Aluminum and Chromium as Gelatin Hardeners

The hardening action of aluminum and chromium upon gelatin is explained on the basis of the formation of chemical compounds between the metal ionr and gelatin protein molecules. The metal ions must possess a positive charge in ordes that they may combine with the negatively charged carboxyl groups. The factors which affect the degree of combination are discussed in detail for both aluminum and chromium, the most important being the pH values of the solutions and the presence of competing anions. — Aluminum fixing baths harden at pH values between 3.5 and 6.0, with the pH of maximum hardening dependent upon the complexing anions that are present. Data are presented that permit selection of the most suitable formulas that can be used under given operating conditions. — Ion migration experiments and other tests are detailed which indicate that chromium fixing baths lose their active hardening properties when the charge on the complex chromium molecules becomes zero or negative. Combination of positively charged chromium and gelatin carboxyl groups is relatively independent of chromium concentration. Complex chromium molecules of zero charge are lightly held by the gelaatin and cause hardening only during and following the drying of the gel. — Chromium fixing baths harden gelatin at pH values from 3 to 6, with the pH of maximum hardening depending upon the sulfite-to-chromium-molal ratio and the age of the solution. Bisulfite ions form complex ions with chromium and increase the rate of hydrolysis of the chromium ions, thereby tending to cause loss of the hardening properties of the solutions with age. By limiting the pH to 4.0 and using a molal ratio of sulfite to chromium less than 2:1, the active hardening properties of a chromium fixing both are at least partially retained. Other conditions are presented under which the hardening properties are also maintained. — Hypo and silver are retained in washed films by both aluminum and chromium if the complex metal molecules still possess a positive charge after combination with the gelatin occurs. The pH values of the solutions and the presence of complex-forming anions are factors which influence the formation of the desired monovalent complex metal ions.

Print ISSN

0097-5834

Published

1946-07

Content type

Original Research

DOI

10.5594/J12806