Moistening the paper
with hydrochloric acid provides in a convenient form for application
Berge's solution for the detection of phosphine (_vide_ Chapter
XIV.). The Keppeler test-papers turn white when the gas contains either
ammonia, phosphine, siliciuretted hydrogen, sulphuretted hydrogen or
organic sulphur compounds, but with carbon disulphide the change is slow.
Thus the paper serves as a test for all the impurities likely to occur in
acetylene. The sensitiveness of the test is such that gas containing
about 0.15 milligramme of sulphur, and the same amount of phosphorus, per
litre (= 0.0655 grain per cubic foot) imparts in five minutes a distinct
white mark to the moistened part of the paper, while gas containing 0.05
milligramme of sulphur per litre (= 0.022 grain per cubic foot) gives in
two minutes a dull white mark visible only by careful inspection. If,
therefore, a distinct white mark appears on moistened Keppeler paper when
it is exposed for five minutes to a jet of acetylene, the latter is
inadequately purified. If the gas has passed through a purifier, this
test indicates that the material is not efficient, and that the purifier
needs recharging.
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