Within a short period sulphurous acid has become an important element in
the preparation of an excellent pyro developer for gelatine plates; and as
it is more or less unstable in its keeping qualities, some easy method of
preparing a small quantity which shall have a uniform strength is
desirable. A method recently described in the _Photographic News_ will
afford the amateur photographer a ready way of preparing a small quantity
of the acid.
[Illustration]
In the illustration given above, A and B are two bottles, both of which can
be closed tightly with corks. A hole is made in the cork in the bottle, A,
a little smaller than the glass tube which connects A and B. It is filed
out with a rat-tail file until it is large enough to admit the tube very
tightly. The tube may be bent easily, by being heated over a common
fish-tail gas burner or over the top of the chimney of a kerosene lamp, so
as to form two right angles, one end extending close to the bottom of the
bottle B as shown.
Having fitted up the apparatus, about two ounces of hyposulphite of soda
are placed in the bottle A, while the bottle B is about three-fourths
filled with water--distilled or melted ice water is to be preferred; some
sulphuric acid--about two ounces--is now diluted with about twice its bulk
of water, by first putting the water into a dish and pouring in the acid in
a steady stream, stirring meanwhile. It is well to set the dish in a sink,
to avoid any damage which might occur through the breaking of the dish by
the heat produced; when cool, the solution is ready for use and may be kept
in a bottle.
The cork which serves to adapt the bent tube to the bottle A is now just
removed for an instant, the other end remaining in the water in bottle B,
and about two or three ounces of the dilute acid are poured in upon the
hyposulphite, after which the cork is immediately replaced.
Sulphurous acid is now evolved by the action of the acid on the hypo, and
as the gas is generated it is led as a series of bubbles through the water
in the bottle B as shown. The air space above the water in bottle B soon
becomes filled by displacement with sulphurous acid gas, which is a little
over twice as heavy as air; so in order to expedite the complete saturation
of the water, it is convenient to remove the bottle A with its tube from
bottle B, and after having closed the latter by its cork or stopper, to
agitate it thoroughly by turning the bottle upside down. As the sulphurous
acid gas accumulated in the air space over the water is absorbed by the
water, a partial vacuum is created, and when the stopper is eased an inrush
of air may be noted. When, after passing fresh gas through the liquid for
some minutes, no further inrush of air is noted on easing the stopper as
before described after agitating the bottle, it may be concluded that the
water is thoroughly saturated with sulphurous acid and is strong enough for
immediate use. More gas can be generated by adding more dilute sulphuric
acid to the hypo until the latter is decomposed; then it should be thrown
aside, and a fresh charge put in the bottle. On preparing the solution it
is well to set the bottles on the outside ledge of the window, or in some
other open situation where no inconvenience will result from the escape of
the excess of sulphurous gas as it bubbles through the water.
The solution of sulphurous acid, if preserved at all, ought to be kept in
small bottles, completely filled and perfectly closed; but as it is very
easy to saturate a considerable quantity of water with sulphurous acid gas
in a short time, there is but little inducement to use a solution which may
possibly have become weakened by keeping.
Care should be taken not to add too much dilute acid to the hypo at a time,
else excessive effervescence will occur, and the solution will froth over
the top of the bottle.
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