Treatise on Poisons by Sir Robert Christison
1. _Hydrosulphuric acid gas_ transmitted in a stream through a solution
871 words | Chapter 112
of corrosive sublimate causes a dark, brownish-black precipitate, the
bisulphuret of mercury. When the solution is not very diluted, the gas
forms a whitish or yellowish precipitate before the blackening
commences,—an effect which, according to Pfaff, distinguishes the salts
of the peroxide of mercury from all other metals that are thrown down
black from their solutions by sulphuretted-hydrogen.[835] The cause of
this is that the particles of sulphuret first formed acquire a thin
covering of corrosive sublimate by that property which chemists of late
have termed superficial attraction. Hydrosulphuric acid is a very
delicate test of the presence of mercury. It will detect corrosive
sublimate, where its proportion is only a 35000th of the solution.[836]
This test is not alone sufficient, unless reliance be placed on Pfaff’s
criterion, which is rather a trivial one; for hydrosulphuric acid
occasions a black precipitate in other metallic solutions, for example,
in solutions of lead, copper, bismuth and silver. In mixed organic
fluids its action is not liable to be prevented; but the precipitate
formed is often kept intimately suspended, as in the instance of milk.
It may be conveniently used in the form of hydrosulphate of ammonia.
This test produces a dark-brown precipitate, which is said to pass
slowly to a bright cinnabar red; but I have not been able to observe any
transformation of the kind.
_Hydriodate of Potass_ causes in solutions of corrosive sublimate a
beautiful pale scarlet precipitate, which rapidly deepens in tint. The
precipitate is the biniodide of mercury. This is a test of great
delicacy when skilfully used, as it acts where the salt forms only a
7000th of the solution (Devergie). Care must be taken, however, not to
add too much of the test, because the precipitate is soluble in an
excess of the hydriodate, or too little, because the precipitate is also
soluble in a considerable excess of corrosive sublimate.
The action of hydriodate of potass is not liable to any important
ambiguity: no other iodide resembles in colour the biniodide of mercury.
It is not a certain test, however, when other salts exist in solution
along with corrosive sublimate. Chloride of sodium, nitrate of potass,
and probably also other neutral salts possess the power of dissolving
the precipitate. Sulphuric and nitric acids, even considerably diluted,
oxidate and dissolve the mercury, and disengage iodine, which colours
the fluid reddish-brown. When corrosive sublimate is dissolved in
coloured vegetable infusions or animal fluids, the hydriodate of potass
cannot be relied on, the colour of the precipitate being altered, as in
infusion of galls, or the action of the test being suspended altogether,
as by milk.
_Protochloride of Tin_ causes first a white precipitate, which, when
more of the test is added, gives place to a grayish-black one. In very
diluted solutions the colour struck is grayish or grayish-black from the
beginning. In such solutions Devergie has found it useful to acidulate
with hydrochloric acid before adding the test. The chemical action here
is peculiar. The white powder thrown down at first is protochloride of
mercury; a part of the chlorine of the bichloride of mercury having been
abstracted by the protochloride of tin, which becomes in consequence the
bichloride. On more of the test being added these changes are repeated,
the chlorine is removed from the protochloride of mercury, and metallic
mercury falls down. This test is one of extreme delicacy, affecting
solutions which contain only an 80,000th of salt. It is prepared by
acting on tin powder or tinfoil with strong hydrochloric acid aided by a
gentle heat. The solution must be kept carefully excluded from the air;
otherwise bichloride of tin is formed, which does not act at all on the
solution of corrosive sublimate.
The protochloride of tin is not liable to any fallacy. Neither is it
liable to be suspended in its action by the co-existence of other saline
substances. It causes precipitates with almost all animal and most
vegetable fluids. But when corrosive sublimate is present, even in very
small proportion, the precipitate is always darker than when no
mercurial salt exists in solution, and frequently has its proper
grayish-black tint. This property, as will presently be seen, is the
foundation of a process for the detection of mercury in all states of
admixture with organic matters.
_Nitrate of Silver_ causes a heavy white precipitate, the chloride of
silver, which darkens under exposure to light. This is a test for the
chlorine of the corrosive sublimate, but not for the mercury, and is a
necessary addition to the three former tests in order to determine how
the mercury is kept in solution. It acts with very great delicacy.
It is of no use, however, when chlorine or hydrochloric acid is present
either free or combined with other bases. It is not of use, therefore,
in animal fluids and vegetable infusions, because very many of them,
besides organic principles which form white precipitates with this test,
contain a sensible proportion of hydrochlorate of soda.
Although the preceding liquid reagents when employed conjunctly are
amply sufficient for determining the presence of corrosive sublimate in
a fluid, many other tests hardly less characteristic and delicate have
been used by medical jurists. These will now be shortly mentioned.
Reading Tips
Use arrow keys to navigate
Press 'N' for next chapter
Press 'P' for previous chapter