be noted. If sterilized by the addition of chloroform,
formaldehyde, mercuric chloride, or some other germicide samples for
sanitary chemical examination may be allowed to stand for longer periods
than those indicated, but as this is a matter which will vary according
to circumstances, no definite procedure is recommended. If unsterilized
samples of sewage, sewage effluents, and highly polluted surface waters
are analyzed after greater intervals than those suggested caution must
be used in interpreting analyses of the organic content, which
frequently changes materially upon standing.
Determinations of dissolved gases, especially oxygen, hydrogen sulfide,
and carbon dioxide, should be made at the time of collection in order to
be reasonably accurate, in accordance with the directions given
hereafter in connection with each determination.