By Dr. J. M. EDER.
Lately I called attention to the metabisulphite of potassium as an
addition to the pyro solution for development, and can give now some
of my experiences with this salt.
The metabisulphite of potassium, which was introduced into the market
by Dr. Schuchardt, and whose correct analysis is not known yet, is a
white crystal, which in a solid condition, as well as in an aqueous
solution, has a strong smell of sulphurous acid. An aqueous 2 per
cent. solution of this salt dissolves pyrogallic acid to a weak
yellowish color, being distinguished from the more light brown
solution of sulphite of soda and pyro. The solution kept very well for
four weeks in half-filled bottles, and showed a better preservation
than the usual solution of pyro and sulphite of soda. More than 2 per
cent. of the metabisulphite of potassium is without any advantage. If
this solution is mixed with soda, a picture will develop rapidly, but
the same will show a strongly yellow coloration in the gelatine film.
Sulphite of soda has to be added to the soda solution to obtain an
agreeable brownish or black tone in the negatives.
If the contents of metabisulphite and pyro-soda developer are
increased, it will act very slowly; larger quantities of the
metabisulphite of potassium, therefore, act like a strong retarder. In
small quantities there is no injurious retarding action, but it will
have the effect that the plates obtain very clear shadows in this
developer, and that the picture appears slower, and will strengthen
more slowly. The strongly retarding action of larger quantities of
metabisulphite might be accounted for in that the bisulphite will
give, with the carbonate of soda, monosulphite and soda bicarbonate,
which latter is not a strong enough alkali to develop the bromide of
silver strongly with pyro. An increase of soda compensates this
retarding action of the metabisulphite of potassium.
Good results were obtained by me with this salt after several tests,
by producing the following solutions: